(PNP)Cobalt-Catalyzed Olefination of Diazoalkanes

Addition of excess diazoalkane to the pincer-supported cobalt(I) dinitrogen complex (^tBumPNP)CoN₂(^tBumPNP = modified 2,6-bis[(ditert-butylphosphino)methyl]pyridine) resulted in the catalytic formation of the homocoupled alkene product with concomitant loss of N₂. Monosubstituted diazoalkanes, trimethylsilyldiazomethane and tolyldiazomethane, generated the olefin product in quantitative yield with exclusive (E)-stereoselectivity. Disubstituted diazoalkanes, diphenyldiazomethane and 9-diazofluorene, also yielded the olefin as the major product along with minor azine coupling. Investigations into the nature of the diazoalkane-cobalt interaction by multinuclear NMR spectroscopy and X-ray diffraction established end-on diazoalkane cobalt complexes as the resting states. The isolated four-coordinate cobalt diazoalkane complexes promoted conversion to the corresponding olefin. The reaction of (^tBumPNP)CoN₂with an α-diazo-β-ketoester resulted in the formation of a five-coordinate Co(I)-diazoalkane complex with a chelating ester unit that was unreactive for olefination.