Benzidine Rearrangements. 19. The Concerted Nature of the One-Proton Rearrangement of 2,2'-Dimethoxyhydrazobenzene

Article abstract of DOI:10.1021/ja00335a035

Kinetic isotope effects (KIE) in the acid-catalyzed, one-proton rearrangement of 2,2'-dimethoxyhydrazobenzene (1) into 3,3'-dimethoxybenzidine (2) have been measured.Nitrogen KIE were determined on the bis(trifluoroacetyl) derivative of 2 which was isolated from low and 100percent conversions of mixtures of 1 and <15N,15N'>1 under kinetic conditions.The ratio of masses M(438)/M(436) as determined by whole-ion mass spectrometric techniques led to the result k(14N)/k(15N)=1.0289 (for two nitrogens).Similar measurements on the derivative of 2 isolated from conversions of mixtures of 1 and <4,4'-13C2>1 led to the result k(12C)/k(13C)=1.0286 (for two carbons).These results show that the formation of 2 is a concerted process and, thus, that the acid-cytylyzed rearrangement of 1 is a 5,5-sigmatropic shift.Rearrangement via a ?-complex intermediate or a solvent-caged, radical/radical-ion pair is excluded.That bonding in the 4,4' positions of 1 is part of the rate-determining step in this rearrangement was confirmed by measurement of an inverse secondary deuterium KIE (k_H/k_D=0.929 for two deuteriums).This result was obtained from ratios of M(244)/M(242) which were measured on 2,2'-dimethoxyazobenzene (3), obtained by oxidizing the hydrazo substrate remaining after high conversions of mixtures of 1 and <4,4'-2H2>1.