New Borrowing Hydrogen Mechanism for Redox-Active Metals

A 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 beta-H as a proton to a ligand or to an external base, rather than as a hydride to the metal via the ubiquitous beta-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 beta-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 beta-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 beta-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, H-1 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 (H-1 and F-19 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 degrees C range.

Automated summary

A 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 reveal the existence of a new pathway for alcohol activation, differing from the existing paradigms of oxidative and ionic activations.