Mechanism‐Guided Development of VO(salen)X Complexes as Catalysts for the Asymmetric Synthesis of Cyanohydrin Trimethylsilyl Ethers

Catalyze this! Detailed study of the mechanism of asymmetric cyanohydrin synthesis catalyzed by VO(salen)X complexes (see figure) led to the development of VO(salen)NCS, as the most active vanadium‐based catalyst yet developed for this reaction. The mechanism by which oxovanadium(V)(salen) complexes...

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Published in:Chemistry : a European journal 2009-02, Vol.15 (9), p.2148-2165
Main Authors: Belokon, Yuri N., Clegg, William, Harrington, Ross W., Maleev, Victor I., North, Michael, Pujol, Marta Omedes, Usanov, Dmitry L., Young, Carl
Format: Article
Language:English
Subjects:
aldehydes
Aldehydes - chemistry
asymmetric catalysis
Catalysis
Crystallography, X-Ray
cyanides
Ethers - chemical synthesis
Ethers - chemistry
Kinetics
Molecular Conformation
Molecular Structure
Nitriles - chemical synthesis
Nitriles - chemistry
Organometallic Compounds - chemical synthesis
Organometallic Compounds - chemistry
reaction mechanisms
Trimethylsilyl Compounds - chemical synthesis
Trimethylsilyl Compounds - chemistry
vanadium
Vanadium - chemistry
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Summary:Catalyze this! Detailed study of the mechanism of asymmetric cyanohydrin synthesis catalyzed by VO(salen)X complexes (see figure) led to the development of VO(salen)NCS, as the most active vanadium‐based catalyst yet developed for this reaction. The mechanism by which oxovanadium(V)(salen) complexes1 VO(salen)X catalyze the asymmetric addition of trimethylsilyl cyanide to benzaldehyde has been studied. The reaction kinetics indicated that the structure of the counterion (X) had a significant influence on the rate, but not on the enantioselectivity of the reaction. The less coordinating the counterion, the lower the catalytic activity; a trend that was confirmed by a Hammett analysis. Variable temperature kinetics allowed the enthalpies and entropies of activation to be determined for some catalysts, and showed that, for others, the overall reaction order changes from second order to zero order as the temperature is reduced. The order with respect to the catalyst was determined for nine of the VO(salen)X complexes and showed that the less active catalysts were active predominantly as mononuclear species whilst the more active catalysts were active predominantly as dinuclear species. Mass spectrometry confirmed the formation of dinuclear species in situ from all of the VO(salen)X complexes and indicated that the dinuclear complexes contained one vanadium(V) and one vanadium(IV) ion. The latter conclusion was supported by cyclic voltammetry of the complexes, by fluorescence measurements and by the fact that catalyst deactivation occurs when reactions are carried out under an inert atmosphere. Based on this evidence, it has been deduced that the catalysis involves two catalytic cycles: one for catalysis by mononuclear VO(salen)X species and the other for catalysis by dinuclear species. The catalytic cycle involving dinuclear species involves activation of both the cyanide and aldehyde, whereas the catalytic cycle involving mononuclear species activates only the aldehyde, thus explaining the higher catalytic activity observed for catalysts which are predominantly active as dinuclear complexes. Based on these mechanistic results, two new VO(salen)X complexes (X=F and NCS) were predicted to form highly active catalysts for asymmetric cyanohydrin synthesis. VO(salen)NCS was indeed found to be the most active catalyst of this type and catalyzed the asymmetric addition of trimethylsilyl cyanide to thirteen aldehydes. In each case, high yields and enantioselectivitie
ISSN:0947-6539
1521-3765
DOI:10.1002/chem.200801679