Effect on chair-chair equilibrium of 3-substituted-1,3,2-oxazaphosphorinanes of replacement of Me2N substituent on phosphorus by isoPr2N

Abstract

The chair-chair conformational equilibria (A ⇌ B) f a series of 1,3,2-dioxaphosphorinanes featuring three-coordinated phosphorus substituted with an isoPr2N group (1–6) have been studied by 1H NMR spectroscopy. Substituents at N(3) included Ph, Me, and isoPr. Compared to the analogous series with an Me2N group on phosphorus, 1–6 populate the chair conformation B with R2N equatorial to a greater extent. This is interpreted to mean that conformer A is more destabilized by the greater steric size of isoPr2N than is conformer B. Thus, the repulsive interactions between equatorial Me2N and the substituent on N3, believed to be responsible for depopulation of B that results in an unexpectedly high population of A with Me2N on phosphorus, is overcome by destabilization of A by the axial isoPr2N. The apparent size effect of substituents on N3 in destabilization of B follows the order Ph > isoPr > Me, as observed earlier for the series with a Me2N group on phosphorus. © 1996 John Wiley & Sons, Inc.