Isomerization of azo compounds. Cleavage recombination mechanism of azosulfides

(Z)-4-Nitrophenyl tert-butylsulfide [(Z)-4-NO2-1], upon formation of its ra dical anion, isomerizes to the (E)-isomer at a very fast rate of 6.3 x 10(6 ) s(-1), about 10(3) times faster than azobenzene, while the 4-fluoro tert- butylsulfide (4-F-1) cleaves without isomerization. The particular behavior of 4-NO2-1 is related to the occurrence of a cleavage recombination mechan ism, different from the usual inversion rotation mechanism, The existence o f a reversible cleavage for the radical anion of 4-NO2-1 bears consequences on the mechanism of S(RN)1 reactions of azosulfides; the attack of the nuc leophile would take place on the intel mediate diazenyl radical and not on the ensuing aryl radical as previously proposed.