DELVING INTO THE WITTIG REACTION-STEREOCHEMISTRY AND MECHANISM. STEREOCHEMICAL IDIOSYNCRASIES AND MECHANISTIC IMPLICATIONS1

Publisher Website
Google Scholar
Abstract
Non-stabilized triphenylphosphorus ylides bearing anionic groups can react with aldehydes to give alkene mixtures anomalously enriched in the E isomer [ref. 7]. To explain this phenomenon, we sought to study both cis and trans oxaphosphetane (OP) intermediates at low temperature. The observation of both intermediates was achieved for the first time by the use of high-field H-1, P-31, and C-13 NMR spectroscopy in various instances. We have monitored some Wittig reactions in detail via NMR-based kinetic measurements of OP's and alkenes. In certain cases, OP's equilibrate, presumably by reaction reversal to aldehyde and ylide, to introduce a measure of thermodynamic control into the Wittig reaction. Thermodynamic control accounts for a large portion of the excess E stereoselectivity observed in going from a triphenyl to trialkyl (i.e., butyl) phosphorus ylide. Quenching experiments with HBr and the deprotonation of erythro and threo beta-hydroxyphosphonium salts are also discussed. Attempts to investigate reactions of stabilized and semi-stabilized ylides in an analogous manner were not fruitful.