The combined catalyst PCl5–HCl shows effective catalysis in the trimerization of some aromatic nitriles or trichloroacetonitrile to s-triazines (III). The scope and limitation of the trimerization, in comparison with other Lewis acids, and its mechanism were studied. It was confirmed that PCl5 is effective both in the formation of an intermediate, nitrile–HCl 2:2 adduct, and in the reaction of the 2:2 adduct with another nitrile to s-triazine.
The compositions and structures of several stable nitrile-HCl salts previously reported were reinvestigated. Most of them have been found to have the composition of 2RC≡N·2HCl and have been identified as N-(α-chloroalkenyl)alkylamidine hydrochlorides (3) by mass and NMR spectra analyses. However, the 2:3 adduct from chloroacetonitrile was confirmed to be N-(α,α,β-trichloroethyl)chloroacetamidine hydrochloride. The scope and limitations of the dimerization reaction of nitriles having α-hydrogen with HCl were studied. Most nitriles having α-hydrogen react with HCl to give N-(α-chloroalkenyl)alkylamidine hydrochlorides (3). Their hydrolysis to diacylamines (4) was also investigated.
The reaction of nitriles having no α-hydrogen, such as aromatic nitriles and thiocyanates, with hydrogen chloride was investigated. Benzonitrile, p-tolunitrile, m-tolunitrile and some thiocyanates gave analytically pure 2:2 adducts in their reactions with HCl in a sealed glass tube at 30–60°C. The structure of the adducts was elucidated on the basis of NMR analysis and the mechanism of the generalized dimerization of nitriles was discussed.
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