Investigation of indenylidene-derivatives of
Grubbs type precatalyst for 1-octene
metathesis
QL Steyl
orcid.org 0000-0003-1120-7632
Dissertation submitted in partial fulfilment of the requirements
for the degree
Masters of Science in Chemistry
at the North West
University
Supervisor:
Dr CGCE van Sittert
Co-supervisor: Prof HCM Vosloo
Table of Contents
List of Figures: ... iii
List of Schemes: ... vi
List of Tables: ... vi
List of Abbreviations: ... viii
Preface ... x
Summary ... xii
Keywords: ... xii
Chapter 1 Introduction and objectives ... 1
1.1. Problem statement and motivation ... 1
1.2. Aim ... 4
1.3. Objectives ... 4
1.4. Method of investigation ... 5
1.5. References ... 5
Chapter 2 Alkene metathesis ... 7
2.1 Introduction ... 7
2.1.1 What is alkene metathesis? ... 7
2.1.2 Discoveries in the field of the alkene metathesis reaction ... 9
2.1.3 Mechanism of metathesis ... 15
2.1.4 Factors influencing metathesis... 27
2.2 References ... 29
Chapter 3 : Experimental ... 33
3.1. Materials and methods ... 33
3.1.1. Analytical instruments ... 34
3.2. Synthesis ... 35
3.2.1. Propargyl alcohols ... 35
3.2.2. Synthesis of the Gr2Ph derivative of ruthenium-indenylidene ... 39
3.3. Metathesis reactions ... 42
3.3.1. General procedures ... 42
3.4 References ... 47
Chapter 4 Results and discussion ... 49
4.1 Synthesis of the Gr2Ph-Ind precatalyst ... 49
4.1.1 Metathesis ... 57
4.1.2 Reaction order for the consumption of 1-octene ... 63
4.1.3 Investigation of product formation ... 65
4.1.4 References ... 78
Chapter 5 Computational study ... 79
5.1. Introduction ... 79
5.2. Grubbs 2-type precatalysts and activation ... 79
5.3. Computational details ... 80
5.4. Results and discussion ... 81
5.5. References ... 88
Chapter 6 : Conclusions and recommendations... 89
6.1. Conclusions ... 89
6.1.1. Synthesis of propargyl alcohols ... 89
6.1.2. Synthesis of the Gr2Ph derivative of ruthenium-indenylidene ... 89
6.1.3. Metathesis ... 89 6.1.4. Computational ... 91 6.2. Recommendations ... 92 6.3. References ... 93 Appendix: ... 94 Metathesis products: ... 94 Acknowledgements ... 105
List of Figures:
Figure 1.1: Grubbs catalysts ... 1
Figure 1.2: Ruthenium carbene complexes (L = neutral ligand, X =anionic ligand) ... 2
Figure 1.3: Ru-indenylidene-derivatives of the Grubbs precatalysts (Ind) [Umicore-M2] ... 2
Figure 1.4: Gr2Ph precatalyst ... 3
Figure 1.5 : Grubbs 2 and Grubbs 2-type precatalysts for comptational study ... 4
Figure 2.1: The timeline of milestones for alkene metathesis ... 9
Figure 2.2: Schrock’s first stable metal alkylidene complex ... 10
Figure 2.3: Alkene metathesis catalysts in history ... 14
Figure 2.4: Carbene and carbenoid species ... 18
Figure 2.5 : Compounds modelled by Herrmann (28) for ligand dissociation energies ... 19
Figure 2.6: Energy profiles of ethene metathesis with 2nd generation benzylidene (Grubbs 2) and indenylidene (2nd generation) from literature results (53)... 24
Figure 2.7: Ethene metathesis mechanism studied with 2nd generation benzylidene (Grubbs 2) and indenylidene (2nd generation) (53) ... 24
Figure 2.8: The dissociative and interchange mechanism studied by Urbina-Blanco (38) ... 25
Figure 2.9: The concept of hemilability ... 26
Figure 3.1 : Setup for the synthesis of the alkyne ligand ... 36
Figure 3.2 : Setup for the synthesis of the pyridinyl alcohol ligand ... 38
Figure 3.3 : Setup for the synthesis of the lithium salt from the pyridinyl alcohol ligand ... 40
Figure 3.4 : Setup for 1-alkene metathesis ... 42
Figure 3.5 : A typical GC chromatogram of a 1-octene metathesis reaction showing the metathesis products identified with GC-MS ... 44
Figure 3.6 : Calibration curve for the GC to determine the RF factor ... 45
Figure 4.1: IR spectrum of 1,1-dipenyl-2-propyn-1-ol ... 49
Figure 4.2: The mass spectrum of 1,1-dipenyl-2-propyn-1-ol ... 50
Figure 4.3: 1H NMR spectrum of 1,1-diphenyl-2-propyn-1-ol ... 50
Figure 4.4: 13C NMR spectrum of 1,1-diphenyl-2-propyn-1-ol ... 51
Figure 4.5: IR spectrum of diphenyl-[2]-pyridyl methanol ... 52
Figure 4.8: 13C NMR spectrum of diphenyl-2-pyridyl methanol ... 53
Figure 4.9: IR spectrum of Gr2Ph-Ind precatalyst ... 54 Figure 4.10: The actual (top) and predicted (bottom) MS spectrum of synthesized GrPh2-Ind precatalyst ... 55 Figure 4.11 : The comparison of the IR spectra of ruthenium-indenylidene, GrPh2-Ind and diphenyl-2-pyridyl methanol for functional group and ligand verification ... 56 Figure 4.12 : The comparison of the IR spectra of GrPh2-Ind, Gr2Ph for functional group and ligand verification ... 57 Figure 4.13: The relationship of mole percentage of the products that formed and catalyst load at 60 °C ... 59 Figure 4.14: Decrease of the n% of 1-octene over time at different temperatures with a catalyst load of 1:9000... 59 Figure 4.15 : Increase of the n% of PMPs over time at different temperatures with a catalyst load of 1:9000 ... 60 Figure 4.16 : Increase of the n% of SMPs over time at different temperatures with a catalyst load of 1:9000 ... 61 Figure 4.17 : Increase of the n% of IPs over time at different temperatures with a catalyst load of 1:9000 ... 61 Figure 4.18 : Summary of results from metathesis experiments with ruthenium-indinylidene at different temperatures with a catalyst load of 1:9000 ... 62 Figure 4.19 : The first order Arrhenius plot for the decrease in 1-octene ... 64 Figure 4.20 : Stacked GC spectra of a 1-octene metathesis reaction with
ruthenium-indenylidene precatalyst at 25 °C ... 66 Figure 4.21 : Stacked GC spectra of a 1-octene metathesis reaction with
ruthenium-indenylidene precatalyst at 35 °C ... 67 Figure 4.22 : Stacked GC spectra of a 1-octene metathesis reaction with
ruthenium-indenylidene precatalyst at 45 °C ... 68 Figure 4.23 : Stacked GC spectra of a 1-octene metathesis reaction with
