Transfer Hydrogenation of Nitriles, Olefins, and N-Heterocycles Catalyzed by an N-Heterocyclic Carbene-Supported Half-Sandwich Complex of Ruthenium

Abstract

In the presence of KOBut, N-heterocyclic carbene-supported half-sandwich complex [Cp­(IPr)­Ru­(pyr)2]­[PF6] (3) (IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene) catalyzes transfer hydrogenation (TH) of nitriles, activated N-heterocycles, olefins, and conjugated olefins in isopropanol at the catalyst loading of 0.5%. The TH of nitriles leads to imines, produced as a result of coupling of the initially formed amines with acetone (produced from isopropanol), and showed good chemoselectivity. Reduction of N-heterocycles occurs for activated polycyclic substrates (e.g., quinoline) and takes place exclusively in the heterocycle. The TH also works well for linear and cyclic olefins but fails for trisubstituted substrates. However, the CC bond of α,β-unsaturated esters, amides, and acids is easily reduced even for trisubstituted species, such as isovaleriates. Mechanistic studies suggest that the active species in these catalytic reactions is the trihydride Cp­(IPr)­RuH3 (5), which can catalyze these reactions in the absence of any base. Kinetic studies are consistent with a classical inner sphere hydride-based mechanism of TH.

Optimum bifunctionality in a 2-(2-pyridyl-2-ol)-1,10-phenanthroline based ruthenium complex for transfer hydrogenation of ketones and nitriles: impact of the number of 2-hydroxypyridine fragments

“…Beller and co-workers demonstrated that Ru(II)-1,4-bis(diphenylphosphino)butane catalyzed TH of aromatic and aliphatic nitriles to amines in 2-butanol at 120°C. 47,48 Both complexes showed excellent catalytic activity but the catalyst loading were relatively high. 46 Reductive amination of the primary amine with acetone, generated from nitrile and 2-propanol respectively during TH, produced the secondary amine.…”

Transfer Hydrogenation of Unsaturated Substrates by Half-sandwich Ruthenium Catalysts using Ammonium Formate as Reducing Reagent

“…Despite a higher load of 5 mol %, they are still outperformed by catalysts 1–6 . Neutral tris(hydride) species 10 and 11 (entries 10 and 11), which were identified as resting states in our previous transfer hydrogenation studies,, are more active than 7–9 but their activity is also inferior to their precursors 1 and 3 , respectively.…”

“…Surprisingly, 4‐cyano‐pyridine remained untouched under these catalytic conditions (entry 9). Attempted reduction of aliphatic nitriles resulted in only moderate to good yields (entries 11–14), which agrees well with the results of our previous studies ,. Noteworthy, a nitrile bearing an alkynyl group was reduced selectively at the C≡N bond (entry 13).…”

“…Transfer hydrogenation of nitriles leads to three types of products: primary amines, imines formed as a result of coupling of amine with the carbonyl by‐product derived from alcohol, and secondary amines formed as a result of transfer hydrogenation of imine intermediates. High temperatures usually favour the latter type of products ,. These complications can be avoided, in principle, by using formates as the hydrogen source.…”

“…Our group has previously reported hydrosilylation and transfer hydrogenation of nitriles, imines, ketones, aldehydes, and olefins catalyzed by a family of half‐sandwich complexes of ruthenium supported by phosphines or NHC ( N ‐heterocyclic carbene) ,. As an extension of our interest in the catalytic reduction, in general, and in finding inexpensive and commercially available reducing agents, in particular, we report herein the use of formates for the ruthenium‐catalyzed transfer hydrogenation of the above substrates.…”