Gold(I) Catalysis
Taschinski, Svenja
DOI:
10.33612/diss.126022756
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2020
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Taschinski, S. (2020). Gold(I) Catalysis: Mechanistic Insights, Reactivity of Intermediates and its
Applications. University of Groningen. https://doi.org/10.33612/diss.126022756
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Chapter 3
Reactivity of Vinyl Gold(I)
Complexes
Abstract: In this chapter, a systematic study of the reaction kinetics for the protodeauration
of vinyl gold(I) complexes with respect to the substituent at the benzofuran core and the ligand was conducted. For this purpose, a series of air-stable vinyl gold(I) complexes was isolated and characterized.
The reactivity of the protodeauration of one complex towards various phenols was explored and the kinetics were monitored by 1H NMR spectroscopy. The results are correlated in a Hammett plot demonstrating the strong dependence of the acidity of the proton source on the protodeauration step.
Moreover, the reactivity of the synthesized complexes was examined towards p-bromophenol using the same method and the results are combined in a Hammett plot, respectively. The correlation demonstrates the major influence of the electronic nature of the benzofuran unit manifested in a negative slope that indicates the building up of positive charge during the reaction at the benzofuran unit. In this particular case, the ligand of the vinyl gold(I) complexes has a negligible influence on the reactivity of the system, which demonstrates that the (final) protodeauration step plays a minor role when choosing a suitable ligand for catalysis. Additionally, the Gibbs energy of activation as well as the Eyring parameters for the reaction were determined.
56
3.1)
Introduction
Over the last decades, gold-catalyzed transformations involving substrates featuring multiple bonds developed into a well-studied research field and several mechanistic studies were conducted to understand the influence of catalyst, ligand, counter-anion, solvent, substrate and substitution pattern, respectively.[1] The characterization of the intermediates in gold catalysis had been challenging due to their high reactivity and only a small number of well-characterized, air-stable vinyl gold(I) complexes have been reported to date (see
Scheme 28).[1a, 2]
Scheme 28: Selected examples for isolated vinyl gold(I) complexes.[2a, 2f-j]
Reactions involving vinyl gold(I) complexes as intermediates start with the activation of the multiple bond of 15 by -coordination of the Lewis acidic gold catalyst 14 followed by an inter- / intramolecular nucleophilic attack to a vinyl gold(I/III) intermediate 16. Subsequently, the catalyst is deaurated by an internal or external electrophile to give product 17 and regenerate catalyst 14 (see Scheme 29).[3]
Scheme 29: Prototypical catalytic cycle of gold-catalyzed reactions involving vinyl gold(I/III)
57
Introduction
Catalytic approaches are reported in which the intermediates are trapped with various groups, such as carbon,[4] silicon,[5] trifluoromethyl,[6] halogens,[2j, 7] tin,[8] or sulfonyl[9] and stoichiometric reactions involving gold complexes as intermediates, such as gold-alkynyl,[10] -vinyl,[7c] -allenyl,[2f] -aryl[7c, 11] and -alkyl[12] complexes, respectively.
One of the key steps of the catalytic transformations involving gold is the electrophilic deauration of substrates which is often reported as the final step in the catalytic cycle. A well-studied electrophilic deauration in catalysis is the protodeauration of vinyl gold(I/III) intermediates. Theoretical[2c, 13] and experimental studies[1a, 1c, 2c, 14] describe the reactivity of isolated[14] or in situ[15] generated complexes depending on their electronic nature towards protodeauration. For example, a systematic study with respect to the electronic nature of the ligand was reported by Hammond and co-workers demonstrating its crucial role in gold catalysis (see Scheme 30).[1a, 1c]
Scheme 30: Reactivity of gold allenoates towards protodeauration. Bottom left: Effects by variation
of the ligand. Bottom right: Hammett plot for different substituted Ph3P-ligands (pictures reprinted with permission from Z. Lu, G. B. Hammond, B. Xu, Acc. Chem. Res. 2019, 52, 1275-1288. Copyright © 2019, American Chemical Society).[1a]
As shown in Scheme 30 and reported earlier, the protodeauration is strongly dependent on the nature of the ligand. The gold center is highly reactive with electron-donating groups or electron-rich ligands, such as Ph3P present and the reactivity drops with electron-withdrawing groups or electron-poor ligands.[1a, 1c, 2c, 13b, 13c] Kinetic studies related to the electronic nature of the substrate and their corresponding vinyl gold(I) complexes have received less attention. Building on the earlier studies by other groups,[1a, 1c, 14] here we develop a deeper understanding for the reactivity of the protodeauration mechanism, a systematic series of substituted vinyl gold(I) complexes was isolated according to a previously published procedure[2d] and their reactivity towards protodeauration was explored. To the best of our knowledge, this is the first reported systematic study on the kinetics of vinyl gold(I) complexes with respect to the electronic nature of the substrate and the variation of the ligand, respectively.
