Activation of carbon dioxide with aluminum porphyrin and reaction with epoxide. Studies on (tetraphenylporphinato)aluminum alkoxide having a long oxyalkylene chain as the alkoxide group

Abstract

(Tetraphenylporphinato)aluminum alkoxide ((TPP)AlOR) having a long oxyalkylene chain as the alkoxide group was prepared by the polymerization of epoxide with (tetraphenylporphinato)aluminum chloride as catalyst [to study artificial CO2 fixation]. The reactivity of this aluminum alkoxide toward CO2 was markedly enhanced in the presence of 1-methylimidazole. CO2 thus trapped with the (porphinato)aluminum alkoxide was activated enough to react with epoxide to yield cyclic carbonate. The spectroscopic investigation of the reaction system, favored by the enhanced solubility of the metalloporphyrin due to the long oxyalkylene chain, indicated that cyclic carbonate was formed by the insertion of CO2 followed by that of epoxide into the Al-O bond of (TPP)AlOR to give the aluminum alkoxide (TPP)Al-O-CHR-CH2-O-CO-OR and subsequent intramolecular attack of this aluminum alkoxide toward the adjacent linear carbonate linkage, regenerating the starting alkoxide, (TPP)AlOR.