ruthenium-indenylidene precatalyst at 60 °C ... 68 Figure 4.24 : Stacked GC spectrums of a 1-octene metathesis reaction with
Figure 6.1 : Rutheniun-indenylidene precatalysts with different alkyne ligands. ... 92
Figure 1: Ratios of isomerisation products at 45 °C at different times during the reaction .... 94
Figure 2: Ratios of isomerisation products at 60 °C at different times during the reaction .... 94
Figure 3: Ratios of nonenes (SMPs) at 45 °C at different times during the reaction ... 95
Figure 4: Ratios of nonenes (SMPs) at 60 °C at different times during the reaction ... 95
Figure 5: Ratios of decenes (SMPs) at 45 °C at different times during the reaction... 96
Figure 6: Ratios of decenes (SMPs) at 60 °C at different times during the reaction... 96
Figure 7: Ratios of cis and trans undecene at 45 °C at different times during the reaction .... 97
Figure 8: Ratios of cis and trans undecene at 60 °C at different times during the reaction ... ... 97
Figure 9: Ratios of cis and trans dodecene at 45 °C at different times during the reaction ... ... 98
Figure 10: Ratios of cis and trans dodecene at 60 °C at different times during the reaction ... ... 98
Figure 11: Ratios of cis and trans tridecene at 45 °C at different times during the reaction ... ... 99
Figure 12: Ratios of cis and trans tridecene at 60 °C at different times during the reaction ... ... 99
Figure 13: Ratios of cis and trans tetradecene (PMPs) at 45 °C at different times during the reaction ... 100
Figure 14: Ratios of cis and trans tetradecene (PMPs) at 60 °C at different times during the reaction………100
List of Schemes:
Scheme 2.1: Alkene metathesis reaction ... 7
Scheme 2.2: Self-metathesis reactions of propene (productive and non-productive) ... 8
Scheme 2.3: Metathesis for cyclic alkenes and acyclic dienes (7) ... 8
Scheme 2.4 : Synthesis of ruthenium-indenylidene precatalyst with internal rearrangement from a to b (34) ... 12
Scheme 2.5: Associative alkene metathesis mechanism (29) ... 15
Scheme 3.1: Synthesis of 1,1-Diphenyl-2-propyn-1-ol ... 35
Scheme 3.2: Synthesis of diphenyl-[2]-pyridinyl methanol ... 37
Scheme 3.3: Synthesis of the lithium salt from diphenyl-[2]-pyridinyl methanol ... 39
Scheme 3.4: Synthesis of the hemilabile precatalyst (Gr2Ph-Ind) ... 41
Scheme 4.1: Proposed reaction pathways of formed products ... 75
Scheme 4.2: Extended reaction pathways for the 1-octene metathesis reaction ... 76
List of Tables:
Table 2.1: Properties of some transition metal precatalysts towards metathesis (36) (37) ... 15Table 3.1: List of chemicals ... 33
Table 4.1: Summary of metathesis reactions of 1-octene with ruthenium-indenylidene at different catalyst concentrations at 25 °C and 60 °C at the reaction time of 1260 min ... 58
Table 4.2: Calculated results for 1-octene metathesis reations with ruthenium-indenylidene at a time of 300 and 1260 min ... 63
Table 4.3: Distribution of observed substrate and metathesis products for the 45 °C metathesis reaction for ruthenium-indenylidene . ... 70
Table 4.4: Ratios of isomers formed for each of the different alkenes at 45 °C for ruthenium-indenylidene. ... 71
Table 4.5: Distribution of observed substrate and metathesis products for the 60 °C metathesis reaction for ruthenium-indenylidene. ... 72
Table 4.6: Ratios of isomers formed for each of the different alkenes at 60 °C for ruthenium-indenylidene. ... 73
Table 4.7: Distribution of substrate and metathesis products for the 80 °C metathesis reaction for ruthenium-indenylidene. ... 74
Table 5.1: Gibbs free energies (ΔG298K) in kcal/mol for the dissociation of the PCy3 ligand from
precatalyst. ... 82 Table 1: Energies of the geometrically optimised structures of Grubbs 2, Ru-Ind and Ru-Ind derivatives, as well as energy PCy3 ... 101
Table 2: LUMO and HOMO energies and visualization of LUMO for Grubbs 2, Ind and Ru-Ind derivatives before and after the dissociation of PCy3, as well as LUMO and HOMO
energies and visualization of HOMO for PCy3 ... 102
Table 3: Energies, LUMO-HOMO energies and visualization of LUMO for closed Gr2Ph, Gr2Ph-indenylidene and Gr2Ph-indenylidene derivatives ... 103 Table 4: Energies, LUMO-HOMO energies and visualization of LUMO for open Gr2Ph, Gr2Ph-indenylidene and Gr2Ph-Gr2Ph-indenylidene derivatives as well as energy, LUMO-HOMO energies and visualization of HOMO 1-octene ... 104
List of Abbreviations:
General abbreviations:ADMET Acyclic diene metathesis polymerization
CM Cross metathesis
13C-NMR Carbon-13 nuclear magnetic resonance spectroscopy
GC Gas Chromatography
GC/MS Gas Chromatography/Mass Spectrometry
1H-NMR Proton nuclear magnetic resonance spectroscopy
IP Isomerisation product
IR Infrared spectroscopy
TLC Thin layer chromatography
NMR Nuclear magnetic resonance
MS Mass spectrometer
M Transition metal atom
PMP Primary metathesis product
RCM Ring-closing metathesis
ROMP Ring-opening metathesis polymerisation
RF Response factor
RT Room temperature
S Selectivity
SMP Secondary metathesis product
SM Self-metathesis
TOF Turnover frequency
Chemical abbreviations: Cy Cyclohexyl EtOH Ethanol Gr2Ph Benzylidene-chloro(1,3-bis-(2,4,6-trimethylphenyl)-2- imidazolidinylidene)-[1-(2'-pyridinyl)-1,1-diphenyl-methanolato]ruthenium Gr2Ph-Ind Indenylidene-chloro(1,3-bis-(2,4,6-trimethylphenyl)-2- imidazolidinylidene)-[1-(2'-pyridinyl)-1,1-diphenyl-methanolato]ruthenium H2IMes 1,3-Bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene Ind Indenylidene NHC N-heterocyclic carbene PCy3 Tricyclohexylphosphine Ph Phenyl Ru-Ind 1,3-Bis(2,4,6-trimethylphenyl)-2-imidazolidinylidene]dichloro- (3-phenyl-1H-inden-1-ylidene)(tricyclohexylphosphine)ruthenium(II) SIMes 1,3-bis-(2,4,6-trimethylphenyl)imidazolinium THF Tetrahydrofuran
Preface
This dissertation is original, unpublished, independent work by the author, Q.L. Steyl under the supervision of Dr. C.G.C.E. van Sittert and Prof. H.C.M. Vosloo.
