Theoretical study of the cyclization of α-iminothioaldehydes into dihydrothiazoles

Abstract

When synthesized by retro-Diels–Alder reaction under flash vacuum thermolysis conditions, α-iminothioaldehydes undergo a rearrangement to 2,3-dihydro-1,3-thiazoles. These secondary enamines in turn are transformed into the more stable 2,5-dihydro-1,3-thiazoles upon standing in CDCl₃ solution. The route from α-iminothioaldehydes to dihydro-1,3-thiazoles has been studied by ab initio molecular orbital theory. Geometries of stationary points on the [C₃H₅NS] potential energy surface were optimized using HF/6-3 1G(d,p) and MP2/6-31(d,p) calculations. Relative energies were estimated using MP4STDQ [and in some cases QCISD(T)] calculations and the 6-31 + G(d,p) basis set and corrected for zero-point vibrational energies. Intrinsic reaction coordinate calculations at HF/6-31 G(d,p) level were performed in order to check the profile of the proposed rearrangement. These calculations support the experimental 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].