An 11B NMR spectroscopy investigation of the mechanism of the reduction of nitriles by BH3 · SMe2

A kinetic and mechanistic investigation of the reduction of acrylonitrile, propionitrile and benzonitrile by BH3 · SMe2 in CH2Cl2 has been conducted using 11B NMR spectroscopy. No reduction of propionitrile and benzonitrile was observed at 25 °C, while the reduction of acrylonitrile was monitored to the vinyliminoborane. The reaction exhibited simple second-order kinetics of the form kobs=k2′[acrylonitrile], with the dissociation of the dimethyl sulfide from borane playing a key role in controlling the reduction process. The activation parameters (ΔS≠ and ΔH≠) for the reaction of BH3 · SMe2 with acrylonitrile were found to be −71 ± 10 J K−1 mol−1 and 58 ± 3 kJ mol−1, respectively. The B3LYP/6-31G∗ results for charge distribution and transfer during the reaction show a reversal of charge in the borane–nitrile adduct for the atoms that make up the borane and the nitrogen and carbon atoms in the nitrile. The activation barriers for the reduction of acrylonitrile were found to be 6–8 kJ mol−1 lower than those of hydrogen cyanide, propionitrile and benzonitrile, with its product iminoborane being 30–130 kJ mol−1 more stable than those of the other nitriles.

Graphical abstractThe reduction of acrylonitrile at 25 °C was monitored by 11B NMR spectroscopy to the vinyliminoborane and exhibited simple second-order kinetics.Figure optionsDownload full-size imageDownload as PowerPoint slide