The article reports the catalytic isomerization of the branched cyanoolefin 2-methyl-3-butenenitrile (2M3BN) to the linear isomer 3-pentenenitrile (3PN) using nickel-phosphine complexes.
The authors report that the optimal catalytic experiment was carried out by adding [Ni(COD)2] (1 eq., 0.020 g, 0.073 mmol) to a mixture of 2M3BN (110 eq., 0.8mL, 7.99 mmol) and PPh3 (10 eq., 0.190 g, 0.73 mmol). Octane was used as an internal standard (5 microL). The mixture was stirred at 100 Celsius degrees for 3h, and after that, GC analysis shows a 96% conversion and the transformation of the substrate to 3PN in 81% yield.
However, when the experiment was reproduced, in our hands and reaction apparatus, we indeed observed the 96% conversion of the substrate, but systematically, only 58% yield of 3PN was observed.
Interestingly, when the PPh3 was substituted by a chelating diphosphine (dppb, diphenylphosphinebutane)and using a [Ni(COD)2]/2M3BN/dppb mixture in a 1/110/1.3 ratio, the GC analysis showed a 96% conversion and the formation of 3PN in 90% yield and 6% of other cyanoolefins. When this experiment was reproduced, in our hands and reaction apparatus, similar results were obtained.
Another point is the 31P NMR analysis of the mixture reaction. The authors mentioned that the 31P NMR spectrum show at room temperature a very broad signal at 2ppm and when the spectrum was recorded at -90 Celsius degrees shows a sharp singlet at -4.6 ppm, assigned to free PPh3 and an AB signal at 32.6 and 23.9 ppm with a J=103 Hz, assigned to the allyl cyanide Nickel complex. The disagreement point is related with the assignation of the species in solution observed at -90C by 31P NMR. Usually, Nickel (II) diphosphine complexes produce J values between 15 and 40 Hz, and Ni (0) diphosphine complexes produce J values above 50 Hz. Probably, the species observed at -90C by the authors, which produces the doublets with J=103 Hz, correspond to a Ni(0) species, the Ni(P-P) complex with the cyanoolefin 3PN coordinated through the C=C bond instead of the allyl cyanide complex as they assigned.
The experimental procedure used to reproduce the experiments is described below:
[Ni(COD)2] was purified prior use from a THF solution, filtered through Celite and vacuum dried to tield yellow crystalline [Ni(COD)2], which was further dried for 3h in vacuo. The PPh3 used, according to 31P NMR spectroscopy, was oxide free.
In the glove box a solution of 2M3BN (110 eq., 0.8 mL, 7.99 mmol) and PPh3 (10 eq., 0.190g, 0.73 mmol) or dppb (1.3 eq., 0.40g, 0.095 mmol) and octane (50 microL) was added to the yellow crystalline [Ni(COD)2] (1eq., 0.20g, 0.073 mmol), 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 for 3h. 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 resonance identical to those for trans-3PN.