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Phosphacycle containing ligands : metal complexes and catalysis
Doro, F.
Publication date 2009
Link to publication
Citation for published version (APA):
Doro, F. (2009). Phosphacycle containing ligands : metal complexes and catalysis.
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Summary
This thesis describes the synthesis of a new class of conformationally constrained aryl-phosphanes and the employment of their metal complexes as catalysts in several Rh, Pd and Ni catalyzed transformations. A strong focus has been directed towards the study of the influence of the catalysts’ structural features on relevant aspects of catalysis (e.g. enantioselectivity, ligand-substrate noncovalent interactions, bite angle effect).
In chapter 1 we reviewed the applications of benzo-fused phosphacycle containing catalysts in several catalytic transformations, highlighting the cases where structural features of the P-heteroles, such as extension of the -delocalization, degree of planarity of the P-ring, orientation of the P-substituents, were shown to have an influence on the catalytic properties.
The building blocks for the synthesis of the ligands to employ in catalysis consist of cyclic aryl-phosphanes in which a hydroxyl moiety, amenable to further functionalization, is attached to one of the aryl groups. The description of the synthetic procedures of these building blocks is reported in chapters 2-4.
P OH P OH O P O OH P OH Chiral Phosphane
The bridging of two different aryl groups brings to the creation of a stereogenic centre on the phosphorus atom, (above, third compound of the series from the left). Chapter 2 describes the synthesis of the racemic mixture of this phenoxaphosphane and the optical resolution through separation of its diastereomeric palladacycle derivatives. The absolute configuration of the enantiopure phosphanes were assigned by means of X-ray crystal structure determination. These building blocks were incorporated into phosphane-phosphite ligands and their metal complexes employed as steric probes in the Rh-catalyzed asymmetric hydrogenation of substituted alkenes. The catalytic results, aided by DFT calculations, showed how the simple alteration of the orientation of the P-aryl groups affects the activity and selectivity of this reaction. These systems
Summary
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showed to be particularly sensitive to the ligand-substrate interface, which makes them promising steric probes for the study of the stereo-recognition in asymmetric catalysis.
This work is described in chapter 3.
P O P O O P O O P O O P O O O OP P O O P O O
Chapter 4 deals with the use of an enlarged series of conformationally constrained phosphane-phosphite ligands as chiral inducers in the rhodium catalyzed asymmetric hydroformylation reaction (AHF) of electronically different styrene derivatives. Catalyst initiators and catalytic intermediates have been thoroughly characterized, in experimental reaction conditions, employing high pressure NMR and FT IR spectroscopy. Only one chiral inducer showed a correlation between the ee and the electronic properties of the substrates. In this case both the catalysis and the spectroscopic analysis of catalytic intermediates showed evidence to support the involvement of aryl-aryl noncovalent interactions between the ligand and substrate. The AHF obeys a kinetic law in accord with the hydrogenolysis of the acyl-Rh complex as rate determining step.
Chapter 5 presents the use of P,O phosphacycle containing nickel- and palladium-complexes in the oligomerization of ethene and in the asymmetric hydrovinylation of styrene. The catalytic results were compared with the performances achieved using open-chain phosphane metal complex analogues.
