Involving Carbene or Not? Mechanism of Corey–Winter Reaction

The mechanism of the Corey-Winter reaction is unclear, even though this reaction, discovered more than 60 years ago, has evolved as an indispensable method of synthesizing olefins from diols. We report a computational study of the mechanism of the Corey-Winter reaction, ruling out three well-known pathways of this reaction and proposing a new pathway consistent with the experimental observations. This new pathway starts from nucleophilic addition of phosphite to the carbon (not the sulfur) atom of the cyclic thionocarbonates. This step is followed by the intramolecular addition of S to P, generating thiaphosphirane intermediates. Elimination of phosphorothioates then takes place, giving oxygen-heterocyclic carbenes (OHCs). Finally, the cycloreversion of the OHCs gives carbon dioxide and the target olefin products. Calculations support that the OHC intermediates are involved in the Corey-Winter reaction. In addition, the Woodward-Hoffmann orbital correlation and electron flow pattern for the cycloreversion process of the OHCs to olefins and carbon dioxide have been revised. Moreover, why diazaphospholidine can accelerate the Corey-Winter reaction has been rationalized in this study by multivariable linear regression analysis.