Pyri diazine

The CNDO method has been modified by substitution of semiempirical Coulomb integrals similar to those used in the Pariser-Parr-Pople method, and by the introduction of a new empirical parameter to differentiate resonance integrals between a orbitals and tt orbitals. The CNDO method with this change in parameterization is extended to the calculation of electronic spectra and applied to the isoelectronic compounds benzene, pyridine, pyri-dazine, pyrimidine and pyrazine. The results obtained were refined by a limited Cl calculation, and compared with the best available experimental data. It was found that the agreement was quite satisfactory for both the n TT and n tt singlet transitions. The relative energies of the tt and the lone pair orbitals in pyridine and the diazines are compared and an explanation proposed for the observed orders. Also, the nature of the lone pairs in these compounds is discussed. 

The reaction of nucleophilic radicals, under acidic conditions, with heterocycies containing an imine unit is by far the most important and synthetically useful radical substitution of heterocyclic compounds. Pyri-dines, quinolines, diazines, imidazoles, benzothiazoles and purines are amongst the systems that have been shown to react with a wide range of nucleophilic radicals, selectively at positions a and y to the nitrogen, with replacement of hydrogen. Acidic conditions are essential because A-protonation of the heterocycle... 

The quatemisation of a diazine naturally increases its propensity for addition of nucleophile for example, Reissert adducts have been described for pyri-dazine and pyrimidine. ... 

Partial reductions of quaternary salts to dihydro compounds can be achieved with borohydride, but such processes are much less well studied than in pyri-dinium salt chemistry. 1,4-Dihydropyrazines have been produced with either silicon" or amide" substitution at the nitrogen atoms, and all the diazines can be reduced to tetrahydro-derivatives with carbamate protection on nitrogen, which aids in stabilisation and thus allows isolation." ... 

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See also in sourсe #XX -- [ ]

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