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Asymmetric cyanohydrin synthesis using heterobimetallic catalysts obtained from titanium and vanadium complexes of chiral and achiral salen ligands

Lookup NU author(s): Emeritus Professor Bill Clegg, Dr Ross Harrington, Dr Carl Young, Professor Michael North

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Abstract

Titanium(IV)(salen) and vanadium(V)(salen) complexes are both known to form catalysts for asymmetric cyanohydrin synthesis. When a mixture of titanium and vanadium complexes derived from the same or different salen ligands is used for the asymmetric addition of trimethylsilyl cyanide to benzaldehyde, the absolute configuration of the product and level of asymmetric induction can only be explained by in situ formation of a catalytically active heterobimetallic complex, and is not consistent with two monometallic species acting cooperatively. Combined use of complexes containing chiral and achiral salen ligands demonstrates that during the asymmetry inducing step of the mechanism, the aldehyde is coordinated to the vanadium rather than the titanium ion. The titanium complexes also catalyse the asymmetric addition of ethyl cyanoformate to aldehydes, a reaction in which vanadium(V)(salen) complexes are not active. For this reaction, use of a mixture of titanium and vanadium(salen) complexes results in a complete loss of catalytic activity, a result which again can only be explained by in situ formation of a heterometallic complex. Both the titanium and vanadium based catalysts also induce the asymmetric addition of potassium cyanide/acetic anhydride to aldehydes. For this reaction, combined use of chiral and achiral complexes indicates that during the asymmetry inducing step of the mechanism, the aldehyde is coordinated to titanium rather than vanadium, a result which contrasts with the observed results when trimethylsilyl cyanide is used as the cyanide source. © 2007 Elsevier Ltd. All rights reserved.


Publication metadata

Author(s): Belokon YN, Clegg W, Harrington RW, Young C, North M

Publication type: Article

Publication status: Published

Journal: Tetrahedron

Year: 2007

Volume: 63

Issue: 24

Pages: 5287-5299

Print publication date: 11/06/2007

ISSN (print): 0040-4020

ISSN (electronic): 1464-5416

Publisher: Elsevier

URL: http://dx.doi.org/10.1016/j.tet.2007.03.140

DOI: 10.1016/j.tet.2007.03.140


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