Effects of Cyclic 8-?-Electron Conjugation in Reductively Silylated N-Heterocycles

Article abstract of DOI:10.1021/ja00342a005

A number of partly reduced N-heterocycles,2-15, have been prepared by reductive silylation of aromatic precursors.The N-silyl substituents stabilize unusual electronic structures such as the 1,4-dihydropyrazine system toward rearrangements.In addition, R3Si substitution is likely to cause planarization at the amino nitrogen atoms.This may lead to cyclic 8-?-electron conjugation, as has been established, e.g., for 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (2).The experimental results obtained for 2 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 the second ionization energy.These effects confirm the predictions made for planar 1,4-dihydropyrazine on the basis of HMO calculations.Corresponding to the very low ionization potentials of most of the reduced compounds, persistent radical cations such as 2⁺. have been readily obtained and were fully characterized by ESR spectroscopy.Modification of the 1,4-dihydropyrazine 2 by methyl substitution or by extension of the ? system results in an attenuation of the 8-?-electron conjugation through steric and/or electronic factors.The flexibility of this system toward steric requirements can be related to the redox behavior of flavoenzymes.