A. Lichtblau et al., N,N'-DISILYLATED 1,4-DIHYDROPYRAZINES - ORGANOSILYL SUBSTITUTION-REACTIONS, STRUCTURAL EFFECTS OF STERIC HINDRANCE, AND ELECTRON-EXCHANGE WITH C-60, Chemische Berichte, 128(8), 1995, pp. 745-750
Organosilyl exchange reactions of the extremely electron-rich and form
ally ''antiaromatic'' 1,4-bis(trimethylsilyl)-substituted 1,4-dihydrop
yrazine 1 with Ph(3)SiCl or Me(5)Si(2)Cl yielded the new compounds 2 a
nd 3 with triarylsilyl and disilanyl substituents. At delta = 4.53 in
CDCl3 compound 3 shows a particularly high-field shifted H-1-NMR reson
ance for the 1,8-dihydropyrazine protons. In contrast to the planar ri
ng arrangement of 1,4-bis(triisopropylsilyl)-1,4-dihydropyrazine 4, th
e crystal structure determination of the 2,5-dimethylated analogue 5 a
s obtained by reductive silylation exhibits a pronounced boat conforma
tion of the heterocycle due to steric repulsion between the substituen
ts, the closest H...H (CH3/CH) contact being 211 pm. Despite the sizea
ble structural differences between 4 and 5 both compounds are reversib
ly oxidized at very negative potentials, at -0.90 and -0.85 V vs. Fc(/0), respectively. It appears that the electrochemical behavior of thi
s class of compounds is determined primarily by the configuration at t
he nitrogen centers and less by the ring conformation. Compound 4 reac
ts with C-60 under EPR-detectable electron exchange to afford 4(+.) an
d C-60(-.). The radical cation 4(+.) generated separately via oxidatio
n with iodine was characterized by low-temperature UV/Vis and ENDOR sp
ectroscopy. An ENDOR spectrum revealing silylalkyl coupling was also o
btained for the related radical cation of reversibly oxidizable (tert-
butyldimethylsilyl)-1,1',4,4'-bipyridylidene (8).