Theoretical study of the Wittig reaction of cyclic ketones with phosphorus ylide.

The Wittig reaction of cyclopropanone, cyclobutanone and cyclopentanone with phosphorus ylide (Me3P = CH2) in gas phase was investigated computationally at B3LYP/6-31G** level of theory. In the Wittig reaction of cyclic ketones, two transition states (TS1 and TS2), corresponding to formation and decomposition of oxaphosphetane (OP) were located and investigated. Two loosely bound intermediates, a reactant complex (RC) and a product complex (PC) were also found. In the reaction of cyclopropanone, cyclobutanone and cyclopentanone, two oxaphosphetanes (OP1 and OP2) were predicted. OP1 initially formed was converted into OP2 by pseudorotation. In contrast to the reactions with cyclobutanone and cyclopentanone, an early TS1 was found in the reaction of cyclopropanone. The order of first activation energy barrier relative to reactant total energy was found to be cyclopropanone (-4.97 kcal mol(-1)) < cyclobutanone (0.30 kcal mol(-1)) < cyclopentanone (3.60 kcal mol(-1)).