A Fluxional α-Carbonyl Diazoalkane Complex of Copper Relevant to Catalytic Cyclopropanation

The neutral copper(I) ethylene complex [ t Bu 2 P(NSiMe 3 ) 2 -κ 2 N ]Cu(η 2 -C 2 H 4 ) ( 1 ) reacts with diazophenanthrone ( 2 ), yielding an equilibrium mixture of 1 and 2 with free ethylene and the novel diazoalkane copper(III) complex 3, which represents a stabilized analogue of early intermediates in the mechanism of copper-catalyzed cyclopropanation reactions of electron-rich olefins with α-carbonyl diazoalkanes. Its X-ray structure determination displays square-planar coordination of the formally d 8 copper(III) center with a κ O:κ N -bound diazophenanthrone. The geometry of the coordinated diazoalkane ligand can be directly compared to that of uncomplexed, cocrystallized diazophenanthrone, indicating a formal two-electron ligand reduction upon coordination. LT NMR spectroscopy reveals facile rotation of the diazoalkane ligand in solution even at temperatures below −110 °C. DFT calculations (B3LYP) on the model [H 2 P(NH) 2 -κ 2 N ]Cu(N 2 CHCHO-κ N:κO) ( 4 ) help to understand the bonding and fluxionality in 3 and predict a small ligand rotation barrier (through the transition state 4 ⧧ ), in accordance with experimental facts. For model 4, a low-energy pathway for dinitrogen extrusion and copper(I) carbene formation exists, involving a π-bound carbonyl diazoalkane complex and a carbon-coordinated diazoalkane copper(I) species. This pathway, and thus also cyclopropanation activity, is sterically blocked for 3, making it an isolable species.