58
3.2)
Results and Discussion
According to a previously published procedure of our group,[2d] a series of complexes were prepared with various substituents at the benzofuran core (see Scheme 31). The gold(I) precursor was activated in situ by treatment with silver tosylate to generate a cationic species with weakly coordinating anion. The pre-activated complex was treated with various alkynes 25R that undergo 5-endo-dig cyclization to intermediate O and deprotonation in the
presence of an excess of base to provide the desired complexes 46R,L (compare Scheme 29).
Scheme 31: Synthesis of different substituted vinyl gold(I) complexes with Ph3P- 46R,Ph3P and
IMes-ligand 46R,IMes.
The isolation of the vinyl gold(I) complexes bearing a Ph3P-ligand was possible in high yields with electron-donating and electron-withdrawing groups, respectively. The yields for the complexes with the IMes-ligand dropped systematically for all substitution patterns, except for the OMe-substituted complex 25, which could be isolated in 72% yield. All reactions were performed without further optimization to increase the yield.
While this type of complexes are known to be unstable at ambient conditions,[2d] the isolated complexes can be stored for several weeks in a freezer (-25 °C). The air-stable complexes
46H,Ph3P and 46H,IMes were characterized by single-crystal X-ray crystallography (Figure 11).
Figure 11: Single-crystal X-ray structure of vinyl gold(I) complex 46H,Ph3P and 46H,IMes. Thermal
ellipsoids are shown at 90% probability, hydrogen atoms are omitted for clarity. Table: Bond length and C-Au-L angle of the complexes 46H,Ph3P and 46H,IMes.
Bond 46H,Ph3P 46H,IMes
C-Au-L 2.036 Å 2.021 Å
C-Au-L 2.286 Å 2.024 Å
59
Results and Discussion
Both complexes show similar bond lengths and C-Au-L angles compared with structural analogues. The bond lengths of complex 46Me,Ph3P are almost identical to the recently
published 4-methylphenylbenzofuranyl (p-CF3Ph)3PAu-complex 46Me,p-CF3Ph3P
[C-Au-PPh3 = 2.036 Å vs. C-Au-P(p-CF3Ph3) = 2.04 Å[2a] and C-Au-PPh3 = 2.286 Å vs.
C-Au-P(p-CF3Ph3) = 2.28 Å[2a]]. The coordination geometry is almost linear with an angle of
178.5°, which is typical for C-Au-P complexes [Ph3PAuCl = 179.68°[16]]. The IMes-gold complex 46H,IMes has almost similar gold-NHC 46H,IMes and gold-vinyl bond lengths 46H,IMes
compared with the previously reported IPr-benzofuranyl gold-complex 46H,IPr
[C-Au-IMes = 2.021 Å vs. C-Au-IPr = 2.042 Å[2d] and C-Au-IMes = 2.024 Å vs. C-Au-IPr = 2.039 Å[2d]]. The NHC-Au bond is slightly longer than in the pre-catalyst IMesAuCl with C-Au-IMes = 1.998 Å.[17] With an almost linear coordination geometry at the gold center, the C-Au-L angle of 46H,IMes is comparable with the IPr-analogue 46H,IPr [C-Au-IMes = 175.8°
vs. C-Au-IPr = 173.96°[2d]]. To probe, how the protodeauration of vinyl gold(I) complexes 46
R,L
is influenced by the acidity of the proton source, the reactivity of the isolated complex 46OMe,Ph3P towards different substituted phenol derivatives was tested.
Table 5: Reaction kinetics of the protodeauration of vinyl gold(I) complex 46OMe,Ph3P using different
substituted p-bromophenols 70 as proton source (21.2 °C).
