The E/Z selectivity in gas phase Wittig reaction of non-stabilized, semi-stabilized and stabilized Me3P and Ph3P phosphorus ylides with monocyclic ketone: A computational study

Abstract

The gas phase Wittig reactions of non-stabilized (X3P=CHMe), semi-stabilized (X3P=CHPh) and stabilized (X3P=CHCO2Me) ylides (X=methyl or phenyl group) with 2,2-di-methyl cyclopentanone have been investigated at B3LYP/6-31G∗∗ level of theory to understand the E/Z selectivity. Our results strongly support the generally accepted salt free oxaphosphetane mechanism. In reactions with all ylides, a high E selectivity was observed. It is shown that the E/Z selectivity cannot be fully understood without considering the energy of the elimination TS in reactions of non- and semi-stabilized Me3P ylides. The reversibility of oxaphosphetane formation is explained by the influence of the ylide stabilization and nature of the substituents on the phosphorus of the ylide and was observed in the reactions of non- and semi-stabilized Me3P ylide reactions only. It is shown that the puckering ability of addition TS doesn’t depend on ylide stabilization but an interplay of 1,2; 1,3 and CH⋯O interactions decides the geometry of TS1s and that the selectivity is determined by the combined effect of 1,2 and 1,3 steric interactions. In all reactions, addition transition states were found to be planar in Z isomers and very slightly puckered in the E isomers. Due to the low reactivity of 2,2-dimethyl cyclopentanone, the first steps are slightly exothermic in reactions with non-stabilized ylides and endothermic in reactions involving semi-stabilized and stabilized ylides.