Mechanistic Aspects of the Metal Catalyzed Alternating Copolymerization of Epoxides and Carbon Monoxide

Abstract

The cobalt-catalyzed alternating copolymerization of epoxides and CO is a novel, direct approach to aliphatic polyesters, such as poly(hydroxybutyrate) (PHB). This reaction was found to be catalyzed by Ph3Si[Co(CO)4] (4) and pyridine affording in a first step the stable mono-insertion product Ph3Si-O-CH(CH3)-CH2-CO-Co(CO)4 (5). However, a profound mechanistic understanding, especially of the role of pyridine as the key component for the polymerization reaction was missing. ATR-IR online monitoring under catalytic conditions and DFT calculations were used to show that an acylpyridinium cation is formed by cleavage of the cobalt-acyl bond of 5 in the presence of pyridine. The Lewis acid thus generated activates the next incoming epoxide monomer for ring opening through [Co(CO)4]-. The catalytic cycle is completed by a subsequent CO insertion in the new cobalt-alkyl bond. The calculations are used to explore the energetic hypersurface of the polymerization reaction and are complemented by extended experimental investigations that also support the mechanistic hypotheses.

Perfectly Alternating Copolymerization of CO and Epoxides to Aliphatic Polyester Oligomers via Cooperative Organoboron–Cobalt Complexes

“…It should be mentioned that, for carbonylative polymerization of CO and epoxides, two previous mechanisms have been proposed: one is the mentioned intermolecular mechanism that was first proposed by Osakada , and the other involves the continuous insertion of CO/epoxides at the cobalt site . The later process requires the participation of the organic base to cleavage the acyl–cobalt for the next epoxide insertion. , Without the additional organic base, the PHA oligomers in this work were anticipated to propagate through the intermolecular attack. Furthermore, the continuous formation of β-lactone provides additional corroboration since the intermolecular reaction would regenerate the catalyst 1a for cycle 1.…”

“…Inspired by the effectiveness of using Al­( i Bu) 3 /H 2 O/cobalt­(III) acetylacetonate for copolymerization of epoxides/CO, a number of catalysts featuring the general formula [Lewis acid] + [Co­(CO) 4 ] − were developed to convert epoxides/CO to PHAs or the corresponding lactones. − The Lewis acid in [Lewis acid] + [Co­(CO) 4 ] − is used for the epoxide activation or stabilization of the alkoxide chain end, and the charge-balancing anionic [Co­(CO) 4 ] − mediates the ring opening of the epoxides and the insertion of CO. Cationic Al, ,− Cr, − Ti, and Co − Lewis acids with elaborate ligands such as salen and porphyrin have been elegantly explored, especially to achieve regio- and/or stereoselective reactions. − In comparison to the above bimetallic catalysts, the monometallic acyl–cobalt complexes have the sustainable benefit of substituting 50% of the metal atom, since they are free of metallic Lewis acid. The successful utilization of monometallic acyl–cobalt complexes for carbonylative polymerization was reported by the research groups of Jia, − Nozaki, and Rieger . In certain circumstances, the utilization of effective and classical cobalt can be circumvented.…”

Lewis Base Catalysis in Organic Synthesis

“…Recent theoretical and experimental studies have supported this mechanism, thus demonstrating the importance of both Lewis acid and base activation in the epoxide opening reaction. [351] Alper and co-workers have extended this method through the combination of organic-soluble cobaltate salts and Lewis acids (LA), such as BF 3 ·Et 2 O.…”

“…Recent theoretical and experimental studies have supported this mechanism, thus demonstrating the importance of both Lewis acid and base activation in the epoxide opening reaction. [351] Alper and co-workers have extended this method through the combination of organic-soluble cobaltate salts and Lewis acids (LA), such as BF 3 ·Et 2 O. [352] Coates and co-workers have introduced a new class of catalysts of the general formula [LA] + [Co(CO) 4 ] À where [LA] + is an aluminum(III)-salen, Cp 2 titanium(III), or chromium(III)porphyrinate species.…”

Application of quantum calculations in the chemical industry—An overview

“…In a third paper, a catalyst system is presented that allows for the direct conversion of the comonomers to poly(hydroxy butyrate) . Like the multisite catalyst for β‐butyro lactone synthesis, it is composed of plus a Lewis acid and, additionally, a pyridine derivative.…”