Selective Hydrosilation of CO2 to a Bis(silylacetal) Using an Anilido Bipyridyl-Ligated Organoscandium Catalyst

A rigid anilido bipyridyl ligand has been designed for use in an organoscandium-based carbon dioxide hydrosilation catalyst. Ligand attachment by alkane elimination results in metalation of an aryl C-H bond in a 3,5-di-tert-butylphenyl group installed on the pyridyl unit, thus rendering the ligand tetradentate. Insertion of CO2 into the newly formed Sc-C bond leads to a kappa(1) carboxylate which, when treated with the borane B(C6F5)(3), becomes hemilabile. In addition to activating the catalyst, the k(1) carboxylate effectively sequesters free B(C6F5)(3) and the ensemble is able to effectively hydrosilate CO2, in the presence of excess Et3SiH, almost exclusively to R3SiOCH2OSiR3. A maximum turnover number of about 3400 (conversion of silane) is observed. Mechanistic experiments suggest that the sequestration of free B(C6F5)(3) by the hemilabile carboxylate contributes to the selectivity observed and prevents over reduction to methane.