Remarkably Stable Copper(I) α-Carbonyl Carbenes: Synthesis, Structure, and Mechanistic Studies of Alkene Cyclopropanation Reactions

The reaction of [tBu(2)P(NSiMe3)(2)-kappa N-2]Cu(eta(2)-C2H4) (1) with various aryl diazo esters [p-X-C6H4]C(N-2)[C(O)R] allowed us to synthesize the corresponding alpha-carbonyl copper(l) carbene complexes [tBu(2)P(NSiMe3)(2)-kappa N-2]Cu=C[p-X-C6H4][C(O)R] (8), where X = OMe, NO2. The rotation barriers around the Cu-C-carbene and C-carbene-C-Ar bonds or their low limits were determined for some of these compounds by H-1-VT-NMR spectroscopy. Carbene 8g with X = OMe and R = OCH(p-Cl-C6H4)(2) was isolated in analytically pure, crystalline form as the first stable representative of this important class of compounds. Its solid-state molecular structure revealed an orthogonal position of the carbene fragment relative to both the ligand plane and the ester C=O group and a remarkably short Cu-C-carbene distance of 1.822(4) angstrom. Compound 8g reacted with styrene stereoselectively to give the corresponding trans-cyclopropane derivative and [tBu(2)P(NSiMe3)(2)-kappa N-2]Cu(eta(2)-CH2=CHPh). The stoichiometric cyclopropanation of styrene with 8g and the previously described diarylcarbene [tBu(2)P(NSiMe3)(2)-kappa N-2]Cu=C[p-NO2-C6H4](2) (6) in toluene-d(8) revealed that the reactions are first order in both the copper carbenes and the alkene. The activation parameters for 8g (Delta H double dagger = 51.5(9) kJ/mol and Delta S double dagger = - 127.1(28) J/(mol K)] and for 6 (Delta H double dagger = 53.4(8) kJ/mol and Delta S double dagger = - 152.1(23) J/(mol K)) were derived from the kinetics of the cyclopropanation processes. Thermal decomposition of carbene 8g in toluene-d8 displayed first-order kinetics until 20-25% conversion with activation parameters Delta H double dagger = 85.5(24) kJ/mol and Delta S double dagger = -49.0(76) J/(mol K). Solutions of 6 in toluene-d(8) also decompose in a first-order fashion with Delta H double dagger = 66.1(20) kJ/mol and Delta S double dagger = - 125.5(56) J/(mol K). A Hammett study employing 8g and para-substituted styrenes afforded rho = - 1.06(19), demonstrating the electrophilic nature of alpha-carbonyl copper(I) carbene (Fischer-type) complexes. The electronic structure of 8 with X = R = OMe was investigated by DFT methods.