The study revolves around the investigation of the synthesis and metathesis activity of a second generation Grubbs type precatalyst. More specifically a ruthenium precatalyst with an indenylidene carbene ligand tested for metathesis activity with 1-octene and an investigation into the formation of various alkenes as products of these metathesis reactions. Furthermore, the formation of the products of metathesis is classified as primary, secondary and isomerisation alkene products. The product formation is compared and discussed at different temperatures with a catalyst concentration determined to be optimal for these reactions. The precatalyst is characterised with respect to isomeric distribution of each observed alkene. Also the possible mechanistic route of metathesis and kinetics for the use of the substrate is investigated.
As part of the dissertation, the activation of the precatalyst is investigated computationally, calculating the energy needed for the activation of the precatalyst to form a vacant site for the coordination of the alkene ligand. The activation energies for the ruthenium-indenylidene catalyst is compared to the second generation Grubbs catalyst as well as for modifications to the indenylidene carbene ligand.
The contents of this dissertation consists of a table of contents, a list of figures, a list of schemes, a list of tables and list of abbreviations. As well as a summary, six chapters and an appendix that forms the main body of this dissertation.
Chapter 1 provides the introduction to the dissertation with a problem statement and a
motivation for the study, which includes a short introduction to Grubbs type catalysts and alkene metathesis. The aims, objectives and the investigation methods are discussed in this chapter.
In Chapter 2 the literature study is given, introducing alkene metathesis. Furthermore the chapter includes a timeline of discovery in alkene metathesis with motivation for the use of ruthenium complexes with alkene metathesis. The mechanistic pathways possible for
The synthesis methods of ligands and precatalyst with graphic illustrations of the experimental setups, instruments and instrumental methodologies is the subject of
Chapter 3. The chapter includes the method of metathesis reactions and equations for
analysis of the obtained data.
Al the results obtained for synthesis reactions and the alkene metathesis reactions are given and discussed in Chapter 4. Metathesis results obtained are compared between concentration and temperature in respect to metathesis activity. As well as an in depth study and discussion into the formation of observable products supplemented by a theoretical discussion and short kinetic investigation.
The computer modelling study as whole forms Chapter 5. The contents of this chapter includes a brief discussion of what computational chemistry consists of, the activation of Grubbs 2 type precatalysts, the details of the software used as well as the hardware and ends with the results and a discussion.
The last chapter is Chapter 6. This is the conclusions of knowledge gathered from the literature and the experimental results. The chapter furthermore contains recommendations for further studies that may follow the work done in this dissertation.
Summary
The ideal metathesis precatalyst is a catalyst with high metathesis activity, high selectivity, enhanced thermal stability, great handling characteristics (high tolerance towards moisture and air) and tolerance to a wide range of functional groups. Ru based precatalysts satisfy most of these requirements. Further improvement of Ru based precatalysts could possibly be obtained by changing the ligands. It has been shown that changing the carbene ligand from a phenylidene to an indenylidene improved the activity, thermal stability and ultimately lifetime of the precatalyst. Another way of changing the ligands is to substitute one of the chlorine ligands and the PCy3 ligand with a hemilabile ligand. In a previous study a Grubbs 2-type precatalyst was developed, which contained a hemilabile pyridinyl alcoholate ligand with two phenyl substituents. This catalyst, referred to as the PUK-Grubbs 2 (Gr2Ph) precatalyst, showed an increase in stability, activity, selectivity and lifetime in comparison with Grubbs 2.
In this study the optimization of a ruthenium-indenylidene precatalyst was done and an in depth investigation into the formation of the metathesis products were conducted. The optimum reaction conditions were obtained, the mechanisms for formation of the observed metathesis products identified as well as possible carbene species that can exist during the reactions.
Variations of the ruthenium-indenylidene precatalyst were investigated via molecular modelling. This includes the addition of groups to the indenylidene carbene species as well as Gr2Ph variations of the ruthenium-indenylidene precatalyst and its derivatives. The change in the energy needed for the activation of the precatalyst was observed.
Keywords:
Ruthenium-indenylidene, Grubbs 2-type, Metathesis, Metathesis products, Activation mechanism, Computational study
Chapter 1 Introduction and objectives
1.1. Problem statement and motivation
Metathesis is a valuable method for the production of new alkenes and in the last 50 years many catalytic systems for alkene metathesis were developed.(1) The ideal metathesis precatalyst is a precatalyst with high metathesis activity, high selectivity, enhanced thermal stability, great handling characteristics (high tolerance towards moisture and air) and tolerance to a wide range of functional groups.(2) Ru-based precatalysts of the Grubbs-type (Figure 1.1) fit these requirements, especially due to their high tolerance towards functional groups, moisture and air.(3)(4)(5)
The 1st-generation Grubbs precatalyst (Grubbs 1) is highly selective during the metathesis of terminal alkenes, but has a limited lifetime at elevated temperatures. By replacing one of the PCy3 groups with an N-heterocyclic carbene (NHC) ligand (Grubbs 2) the activity, lifetime and tolerance to a wide range of functional groups were improved, but the catalyst still has a limited lifetime and low selectivity at elevated temperatures.
Figure 1.1: Grubbs catalysts
Other means of improving the properties of catalysts, is by modifying the carbene ligand. In the light of the possible changes to the ligands of the ruthenium catalysts, only a few have displayed effective metathesis activity. Such 16-electron Ru-carbene complexes includes vinylidene, allenylidene and indenylidene complexes (Figure 1.2).(6)
Ru L1
L2 Cl Cl
Grubbs 1: L1 and L2 = PCy3
Figure 1.2: Ruthenium carbene complexes (L = neutral ligand, X =anionic ligand)
Compared to the Grubbs precatalyst, ruthenium-indenylidene is found to be especially striking due to the uncomplicated synthesis of the precatalysts. The development of indenylidene-derivatives (Ind), of the Grubbs precatalysts (Figure 1.3) has improved the activity, thermal stability and ultimately lifetime of the prectalysts in the harsh reaction conditions of metathesis.(3)(7)(8) Some of the developed ruthenium-indenylidene complexes are even commercially available.
Ru Cl Cl
L1
L2
Grubbs 1: L1 and L2 = PCy3
Grubbs 2: L1 = NHC group and L2 = PCy3
Figure 1.3: Ru-indenylidene-derivatives of the Grubbs precatalysts (Ind) [Umicore-M2]
Another way of stabilizing these Grubbs-type precatalysts is to slow down or prevent the dissociation of the L2 ligand at room temperature. In an earlier study in our laboratory a Grubbs 2-type precatalyst was developed, which contained a hemilabile pyridinyl alcoholate chelate ligand with two phenyl substituents (Figure 1.4).(9)(10) This catalyst, referred to as Gr2Ph precatalyst, showed an increase in stability, activity, selectivity and lifetime in comparison to Grubbs 2. (10) Ru L X Cl C C Ph H Ru L X Cl C C C Ph Ph Ru L L Cl Cl Ph Vinylidene Allenylidene Indenylidene
L1 = NHC group and R1,R2 = Ph O N Ru Ph R1 R2 L1 Cl Figure 1.4: Gr2Ph precatalyst
Reaction conditions for ruthenium-indenylidene has hardly been investigated and optimised, especially for metathesis reactions involving terminal alkenes as most investigations involves ring closing metathesis (1) (5) (6). Neither the effect a hemilabile ligand such as the hemilabile pyridinyl alcoholate chelate ligand of Gr2Ph will have on the activity of the precatalyst.
