N,N′ ‐Disilylated 1,4‐Dihydropyrazines: Organosilyl Substitution Reactions, Structural Effects of Steric Hindrance, and Electron Exchange with C₆₀

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Organosilyl exchange reactions of the extremely electronrich and formally “antiaromatic” 1,4‐bis(trimethylsilyl)‐substituted 1,4‐dihydropyrazine 1 with Ph 3 SiCl or Me 5 Si 2 Cl yielded the new compounds 2 and 3 with triarylsilyl and disilanyl substituents. At δ = 4.53 in CDCl 3 compound 3 shows a particularly high‐field shifted 1 H‐NMR resonance for the 1,4‐dihydropyrazine protons. In contrast to the planar ring arrangement of 1,4‐bis(triisopropylsilyl)‐1,4‐dihydropyrazine 4 , the crystal structure determination of the 2,5‐dimethylated analogue 5 as obtained by reductive silylation exhibits a pronounced boat conformation of the heterocycle due to steric repulsion between the substituents, the closest H…H (CH 3 / CH) contact being 211 pm. Despite the sizeable structural differences between 4 and 5 both compounds are reversibly oxidized at very negative potentials, at ‐0.90 and ‐0.85 V vs. Fc + /0, respectively, It appears that the electrochemical behavior of this class of compounds is determined ***primarly by the configuration at the nitrogen centers and less by the ring conformation. Compound 4 reacts with C 60 under EPR‐detectable electron exchange to afford 4 +* and C . The radical cation 4 +* generated separately via oxidation with iodine was characterized by low‐temperature UV/Vis and ENDOR spectroscopy. An ENDOR spectrum revealing silylalkyl coupling was also obtained for the related radical cation of reversibly oxidizable 1,1′ ‐bis( tert ‐butyldimethylsilyl)‐1,1′,4,4′ ‐bipyridylidene (8 ).