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Investigation of Lewis Acid versus Lewis Base Catalysis in Asymmetric Cyanohydrin Synthesis

Lookup NU author(s): Professor Michael North, Marta Omedes Pujol, Courtney Williamson


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The asymmetric addition of trimethylsilyl cyanide to aldehydes can be catalysed by Lewis acids and/or Lewis bases, which activate the aldehyde and trimethylsilyl cyanide, respectively. It is not always apparent from the structure of the catalyst whether Lewis acid or Lewis base catalysis predominates. To investigate this in the context of using salen complexes of titanium, vanadium and aluminium as catalysts, a Hammett analysis of asymmetric cyanohydrin synthesis was undertaken. When Lewis acid catalysis is dominant, a significantly positive reaction constant is observed, whereas reactions dominated by Lewis base catalysis give much smaller reaction constants. [{Ti(salen)O}(2)] was found to show the highest degree of Lewis acid catalysis, whereas two [VO(salen)X] (X=EtOSO3 or NCS) complexes both displayed lower degrees of Lewis acid catalysis. In the case of reactions catalysed by [{Al(salen)}(2)O] and triphenylphosphine oxide, a non-linear Hammett plot was observed, which is indicative of a change in mechanism with increasing Lewis base catalysis as the carbonyl compound becomes more electron-deficient. These results suggested that the aluminium complex/triphenylphosphine oxide catalyst system should also catalyse the asymmetric addition of trimethylsilyl cyanide to ketones and this was found to be the case.

Publication metadata

Author(s): North M, Omedes-Pujol M, Williamson C

Publication type: Article

Publication status: Published

Journal: Chemistry: A European Journal

Year: 2010

Volume: 16

Issue: 37

Pages: 11367-11375

Print publication date: 16/08/2010

ISSN (print): 0947-6539

ISSN (electronic): 1521-3765

Publisher: Wiley - VCH Verlag GmbH & Co. KGaA


DOI: 10.1002/chem.201001078


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