Kinetics and Mechanism of Iridium-Catalyzed Dehydrogenation of Primary Amines to Nitriles

Abstract

A family of primary amines has been catalytically dehydrogenated to nitriles by [C6H3-2,6-(OP t Bu2)2]IrH2 using tert-butylethylene as a hydrogen acceptor. The catalytic mechanism has been investigated by a series of kinetic and isotopic labeling experiments, in addition to the isolation of intermediates along the reaction pathway. The turnover frequency exhibits a first-order dependence on the concentration of amine, an inverse first-order dependence on nitrile, and a zero-order dependence on tert-butylethylene. The mechanism of amine dehydrogenation is proposed to proceed from an iridium(I) nitrile complex, the catalyst resting state, via two preturnover limiting equilibria, followed by a slow β-hydride elimination event from a transient iridium(III) amido hydride species. Measuring rate constants for the stoichiometric dehydrogenation of isobutylamine over a 41 °C temperature range established activation parameters for β-hydride elimination of ΔH ⧧ 24.8(9) kcal/mol and ΔS ⧧ = −10(2) eu. Dehydrogenation of a series of para-substituted benzyl amines indicated enhanced conversions to nitriles for substrates bearing electron-donating substituents.

Structures, relative energies, and ligand dissociation energies of iridium carbonyl phosphine clusters

“…Most of these phosphines and phosphites are poor p-acceptors as compared with carbonyl. Our calculations show that the T d structure of Ir 4 PH 3 (CO) 11 is lower in energy by only 0.3 kcal/mol than the C 3v structure. As more CO ligands are replaced by PH 3 ligands, the C 3v structure becomes more favoured.…”

“…The C 3v structures of Ir 4 (PH 3 ) y (CO) z clusters have similar energies with respect to the T d isomers for Ir 4 PH 3 (CO) 11 and Ir 4 (PH 3 ) 2 (CO) 10 , and the C 3v structures are of even lower energy for Ir 4 (PH 3 ) 3 (CO) 9 and Ir 4 (PH 3 ) 4 (CO) 8 . This conclusion is consistent with observations of C 3v structures for mono-phosphine Ir 4 clusters with larger and more electron-withdrawing R groups on the phosphorus [40].…”

“…Because it has been shown experimentally [40] that no more than four carbonyls can be substituted by phosphines, only the Ir 4 PH 3 (CO) 11 , Ir 4 (PH 3 ) 2 (CO) 10 , Ir 4 (PH 3 ) 3 (CO) 9 , and Ir 4 (PH 3 ) 4 (CO) 8 clusters are considered here. Besides the lowest energy isomers characterized by experiment, other isomers with substitution at various positions were also investigated, including both C 3v and T d structures and terminal and bridging position substitution and dissociation.…”

“…Iridium complexes play important roles in oxidation (e.g., of alcohols [4][5][6], phenols [7][8][9], and amines [10,11]), hydrogenation [12,13], C-H activation [14][15][16], cycloaddition (e.g., [2 + 2 + 2] [17][18][19], [2 + 2 + 1] [20,21], and [4 + 2] [22,23]), cycloisomerization [1,2], and ring-opening reactions [24]. The richness of this chemistry explains the broad interest in the properties of the family of iridium complexes and clusters [25].…”

“…Electronic effects may also be significant in this chemistry, reflecting the electron withdrawing or donating tendencies of the groups bonded to the phosphorus [38]. When L is a phosphine or phosphite, its bulk also strongly influences the degree of ligand substitution in the parent cluster Ir 4 (CO) 12 -that is, determining whether Ir 4 (CO) 8 L 4 or Ir 4 (CO) 9 L 3 is the final product [41] -but synthesis of Ir 4 (CO) 10 L 2 or Ir 4 (CO) 11 L directly from the parent through direct substitution is difficult [41]. Okrut et al [42] demonstrated that as the number of strong r-donor phosphine ligands increases, the thermomechanical stability against aggregation increases, whereas the thermochemical stability against decarbonylation varies inversely.…”