Theory Rules Out a [2 + 2] Addition of Osmium Tetroxide to Olefins as Initial Step of the Dihydroxylation Reaction

摘要

The addition of OsO_4 to olefins yielding cis-diois has become an intensively investigated reaction in stereoselective synthesis since Sharpless developed the catalytic version of the reaction into a powerful method for highly enantioselective functionalization of olefins using cinchona alkaloids as ligands. The transfer of the stereochemical information of thechiral ligand to the substrate was explained by Sharpless with a two-step mechanism for the addition reaction (Scheme 1). which should occurrather than a concerted [3 + 2] addition as originally proposed by Criegee. It was suggested that the initial step of the reaction course is a [2 + 2] addition yielding the metallaoxetanc 1, which then rearranges to the cyclic ester 2. The barrier for thebond migration 1 --> 2 must be very low, because intermediates of structure 1 have never been observed, whereas base adducts of 2 arc well known.