AN AB-INITIO STUDY OF THE THERMAL FRAGMENTATION OF N-ACYLOXYPYRIDINE-2-THIONES - A [3,3]SIGMATROPIC PROCESS INITIATES THE RADICALAR CHAIN

The possibility of [3,3]sigmatropic rearrangement in thiohydroxamic ac id esters has been studied using ab initio MO calculations. The rearra ngement in N-acyloxypyridine-2-thiones is predicted to be feasible wit h an activation energy (DELTAH(double dagger)) of 23 kcal mol-1 and en thalpy change (DELTAH) = -22 kcal mol-1. The rearrangement proceeds vi a an early chair-like transition state (TS) in a concerted manner. The TS is similar to the TS observed for the rearrangement in allyl vinyl ether. The perester, thus formed, cleaves homolytically to furnish, a fter decarboxylation, the necessary alkyl radical R which is believed to propagate the decomposition chain in Barton fashion. It is, therefo re, proposed that a [3, 3] sigmatropic rearrangement precedes any radi calar fragmentation of N-acyloxy pyridine-2-thiones. The present study provides a 'new outlook' to the thermal fragmentation of thiohydroxam ic acid esters.