An Uncanny Dehydrogenation Mechanism: Polar Bond Control over Stepwise or Concerted Transition States

The mechanism of the dehydrogenation of N-heterocycles with the recently established bifunctional catalyst (iPrPNP)­Fe­(CO)­(H) was investigated through experiments and density functional theory calculations (iPrPNP = iPr2PCH2CH2­NCH2­CH2PiPr2). In this system, the saturated N-heterocyclic substrates are completely dehydrogenated to the aromatic products. Calculations indicate that dehydrogenation barriers of the C–C bonds are very high in energy (ΔG ‡ = 37.4–42.2 kcal/mol), and thus dehydrogenation only occurs at the C–N bond (ΔG ‡ = 9.6–22.2 kcal/mol). Interestingly, substrates like piperidine with relatively unpolarized C–N bonds are dehydrogenated through a concerted proton/hydride transfer bifunctional transition state involving the nitrogen on the PNP ligand. However, substrates with polarized C–N bonds entail stepwise (proton then hydride) bifunctional dehydrogenation.