In this study, ruthenium-indenylidene was tested for metathesis activity towards 1-octene and a detailed investigation was done on the product distribution during the formation of the metathesis products. The mechanisms for the formation of the different formed metathesis products were also theoretically investigated and compared to the observable experimental results. Also in this study, the synthesis of the Gr2Ph derivative of the Ru-indenylidene catalyst was attempted and tested for metathesis activity with 1-octene. Grubbs 2, Ru-indenylidene, Gr2Ph-type indenylidene and derivatives thereof (Figure 1.5) were studied computationally to determine the possible activity and stability of the precatalysts.
Ru SImes Cl Cl PCy3 Ru Cl Cl PCy3 SImes Ru SImes Cl O N Ph Ph Ru Cl O SImes N Ph Ph R Grubbs 2 Ruthenium-indenylidene Gr2Ph Gr2Ph-type indenylidene R = H, F, CH3, NO2
Figure 1.5 : Grubbs 2 and Grubbs 2-type precatalysts for comptational study
1.2. Aim
The aim of the study is the optimisation and improvement of the commercially available ruthenium-indenylidene precatalyst for 1-octene metathesis, as well as an in-depth investigation of the product formation.
1.3. Objectives
Experimental:
• Test the ruthenium-indenylidene precatalyst for metathesis activity of 1-octene. • Optimize metathesis reaction conditions.
• Synthesis of the Gr2Ph indenylidene derivative.
• Test the Gr2Ph indenylidene derivative for metathesis activity of 1-octene.
Computational study:
The first step in the dissociative mechanism of the metathesis of 1-octene in the presence of the ruthenium phenylindenylidene-derivative precatalysts will be investigated using Density Functional Theory.
1.4. Method of investigation
All experiments were done under inert conditions. Schlenk and vacuum techniques were used in the synthesis and purification of the compounds. All compounds were synthesised from commercially available chemicals. Synthesises compounds were analysed with FT-IR, MS and NMR whereas most of the metathesis results were obtained from GC and GC/MS. The computational study was done on Materials Studio 2016, using DMol3 calculations with the GGA/PW91 functional and DNP numeric basis set.
1.5. References
1. Krehl, S.; Geisler, D.; Hauke, K. O.; Staude, L.; Schmidt, B. Beilstein, J. Org. Chem. 2010, 6, 1188. 2. Dragutan, V.; Dragutan, I.; Verpoort, F. Platinum Met. Rev. 2005, 49, 33.
3. Boeda, F.; Clavier, H.; Nolan, S. Chem. Commun. 2008, 2726.
4. Grubbs, R. H. Handbook of Metathesis; Wiley: New York, 2003; Vol. 1. 5. Schrodi, Y.; Peterson, R. L. Aldrichim. Acta. 2007, 40, 45.
6. Clavier, H.; Cesar, A.; Urbina-Blanco; Nolan, S. Organometallics 2009, 28, 2848. 7. Katayama, H.; Ozawa, F. Coord. Chem. Rev. 2004, 248, 1073.
8. Opstal, T.; Verpoort, S. New. J. Chem. 2003, 27, 257.
9. Jordaan, M. Experimental and Theoretical investigation of New Grubbs-type Catalysts for the
Metathesis of Alkenes; PhD Thesis; North-West University: Potchefstroom, 2007.
10. Jordaan, M.; Van Helden, P.; Van Sittert, C. G. C. E.; Vosloo, H. C. M. J. Mol. Cat. A. 2006, 254, 145.
Chapter 2 Alkene metathesis
2.1 Introduction
To understand alkene metathesis we will have to know what alkene metathesis is, events leading up to the understanding of alkene metathesis, the mechanisms which function in alkene metathesis and the factors that influences the formation of alkene metathesis products.
2.1.1
What is alkene metathesis?
The disproportion of alkenes was introduced by Calderon (1) in 1967 as alkene metathesis, and can be explained as a reaction where alkenes are converted to form new products via simultaneously breaking and reforming C-C double bonds (2) (3) (Scheme 2.1).
R, R', R'', R''' = H, alkyl, Aryl Catalyst R R' R'' R''' + R' R''' R R'' +
Scheme 2.1: Alkene metathesis reaction
Alkene metathesis can be divided into different categories: acyclic cross metathesis (ACM), cross metathesis (CM), acyclic diene metathesis (ADMET), ring closing and ring opening metathesis (RCM and ROM) and ring opening metathesis polymerization (ROMP). (4) (5) (6)
With terminal alkenes (1-alkenes) there are two ways that the 1-alkenes can undergo metathesis. One is cross metathesis (where two different 1-alkenes react with each other) and the other is self-metathesis (where two of the same 1-alkenes react). Self-metathesis can be productive or non-productive (Scheme 2.2).
Scheme 2.2: Self-metathesis reactions of propene (productive and non-productive)
For cyclic alkenes and acyclic dienes there are two possible pathways (Scheme 2.3). Cyclic alkenes can either be opened to form dienes (ROM), or they can be opened and polymerized (ROMP). Acyclic dienes can either be closed to form cyclic alkenes (RCM), or they can be polymerized (ADMET).
Scheme 2.3: Metathesis for cyclic alkenes and acyclic dienes (7)
Metathesis reactions as described above occur in the presence of catalysts. Catalysts are needed in many processes of industry to lower the energy needed to get the reactions going. Only a small amount is needed in most cases to have control over the formation of products. CH3CH CH2 + CH3CH CH2 CH3CH CHCH3 + CH2 CH2 CH3CH CH2 + CH3CH CH2 + CH3CH CH2 CH3CH CH2 Productive: Non-productive:
2.1.2
Discoveries in the field of the alkene metathesis reaction
During a time period of 60 years, there have been some great discoveries in the field of alkene metathesis. Alkene metathesis catalysts, the mechanism of metathesis and the properties affecting catalytic performance are just some of these discoveries.In the rest of this section some of these milestones over the 60 year time period leading up to the use of ruthenium catalysts for alkene metathesis will be discussed.
