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.