Synthesis, reactions and DFT study of tropolone N,N-dimethylthiocarbamate

A new tropolone derivative namely O-(7-oxocyclohepta-1,3,5-trienyl) dimethylthiocarbamate was synthesized from tropolone and N,N-dimethylthiocarbamoyl chloride and its reactions with lithium halides in acidic media were examined. The product of each lithium halide treatment was identical and identified as S-(3-oxocyclohepta-1,4,6-trienyl) dimethylcarbamothioate. Eventually it was converted to 3-mercapto 2,4,6-cycloheptatrien-1-one on treatment with base. DFT calculations at the B3LYP/6-311++G∗∗level were performed on the key elimination step of the mechanism, which was proposed by Ponaras et al. on analogous five-membered ring systems. The experimentally observed regioselectivity was correlated to the calculated activation barriers on five and seven-membered systems in the elimination step such that in both cases, experimentally observed product was favored over the alternative one. Thus, quantum mechanical calculations have provided insights into the synthetic pathway and regioselectivity of the reaction in this study, in comparison to the analogous five-membered system.