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