Preferred conformations of stabilized phosphorus ylides

Abstract

The stabilized phosphorus ylides, Ph3P=C(CO.R')CO.OR, 1, R=Et, R' =CH2P+Ph3; 2, R=R'=Me; 3, R=Et, R'=Me; 4, R=Pr-i; R'=Me, 5, R=Bu-t; R'=Me, adopt a near planar conformation in the crystal which allows extensive electronic delocalization. The keto and alkoxylic oxygens are oriented and align favorably with the cationoid phosphorus. These conformations bring methyl hydrogens in the ester residue into proximity with the face of a phenyl group and lead to pi-shielding and upfield shifts of the (HNMR)-H-1 signals of 3 over a wide temperature range (-50-95degreesC) in (CD3)(2)CO, CDCl3 and DMSOd d.6. Geometries of 2 and 3, optimized by using the HF 3-21 (G*) or 6-31 (G*) basis sets are very similar to those in the crystal, but semiempirical treatments generate structures in which either the ester or keto moiety is twisted out of plane.