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NMR study and quantum mechanical calculations for determination of more stable isomer of the E and Z isomers as a major or minor form in stable phosphorus ylide involving a 2-mercapto pyrimidine

Hojjat Ghasempour*, Gholamreza Foroghi-Nematallahi

Stable crystalline phosphorus ylides were obtained in excellent yields fromthe 1:1:1 addition reactions between triphenylphosphine and dialkyl acetylenedicarboxylates, in the presence of SH-heterocyclic compound, such as 2-mercapto pyrimidine. 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 the E- isomers were undertaken for the two rotamers of phosphorus ylides involving a 2- mercapto pyrimidine [namely dimethyl 2-(pyrimidine-2-sulfanyl)-3- (triphenyl phosphanylidene) butanedioate] by natural population analysis (NPA) and atoms in molecules (AIM) methods.