Multisite Catalysis: A Mechanistic Study of β‐Lactone Synthesis from Epoxides and CO—Insights into a Difficult Case of Homogeneous Catalysis

Abstract

Carbonylation of epoxides with a combination of Lewis acids and cobalt carbonyls was studied by both theoretical and experimental methods. Only multisite catalysis opens a low-energy pathway for trans opening of oxirane rings. This ring-opening reaction is not easily achieved with a single-site metal catalyst due to structural and thermodynamic constraints. The overall reaction pathway includes epoxide ring opening, which requires both a Lewis acid and a tetracarbonylcobaltate nucleophile, yielding a cobalt alkyl-alkoxy-Lewis acid moiety. After CO insertion into the Co-C(alkyl) bond, lactone formation results from a nucleophilic attack of the alkoxy Lewis acid entity on the acylium carbon atom. A theoretical study indicates a marked influence of the Lewis acid on both ring-opening and lactone-formation steps, but not on carbonylation. Strong Lewis acids induce fast ring opening, but slow lactone formation, and visa versa: a good balance of Lewis acidity would give the fastest catalytic cycle as all steps have low barriers. Experimentally, carbonylation of propylene oxide to beta-butyrolactone was monitored by online ATR-IR techniques with a mixture of tetracarbonylcobaltate and Lewis acids, namely BF(3), Me(3)Al, Et(2)Al(+).diglyme, and a combination of Me(3)Al/dicobaltoctacarbonyl. We found that the last two mixtures are extremely active in lactone formation.

On the Formation of Aliphatic Polycarbonates from Epoxides with Chromium(III) and Aluminum(III) Metal–Salen Complexes

“…The function of the chromium complex is of a dedicated Lewis acid that polarizes and activates the epoxide, and is capable of forming a stable metalate complex. The reaction has major similarities to what has been described for the formation of lactones from epoxides and CO. [67] The activation barrier for the nucleophilic attack is negligible ( (Table 1). Likewise, it was found that the ring-opening event occurred easily.…”

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

“…We have previously reported in experimental and theoretical studies on the control of the product distribution by choice of the proper Lewis acid. [7,9] The intermediate species LA-O-CH(CH 3 )-CH 2 -CO-Co(CO) 4 (2), that 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 Ph 3 Si-[Co(CO) 4 ] (4) and pyridine affording in a first step the stable mono-insertion product Ph 3 Si-O-CH(CH 3 )-CH 2 -COCo(CO) 4 (5).…”

“…[9] At this stage, for many systems, lactone formation is the next step in the catalytic cycle. However, in the presence of a base/nucleophile such as pyridine, the situation is different and polymer is produced.…”

“…In the presence of other (permanent) Lewis acids, lactone formation would be the main route. [9] A second, important factor for the suppression of the lactone formation cycle is a stable chain end of the growing polymer chain (Ph 3 Si-O-). [7] …”

“…Based on the mechanism of lactone formation from epoxides and CO, as described in ref. [9] a Lewis acid is required to activate the epoxide prior to the nucleophilic attack. Therefore, an acyl-pyridinium species was proposed to be a good candidate for the Lewis acid able to activate PO.…”