Stereochemistry of substitution at trico-ordinate phosphorus

Abstract

A series of reactions of ring-substituted 1,3,2-dioxaphospholanes, 1,3,2-oxazaphospholanes, 1,2-oxaphospholes, and phosphetanes bearing the leaving groups Cl, OR, or NR₂ on phosphorus, with the nucleophiles HCI, MeO^–, MeOH, PhOH, Me₂NH, Et₂NH, and [CH₂]₅NH have been studied. N.m.r. signals (¹H and ³¹P) from reactant and product diastereoisomers have been assigned, and the stereochemistry of the substitution reactions have been determined by ³¹P n.m.r. monitoring. The outcome varies from complete inversion to complete lack of steroselectivity. During the initial stages many of the non-selective reactions proceed with predominant inversion, and most of the results may be interpreted by assuming that the actual substitution step occurs with inversion, and that the lack of stereoselectivity is due to competing isomerizations of products or reactants. Exceptions are certain reactions where the leaving group is Cl; these appear to involve a non-selective substitution step.