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

Abstract

The reaction of [tBu 2 P(NSiMe 3 ) 2 -κ 2 N]Cu(η 2 -C 2 H 4 ) (1) with various aryl diazo esters [p-X-C 6 H 4 ]C(N 2 )[C(O)R] allowed us to synthesize the corresponding R-carbonyl copper(I) carbene complexes 8), where X ) OMe, NO 2 . 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 1 H-VT-NMR spectroscopy. Carbene 8g with X ) OMe and R ) OCH(p-Cl-C 6 H 4 ) 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 CdO group and a remarkably short Cu-C carbene distance of 1.822(4) Å. Compound 8g reacted with styrene stereoselectively to give the corresponding trans-cyclopropane derivative and [tBu 2 P(NSiMe 3 ) 2 -κ 2 N]Cu(η 2 -CH 2 dCHPh). The stoichiometric cyclopropanation of styrene with 8g and the previously described diarylcarbene [tBu 2 P(NSiMe 3 ) 2 -κ 2 N]CudC[p-NO 2 -C 6 H 4 ] 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 (∆H q ) 51.5(9) kJ/mol and ∆S q ) -127.1(28) J/(mol K)] and for 6 (∆H q ) 53.4(8) kJ/mol and ∆S q ) -152.1(23) J/(mol K)) were derived from the kinetics of the cyclopropanation processes. Thermal decomposition of carbene 8g in toluene-d 8 displayed first-order kinetics until 20-25% conversion with activation parameters ∆H q ) 85.5(24) kJ/mol and ∆S q ) -49.0(76) J/(mol K). Solutions of 6 in toluene-d 8 also decompose in a first-order fashion with ∆H q ) 66.1(20) kJ/mol and ∆S q ) -125.5(56) J/(mol K). A Hammett study employing 8g and para-substituted styrenes afforded F ) -1.06(19), demonstrating the electrophilic nature of R-carbonyl copper(I) carbene (Fischer-type) complexes. The electronic structure of 8 with X ) R ) OMe was investigated by DFT methods.

Synthesis and structures of gold and copper carbene intermediates in catalytic amination of alkynes

“…Therefore, copper complex 7 could be described as a copper carbene complex 7' bearing a mesoionic imidazolium-4-olate ring. The Cu1-C1 bond (1.924(6) Å) in 7 is similar to that (1.882(3) Å) for a three-coordinate cationic copper carbene [Cu{=CR 1 (OR 2 )}(MeCN)(OEt 2 )] + (R 1 =( E )-CH=CH-2-furyl; R 2 =menthyl)34, and longer than those (1.822–1.834 Å) observed for the copper carbenes obtained from either diphenyldiazomethane or α-carbonyl diazo compounds30313233. The imidazolium-4-olate ring in 7 is nearly co-planar with the carbene centre, and the neighbouring phenyl ring is tilted with a slightly contracted bond (1.470(8) Å) between the carbene centre and the C ipso (Ph) carbon.…”

Organometallic chemistry

“…In the mechanism of the Dötz reaction, he found that chromacyclobutene structures were unrealistic intermediates and instead proposed vinylcarbene complexes as much more stable isomers [64,68,88]. Other research projects of his group included the synthesis of ruthenium–carbene complexes with cis -dichloro ligands for olefin metathesis catalysis [86–87 90–91 98–100 108,132,138,147], the characterization of copper–carbene complexes as intermediates in the cyclopropanation of alkenes [102,106,113,119–120], as well as detailed studies into the mechanism of the hydroformylation of alkenes. An improved understanding of this industrially important process was achieved by a combination of ligand design for the rhodium catalysts, kinetic studies, and high-level quantum-chemical calculations [126,135,142,145–146 152,155].…”

Alkane Catalytic Functionalization by Carbene or Nitrene Insertion Reactions

“…Copper has also provided some examples for this class of catalytic intermediates. Hofmann first described [44,45,46] the formation in solution of such species upon reacting a Cu(I)-containing iminophosphanamide complex with PhC(N 2 )CO 2 Et (Scheme 6.7a). Later, Warren [47] employed the same strategy starting from a b-diketiminate Cu(I) ethylene complex and diphenyldiazomethane: dinuclear and mononuclear copper-carbene (Scheme 6.7b) complexes were obtained.…”