Homogeneous nickel(II) catalysed transfer hydrogenation of unsaturated compounds

Abstract

A variety of well-known N-donor ligands, some synthesised, i.e. bis(pyrazolyl-1-methyl)pyridine (L3.1, L4.1, L5.1) and bis(2,5-dimethylpyrazolyl-1-methyl)pyridine (L3.2, L4.2), and others commercially sourced, i.e. phenanthroline (L3.3), bipyridine (L3.4), 2,2′-dipyridylamine (L3.5), ethylene diamine (L3.6, L4.4, L5.2), 1,2-diaminocyclohexane (L5.3) and ortho-phenylenediamine (L5.4) were combined with NiCl2·6H2O and other nickel(II) salts to generate in situ Ni(II) precatalysts. All ligands synthsised in this study were characterised with Fourier transform infrared (FT-IR) and nuclear magnetic resonance (NMR) spectroscopy as well as atmospheric pressure chemical ionisation mass spectrometry (APCI-MS). The in situ Ni(II) pre-catalysts were used in the transfer hydrogenation (TH) of quinolines, nitriles and alkenes with ammonia borane (AB) serving as the source of H2. Optimisation of the reaction conditions for quinoline TH showed that 1 mol% of Ni/L3.1 combined with one equivalent of AB could successfully convert quinoline to 1,2,3,4- tetrahydroquinoline (97% conversion, 90% isolated yield) in 30 minutes at 25 °C. Several quinoline derivatives formed part of the study with moderate to high yields (26 - 90%) obtained for the desired products. Investigation into the TH mechanism of quinoline illustrated the presence of a 1,4-dihydroquinoline intermediate with Ni(II)-hydride species responsible for the high activity observed. Further evaluation, using the same optimised reaction conditions as mentioned above, for the TH of nitriles revealed the versatility of the in situ prepared Ni/L4.1 and Ni/L4.4 precatalysts. Various nitriles could successfully undergo chemoselective TH to their secondary amine products (40 - 97% yields). A very high turnover number (TON) of 5100 was achieved with Ni/L4.1 using only 0.01 mol% of the pre-catalyst. Using α-picoline borane (without dissociable protons) and external dihydrogen (H2) during mechanistic elucidation, enabled the postulation of a two-step TH mechanism of benzonitrile, i.e. i) AB dehydrogenation followed by ii) benzonitrile hydrogenation. The dehydrogenation of AB to determine the potential amount of H2 that can be released was investigated and revealed that an almost quantitative amount (2.96 equivalents) of H2 was produced using 1 mol% of Ni/L4.1. Finally, the TH of aromatic- and aliphatic alkenes along with α,β-unsaturated esters were investigated using 1 mol% of Ni/L5.2. Successful conversion of aromatic alkenes (97 - 100% yield) was observed at 50 °C in 30 minutes, whereas the aliphatic alkenes only required 25 °C to be converted in 75 to 99% yields after one hour. Chemoselective TH of α,β-unsaturated esters to saturated esters (35 - 100% yield) was achieved under the same reaction conditions as those used for the aromatic alkenes. Mechanistic elucidation of the TH of styrene was conducted in the same manner as was done for benzonitrile. It was again concluded that borane activates H2 leading to the postulation of a two-step TH mechanism, which is responsible for the conversion of styrene to ethylbenzene using AB as the soure of H2.