Entry R pKa[18] kR [s-1] kR [s-1] log (kR/kH) σtheo[19]
1 OMe 10.20 5.50E-05 + 0.02E-05 - 0.14 - 0.27
2 N(CH3)2 - 1.23E-05 + 0.01E-05 - 0.79 - 0.17
3 H 9.99 7.64E-05 + 0.03E-05 0 0
4 F 9.92 6.85E-04 + 0.05E-04 0.95 0.06
5 Cl 9.38 5.19E-03 + 0.22E-03 1.83 0.24
6 Br 9.34 6.10E-03[a] + 0.66E-03 1.90 0.26
General conditions: Au(I) complex (5.02 mmol/L in CDCl3), 10.0 eq p-bromophenol in CDCl3, determined with 1H NMR spectroscopy (500 MHz, 21.2 °C) using tetrachloroethane as internal standard. [a] Reaction was carried out in triplicate and the average is given.
The acidity of the proton source is related to the reaction rates of protodeauration which can be seen clearly by comparing kobs for the protodeauration with various phenols (see Table 5). The more electron-withdrawing the phenyl moiety, the faster the protodeauration proceeds, as seen for entry 4-6, and the larger the Hammett parameter, the faster the reaction. While phenol 70H has a pKa of 9.99 and and a Hammett value set to 0 (reference),
60
4-fluorophenol 70F with a pKa value of 9.92 and = 0.06 reacts faster. The negative charge formed during the reaction at the phenolate anion can be stabilized more efficient. 4-Dimethylaminophenol (DMAP) 70N(CH3)2 is supposed to react faster than
4-methoxyphenol 70OMe according to its Hammett parameters but the reaction proceeded
slower than expected with kOMe = 5.50 10-05 s-1 and kN(CH3)2 = 1.23 10-05 s-1 (see entry 1
and 2). Although the kinetics for DMAP were not fully followed until the end of reaction, log (kN(CH3)2/kH) was determined with a value of - 0.79. The results are shown in a Hammett plot in which the values for entries 1 and 3-6 can be connected with a linear slope of 6.47 (see
Figure 12). The value for DMAP is excluded due to the incomplete monitoring of the reaction.
The positive slope confirms that a negative charge at the phenol is build after deprotonation and it can be seen that the higher the acidity of the phenol, the faster the protodeauration proceeds.
Figure 12: Hammett plot for the protodeauration of vinyl gold(I) complex 46OMe,Ph3P.
With the previous results from the protodeauration of the OMe-substituted vinyl gold(I) complex 46OMe,Ph3P towards different phenols, the reactivity of the isolated complexes was
investigated. The protodeauration of the different substituted vinyl gold(I) complexes 46 towards p-bromophenol 70Br as proton source was monitored by 1H NMR spectroscopy and
the reaction rates kR are given in Table 6.
The reaction proceeds faster with electron-donating groups at the para-position of the substrate because of their higher -(hyper)conjugation (E) which is part of the stabilizing orbital interactions EOrb for bond formation between the interacting fragments.[20] The highest reaction rates were determined with the OMe-substituted complexes 46OMe,Ph3P
kOMe,Ph3P = 6.10 10-3 s-1 (Table 6, entry 1) and the IMes-complex 46OMe,IMes, giving a
comparable reaction rate of kOMe,IMes = 2.31 10-3 s-1. The reaction rates are strongly dependent on the substitution pattern of the complexes, which can be observed when changing the electronic nature of the complexes at the para-position of the benzofuran core from OMe- to Me-substitution (see Table 6, entry 2). Electron-withdrawing groups decrease the reaction rate as seen for the F-substituted complexes with reaction rates of
kF,Ph3P = 7.51 10-4 s-1 and kF,IMes = 3.37 10-4 s-1 (see Table 6, entry 4). The complexes bearing a CF3-group were the least reactive as expected with rates of kCF3,Ph3P = 9.95 10-5 s-1 and kCF3,IMes = 2.77 10-5 s-1 (see Table 6, entry 5).
Intercept 0.27 + 0.12 Slope 6.47 + 0.65 R2 0.9702 Br Cl H F OMe N(CH3)2
61
Results and Discussion
Table 6: Reaction kinetics of the protodeauration of substituted vinyl gold(I) complexes 46R,L using
p-bromophenol 70Br as proton source.
