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