Heavy-Atom Kinetic Isotope Effects in the Acid-Catalysed and Thermal Rearrangements of 2,2'-Hydrazonaphthalene. Transition-State Differences in their Concerted Rearrangements

Article abstract of DOI:10.1021/ja00297a030

Acid-catalyzed (70percent aqueous dioxane at 0 deg C) and thermal rearrangement (95percent ethanol at 80 deg C) of 2,2'-hydrazonaphthalene (1) into 2,2'-diamino-1,1'-binaphthyl (2) have been shown to be concerted, <3,3>-sigmatropic shifts.This was accomplished by measuring the nitrogen and carbon kinetic isotope effects (KIE), for which purpose mixtures of 1 with <15N,15N>1 and <1,1'-13C2>1 were used.KIE were calculated from whole-molecule mass ratios, measured by multiscan mass spectrometry, in the bis(trifluoroacetyl) derivative of product 2, isolated after low and high conversions.The results (averaged) were, for two isotopic atoms, k(14N)/k(15N') = 1.0904, k(12C)/k(13C) = 1.0086 in the one-proton, acid-catalyzed rearrangement and 1.0611 and 1.0182, respectively, in the neutral, thermal rearrangement.These results indicate that although the rearrangements are concerted processes, the breaking of the N-N bond and the forming of the C-C bond proceed to different extents in the transition states.Furthermore, the differences in the timing of these events is greater in the acid-catalyzed than in the thermal rearrangement, a difference which may be attributable to earlier C-C bonding in the polar transition state of the former.