Remarkably Stable Copper(I) α-Carbonyl Carbenes: Synthesis, Structure, and Mechanistic Studies of Alkene Cyclopropanation Reactions
journal contributionposted on 23.02.2009, 00:00 by Igor V. Shishkov, Frank Rominger, Peter Hofmann
The reaction of [tBu2P(NSiMe3)2-κ2N]Cu(η2-C2H4) (1) with various aryl diazo esters [p-X-C6H4]C(N2)[C(O)R] allowed us to synthesize the corresponding α-carbonyl copper(I) carbene complexes [tBu2P(NSiMe3)2-κ2N]CuC[p-X-C6H4][C(O)R] (8), where X = OMe, NO2. The rotation barriers around the Cu−Ccarbene and Ccarbene−CAr bonds or their low limits were determined for some of these compounds by 1H-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 CO group and a remarkably short Cu−Ccarbene distance of 1.822(4) Å. Compound 8g reacted with styrene stereoselectively to give the corresponding trans-cyclopropane derivative and [tBu2P(NSiMe3)2-κ2N]Cu(η2-CH2CHPh). The stoichiometric cyclopropanation of styrene with 8g and the previously described diarylcarbene [tBu2P(NSiMe3)2-κ2N]CuC[p-NO2-C6H4]2 (6) in toluene-d8 revealed that the reactions are first order in both the copper carbenes and the alkene. The activation parameters for 8g (ΔH⧧ = 51.5(9) kJ/mol and ΔS⧧ = −127.1(28) J/(mol K)] and for 6 (ΔH⧧ = 53.4(8) kJ/mol and ΔS⧧ = −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 ΔH⧧ = 85.5(24) kJ/mol and ΔS⧧ = −49.0(76) J/(mol K). Solutions of 6 in toluene-d8 also decompose in a first-order fashion with ΔH⧧ = 66.1(20) kJ/mol and ΔS⧧ = −125.5(56) J/(mol K). A Hammett study employing 8g and para-substituted styrenes afforded ρ = −1.06(19), demonstrating the electrophilic nature of α-carbonyl copper(I) carbene (Fischer-type) complexes. The electronic structure of 8 with X = R = OMe was investigated by DFT methods.