Asymmetric Addition of Carbon Dioxide

Carbon dioxide is one of the most abundant carbon resources on earth. It reacts with an epoxide to give either a cyclic carbonate or a polycarbonate depending on the substrates and reaction conditions. Kinetic resolution of racemic propylene oxide is reported in the formation of both cyclic carbonate and polycarbonate. The fe ei value defined as ln[l-(conversion)(l+%ee)]/ln[l-(conversion)(l% ee)] reached 6.4 or 5.6 by using a Co(OTs)-salen complex with tetrabutylammonium chloride under neat propylene oxide or using a combination of a Co-salen complex and a chiral DMAP derivative in dichloromethane, respectively. 

Kinetic resolution of propylene oxide in its alternating copolymerization with CO2 is performed using similar Co-salen complexes. Reaction conditions, 

Co(OAr)-salen complex [Ar = 2,4-(N02)2CeH3] with tetrabutylammonium chloride under neat propylene oxide, quite similar to the conditions for the cyclic carbonate synthesis, give polycarbonate with fe ei of 3.5. ° Without any additives, the use of Co(OAc)-salen provides the polycaronate with fe ei of 2.8.  

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Berkessel, A. and Brandendurg, M. (2006) Catalytic asymmetric addition of carbon dioxide to propylene oxide with unprecedented enantioselectivity. Organic Letters, 8, 4401 1404. 

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