MECHANISM OF THE CHEMOSELECTIVE AND STEREOSELECTIVE RING-OPENING OF OXATHIAPHOSPHOLANES - AN AB-INITIO STUDY

Ab initio investigations of the reaction profiles for the base-catalyz ed methanolysis of amino-2-thiono-1,3,2-oxathiaphospholane suggested t hat ring opening with a retention of configuration at phosphorus would be energetically favorable, which provides a reasonable interpretatio n for the chemo-and stereoselectivity that have been determined experi mentally for the reaction of diastereomerically pure yl)ethyl)amino]-2 -thiono,1,3,2-oxathiaphospholane. Nucleophilic attack at phosphorus wi ll lead to a trigonal bipyramidal (TBP) pentacoordinate phosphorane in termediate, and pseudorotation will then occur concomitantly with coll apse of the TBP intermediate. Thus, pseudorotation is strongly coupled with the reaction coordinate for the substitution pathway. This findi ng suggests that substitution with a retention of configuration can oc cur even though a TBP species resulting from pseudorotation fails to e xist as an intermediate.