Tekla Bottin-Strzalko

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

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.

Nuclear magnetic resonance structural study of phosphonate anions

It is shown by 31P, 13C, and 1H n.m.r. spectroscopy that BunLi and the phosphonates (1) and (2) in tetrahydrofuran give a single, planar, charge-delocalized species (A), whereas ButOK and ButOLi and (1) give another species (B) in addition to (A); the structure proposed for (B) is analogous to that of the lithium adducts of phosphorus ylides.