This article reports the catalytic isomerization of the branched isomer 2-methyl-3-butenenitrile (2M3BN) to the linear isomer 3-pentenenitrile (3PN) by DPEphos nickel species.
The species involved in the isomerization reaction were investigated by variable-temperature NMR experiments, and also kinetic studies are presented. The authors also report the isolation and characterization of an intermediate in this reaction, which they characterized in solution by NMR experiments and by X-ray diffraction of single crystal. This nickel complex is formed via a C-CN bond activation reaction.
The authors mention at the beginning of the manuscript, that they were moved to report their results because of recent contributions in the field. However, even that they carried out catalytic experiments to obtain kinetic data, they never reported results for the conversion and yield of the desired product, in order to make a more complete comparison with the previous reports that motivated them to publish this paper. In the experimental section , in the supporting information, the authors just say that once the mixture reaction was prepared adding DPEphos (201 mg, 0.374 mmol) to a solution of [Ni(COD)2] (98.3 mg, 0.359 mmol) in aprox. 20 mL of toluene, and then adding 2M3BN (6.3158 g, 77.915 mmol). From this mixture reaction aliquots of 1300 microL were taken as starting volume and 6 different temperatures were studied. The reaction was followed up to 75% conversion, and the rate was determined from GC analysis of 4 points measured at time.
The experiment was reproduced only at 100 Celsius degrees for comparison purposes with other reports. In our hands and reaction apparatus, the same experiment was followed up to 90% conversion, which was reached after 16h, with a yield of 82% of 3PN and 8% of other isomers of the cyanoolefin.
The experimental procedure used to reproduce the experiment is described below:
[Ni(COD)2] was purified prior use from a THF solution, filtered through Celite and vacuum dried to yield yellow crystalline [Ni(COD)2], which was further dried for 3h in vacuo. The DPEphos used, according to 31P NMR spectroscopy, was oxide free.
Using a tenth of the reported amounts, in the glove box a solution of 2M3BN (0.631 g, 7.79 mmol) and DPEphos (20.1 mg, 0.0374 mmol) was added to the yellow crystalline [Ni(COD)2] (9.83 mg, 0.0359 mmol) in 2 mL of toluene, producing a red solution. The mixture was transferred to an NMR tube with a J. Young valve and the tube was heated at 100 Celsius degrees in an oil bath with stirring. Samples of the mixture reaction were taken after 3, 6, 10 and 16h. After cooling down to room temperature, a sample of the reaction mixture was dissolved in THF inside the glove box and analyzed by GC-MS. A second sample was dissolved in toluene-d8 and analyzed by 1H NMR spectroscopy, showing the resonances identical to those for trans-3PN.