Reactions of Diazomethanes with 5‐Benzylidene‐3‐phenylrhodanine – A Computational Study

Abstract

AbstractIt has been shown previously that the reaction of diazomethane with 5‐benzylidene‐3‐phenylrhodanine (1) in THF at −20° occurs at the exocyclic CC bond via cyclopropanation to give 3a and methylation to yield 4, respectively, whereas the corresponding reaction with phenyldiazomethane in toluene at 0° leads to the cyclopropane derivative 3b exclusively. Surprisingly, under similar conditions, no reaction was observed between 1 and diphenyldiazomethane, but the 2‐diphenylmethylidene derivative 5 was formed in boiling toluene. In the present study, these results have been rationalized by calculations at the DFT B3LYP/6‐31G(d) level using PCM solvent model. In the case of diazomethane, the formation of 3a occurs via initial Michael addition, whereas 4 is formed via [3+2] cycloaddition followed by N2 elimination and H‐migration. The preferred pathway of the reaction of 1 with phenyldiazomethane is a [3+2] cycloaddition, subsequent N2 elimination and ring closure of an intermediate zwitterion to give 3b. Finally, the calculations show that the energetically most favorable reaction of 1 with diphenyldiazomethane is the initial formation of diphenylcarbene, which adds to the S‐atom to give a thiocarbonyl ylide, followed by 1,3‐dipolar electrocyclization and S‐elimination.