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

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 CoC 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 β ‐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.