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 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.