Complex [RhCl(κ 3-P,O,P-{xant(P i Pr 2) 2 })] (1; xant(P i Pr 2) 2 = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) activates C(sp 3)-Cl bonds of mono-and dichloroalkanes and catalyzes the dehalogenation of chloroalkanes and the homocoupling of benzyl chloride. Complex 1 reacts with chlorocyclohexane to give [RhHCl 2 (κ 3-P,O,P-{xant(P i Pr 2) 2 })] (2) and cyclohexene and promotes the dehalogenation of the chlorocycloalkane to cyclohexane using 2-propanol solutions of sodium formate as reducing agent. The oxidative addition of benzyl chloride to 1 leads to [Rh(CH 2 Ph)Cl 2 (κ 3-P,O,P-{xant(P i Pr 2) 2 })] (4). The dehalogenation of this chloroalkane with 2-propanol solutions of sodium formate, in the presence of 1, gives toluene and 1,2-diphenylethane. The latter is selectively formed with KOH instead of sodium formate. Complex 1 also reacts with trans-1,2-dichlorocyclohexane and dichloromethane. The reaction with the former gives [RhCl 3 (κ 3-P,O,P-{xant(P i Pr 2) 2 })] (5) and cyclohexene, whereas complex 1 undergoes oxidative addition of dichloromethane to afford cis-dichloride-[Rh(CH 2 Cl)Cl 2 (κ 3-P,O,P-{xant(P i Pr 2) 2 })] (6a), which evolves into its isomer trans-dichloride 6b. The kinetic study of the overall process suggests that the oxidative addition is cisconcerted and the isomerization an intramolecular reaction which takes place through a σ-C-Cl intermediate with two conformations. RESULTS AND DISCUSSION Monochloroalkanes. Complex 1 activates the C-Cl bond of chlorocyclohexane. However, the presence of four β-hydrogen atoms in the substrate destabilizes the resulting alkyl intermediate, which undergoes a β-hydride elimination reaction (Scheme 2). Thus, complex 1 affords cyclohexene and the rhodium(III) monohydride [RhHCl 2 (κ 3-P,O,P-{xant(P i Pr 2) 2 })] (2) in chlorocyclohexane as solvent. According to the 31
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 3 -P,O,P-[xant(P i Pr 2 ) 2 ]} (xant(P i Pr 2 ) 2 = 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 3 -P,O,P-[xant(P i Pr 2 ) 2 ]} and the C–C reductive eliminations of biphenyl and benzylbenzene from [RhPh 2 {κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]}]BF 4 and [RhPh(CH 2 Ph){κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]}]BF 4 , 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 3 )–C(sp 2 ) coupling to give benzylbenzene is faster than the C(sp 2 )–C(sp 2 ) bond formation to afford biphenyl. In spite of that a most demanding orientation requirement is needed for the C(sp 3 )–C(sp 2 ) coupling than for the C(sp 2 )–C(sp 2 ) bond formation, the energetic effort for the pregeneration of the C(sp 3 )–C(sp 2 ) bond is lower. As a result, the weakest C–C bond is formed faster.
Reactions of the aryl complexes Rh(aryl){κ3-P,O,P-[xant(PiPr2)2]} (1; aryl = 3,5-Me2C6H3 (a), C6H5 (b), 3,5-Cl2C6H3 (c), 3-FC6H4 (d); xant(PiPr2)2 = 9,9-dimethyl-4,5-bis-(diisopropylphosphino)xanthene) with O2, CO, and MeO2CC≡CCO2Me have been performed. Under 1 atm of O2, the pentane solutions of complexes 1 afford the dinuclear peroxide derivatives [Rh(aryl){κ2-P,P-xant(PiPr2)2}]2(μ-O2)2 (2a–2d) as yellow solids. In solution, these species are unstable. In dichloromethane, at room temperature, they are transformed into the dioxygen adducts Rh(aryl)(η2-O2){κ3-P,O,P-[xant(PiPr2)2]} (3a–3d), as a result of the rupture of the double peroxide bridge and the reduction of the metal center. Complex 3b decomposes in benzene, at 50 °C, to give diphosphine oxide, phenol, and biphenyl. Complexes 1 react with CO to give the square-planar mono carbonyl derivatives Rh(aryl)(CO){κ2-P,P-[xant(PiPr2)2]} (4a–4d), which under carbon monoxide atmosphere evolve to benzoyl species Rh{C(O)aryl}(CO){κ2-P,P-[xant(PiPr2)2]} (5a–5d), resulting from the migratory insertion of CO into the Rh-aryl bond and the coordination of a second CO molecule. The transformation is reversible; under vacuum, complexes 5 regenerate the precursors 4. The addition of the activated alkyne to complexes 1b and 1d initially leads to the π-alkyne intermediates Rh(aryl){η2-C(CO2Me)≡C(CO2Me)}{κ3-P,O,P-[xant(PiPr2)2]} (6b, 6d), which evolve to the alkenyl derivatives Rh{(E)-C(CO2Me)=C(CO2Me)aryl}{κ3-P,O,P-[xant(PiPr2)2]} (7b, 7d). The diphosphine adapts its coordination mode to the stability requirements of the different complexes, coordinating cis-κ2-P,P in complexes 2, fac-κ3-P,O,P in compounds 3, trans-κ2-P,P in the mono carbonyl derivatives 4 and 5, and mer-κ3-P,O,P in products 6 and 7.
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