Dramatic Behavioral Differences of the Copolymerization Reactions of 1,4-Cyclohexadiene and 1,3-Cyclohexadiene Oxides with Carbon Dioxide

The copolymerization of 1,3-cyclohexadiene oxide (1,2-epoxy-3-cyclohexene) with CO2 in the presence of the binary catalyst (salen)­CoX or (salen)­CrX and onium salts was shown to selectively afford the completely alternating copolymer poly­(1,3-cyclohexadiene carbonate) in good yield. In the process catalyzed by the cobalt­(III) system, the reaction was 100% selective for copolymer, whereas employing the higher temperature chromium­(III) catalyst, the reaction yielded in addition to copolymer a significant quantity of the cis-1,3-cyclohexadiene carbonate. Importantly, no corresponding trans-1,3-cyclohexadiene carbonate was produced. The reactivity of 1,3-cyclohexadiene oxide in coupling reactions with CO2 was strikingly greater than that of 1,4-cyclohexadiene oxide under either catalytic conditions. Authentic samples of the cis-cyclic carbonate and trans-cyclic carbonate were synthesized from epoxide and CO2 using ZnCl2/PPNI catalyzed and trans-diol/ethyl chloroformate routes, respectively. trans-1,3-Cyclohexadiene carbonate was fully characterized by X-ray crystallography. Unlike the copolymer derived from the symmetrical 1,4-cyclohexadiene oxide (1,2-epoxy-4-cyclohexene), deprotonation of poly­(1,3-cyclohexadiene carbonate) by a strong base did not lead to depolymerization with formation of the trans-cyclic carbonate. Computational studies revealed the trans-cyclic carbonate was thermodynamically unstable relative to the polycarbonate, with the enthalpy of reaction being +12.8 kcal/mol. The enhanced reactivity of the 1,3-isomeric epoxide versus that of its 1,4-isomer was further demonstrated by the facile terpolymerization reaction of 1,3-cyclohexadiene oxide with propylene oxide and CO2. This latter process is useful for the preparation of cross-linked or functionalized polycarbonates via thiol–ene chemistry.