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

Abstract

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)(2) 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)(2) 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-(t)()Bu-C(6)H(4)Br or the ligand displacement reaction of {Pd[P(o-tolyl)(3)](4-(t)()Bu-C(6)H(4))(&mgr;-Br)}(2) with ligand 2 affords the bis-phosphine complex L(2)Pd(4-(t)()Bu-C(6)H(4))(Br) (13, L = 2). 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 mono-phosphine P,O-chelating Pd/L intermediates, which appear to be most suitable for Suzuki arylation reactions involving certain substrates and conditions.

Recent applications of the Suzuki–Miyaura cross-coupling reaction in organic synthesis

“…Guram and co-workers have shown that a combination of the phenyl backbone-derived P -O ligand 24 and Pd(dba) 2 gives an efficient catalyst system for the SM cross-coupling of a variety of aryl chlorides and arylboronic acids using CsF/dioxane/100 -1108C conditions. 41 This efficient catalytic system has also permitted o-substitution on one or both coupling partners and electron-rich and EWGs were tolerated. 42 The use of the catalytic cross-coupling reaction for the preparation of aryl-functionalised heterocycles with multiple applications is increasing at an accelerating phase.…”

Resorcin[4]arene‐Derived Mono‐ and Diphosphines in Suzuki Cross‐Coupling

“…In the corresponding reports, the efficiency was assigned to the ability of such phosphines to form P,O-chelated monophosphine intermediates. [37] According to more recent investigations on Suzuki crosscoupling reactions, the formation of a Pd(0)-monophosphine complex appears to be a key step that precedes the oxidative addition step of aryl bromides. [38][39][40][41][42] On the other hand, the transient binding of an oxygen atom is expected to increase the electronic density of the metal centre, thereby favouring the oxidation addition step, irrespective of whether the active species is a bis-or a mono-phosphine complex.…”

Aminophosphine complex as a catalyst precursor in Suzuki coupling reactions

“…Iit has been shown that a catalytic system comprising Pd 2 (dba) 3 and semi-labile aminophosphines exhibited superior catalytic activity in C-C coupling reactions compared to those using traditional phosphines [16][17][18][19]. In particular, functionalized aminophosphines can significantly alter the properties of the transition-metal complex [3], thereby providing a strategy to enhance the activity of such catalytic systems [4].…”