BIFUNCTIONAL CATALYSIS OF THE REACTION OF CARBON-DIOXIDE WITH OXIRANES

Bifunctional Catalysis of the Reaction of Carbon Dioxide with Oxiranes Kinetic investigation of the Lewis acid/Lewis base‐catalyzed reaction of carbon dioxide with monosubstituted oxiranes 1 to cyclic carbonates 2 at atmospheric pressure reveals that the rate depends first order on substrates and the catalyst components ZnCl2 and (n‐Bu)4NI. Electron‐releasing substituents accelerate the reaction, except they are sterically demanding like the tert‐butyl group in which case complete inhibition occus; 2,2‐disubstituted oxiranes are inert. The activation parameters are ΔH≠ = (71.4 ± 3.7) kJ ± mol‐1 und ΔS≠ = (–49.2 ± 9.3) J ± K‐1. A rate law is established for the multi‐step reaction mechanism and simplified for the extremes of high and low pressure of carbon dioxide. The reactivity pattern of several catalyst combinations suggests that solvated zinc chloride and trihalogenozincates are the Lewis acids which catalyze the nucleophilic ring opening of the oxirane by iodide to the corresponding iodo alcoholate. This is stabilized by the Lewis acid as indicated by the investigation of the stereochemistry at the unsubstituted ring carbon atom by the use of [2‐D]‐2‐phenyloxirane. Nucleophilic attack of te alcoholate at carbon dioxide leads t the linear carbonate which is transformed into the product by intramolecular substitution of iodide. Copyright © 1990 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim