THEORETICAL-STUDY OF THE CYCLIZATION OF ALPHA-IMINOTHIOALDEHYDES INTODIHYDROTHIAZOLES

When synthesized by retro-Diels-Alder reaction under flash vacuum ther molysis conditions, alpha-iminothioaldehydes undergo a rearrangement t o 2,3-dihydro-1,3-thiazoles. These secondary enamines in turn are tran sformed into the more stable 2,5-dihydro-1,3-thiazoles upon standing i n CDCl3 solution, The route from alpha-iminothioaldehydes to dihydro-1 ,3-thiazoles has been studied by ab initio molecular orbital theory. G eometries of stationary points on the [C3H5NS] potential energy surfac e were optimized using HF/6-31G(d,p) and MP2/6-31(d,p) calculations. R elative energies were estimated using MP4STDQ [and in some cases QCISD (T)] calculations and the 6-31 + G(d,p) basis set and corrected for ze ro-point vibrational energies. Intrinsic reaction coordinate calculati ons at HF/6-31G(d,p) level were performed in other to check the profil e of the proposed rearrangement. These calculations support the experi mental findings. The most energy requiring step was found to be trans- ->cis isomerization of the methylimino group [27.8 kcal mol(-1) at the QCISD(T)/6-31 + G(d,p) level].