The dimerization of thiophosgene and trichlorothioacetyl chloride S-imides – Head-to-head and/or head-to-tail?

A DFT study on the mechanism of the formation of 1,4,2,3‐dithiadiazinanes by head‐to‐head [3 + 3] cyclodimerization of thiocarbonylS‐imides

“…In continuation of our formerly published semiempirical PM7 [ 18 ] results concerning the observed formation of 5,5,6,6‐tetrachloro‐1,4,2,3‐dithiadiazinanes 2 instead of the expected 3,3,6,6‐tetrachloro‐1,4,2,5‐dithiadiazinanes 3 , [ 9,10 ] we now conducted more sophisticated DFT calculations on the reaction mechanism.…”

“…This dimerization could occur either as a head‐to‐head ( → 1,4,2,3‐dithiadiazinane) or as a head‐to‐tail ( → 1,4,2,5‐dithiadiazinane) process. [ 9,10 ]…”

“…Interestingly and in contrast to the expected head‐to‐tail 33CD, preferably the regioselective head‐to‐head 33CD to form the corresponding 1,4,2,3‐dithiadiazinanes 2 (R 1 = Cl, R 2 = Ar) occurs with thiophosgene S ‐( N ‐aryl)imides 1 (R 1 = Cl, R 2 = Ar). [ 9,10 ] This reaction course is typical of the 33CD reaction of another type of sulfur‐centered TACs, the better known thiocarbonyl S ‐methanides Ar 2 C=S=CR 2 , which leads to 4,4,5,5‐tetraaryl‐1,4‐dithianes. [ 11,12 ] The aim of our computational study was thus to propose an explanation of this counter‐intuitive regioselectivity.…”