Mechanism of the Chemoselective and Stereoselective Ring Opening of Oxathiaphospholanes: An Ab Initio Study

Ab initio investigations of the reaction profiles for the base-catalyzed methanolysis of amino-2-thiono-1,3,2-oxathiaphospholane suggested that ring opening with a retention of configuration at phosphorus would be energetically favorable, which provides a reasonable interpretation for the chemo- and stereoselectivity that have been determined experimentally for the reaction of diastereomerically pure 2-[(1-(α-naphthyl)ethyl)amino]-2-thiono-1,3,2-oxathiaphospholane. Nucleophilic attack at phosphorus will lead to a trigonal bipyramidal (TBP) pentacoordinate phosphorane intermediate, and pseudorotation will then occur concomitantly with collapse of the TBP intermediate. Thus, pseudorotation is strongly coupled with the reaction coordinate for the substitution pathway. This finding suggests that substitution with a retention of configuration can occur even though a TBP species resulting from pseudorotation fails to exist as an intermediate.