Cyano triphenyl phosphonium ylides stabilized by ester or keto groups: Computed conformations and crystal structures

Abstract

The mixed stabilized triphenyl phosphonium ylides Ph3P= C(CN)CO-R and Ph3P= C(CN)COOR, R= CH3, C2H5, CH(CH3)(2), 1a, c and 2a, b, respectively, have sharp H-1 and C-13 NMR signals, except for 2b in some conditions. Ab initio geometrical optimizations indicate that in the preferred conformer the acyl oxygen is syn- to phosphorus, with barriers to rotation about the ylidic bond of 15-18 kcal. mol(-1), and differences between the syn- and anti-conformers of ca. 4 and 11 kcal. mol(-1) for the keto and ester derivatives, respectively. These energetic differences are not very sensitive to changes in the alkyl group, R, or the level of computation. The computed geometries indicate that the ylidic moieties are approximately planar with extensive electronic delocalization, as in crystalline 1a and 2a. The C-13 NMR signals of the ylidic carbons are shifted strongly up field, consistent with their anionoid character.