A DFT Study on the Barton–Kellogg Reaction – The Molecular Mechanism of the Formation of Thiiranes in the Reaction between Diphenyldiazomethane and Diaryl Thioketones

Abstract

The mechanism of the reaction of diphenyldiazomethane with thiobenzophenone and related hetaryl thioketones leading to the corresponding tetrasubstituted thiiranes was studied by means of DFT computational methods at the M06-2X/6-311+G(d) level of theory. The study showed that the initial step of this conversion is a classic one-step (3+2) cycloaddition to give 1,3,4-thiadiazolines in a regioselective manner. These initially formed products undergo a spontaneous extrusion of N2 resulting in the formation of tetraaryl-substituted thiocarbonyl ylides as reactive intermediates. The calculated parameters of the activation for the subsequent 1,3-dipolar electrocyclizations are low (ΔG = 10.8–11.9 kcal/mol) and the only products formed in these reactions are the corresponding thiiranes. The alternative pathways leading to thiocarbonyl ylides via stepwise mechanisms involving zwitterionic or biradical nitrogen-containing intermediates were ruled out.