L = Ph3P[a,b] L = IMes[a,c]
Entry R kR [s-1] k
R [s-1] kR [s-1] kR [s-1]
1 OMe 6.10E-03 + 0.81E-03 2.31E-03 + 0.23E-03
2 Me 2.53E-03 + 0.04E-03 9.75E-04 + 0.48E-04
3 H 1.03E-03 + 0.02E-03 4.21E-04 + 0.56E-04
4 F 7.51E-04 + 0.44E-04 3.37E-04 + 0.26E-04
5 CF3 9.95E-05 + 1.32E-05 2.77E-05 + 0.24E-05
[a] All reactions were carried out in triplicate and the average is given. [b] General conditions: Au(I) complex (5.00 mmol/L in CDCl3), p-bromophenol (50.1 mmol/L in CDCl3), determined with 1H NMR spectroscopy (500 MHz, 21.4 °C) using tetrachloroethane as internal standard. [c] General conditions: Au(I) complex (5.00 mmol/L in CDCl3), p-bromophenol (50.1 mmol/L in CDCl3), determined with 1H NMR spectroscopy (500 MHz, 24.5 °C) using tetrachloroethane as internal standard.
When comparing the reactions with same substitution pattern but different ligand, a trend can be observed: complexes bearing Ph3P-ligands react slightly faster than those having an IMes-ligand (see Table 6), a trend that was previously noted.[1a, 1c, 2c, 13b, 13c] When correlating the reaction rates as their experimental values[19b] (see Table 7) in a Hammett plot[19] both ligands show linear behavior (see Figure 13). The slopes of Ph3P = -2.2 and IMes = -2.4 are almost identical. The negative values indicate the formation of a positive charge during the prodeauration step.
Table 7: Experimental -values for the protodeauration of substituted vinyl gold(I) complexes 46R,L
using p-bromophenol 70Br as proton source. Entry R log (kR/kH) log (kR/kH) log (kR/kH) log (kR/kH) σtheo[19]
L = Ph3P[a,b] L = IMes[a,c]
1 OMe 0.77 + 0.06 0.74 + 0.04 - 0.27
2 Me 0.39 + 0.01 0.36 + 0.02 - 0.14
3 H 0.00 - 0.00 - 0.00
4 F - 0.14 + 0.03 - 0.10 + 0.03 0.06
62
Figure 13: Hammett plot for substituted vinyl gold(I) complexes with Ph3P- 46R,Ph3P (red) and
IMes-ligand 46R,IMes (black).
These data indicate, that the protodeauration of substituted benzofuranyl gold(I) complexes is mainly influenced by the substitution pattern of the benzofuran core and hardly influenced by the nature of the ligand. This also suggests that choosing a suitable ligand for reactions involving an inter- / intramolecular nucleophilic attack should not depend on the protodeauration step due to the negligible effect of the ligands as shown before. This is in contrast to previous reports by Hammond and co-workers, where IPr-vinyl gold(I) allenoates are deaurated faster by trifluoroacetic acid (TFA) than Ph3P-vinyl gold(I) allenoates demonstrating that NHC and phosphine ligands may react in a different way.[1a, 1c] Another example is shown in Chapter 2, where the variation of the ligand and the conditions can lead to different products (substituted benzofurans 37 and azobenzofurans 32) but can also lead to the same product as shown for the formation of azobenzofuran 32Me,H (Ph3PAuNTf2, NaHCO3, CH2Cl2 22% see Table 1, entry 2 and IMesAuNTf2, NaHCO3, CH2Cl2 10% see Table
4, entry 1, Chapter 2).
To determine the second order rate constant and from this the Gibbs free energy of activation, the protodeauration of the F-substituted Ph3PAu-complex 46F,Ph3P was monitored regarding
different concentrations of p-bromophenol (see Table 8). The value for the reaction with 250 eq p-bromophenol is shown but excluded from the trendline, because the reaction proceeded to fast to monitor it reproducibly (see Figure 14, Figure S 7, and Table S 26).
Ph3P IMes + 0.16 - 2.20 + 0.07 - 2.38 R2 0.9891 0.9983 OMe Me H F CF3
63
Results and Discussion
Table 8: Reaction rates of the protodeauration of F-substituted Ph3PAu-complex 46F,Ph3P towards
different concentrations of p-bromophenol.
