C-Cl Oxidative Addition and C-C Reductive Elimination Reactions in the Context of the Rhodium-Promoted Direct Arylation.

A cycle of stoichiometric elemental reactions defining the direct arylation promoted by a redox-pair Rh(I)-Rh(III) is reported. Starting from the rhodium(I)-aryl complex RhPh{κ-P,O,P-[xant(PPr)]} (xant(PPr) = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene), the reactions include C-Cl oxidative addition of organic chlorides, halide abstraction from the resulting six-coordinate rhodium(III) derivatives, C-C reductive coupling between the initial aryl ligand and the added organic group, oxidative addition of a C-H bond of a new arene, and deprotonation of the generated hydride-rhodium(III)-aryl species to form a new rhodium(I)-aryl derivative. In this context, the kinetics of the oxidative additions of 2-chloropyridine, chlorobenzene, benzyl chloride, and dichloromethane to RhPh{κ-P,O,P-[xant(PPr)]} and the C-C reductive eliminations of biphenyl and benzylbenzene from [RhPh{κ-P,O,P-[xant(PPr)]}]BF and [RhPh(CHPh){κ-P,O,P-[xant(PPr)]}]BF, respectively, have been studied. The oxidative additions generally involve the cis addition of the C-Cl bond of the organic chloride to the rhodium(I) complex, being kinetically controlled by the C-Cl bond dissociation energy; the weakest C-Cl bond is faster added. The C-C reductive elimination is kinetically governed by the dissociation energy of the formed bond. The C(sp)-C(sp) coupling to give benzylbenzene is faster than the C(sp)-C(sp) bond formation to afford biphenyl. In spite of that a most demanding orientation requirement is needed for the C(sp)-C(sp) coupling than for the C(sp)-C(sp) bond formation, the energetic effort for the pregeneration of the C(sp)-C(sp) bond is lower. As a result, the weakest C-C bond is formed faster.