Catalytic Cycle for the Second Stage

As mentioned earlier, the first reaction in the second stage is the isomerization of 3PN to 4PN, reaction 7.18. The mechanism of this reaction is very similar to the mechanism of alkene isomerization discussed in Section 7.2.1, and shown by Fig. 7.14. The following points need attention. 

The nickel-hydride complex that acts as a precatalyst for this isomerization reaction is thought to be the cationic part of 7.52. The evidence for the existence and participation of a cationic species such as 7.52 comes from multi-nuclear NMR and IR data. An equilibrium as shown by 7.23 exists, and the cation [HNiL4]+ is the dominant precatalyst for the isomerization reaction. The cation is an 18-electron complex. It undergoes ligand dissociation to give 7.53 before alkene coordination takes place. A ligand dissociated species such as 7.53 with L = p-tolylphosphite has been observed spectroscopically at low temperatures. 

The isomerization of 3PN can lead to two possible products, 2-pentenenitrile (2PN), the unwanted isomer, and 4PN, the desired isomer. The former does not undergo hydrocyanation and thermodynamically is the most stable isomer. If the isomerization of 3PN were allowed to reach thermodynamic equilibrium, the concentrations of the three isomers 2PN, 3PN, and 4PN would be approximately 78 20 2. Fortunately the isomerization of 3PN to 4PN is about 70 times as fast as that of 3PN to 2PN. In other words, although 4PN is thermodynamically the less stable isomer, the favorable kinetics allows its preferential formation. 

The catalytic cycle for the hydrocyanation of 4PN to desired adiponitrile and undesired 2-methyl glutaronitrile (MGN) is shown by Fig. 7.15. The intermediates that lead to the formation of 7.56 or 7.57 from NiL3 are not shown 

The left- and right-hand loops of the catalytic cycle involves anti-Markov-nikov and Markovnikov additions, giving 7.56 and 7.57, respectively. The presence of the Lewis acid ensures that the former pathway is favored. Spectroscopic evidence for species 7.56 or 7.57 have so far not been reported. However, dissociation of two moles of ligand and involvement of the Lewis 

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