Abstract
A no. of partly reduced N-heterocycles ,e.g., I R = H (II), 2-Me, 2,3,5-Me3, etc.), III, have been prepd. by reductive silylation of arom. precursors. The N-silyl substituents stabilize unusual electronic structures such as the 1,4-dihydropyrazine system toward rearrangements. In addn., Me3Si substitution is likely to cause planarization at the amino N atoms. This may lead to cyclic 8p-electron conjugation, as has been established, e.g., for II. The exptl. results obtained for II by comparative 1H NMR and photoelectron spectroscopic studies are a distinct paratropism, an exceptionally low ionization potential, and an enormous difference between the first and second ionization energies. These effects confirm the predictions made for planar 1,4-dihydropyrazine on the basis of HMO calcns. Corresponding to the very low ionization potentials of most of the reduced compds., persistent radical cations such as II+.bul. have been readily obtained (ESR). Modification of II by Me substitution or by extension of the p system results in an attenuation of the 8p-electron conjugation through steric and/or electronic factors. on SciFinder(R)
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