title: Two [1,2,4-(Me3C)3C5H2]2CeH molecules are involved in hydrogenation of pyridine to piperidine as shown by experiments and computations.
abstract: Hydrogenation of pyridine to piperidine catalyzed by [1,2,4-(Me3C)3C5H2]2CeH, abbreviated as Cp'2CeH or [Ce]'-H, is reported. The reaction proceeds from Cp'2Ce(2-pyridyl), isolated from the reaction of pyridine with Cp'2CeH, to Cp'2Ce(4,5,6-trihydropyridyl), and then to Cp'2Ce(piperidyl). The cycle is completed by the addition of pyridine, which generates Cp'2Ce(2-pyridyl) and piperidine. The net reaction depends on the partial pressure of H2 and temperature. The dependence of the rate on the H2 pressure is associated with the formation of Cp'2CeH, which increases the rate of the first and/or second additions of H2 but does not influence the rate of the third addition. Density functional theory calculations of several possible pathways are consistent with three steps, each of which are composed of two elementary reactions, (i) heterolytic activation of H2 with a reasonably high energy, ΔG(⧧) = 20.5 kcal mol(-1), on Cp'2Ce(2-pyridyl), leading to Cp'2CeH(6-hydropyridyl), followed by an intramolecular hydride transfer with a lower activation energy, (ii) intermolecular addition of Cp'2CeH to the C(4)═C(5) bond, followed by hydrogenolysis, giving Cp'2Ce(4,5,6-trihydropyridyl) and regenerating Cp'2CeH, and (iii) a similar hydrogenation/hydrogenolysis sequence, yielding Cp'2Ce(piperidyl). The calculations reveal that step ii can only occur in the presence of Cp'2CeH and that alternative intramolecular steps have considerably higher activation energies. The key point that emerges from these experimental and computational studies is that step ii involves two Cp'2Ce fragments, one to bind the 6-hydropyridyl ligand and the other to add to the C(4)═C(5) double bond. In the presence of H2, this second step is intermolecular and catalytic. The cycle is completed by reaction with pyridine to yield Cp'2Ce(2-pyridyl) and piperidine. The structures of Cp'2CeX, where X = 2-pyridyl, 4,5,6-trihydropyridyl, and piperidyl, are fluxional, as shown by variable-temperature (1)H NMR spectroscopy.
authors.affiliation: Université de Toulouse, INSA, UPS, LPCNO , 135 avenue de Rangueil, F-31077 Toulouse, France , and CNRS , F-31077 Toulouse, France.
authors.forename: Lionel
authors.initials: L
authors.lastname: Perrin
authors.forename: Evan L
authors.initials: EL
authors.lastname: Werkema
authors.forename: Odile
authors.initials: O
authors.lastname: Eisenstein
authors.forename: Richard A
authors.initials: RA
authors.lastname: Andersen