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Herein, a series of separate dynamic (1)H NMR effects are reported at different temperatures within a particular enaminoester involving a phenanthridine. These effects are attributed to restricted rotation around the two single bonds such as carbon-carbon (H(a)-C-C-H(b)) and nitrogen-carbon (NCCOOCH(3)). Activation energies (E(a)) for these interconversion processes in their rotational isomers are equal to 20 and 20 ± 1 kJ mol(-1), respectively. In addition, three dynamic (1)H NMR effects are investigated at different temperatures for a particular phosphorus ylide involving a 2-indolinone around the carbon-carbon single bond (H-C-C-PPh(3)) within the two Z- and E-rotational isomers and partial carbon-carbon double bond (OCH(3)-C=C-PPh(3)). Activation energies (E(a)) for these interconversion processes in rotational isomers are equal to 53, 63 and 73 ± 1 kJ mol(-1) , respectively. This behavior was also observed for other phosphorus ylide containing 2-mercaptobenzoxazole around the carbon-carbon single bond and partial carbon-carbon double bond with their relevant activation energies containing 13, 10 and 75 ± 1 kJ mol(-1), respectively.

ABSTRACT:In the present work, NMR, theoretical, kinetics, and mechanism investigations were undertaken for a one-pot condensation reaction between 2-mercapto-4,6-dimethyl pyrimidine and dialkyl

Triphenylphosphine reacts with dialkyl acetylenedicarboxylates in the presence of methyl carbamate to generate stable phosphorus ylides. These ylides exist in solution as a mixture of 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. Quantum mechanical calculations were employed to check the stability of the two geometrical isomers. To determine the kinetic parameters and mechanistic investigation of the reactions, they were monitored by UV spectrophotometry. Useful information was obtained from studies of the effect of solvent, the structure of reactants (different alkyl groups within the dialkyl acetylenedicarboxylates), and also the concentration of reactants, on the rate of reactions. The proposed mechanism was confirmed according to the obtained experimental results, and a steady state approximation. The first (k 2 ) and third (k 3 ) steps of the reactions were recognized as the ratedetermining and fast steps, respectively, on the basis of experimental data.