on the occasion of his 65th birthday
The Suzuki cross-coupling reaction, which involves the coupling of aryl boronic acid with an organohalide, has proven to be a versatile tool in organic synthesis. [1] Recent developments have led to catalysts based on sterically demanding, basic phosphanes allowing even the conversion of unreactive aryl chlorides. Nevertheless, these reactions still proceed very slowly at room temperature. As N-heterocyclic carbenes (NHC) are sometimes better ligands than phosphanes in homogeneous catalysis, [4] they were also tested in crosscoupling chemistry. [5±7] However, for the effective activation of aryl chlorides the known NHC-based catalysts require temperatures above 80 8C to yield reasonable activities. We now report on palladium(0) catalysts for the Suzuki crosscoupling at ambient temperatures.
Recently we established a synthetic procedure for homoleptic bis(NHC)-complexes of palladium(0) by ligand exchange in bis(tri-ortho-tolylphosphane)palladium(0) (1 a). [6] The catalytic activity of these complexes strongly depends on the steric bulk of the NHC ligand. 1,3-Bisadamantylimidazolin-2-ylidene (2), as one of the most bulky NHC ligands, [8] represents a good candidate for palladium(0) catalysts of high activities in the Suzuki cross-coupling. However, even in presence of an excess of 2 only one phosphane ligand of 1 a was exchanged, which resulted in the formation of complex 3 a [Eq. ( )]. mixture was stirred at ambient temperature for 24 ± 48 h during which a yellow solid precipitated. After filtration and drying in vacuo the product was obtained as an analytically pure bright yellow solid. X-ray suitable crystals were obtained by recrystallization from diethyl ether. Yield: 1177 mg, 1.51 mmol, 83 %; m.p. > 285 8C (decomp.); 1 H NMR (400 MHz, [D 8 ]toluene, 25 8C): d 1.47 ± 1.80 (m, 24 H, CH 2 of C 10 H 15 ), 2.06 ± 2.12 (m, 12 H, CH of C 10 H 15 ), 3.05 (s, 24 H, CH 2 of C 10 H 15 ), 6.69 (s, 4 H, NCHCHN); 13 C{ 1 H} NMR (100.5 MHz, [D 8 ]toluene, 25 8C): d 31.0, 36.8, 44.0 (C 10 H 15 ), 57.2 (ipso-C of C 10 H 15 ), 112.7 (NCHCHN), 191.8 (NCN); CI-MS: m/z (%): 778 (3) [M ], 336 (100) [NHC ], 281 (33), 207 (27), 203 (40); elemental analysis calcd (%) for C 46 H 64 N 4 Pd (779.44): C 70.88, H 8.28, N 7.19; found: C 70.80, H 8.24, N 7.22.
Alternative synthesis: 2 (343 mg, 1.02 mmol), [(h 3 -C 3 H 5 )PdCl] 2 (100 mg, 0.254 mmol), and sodium dimethylmalonate (78 mg, 0.506 mmol) were suspended in toluene (30 mL) in a Schlenk tube. The stirred mixture was heated at 90 8C for 16 h after which the solution was brown. After cooling to room temperature, the mixture was filtered to remove some free palladium metal and the solution was concentrated. Crystallization at À 50 8C yielded the product as bright yellow solid. Yield: 219 mg, 0.28 mmol, 55 %.