Metathesis via catalysis was discovered around 1950 with an observation by Ziegler (8) in the industry where ethylene was polymerized. In 1957 Eleuterio described the formation of unsaturated polymers from norbornene, when it reacted in the presence of molybdenum oxide on alumina and lithium aluminium hydride. (9) In 1964 Maasböl and Fischer reported the formation of stable carbene complexes by treating hexacarbonyl tungsten with phenyl and phenyl lithium in ether. (10) They however soon realised that these complexes were not stable as the carbene ligand cleaved and formed aldehydes. (10) (11) Fisher carbenes, although shown to have metathesis activity, were not energetically favoured for alkene reactions and resulted in cyclopropanation (10). However, the research into these complexes did result in the identification of basic processes involving organometallics. (10)
Natta and co-workers showed in 1966 the polymerization of cycloheptene, cyclooctene and cyclododecene with the combination of tungsten hexachloride and triethylaluminum or diethylaluminum chloride. (12) The following year Calderon extended the findings with other cycloalkenes, and he called the reactions alkene metathesis. (13) The result of these findings led researchers to realize the potential there might be in alkene metathesis, but the mechanism responsible for alkene metathesis and the activity of catalysts were still a mystery. The application of alkene metathesis in the presence of catalysts was difficult due to sensitivity towards air and moisture, short lifetimes and side-reaction of catalysts. Thus the need to identify catalysts that could be manipulated, which would be stable was ever growing. In the 1970s Schrock synthesized the first stable metal alkylidene complex, namely Ta(CH2CMe3)3(=CHCMe3) (Figure 2.2). (14)
Figure 2.2: Schrock’s first stable metal alkylidene complex
Scrock also synthesized other Ta-alkylidene complexes, which included the first methylene (CH3)3CCH2
(CH3)3CCH2
Ta C C(CH3)3
H (CH3)3CCH2
metathesis. (15) In 1978 Tebbe developed the Tebbe reagent (Figure 2.3), which was the first well-defined alkylidene complex which was active for metathesis. (16)
In 1980 Schrock and his group synthesized a tantalum-alkylidene complex Ta(=CHC(CH3)3)Cl(PMe3)(O-C(CH3)3)2, (17) which was active for the metathesis of
cis-2-pentene. This complex was active because of the presence of alkoxide ligands. The hunt for more stable alkylidene and alkylidyne complexes led to the molybdenum-alkylidene and tungsten-alkylidene complexes, which have the general formula of M(=CHMe2Ph)(=N-Ar)(OR2), where R is bulky groups (Figure 2.3). (18) (19) (17) (20)
Although Natta had already used ruthenium chloride as a catalyst for the polymerization of cyclobutene by ROM, (21) it was only in the 1980s that Grubbs (22) and his co-workers found that ruthenium chloride polymerized alkenes even in water.
The result of Grubbs’s studies led to the development of catalysts that were well defined and tolerant to a wide range of functional groups that could be used with standard organic techniques. The work done by Grubbs and co-workers resulted in the development of their first well-defined ruthenium carbene complex (Figure 2.3) in 1992 that was active for the polymerization of norbornene and stable in protic solvents. (23)
In 1995 Grubbs reported a new well-defined catalyst Ru(=CHPh)Cl2(PR3)2 where R is either Ph or Cy (cyclohexyl). (24) (25) The complex with the formula Ru(=CHPh)Cl
2(PCy3)2 is commercially known as the 1st generation Grubbs catalyst (Figure 2.3). The 1st generation Grubbs precatalyst is stable towards moisture and air, but at increased temperatures the PCy3-ligand degrades and the precatalyst’s lifetime decrease. Because of this catalyst’s compatibility with a variety of functional groups and stability in air, it is preferred for metathesis by organic chemists. However, the lifetime of the catalyst was insufficient when scientists tried to get high yields with ring-closing reactions. (26) It was clear that catalysts with improved properties were needed, and that the way forward for the Grubbs type catalysts was to focus on the carbene ligand species of the catalysts. This swiftly led to the development of a ruthenium catalyst that is reported to be powerful and robust. (27)
In 1998 Herrmann synthesised ruthenium complexes (Figure 2.3) in which he substituted (28)
of the phosphine ligands with such a carbene ligand, and this increased the dissociation rate of the phosphine ligand, which led to higher metathesis activity. (29) The decomposition of the 1st generation Grubbs precatalyst was slowed with the substitution of one of the PCy3-ligands with a NHC ligand. (30) This new ruthenium catalyst is called the 2nd generation Grubbs catalyst and is the most used catalyst for cross-metathesis. (30) This new generation precatalyst (Grubbs 2, Figure 2.3) has improved stability at higher temperatures.
The success of the Grubbs catalyst led to inspiration for researchers to develop ruthenium catalysts that could be used for other applications. One of these researcher teams, Hoveyda and his group (31), developed a precatalyst in 1999 that is similar to the Grubbs catalyst by replacing a PCy3 ligand with an isopropoxystyrene ligand.
The Hoveyda-Grubbs precatalyst has similar efficiency to the Grubbs catalyst, but the substrate specificity is a little different with efficiency towards substrates such as fluorinated alkenes and acrylonitrile. (31) (32) (33)
In the same year that the Hoveyda group discovered the Hoveyda-Grubbs precatalyst, another precatalyst emerged, known as ruthenium-indenylidene (Figure 2.3). Ruthenium-indenylidene was originally identified as a diphenylallenylidene complex (Scheme 2.4, a), but after detailed studies the precatalyst was identified as a rearranged indenylidene ruthenium complex (27) (34) (Scheme 2.4, b).
Rearrangement RuCl2(PPh3)3 OH Ph Ph THF a Ru PPh3 PPh3 Cl Cl Ph b Ru PPh3 PPh3 C C C Ph Ph Cl Cl
Scheme 2.4 : Synthesis of ruthenium-indenylidene precatalyst with internal rearrangement
from a to b (34)
However, this rearrangement does not always occur. When the synthesis is performed with RuCl2(PPh3)3 and 1,1-diphenyl-propagyl alcohol in the presence of tricyclohexylphosphine (PCy3), no indenylidene is formed and only allenylidene is formed. (27)
The ruthenium-indenylidene class of ruthenium precatalyst displayed a wide application in metathesis, such as ring-closing metathesis with substituted linear dienes, enyne metathesis, acyclic diene metathesis and ring-opening metathesis polymerisation.
In 2007 a new second generation Grubbs type precatalyst with a hemilabile pyridinyl alcoholate chelate ligand, named the PUK-Grubbs precatalyst, was synthesised. This catalyst is also referred to as Gr2Ph. The Gr2Ph (Figure 2.3) precatalyst showed increased metathesis activity, as well as longer lifetime compared to the second generation Grubbs precatalyst.
Figure 2.3: Alkene metathesis catalysts in history
With all these advances in metathesis catalysts and developments for a magnitude of applications, we can find ourselves asking the question: Why ruthenium catalysts when it
tolerance for four transition metal precatalysts. Titanium has the lowest tolerance and activity towards alkenes, and it would not be wise to use it in alkene metathesis reactions as it will deactivate. Molybdenum precatalysts have the advantages of greater tolerance than titanium and tungsten, but the extreme sensitivity towards O2 and H2O is a definite disadvantage. The reactivity towards water and alcohols is high. (35) These disadvantages can lead to lower activity during reactions. Ruthenium has the highest activity and tolerance towards alkenes. Ruthenium’s reactivity towards alcohols and water is lower than that of the other transition metal systems. These properties of ruthenium mean that ruthenium catalyst systems will be able to give good results during alkene metathesis.
Table 2.1: Properties of some transition metal precatalysts towards metathesis (36) (37)
2.1.3
Mechanism of metathesis
The mechanism of alkene metathesis is either associative (Scheme 2.5) or dissociative (Scheme 2.6). A third possibility exists, namely the interchange mechanism (Scheme 2.7).
