REGIOCHEMISTRY IN CYCLOADDITIONS OF DIAZOMETHANE TO THIOFORMALDEHYDE AND THIOKETONES

In cycloadditions of open-chain aliphatic thiones (R2C=S, R = ethyl, n-propyl, isopropyl, and tertbutyl) to diazomethane mixtures of regioisomers were obtained; the ratio depends on R and solvent polarity. Ab initio RHF and CASSCF calculations (3-21G*,6-31G*, CAS/3-21G*) and semiempirical calculations (AM1 and MNDO-PM3) are carried out on ground states, possible transition structures, and intermediates. Extensive ab initio calculations on the reaction of diazomethane with thioformaldehyde suggest that both regioisomers should be formed via concerted pathways. In contrast, AMI and MNDO-PM3 advocate a concerted formation of the 1,3,4-thiadiazoline and a two-step reaction to the 1,2,3-isomer involving the electrocyclic ring closure of an intermediate of pentadienyl anion type. The ab initio calculations (3-21G*) favor the 1,2,3-over the 1,3,4-thiadiazoline for thioformaldehyde as dipolarophile by DELTADELTAH(double dagger) = 2.4 kcal mol-1, for dimethyl thioketone the DELTADELTAH(double dagger) value decreases to 0.5 kcal mol-1, and for the diethyl compound both DELTAH(double dagger) values are identical. A similar trend is observed for the 6-31G* basis set: the CASSCF single-point calculations of the 3-21G* transition structure for the reaction of thioformaldehyde reduces the difference in favor of the 1,2,3-isomer to 0.5 kcal mol-1. The preference for the 1,2,3-thiadiazoline structure in more polar solvents is explained by the higher dipole moment (ca. 5 D as compared to ca. 2 D) of the transition structure. The ab initio calculations seem to be in better agreement with experiment than the AM1 or MNDO-PM3 results.