Dependence of Ring-Opening Reaction of Five-Membered Dithiocarbonates on Cationic Catalyst: Control of Isomerization and Polymerization

The selective cationic isomerizations and ring-opening polymerizations of cyclic five-membered dithiocarbonates ( 1a, 5-phenoxymethyl-1,3-oxathiolane-2-thione; 1b, 5-methyl-1,3-oxathiolane-2-thione; 1c, 5-ethyl-1,3-oxathiolane-2-thione; 1d, 5-chloromethyl-1,3-oxathiolane-2-thione) have been achieved by the selection of the catalysts, which have enabled the first facile preparations of 1,3-dithiolan-2-one derivatives ( 2a − d ) and polydithiocarbonates ( P1a − d ). The mechanism of the selectivity was elucidated by the 1 H and 13 C NMR spectroscopic analyses for their reaction intermediates. The initiation steps of the reactions of 1a with TfOEt or TfOH were different, wherein the formation of the cyclic carbenium ion ( 6 ) via the ethylation of TfOEt on the thiocarbonyl sulfur and the oxonium cation ( 7 ) via the protonation of TfOH on the ether oxygen were observed, respectively. The cation 7 might not be electrophilic enough to be attacked by the monomer ( 1 ), which could explain the fact that the results in the reaction of 1 selectively gave the isomer with TfOH. The polymerization and isomerization should proceed predominantly via 6 and 7, respectively. The reactivities of 1a − d decreased in the order of 1b ≅ 1c > 1a > 1d, which showed good agreement with the 1 H NMR chemical shifts of the methine protons of 6a − d .