A new borrowing hydrogen mechanism for redox-active metals

International audienceA combined computational and experimental investigation of three different Cp*CoIII systems used in combination with a base as catalysts in “borrowing hydrogen” alkylations by secondary alcohols (ROH), resulting in C–N or C–C bond formation, reveal the existence of a new pathway for alcohol activation, differing from the existing paradigms of oxidative and ionic activations. The metal-coordinated alkoxide (OR) generates a ketone by transfer of β-H as a proton to a ligand or to an external base, rather than as a hydride to the metal via the ubiquitous β-H elimination, and of two electrons to the metal complex. For [Cp*CoI(Oquin)]/KOtBu/ROH (Oquin = 8-oxoquinolinato), after iodide exchange to yield [Cp*CoI(Oquin)(OR)], the β-H of R is transferred to the Oquin ligand, which becomes hydroxyquinoline (quinOH) in [Cp*CoI(quinOH)(ketone)]. For [Cp*CoI2]/KOtBu/ROH, the intermediate [Cp*Co(OR)2] undergoes a β-H transfer from R to the second alkoxide to yield [Cp*Co(ROH)(ketone)]. Finally, in the [Cp*CoI2]/PhNH2/ROH system, access to [Cp*Co(OR)2] is energetically too costly, but the external base is able to accept the β-H from the [Cp*CoI(OR)] intermediate, yielding PhNH3+[Cp*CoI(ketone)]−. Stoichiometric reactions of [Cp*CoI(Oquin)]/KOtBu with a variety of secondary alcohols lead to the formation of ketone (isolated in the case of PhCOPh) and uncharacterizable paramagnetic solutions, though further exposure to CO allowed the spectroscopic identification (IR, 1H NMR) of a paramagnetic CO adduct, which is proposed to be the triplet 20-electron [Cp*Co(quinOH)(CO)]. A diamagnetic unstable [Cp*Co(Oquin){OCH(CF3)2}] was generated and characterized (1H and 19F NMR) by using hexafluoroisopropanol (HFIP). The DFT calculations reproduce the observed relative stabilities and the ground states of all molecules and suggest that several key “CpCoI” intermediates involve ligand non-innocence and are better described as CpCoII complexes with either a ferromagnetic (1/2, 1/2) or an antiferromagnetic (3/2, −1/2) coupling of the metal with a ligand anion radical. The DFT calculations also pinpoint the rate-determining steps of the catalytic cycles, rationalizing the observed selectivities. The calculated cycle span parameters for a model system of the C6Me5COMe/PhCH(Me)OH reaction, which yields C6Me5COCH2CH(Me)Ph, are in good agreement with those obtained from the Eyring analysis of the rate constant in the 120–150 °C range