Abstract
A comparison of the diastereoselective addition of allylMgCl to differently O-protected N-trialkylsilyllactaldimine and to N-benzyllactaldimines is reported. The stereoselectivity strongly depends on the O- and N-protecting groups, on the metal cation, and on the reaction temperature, and varies from predominance of the anti diastereoisomer to predominance of the syn one. O-TBS- and O-TIPS-silyl imines displayed better anti diastereoselectivity than their O-alkyl counterparts. The N-protection produced unexpected results: in particular, the bulky, achiral, N-TIPS group favoured the predominance of the syn diastereoisomer. The temperature dependence of the stereochemical outcome provides evidence of the important role of differential entropies of activation (ΔΔS≠), and the presence of inversion temperatures (Tinv) related to dynamic solvation phenomena. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)
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