Oxygen/Sulfur Scrambling During the Copolymerization of Cyclopentene Oxide and Carbon Disulfide: Selectivity for Copolymer vs Cyclic [Thio]carbonates

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 [thio]­carbonates 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 [thio]­carbonate
compounds displaying both cyclopentyl and cyclohexyl backbones were
produced. A proposed mechanistic pathway for atom scrambling involves
nucleophilic alkoxide attack at a [thio]­carbonyl center to induce
oxygen/sulfur atom exchange.