Efficient Hydrogenation of N-Heterocycles Catalyzed by NNP-Manganese(I) Pincer Complexes at Ambient Temperature

Abstract

Manganese-catalyzed hydrogenation reactions have aroused widespread interest in recent years. Among the catalytic systems described, especially PNP- and NNP-Mn pincer catalysts have been reported for the hydrogenation of aldehydes, ketones, nitriles, aldimines and esters. Furthermore, NNP-Mn pincer compounds are efficient catalysts for the hydrogenolysis of less reactive amides, ureas, carbonates, and carbamates. Herein, the synthesis and application of specific imidazolylaminophosphine ligands and the corresponding Mn pincer complexes are described. These new catalysts have been characterized and studied by a combination of experimental and theoretical investigations, and their catalytic activities have been tested in several hydrogenation reactions with good to excellent performance. Especially, the reduction of N-heterocycles can be performed under very mild conditions.

Keywords: hydrogenation; manganese; nitrogen heterocycles; pincer ligands; reaction mechanisms.

Figure 1

Reported Mn‐based catalytic systems for the hydrogenation of N‐heterocycles.

Scheme 1

Preparation of N^ImN^HP−Mn(I) pincer complexes; molecular structures drawn with thermal ellipsoids at the 30 % probability level. C‐bonded hydrogen atoms and solvent molecules omitted for clarity.

Figure 2

Sections of ¹H NOESY NMR spectra of Mn‐2 b (left; methanol‐d₄ , 298 K) and Mn‐N‐2 b (right; benzene‐d₆ , 298 K).

Scheme 2

Catalytic applications of the NNP−Mn complexes in different hydrogenation reactions.

Scheme 3

Reaction of Mn‐1 b and Mn‐2 b with KO^tBu.

Figure 3

Comparison of ¹H and ³¹P NMR spectra (benzene‐d₆ , 333 K) of quinoline (a), Mn‐N‐2 b (b) and a mixture thereof in the absence (c) and presence (d) of 1 bar of hydrogen (after 24 h). * Recorded at 298 K.

Scheme 4

a) In situ IR measurements monitoring the formation of the amido species Mn‐N‐2 b. b) Proposed pathway for the formation of the amido species.

Scheme 5

Computed Gibbs free energy for the transformation between Mn alkoxide, Mn amido and Mn hydrido complexes.

Figure 4

Computed Gibbs free energies (kcal/mol) for the interconversion between the hydrido and amido complexes.

Figure 5

Computed Gibbs free energy barrier (kcal/mol) for the first Mn−H transfer and the reaction free energy of the N=C hydrogenation.

Figure 6

Computed Gibbs free energy barrier (kcal/mol) for ligand exchange.