Wittig–Horner reagents: structural and conformational study of five- and six-membered cyclic phosphonates and their carbanions. Evidence of anomeric effect in anionic species

Abstract

A structural and conformational study of the electron-withdrawing substituted phosphonates 2-oxo-1,3,2-dioxaphosphorinanes (1) and (2) and of the related anionic species (3, K⁺) and (4, K⁺) in dimethyl sulphoxide has been made. Phosphonates (1) and (2) exist predominantly in the chair form with the PO bond equatorial. The anion (3, K⁺) gives rise to three rapidly interconverting species, in the chair form, and (4, K⁺) to two interconverting chairs, chelated to K⁺. In each anionic species (3) and (4), the PO axially substituted conformer is significantly populated, due to an anomeric effect. The 2-oxo-1,3,2-dioxaphospholane (5) is very sensitive to moisture, and open-chain compounds can be found in solutions of (5), if the solvent is not carefully dried. The related anionic species (6, K⁺) is not very stable; however, its ¹H and ³¹P n.m.r. data show some similarities with those of the related acyclic compound.