Germanium(II)-Mediated Reductive Cross-Aldol Reaction of Bromoaldehydes with Aldehydes: NMR Studies and ab Initio Calculations.

[摘要]：A highly practical reductive cross-aldol reaction of a-bromoaldehydes with various aldehydes has been developed using Ge(II)Cl2 to produce aldehyde germanium(IV) aldolates, which were directly transformed to various multifunctionalized compds. A remarkable change in stereoselectivity depended on the a-bromoaldehydes employed; secondary a-bromoaldehydes gave syn selectivities, while tertiary a-bromoaldehydes accomplished the synthesis of anti-selective aldol products with a quaternary carbon center. NMR studies and X-ray anal. strongly suggested the formation of germanium enolate in the reaction of a-bromoaldehyde 2h with GeCl2-dioxane. Detailed mechanistic studies, including NMR anal. and ab initio calcns., revealed the generation of stable germanium aldolates, which was due to the remarkably low Lewis acidity of the germanium(IV).

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