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

Keywords

Boron Aldehyde Enol Acetophenone Silyl 

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