Stereochime de reactions au niveau des liaisons silicium (ou germanium)— phosphore dans le systeme dimethyl-1,2 sila (ou germa)) cyclopentanique

The stereochemistry of nucleophilic displacement of diethylphosphino leaving group from geometrical isomers of 1,2-dimethy -1-diethylphosphino-1-sila (or germa) cyclopentane 1 (or 2 has been studied. Retention of configuration at Si (or Ge) atom was found in reduction with LiAlH₄(Et₂OorTHF), and inversion with bdLiBH₄. With reagents like n-butyllithium, allyllithium or allylmagnesium bromide, 1 (or 2) gives retention of configuration. Alcohols and acidic reagents react far more rapidly, but reactions are not (or weakly) stereospecific. PR₂; is a poor leaving group, neighbouring hydrogen atom in the Sommer-Corriu's scale.The stereochemistry of addition reactions of 1 (or 2) with various unsaturated derivatives (carbonyl and dissymetric ethylenic derivatives, ambident nucleophiles) has been also investigated. It was shown that these reactions are both regiospecitic and stercospecific, and proceed with retention at the silicon (or germanium) centre.The stereochemical results are consistent with recent conclusions that S_N-Si retention reactions involve frontier orbital interactions.Relative configurations of these geometrical isomers, and the stereochemistry of their reactions, were assigned by pmr and cmr spectroscopy.