Pyridines from Diynes and Nitriles

A Ni/SIPr (1 2) combination catalyzed the cycloaddition of diynes and nitriles into pyridines. Aryl, heteroaryl as well as alkyl nitriles were reactive under ambient conditions (Equation (10.23)). In contrast with aryl isocyanates vide supra), the reaction was not dependent on the electronic demand on the aryl nitrile since both electron-rich and electron-poor substrates gave similar results. Finally, and similarly to isocyanates reactions, the larger substituent was introduced in the 3-position of the pyridine ring. 

4 Pyrans from Unsaturated Hydrocarbons and Carbonyl Subsfrates 

Unactivated ketones such as cyclohexanone produced spiropyrans in good yields (Equation (10.24)). The authors envisioned that this surprising reactivity was probably due to the ability of the NHC ligand to enhance C-O bondforming reductive elimination. 

It is worth noting that [2 + 2 + 2] cycloadditions of imines and alkynes have not been reported in the presence of NHC-Ni systems. These reactions seem to fail since Ogoshi et al. noted that the reaction of 2-butyne and A -benzene-sulfonylbenzaldimine did not proceed in the presence of a NHC-Ni catalyst (IMes or SIMes). This result contrasted with the results obtained by the same authors for the reaction catalyzed by Ni/phosphine systems. 

Sato and co-workers described the nickel-catalyzed [2 + 2 + 2] cycloaddition of arynes and an unactivated alkene allowing for the original preparation of 

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Kase et al. s instant catalyst based on the catalytic mixture IX (Figure 1.3) was tested for the construction of pyridines from diynes and nitriles but proved inefficient [40], Oligomers of the starting diynes were obtained instead of the products desired. To circumvent this problem, the Dipimp ligand was replaced by bis(diphenylphosphino)ethane (DPPE), and the reaction was carried out in At-methylpyrrolid-2-one (NMP) at room temperature. In this case, diynes 81 and nitriles 82 co-cyclized to give the expected bicyclic pyridines 83 in good yield (Scheme 1.21). 

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