C(sp3)-Cl Bond Activation Promoted by a POP-Pincer Rhodium(I) Complex

The complex [RhCl(kappa P-3,O,P-{xant((PPr2)-Pr-i)(2)})] (1; xant (PiPr(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 [RhHCl2(kappa P-3,O,P-{xant((PPr2)-Pr-i)(2)})] (2) and cyclohexene and promotes the dehalogenation of the chlorocycloalkane to cyclohexane using 2-propanol solutions of sodium formate as the reducing agent. The oxidative addition of benzyl chloride to 1 leads to [Rh(CH2Ph)Cl-2(kappa P-3,O,P-{xant((PPr2)-Pr-i)(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 [RhCl3(kappa P-3,O,P-{xant((PPr2)-Pr-i)(2)})] (5) and cyclohexene, whereas complex 1 undergoes oxidative addition of dichloromethane to afford cis-dichloride-[Rh(CH2Cl)-Cl-2(kappa P-3,O,P-{xant((PPr2)-Pr-i)(2)})] (6a), which evolves into its trans-dichloride isomer 6b. The kinetic study of the overall process suggests that the oxidative addition is cis-concerted and the isomerization an intramolecular reaction which takes place through a sigma-C-Cl intermediate with two conformations.