Investigation of the Kinetic Resolution of Terminal Epoxides by Al(III), Co(III), and Cr(III)salen Complexes

Although the industrial era has undoubtedly improved standards of living, it has also resulted in a gradual but definite increase in atmospheric carbon dioxide levels. Consequently, undesirable weather events have intensified. Thus, the use of carbon dioxide as a feedstock is attractive, particularly if the reaction takes place under mild conditions. In this work, we utilise carbon dioxide in the conversion of epoxides to enantio-enriched cyclic carbonates, which may be used en route to the production of valuable pharmaceuticals. The application of aluminium(salen), chromium(salen), and cobalt(salen) catalysts enables this 100 % atom-economical process to take place under 1 bar of CO2.
The kinetic resolution of epoxides via the insertion of carbon dioxide has hitherto been reported only for cobalt(salen) complexes. Jacobsen’s study of kinetic resolution of epoxides using water as the ring-opening agent in the presence of the same cobalt(salen) catalysts showed that cobalt(salen) complexes form a “stepped” bimetallic transition state, which presents as a second-order rate dependence on catalyst. In this thesis, we show that ring-opening with carbon dioxide may operate via a different pathway as a second-order rate dependence on catalyst was not observed. Investigation of existing crystal structures of aluminium(salen), cobalt(salen), and chromium(salen) complexes in the Cambridge structural database were similarly inconclusive in demonstrating the relationship between the “step” of the complex and the enantioselectivity.
Furthermore, we demonstrate the successful kinetic resolution of both phenyl glycidyl ether and N-(2,3-epoxypropyl)diphenylamine in the presence of aluminium(salen) and chromium(salen) complexes, which has not previously been reported. The maximum krel value obtained for N-(2,3-epoxypropyl)diphenylamine as substrate was 15.38 with our aluminium(salen) catalysts, and 7.32 with our chromium(salen) catalysts. In contrast, the maximum krel value obtained using our cobalt(salen) complexes was 2.26.