Entry eq. k F, Ph3P [s-1] k F, Ph3P [s-1]
1 50.1 8.32E-04 + 0.95E-04
2 75.0 2.46E-03 + 0.06E-03
3 100.1 5.41E-03 + 0.25E-03
4 125.1 1.22E-02 + 0.34E-02
All reactions were carried out in triplicate and the average is given. General conditions: Au(I) complex (5.00 mmol/L in CDCl3), p-bromophenol (in CDCl3), determined by 1H NMR spectroscopy (500 MHz, 23.1 °C) using tetrachloroethane as internal standard.
Figure 14: Reaction rates of the protodeauration of F-substituted Ph3PAu-complex 46F,Ph3P towards
different concentrations of p-bromophenol.
The second order rate constant can be determined from the slope of the graph to be
k2 = 1.48E-01 + 0.34E-01 s-1 and from this G‡ was estimated to be G‡
296 K = 77.3 + 0.6 kJ mol-1 (G‡296 K = 18.5 + 0.1 kcal mol-1). Comparable studies based on DFT calculations by Ariafard and co-workers support this value by calculating free activation energies in a range of G‡ = 3.1-25.8 kcal mol-1 for the protodeauration of different substituted alkenyl gold(I) complexes.[13c] The protodeauration of the non-substituted alkenyl gold(I) complex with Ph3P as ligand was calculated with G‡ = 17.2 kcal mol-1[13c] which is in the same magnitude than the explored value for the protodeauration of the F-substituted benzofuranyl gold(I) complex 46F,Ph3P with G‡296 K = 18.5 + 0.1 kcal mol-1.
Intercept - 7.76E-03 + 3.08E-03 Slope 1.48E-01 + 0.34E-01
64
3.3)
Conclusions
In this chapter, differently substituted vinyl gold(I) complexes with respect to the benzofuran core and the ligand were synthesized in moderate to high yields. The non-substituted vinyl gold(I) complexes with Ph3P- and IMes-ligand were characterized by single-crystal X-ray crystallography. Although this type of vinyl gold(I) complexes are known as unstable intermediates which can be stored after isolation for a limited time only,[2d] the synthesized complexes shown in this chapter are air-stable and can be stored for several weeks in a freezer.
The kinetics of the protodeauration of the OMe-substituted vinyl gold(I) complex 46OMe,Ph3P
towards different substituted phenols were determined and the results are correlated in a Hammett plot. With a linear slope of Phenol = 6.47, the protodeauration is strongly dependent on the acidity of the phenol.
To investigate the kinetics of the different substituted vinyl gold(I) complexes 46R,L the
previously synthesized complexes were treated with p-bromophenol as proton source. The results of the systematic study are shown in a Hammett plot. Both complex types with Ph3P- and IMes-ligand are deaurated in the same fashion with slopes of Ph3P = - 2.2 and IMes = - 2.4. The negative values indicate the formation of a positive charge at the vinyl gold(I) complexes during deauration. The examined reactivity also demonstrates that the substitution pattern of the benzofuran core plays a major role in the protodeauration of the synthesized vinyl gold(I) complexes 46R,L while the ligand in this specific case has a negligible
impact. Substitutents at the benzofuran core with an electron-donating effect, such as OMe, accelerate the protodeauration, electron-withdrawing groups, such as CF3, decrease the reactivity at the active position. Our study demonstrates that the choice of ligand has a minor influence on the (final) protodeauration step in catalysis than expected.
The Gibbs free energy of activation was determined by treating the F-substituted Ph3PAu-complex 46F,Ph3P with different equivalents of p-bromophenol. The slope of the plotted
graph was used to calculate G‡, giving a value of G‡296 K = 77.3 + 0.6 kJ mol-1 (G‡
296 K = 18.5 + 0.1 kcal mol-1). This value is comparable with theoretical studies previously reported for the protodeauration of alkenyl gold(I) complexes.[13c]
65
Author Contributions and Acknowledgements
3.4)
Author Contributions and Acknowledgements
The method to measure the kinetics was developed together with Christina Bauer (Master-Internship 2019). The Me-substituted Ph3P-Au complex 46Me,Ph3P was previously synthesized
and the protodeauration towards p-bromophenol monitored with 1H NMR spectroscopy to evaluate the method together with Christina Bauer. Data plotting with Origin was done with the help of Christina Bauer. NMR measurements were performed under the supervision and with assistance of Ing. Pieter van der Meulen and Dr. Johan Kemmink. Crystallographic measurements were performed and evaluated by Folkert de Vries.
3.5)
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