Low-Coordinate German ium( II) Centers Within Distorted Axially Chiral Seven-Membered Chelates: Stereo- and Enantioselective Cycloadditions

摘要

The heavier group 14 analogues of carbenes (R2E:) have attractive structures and interesting reactivity toward unsaturated compounds.R2E: compounds have two covalent bonds with high p character, a single electron lone pair with high s character, and a single vacant p orbital in the singlet ground state. The intrinsic ambiphilic nature of R2E: compounds allows their participation in a variety of cycloaddition reactions with compounds containing unsaturated bonds. (2 + 2 + 1) Cycloaddition reactions of divalent group 14 element species provide one of the most interesting ways for synthesizing a heterocycle in one step but have been limited to a number of transient silylenes and Si(Cp*)2 (Cp* = pentamethylcyclopentadienyl).The reactions of these silylenes with a ketone or a aldehyde afforded the corresponding 1:2 cycloadducts, although different reaction intermediates were proposed for the used silylenes. In both cases, the high Lewis acidity of the group 14 element atom was believed to be an essential factor in promoting the formation of these intermediates.