Boron Lewis Acids

The classical boron Lewis acids—BF₃ or BCl₃—were used in stoichiometric amounts in Mukaiyama aldol additions under anhydrous conditions. Though TiCl₄ is the more effective Lewis acid in Mukaiyama reactions with aldehydes there are several useful applications of BF₃ ∙ Et₂O. BF₃ is known to reverse diastereofacial selectivities in several aldol additions of aldehydes with silyl enol ethers compared with corresponding enolate or Lewis acid-mediated aldol additions. For comprehensive overview of applications of boron Lewis acids in aldol additions see references.^1,2 Much more informations and correlations between substrates and Lewis acids used and stereochemical results are given in Mahrwald.³ Also in view of this chapter, results of BF₃-mediated aldol additions compared with other Lewis acids used in Mukaiyama aldol reactions were discussed. An explanation for this outstanding behaviour of BF₃ was often given by the non-chelation control of these reactions.

BF₃ was originally suspected to be unable of chelation during aldol additions. Later investigations indicated that the level of 1,2-asymmetric induction in BF₃-mediated aldol additions is also affected by the bulk of the silyl group in the substrate (Scheme 3.1.3.1).⁴ In aldol additions of tetrasubstituted silyl enol ether with oxygen-containing aldehydes a reversal of diastereoselectivity is observed by deployment of Et₂BOTf or BF₃ (Scheme 3.1.3.2).⁵