Ru Cl Cl L PCy3 Ph + ethene - ethene Ru L PCy3 Ph Cl Cl Cl Ru L Cl PCy3 Ph L= PCy3, H2IMes
The associative mechanism is where ethene, in the case shown in Scheme 2.5, coordinates to the catalyst complex. This coordination leads to the formation of the metal cyclobutane intermediate. In this mechanism the phosphine ligand does not dissociate from the complex and is carried through the whole metathesis mechanism.
Ru Cl Cl L PCy3 Ph + PCy3 - PCy3 Ru L Ph Cl Cl L= PCy3, H2IMes + ethene - ethene Ru L Ph Cl Cl Ru Cl Cl L Ph
Scheme 2.6 : Dissociative alkene metathesis mechanism (29)
However, in the dissociative mechanism the PCy3 phosphine ligand dissociates and a coordination site is created where ethane coordinates to the catalyst complex (Scheme2.6). This coordination leads to the formation of the metal cyclobutane intermediate.
Ru L R Cl Cl PCy3 + ethene - ethene Ru L PCy3 Cl R Cl Ru L R Cl Cl Ru L Cl Cl R - PCy3 + PCy3 L = PCy3, NHC
Scheme 2.7 : Interchange alkene metathesis mechanism (38)
The interchange mechanism is an intermediate pathway between fully associative and fully dissociative mechanisms. The ligand first coordinates to the complex via an intermediate as it would with the associative mechanism before the dissociation of the phosphine ligand to the formation of the cyclobutane intermediate.
There are studies stating the real possibility of metathesis initiation occurring via associative (39) and interchange mechanisms (38), and research that can’t distinguish between interchange and dissociative mechanisms (40). Nevertheless, most research have found metathesis reactions to occur via the dissociative mechanism (41).
In an effort to understand the dissociative mechanism of alkene metathesis, Chauvin (42) combined the findings of various researchers e.a. findings of Fischer on the synthesis of tungsten-carbene complexes (43), the polymerization of cyclopentene by Natta through ROM catalysed with a WCl6 and AlEt3 mixture (44), and the formation of ethylene and 2-butene catalysed with W(CO)6 on alumina from propene by Banks and Bailey. (26) From these findings Herrisson and Chauvin (42) suggested in 1971 that the alkene metathesis reaction is initiated via a metal carbene that reacts with an alkene to form a metal cyclobutane intermediate that breaks apart to form a new alkene and a new metal carbene (Scheme 2.8). The mechanism involves the coordination of an alkene to the metal centre, [2+2] cycloaddition between the metal carbene and alkene to form a metal cyclobutane, (3) (45) rupture of the metal cyclobutane to form a new carbene and alkene, and displacement of the coordinated alkene with a new alkene to restart the cycle. (3) (45) (46)
R R2 (M) R1 R2 R1 + (M) R (M) R R2 R1 (M) R R2 R1 + (M) R1 R R2
Scheme 2.8: Chauvin's proposed mechanism for metal cyclobutane formation (42)
This mechanism first received little support, but additional support for the Chauvin mechanism was gained when the well-defined carbene complexes of Ta, Mo, W, Re and Ru were discovered. (3) With spectroscopic techniques, the detection of the propagating metal carbene and the intermediate metal cyclobutane complexes in some of these systems could be detected. This provided the needed additional support to validate the Chauvin mechanism. (3)
Work done by Bernardi and co-workers (47) showed the possibility of two species responsible for the catalytic activity. The first is the (PH3)2Cl2Ru=CH2 species investigated by Grubbs, which is a metal carbene species, and the other is the (PH3)2ClRu-CH2Cl carbenoid complex (Figure 2.4). However the energy of the carbenoid species (19.26 kcal/mol) is higher than the carbene specie’s energy (18.45 kcal/mol). For this reason it could not be the active starting complex. Ru PH3 PH3 Cl Cl CH2 Ru PH3 PH3 Cl CH2Cl
Carbene specie Carbenoid specie
Figure 2.4: Carbene and carbenoid species
The nature of the metal cyclobutane ring as either a transition state or an intermediate was also investigated. (47) It was concluded that the metal carbene species (PH
3)Cl2Ru=CH2 is the active species and that this species is only involved during a dissociative mechanism.
In 1999 Herrmann investigated theoretically the ligand dissociation energies of NHC and phosphanes of highly active ruthenium catalysts (Figure 2.5). (28) He raised the question whether the dissociation mechanism would also apply to the 2nd generation complexes. He found that the dissociation energies for the ligands increased in the order PH3 < PMe3 < NHC. It was also observed that with the mixed NHC/phosphine (2nd generation) complexes the dissociation energies were very close to that of the 1st generation, meaning that the dissociation pathway could be taken. In a study in which the activities of several ruthenium catalysts were studied by Dias (29) , the most active catalysts were the catalysts with Cl and PCy3 ligands. It was postulated that the dissociation of the phosphine ligand was a key step in the catalytic mechanism. (29)
Ru PH3 PH3 CH2 Cl Cl Ru CH2 Cl Cl N N N N H H H H Ru PH3 CH2 Cl Cl N N H H R = H, Me 1 2 3
Dissociation energies for ligands: 1: 18.2 kcal/mol (PH3)
2: 45.0 kcal/mol (NHC)
3: 18.7 kcal/mol (PH3) ; 46.9 kcal/mol (NHC)
Figure 2.5 : Compounds modelled by Herrmann (28) for ligand dissociation energies
In 2001 Cavallo (48) studied the ligand binding energies for 1st generation Grubbs-type precatalysts and for 2nd generation Grubbs-type precataysts. A few main conclusions were made namely: (1) the binding energies of the coordinated phosphine to ruthenium for different precatalysts were comparable with experimental activation energy ΔE and free energy ΔG of phosphine exchange (ΔG = 21.3 kcal/mol for 1st generation and 25.2 kcal/mol for 2nd generation); (2) the binding energies for the coordination of ethene to the different ruthenium catalysts displayed the same trend as for phosphines; (3) the solvent effect reduced the absolute binding energies for the coordination of phosphines in solution resulting in higher initiation rates; (4) the systems with NHC ligands had lower metathesis insertion barriers compared to 1st generation Grubbs catalysts, which was in agreement with higher activity; and (5) the bulkiness of the NHC ligands exerted strong steric pressure on the alkylidene component because of the presence of the Mes groups. This means that phosphine dissociation is not promoted and thus slow precatalyst activation occurs. As a result of the slow activation alkene coordination is promoted, the metathesis reaction barrier is lowered and the metallacycle intermediate is stabilised. Adlhart (46) also investigated the role of PCy3 and H2IMes ligands of the Grubbs-type complexes. He also concluded that for the catalyst containing the NHC ligand, the metallacyclobutane 14-electron intermediate led to a greater rate enhancement in the formation of metallacylobutane than the catalyst with the PCy3. This is because the NHC ligand is a weak
π-acid and a strong σ-donor. The PCy3 ligand destabilises the metallacyclobutane, because of the stronger π-acid. It
in which the alkene coordinates trans during the dissociative mechanism (Scheme 2.8) and that this also applied to the second generation catalysts. They also concluded that the rate-limiting step for 1st generation catalyst was the formation of the metal cyclobutane intermediate and for the 2nd generation catalyst it was the dissociation of the phosphine ligand.
