Boron III Lewis Acids

Boron Lewis acids have been used routinely in dealkylation of aromatic and alkyl ethers, and as catalysts for Friedel-Crafts type electrophillic aromatic substitution. 

Acid CaUdysK in Modem Organic Syndi K. Editedby H. Yamamoto and K. Ishihara Copyright 2008 WILEY-VCH Vexlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31724-0 

Recently, designer boron Lewis acids have emerged and it has been shown that the Lewis acidity of the boron center can be manipulated by modification of the electronics by taking advantage of the geometry of the boron center and its substituents. 

This chapter will focus on the developments in the applications of boron Lewis acids from 1998 to 2006 and at the same time highlight key benchmarks prior to 1998. Both achiral and chiral boron Lewis acids are discussed in each section. 

Cycloaddition reactions provide the opportunity to build at least one ring and several stereocenters in one operation and are therefore desirable in a synthetic scheme. This section describes various examples, both chiral as well as achiral, of stereoselective cycloaddition reactions in which boron Lewis acids were employed. 

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Chiral boron(III) Lewis acid catalysts have also been used for enantioselective cycloaddition reactions of carbonyl compounds [17]. The chiral acyloxylborane catalysts 9a-9d, which are also efficient catalysts for asymmetric Diels-Alder reactions [17, 18], can also catalyze highly enantioselective cycloaddition reactions of aldehydes with activated dienes. The arylboron catalysts 9b-9c which are air- and moisture-stable have been shown by Yamamoto et al. to induce excellent chiral induction in the cycloaddition reaction between, e.g., benzaldehyde and Danishefsky s dienes such as 2b with up to 95% yield and 97% ee of the cycloaddition product CIS-3b (Scheme 4.9) [17]. 

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