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

GRAPHICAL ABSTRACT

ABSTRACT

It has been shown that certain N-aryltrichoromethanesulfenamides Cl₃C−S−NHAr can be dehydrochlorinated to form red-colored thiocarbonyl S-imides CCl₂=S=NAr which dimerize spontaneously. The dimers so far obtained were first assigned the 1,4,2,5-dithiadiazinane structure but later shown to be the isomeric 1,4,2,3-dithiadiazinanes. It is proposed that both the so far unknown 1,4,2,5-dithiadiazinanes and the so far elusive thiocarbonyl S-imides R¹R²C=S=NR³ might be made directly observable by introducing massive steric strain via bulky R in the starting chlorinated sulfenamides such as Cl₃C−S−NHAr and CCl₃CCl₂−S−NHR. – According to PM7 and DFT type MO calculations of the enthalpies, the formation of the 1,4,2,5-dithiadiazinanes is thermodynamically favored over that of the 1,4,2,3-isomers. However, the activation energies for the cyclization reaction are lower for the head-to-head dimerization. Thus, the observed dimerization of thiocarbonyl S-imides under formation of 1,4,2,3-dithiadiazinanes is a kinetically controlled reaction.

KEYWORDS:

• 1,3-Dipoles, 1,4,2,3-dithiadiazinanes
• 1,4,2,5-dithiadiazinanes
• head-to-head dimerization
• head-to-tail dimerization
• MO calculations
• N-monosubstituted pentachloroethanesulfenamides
• N-monosubstituted trichloromethanesulfenamides
• 2-thiacumulenes
• thiocarbonyl S-imides

Acknowledgments

We thank Dr. James Stewart and Dr. Alex Granovsky for providing us with the latest versions of Mopac 2012 and Firefly 8.1.0, respectively.

Funding

A travel grant to A. S. from Kemisk Forenings Rejsefond (the Danish Chemical Society's Travel Fund) is gratefully acknowledged.