Oxygen/Sulfur Scrambling During the Copolymerization of Cyclopentene Oxide and Carbon Disulfide: Selectivity for Copolymer vs Cyclic Thiocarbonates

The catalytic coupling of cyclopentene oxide with carbon disulfide has been investigated utilizing (salen)CrCl in the presence of added onium salts. Both polymeric and cyclic materials were produced, with oxygen/sulfur atom scrambling observed in both instances. This atom redistribution process was found to require (salen)CrCl and excess epoxide, though an increase in the rate of atom scrambling was noted upon the addition of the onium salt. Cyclopentene sulfide was observed as a side product of the coupling reaction and was found to be unreactive toward both CS2 and CO2, instead undergoing desulfurization to cyclopentene under the conditions of the reaction. Of the 12 cyclic cyclopentene thiocarbonates possibly produced by this coupling reaction, eight were observed, and the crystal structure of trans-cyclopentene trithiocarbonate is reported herein. Computational studies reveal that the cis- cyclic materials are more stable than their trans-counterparts by >5 kcal/mol of enthalpy, and there is a 10–25 kcal/mol preference for the formation of a C═O vs C═S double bond. When trans-cyclohexene trithiocarbonate was exposed to the catalyst system in the presence of excess cyclopentene oxide, mixed-species scrambling was observed, whereby cyclic thiocarbonate compounds displaying both cyclopentyl and cyclohexyl backbones were produced. A proposed mechanistic pathway for atom scrambling involves nucleophilic alkoxide attack at a thiocarbonyl center to induce oxygen/sulfur atom exchange.