Jordaan (49) studied the mechanism of alkene metathesis with 1-octene as substrate, and she found that electron withdrawing phosphine ligands destabilised the metal cyclobutane intermediate. She also concluded that the formed heptylidene species was kinetically and thermodynamically favoured, and that the heptylidene species was the catalytically active species. She stated that trans-tetradecene is the thermodynamically favoured primary product that forms. Through these kinetic and mechanistic studies of the Grubbs 1st and 2nd generation catalysts, the mechanism was shown to be dissociative. (50) (51)
Scheme 2.8: Pathway for the 1st and 2nd activation of the catalyst for alkene metathesis
From the work of Jordaan(47) it was shown that there were various activation pathways.
Scheme 2.8 represents two possible activation pathways with the metal cyclobutane
intermediate. From step a to b the precatalyst loses one phoshine ligand, creating a vacant active site. From step b there are two different directions to go. The first direction (c1) is where an alkene binds cis to the phenylidene ligand and the tail of the alkene is bent downwards. The second direction (c2) is where the alkene is also cis but the tail of the alkene is pointed upwards. For both pathways the metal cyclobutane intermediate is formed in steps d1 and d2, where after the intermediate is rearranged to form new carbenes and simultaneously the dissociation of a new alkene. The newly formed
methylidene ruthenium carbene species is now the new carbene (f) with the vacant active site.
A third possible activation of the precatalyst is represented by Scheme 2.9. In steps a to b an active vacant site is created when a phosphine ligand dissociates. An alkene binds trans to the catalyst with the tail of the alkene pointing downwards in step c1. From c1 to e1, the metal cyclobutane intermediate is formed and dissosiates forming a new alkene and an active heptylidene ruthenium carbene catalyst (f1).
Scheme 2.9: Pathway for the 3rd and4th activation of the catalyst for alkene metathesis
The fourth possible activation mechanism shown in Scheme 2.9 is almost the same as for activation 3. The alkene also binds trans to the catalyst, but the tail of the alkene points upwards (c2). The metal cyclobutane forms and dissociates to again form a new alkene
All of the four possible activation mechanisms result in metal carbene complexes with active vacant sites. These vacant sites are available for a substrate to coordinate and undergo a reaction. During the metathesis reaction the alkene gets converted to primary metathesis, isomerisation and secondary metathesis products.
Scheme 2.10: The catalytic cycle for 1-octene metathesis
As seen from Scheme 2.10 there are three possible pathways in the catalytic cycle. In the first pathway 1-octene binds to the vacant site of the active catalyst complex and is converted to ethene and the active heptylidene ruthenium carbene species. The second pathway forms trans tetradecene and the third pathway cis tetradecene. In both the second and third pathway the active methylidene ruthenium carbene species is also formed. Although many theoretical studies (52) were done on ruthenium catalysts, these studies
were mostly on the 1st and 2nd generation Grubbs catalysts. More recently other systems have been investigated, such as the ruthenium-indenylidene complexes. In a specific study ruthenium-alkylidene groups were compared and it was concluded that there is very similar behaviour between benzylidene and indenylidene (Figure 2.6, Figure 2.7) (53). Despite the differences between the natures of the two groups, both groups favour the dissociation of PCy3. For both benzylidene and indenylidene reversion back to the starting catalyst complex (known as backward opening of metallacycle) is favoured, over moving forward with the activation process. This could mean that activation is slowed down.
Figure 2.6: Energy profiles of ethene metathesis with 2nd generation benzylidene (Grubbs 2)
and indenylidene (2nd generation) from literature results (53)
Figure 2.7: Ethene metathesis mechanism studied with 2nd generation benzylidene
Up to now the mechanisms for all ruthenium catalysts was assumed to be the same, namely dissociative. A very recent study by Urbina-Blanco focused on the mechanism of activation for a few ruthenium-indenylidene complexes. (38) Because it was assumed that all ruthenium catalysts followed the dissociative pathway, he evaluated this assumption for a few ruthenium-indenylidene precatalyst and compared it to benzylidene catalysts. The study was divided into an experimental study, dissociation of phosphine via magnetization transfer experiments, and a DFT study. Experimentally he found that the Grubbs 2-type indenylidene precatalyst had a negative value for the entropy of activation (ΔS‡). He also observed that the phosphine exchange rate was dependent on the concentration of the phosphine and not independent as Grubbs reported for 2nd generation benzylidene complexes. (51) (50) He concluded with a hypothesis that another initiation mechanism existed for the Grubbs 2-type indenylidene precatalyst. With the DFT study, dissociative and interchange mechanisms were focused on. (Figure 2.8). The conclusion is that, for Grubbs 2nd generation precatalyst and 1st generation indenylidene precatalysts, a dissociative initiation is favoured. For the 2nd generation indenylidene precatalysts, however, an interchange initiation mechanism is favoured, thus supporting Urbina-Blanco’s experimental observations.
However, for catalysts with a hemilabile (chelating) ligand such as the PUK-Grubbs 2-type precatalyst (Figure 2.3), the activation and catalytic cycle mechanisms will change. The reason for this is that due to the hemilability in ligands (Figure 2.9), where one atom is tightly bound and the other is softly bound, a vacant coordination site could be made available at the metal centre on demand.
M R A Z Z M R + S - S A M R Z S A Z = Tightly bound atom
A = Labile atom S = Substrate Figure 2.9: The concept of hemilability
After the dissociation of the labile atom, either re-coordination of the dissociated atom can take place or another ligand can bind to the open coordination site. In terms of organometallic catalysts and the selectivity of the catalysts, hemilability leads to the balance between high activity and stability of the precatalysts. (51) (54) (55) (56) Experimentally the increase in stability was observed by an increase in lifetime and activity of the Grubbs 1st and 2nd generation complexes.
Because the hemilabile ligands seemed to increase stability, research groups (57) (31) (58) (59) started to investigate these ligands on catalysts. As discussed previously, the mechanism of initiation for catalysts with hemilabile ligands is also assumed to be the same with the dissociation of the softly bound atom first and then the coordination of the alkene. However, a theoretical study by Raymakers (60) concludes that for the PUK-Grubbs 2-type catalysts an associative mechanism has a high possibility.
Jordaan (59) investigated several hemilabile ligands and found higher stability, activity, lifetime and selectivity of Grubbs 2-type catalysts due to aromatic R groups coordinated to the hemilabile pyridinyl alcoholate ligand.
2.1.4
Factors influencing metathesis
There is a range of factors that can influence the product composition in the metathesis reactions. Some of these factors are: Temperature, concentration of precatalyst, pressure due to the formation of ethylene, the reaction time and the substrates used.
