Crotylation of Aldehydes by Crotyltins: Discrimination between Mechanisms Involving Transmetallation or Simple Lewis Acid Assistance through the Consideration of the Stereochemistry of the Corresponding Homoallylic Alcohols

In the reaction of crotyltins with aldehydes in the presence of metal salts, the double consideration of the syn/anti ratio of the branched homoallylic alcohols and the Z/E ratio of their linear regioisomers is proposed as a way to discriminate between a reaction mechanism involving a transmetallation step and a reaction mechanism involving simple Lewis acid activation of the aldehyde. The formation of branched syn isomers along with Z-linear isomers as major compounds is considered to be indicative of a reaction occurring under Lewis acid assistance, whereas preference for the branched anti isomers together with E-linear isomers is considered to be indicative of a transmetallation step prior to crotylation. For reactions performed in the presence of CeCl3 center dot 7H(2)O/NaI, the Lewis acid assistance was shown to be the exclusive or highly prevalent pathway. Moreover, in regards to the selectivity, the regiopreference depends on the nature of the crotyltin. Whereas soluble crotyltin preferentially leads to Z-linear adducts, polymer-supported crotyltin affords the syn-branched adducts probably due to a lower 1,3-metallotropy, For reactions performed in the presence of InX3, simple Lewis acid assistance and transmetallation appear to be competitive processes; the first one is favoured with aromatic aldehydes especially in dichloromethane, whereas transmetallation appears to be prevalent with poorly reactive aldehydes especially with InBr3 in acetonitrile. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).