AIM analysis for assignment of the two Z- and E-isomers in phosphorane containing a dimedon-1-yl

Triphenylphosphine reacts with dialkyl acetylenedicarboxylates in the presence of a SH-heterocyclic compound such as dimedon-1-yl to generate stable phosphorus ylides. These stable ylides exist in solution as a mixture of the two geometrical isomers as a result of restricted rotation around the carbon-carbon particle double bond resulting from conjugation of the ylide moiety with the adjacent carbonyl group. In the recent work, NMR study and the assignment of more stable Z- or E- isomers as the major form was investigated using AIM theoretical calculations.