Temperature will influence the rate of the reaction, for example if the reaction is carried out at a low temperature the precatalyst can take longer to activate and the frequency with which the catalyst converts the substrate into product will be lower. At higher temperature the precatalyst will activate much faster and the frequency of substrate conversion into products will be high. It is also important to note that the rate at which products are formed will also determine which products are formed. For example, at a lower conversion rate, the number of side reactions that can take place could be lower, decreasing the chance of unwanted products. At high conversion rate, meaning higher temperature, the chance of side reactions occurring is higher and consequently a higher chance of forming unwanted products. This affects the selectivity of the catalyst. If small amounts of unwanted products form, the catalyst has a high selectivity towards forming the products that are wanted. Conversely, the selectivity is low when many of the unwanted products are formed.
Concentration can influence the metathesis reaction in more than one way. If there is a low concentration of the catalyst the reaction can be slow because the catalyst has to perform more catalytic cycles. This could also mean that the catalyst will not be able to convert all of the substrate into products. A too high concentration could mean that side reactions can take place faster.
During the metathesis reaction in a closed system, ethylene is formed as part of the primary product. Due to the gaseous nature of ethylene the pressure increases in the closed system. The negative side of the formation of ethylene is that it is known to form more secondary and isomerisation metathesis products, and could deactivate the catalyst. The ethylene reacts with the metal carbenes, which leads to the formation of hydride species. (61) (62)
Metathesis reactions use unsaturated compounds as substrates, for example alkenes, alkynes and cyclic alkenes. For different substrates, different products will form. For example, if 1-octene is used as substrate (C=C7) the only primary products that will form during productive metathesis are ethylene (C=C) and tetradecene (C7=C7).
However, isomerisation, secondary metathesis and dimerization could also take place as shown in Scheme2.11. The formation of the primary metathesis products (PMPs) and the different secondary metathesis products (SMPs) is described as self-metathesis and cross-metathesis. Dimerization can also take place with two 1-octene molecules reacting with each other, while isomerisation entails migration of the double bond.
Primary metathesis: self-metathesis : Isomerisation : Secondary metathesis : self-metathesis : cross-metathesis : Dimerisation : C C7 C2 C6 C3 C5 C4 C4 2 C3 C5 C3 C3 + C5 C5 2 C C7 C C + C7 C7 2 C2 C6 C2 C2 + C6 C6 C C7 + C2 C6 C C2 + C C6 + C2 C7 + C6 C7 C C7 + C4 C4 C C4 + C4 C7 C C7 + C3 C5 C C3 + C C5 + C3 C7 + C5 C7 C2 C6 + C3 C5 C2 C3 + C2 C5 + C3 C6 + C5 C6 C2 C6 + C4 C4 C2 C4 + C4 C6 C3 C5 + C4 C4 C3 C4 + C4 C5 2 C C7 C16
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Chapter 3 : Experimental
3.1. Materials and methods
All solvents were purified by distillation. THF and 1,4-dioxane were dried with sodium and benzophenone as indicator, toluene and pentane were dried with CaH2. The table below is a list of chemicals used in the experiments to follow, indicating where they were sourced and the purity of the chemicals.
Table 3.1: List of chemicals
Chemical Supplier Purity
Benzophenone Sigma Aldrich 99.0%
n-Butyllithium solution (2.5 M in hexanes) Sigma Aldrich -
2-Bromopyridine Sigma Aldrich 99%
Diethyl ether Sigma Aldrich 99%
Ethynylmagnesium bromide solution (0.5 M in
THF) Sigma Aldrich -
Nonane Sigma Aldrich 99%
1-Octene Sigma Aldrich 98%
Pentane Sigma Aldrich 99%
Toluene Sigma Aldrich 99%
Tetrahydrofurane (THF) Sigma Aldrich 99.5%
Tert-Butyl Hydrogenperoxide solution (5.5 M in
decane) Sigma Aldrich -
Umicore M2 catalyst Umicore Metal purity 99.95%, 11%
Ru
All glassware was pre-dried in an oven. Schlenk and vacuum techniques were used in the synthesis and purification of the compounds. The progress of reactions and the formation of compounds were monitored by GC and GC-MS. The compounds were analysed by Bruker Advance III ultra shield 600 MHz NMR, Bruker Alpha-p ATR-IR, HP 6890N GC, GC-MS and MS. The spectra of analysed compounds are listed in the Appendix.
3.1.1. Analytical instruments
Infrared spectroscopy (IR):
Infrared spectra were obtained with a Bruker Alpha-p ATR-IR spectrometer. The samples were placed directly on the single reflection diamond ATR module without any prior sample preparation.
Nuclear Magnetic Resonance (NMR):
A Bruker Advance III ultra shield 600 MHz NMR spectrometer was used for NMR spectra (1H and 13C). Samples were prepared by dissolving 20 mg of the samples in 1.5 mL of deuterated chloroform (CDCl3) under an inert atmosphere and filtered into a NMR tube.
Gas Chromatography (GC):
The progress of the metathesis reactions was followed on a HP 6890N gas chromatograph. The GC was equipped with a HP-5 5% phenyl methyl siloxane capillary column (30 m x 320 µm x 1.00 µm) and with a flame ionisation detector (FID). Samples were prepared by drawing 0.2 mL of metathesis reaction mixture and injecting the sample into a GC vial containing 0.2 mL toluene and 2 drops tert-buthyl hydrogen peroxide.
Gas Chromatography/Mass Spectrometry (GC/MS):
For the identification of the products of metathesis the GC/MS was used. The instrument is a HP 6890N GC with a ZB-1 100% methyl siloxane capillary (30 m x 320 µm x 1.00 µ) column. The mass detector was an Agilent 5973. Samples were prepared by drawing 0.2 mL of metathesis reaction mixture and injecting the sample into a GC vial containing 0.2 mL toluene and 2 drops tert-buthyl hydrogen peroxide. The samples were injected with an Agilent Technologies 7683B Series auto injector.
Mass Spectrometry (MS):
Mass spectrometry of the precatalysts was done with a Bruker micrOTOF-Q II 10390 spectrometer. The technique used was Electron spray ionization; the sample was dissolved
Melting point instrument:
To determine the melting points a Buchi B-540 apparatus was used. The sample to be tested was placed in the sample tube and inserted in the instrument.
3.2. Synthesis
3.2.1. Propargyl alcohols
To succeed in the synthesis of the alcohols note has to be taken of a few prerequisites. For the synthesis of the alkyne alcohol ligand the ethynylmagnesium bromide needs to be added dropwise to the reaction mixture to prevent the temperature from rising too fast. The THF needs to be dry because the Grignard reagent dissociates in water. Synthesis of 1,1-Diphenyl-2-propyn-1-ol:
Scheme 3.1: Synthesis of 1,1-Diphenyl-2-propyn-1-ol
The synthesis method of 1,1-diphenyl-2-propyn-1-ol was adapted from a method reported in literature. (1) For example, in literature the reported synthesis was done at room temperature for 16 hours. However, these results could not be repeated after several attempts. With some experimentation the ligand could be synthesized at a temperature of 50 °C and in a time of 4 hours.
