| Summary: | Stable crystalline phosphorus ylides were obtained in excellent yields from the 1:1:1 addition reactions between triphenylphosphine and dialkyl acetylenedicarboxylates, in the presence of NH-heterocyclic compound, such as 6-chloro-2-benzoxazolethiol. These stable ylides exist in solution as a mixture of the two geometrical isomers as a result of restricted rotation around the carbon–carbon partial double bond resulting from conjugation of the ylide moiety with the adjacent carbonyl group. In the recent work, NMR study and the stability of the Z- and E-isomers were undertaken for the two rotamers of phosphorus ylides involving 6-chloro-2-benzoxazolethiol by atoms in molecules (AIM) and natural population analysis (NPA) methods. The relative energy for the two Z and E isomers was calculated at both HF/6-31G (d,p) and B3LYP/6-311++G (d,p) in the presence of a solvent medium (ethyl acetate) and gas phases. The results were in good agreement with those that obtained by the 1H, 31P and 13C NMR experimental data.
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