Palladium/P,O-ligand-catalyzed Suzuki cross-coupling reactions of arylboronic acids and aryl chlorides. Isolation and structural characterization of (P,O)-Pd(dba) complex

The phenyl backbone-derived P,O-ligands 1 and 2 were investigated for their utility as ligands in palladium/ligand-catalyzed Suzuki reactions. The 2-(2'-dicyclohexylphosphinophenyl)-2-methyl-1,3-dioxolane (ligand 1) in combination with Pd(dba)a affords an efficient catalyst for general Suzuki reactions of a wide variety of arylboronic acids and aryl chlorides, bromides, and iodides to afford the desired biaryl products in high isolated yields. Arylboronic acids and aryl chlorides containing electron-poor, electron-rich, and ortho substituents participate effectively. In contrast, the structurally related ligand 2-(2'-dicyclohexylphosphinophenyl)-1,3-dioxolane (ligand 2) was found to be less efficient under similar conditions. The reaction of ligand 1 with Pd(dba)P affords the complex LPd(dba) (14, L = 1). The NMR spectroscopic and X-ray crystallographic data of complex 14 establish that ligand 1 functions as a P,O-chelating ligand in complex 14. The reaction of ligand 2 (2 equiv) with Pd(dba)(2) and excess 4-Bu-t-C6H4Br or the ligand displacement reaction of {Pd[P(o-tolyl)(3)](4-Bu-t-C6H4)(mu-Br)}(2) with ligand 2 affords the bis-phosphine complex L2Pd(4-Bu-t-C6H4)(Br) (13, L = a). The NMR spectroscopic data of complex 13 establish that ligand 2 in complex 13 functions as a nonchelating ligand. Thus, the higher efficiency of ligand 1 over ligand 2 in Pd/L-catalyzed Suzuki arylation of aryl chlorides can be ascribed to the ability of ligand 1 to generate and stabilize monophosphine P,O-chelating Pd/L intermediates, which appear to be most suitable for Suzuki arylation reactions involving certain substrates and conditions.