N,N'-DISILYLATED 1,4-DIHYDROPYRAZINES - ORGANOSILYL SUBSTITUTION-REACTIONS, STRUCTURAL EFFECTS OF STERIC HINDRANCE, AND ELECTRON-EXCHANGE WITH C-60

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).