Heavy-Atom Kinetic Isotope Effects and Mechanism of the Acid-Catalyzed o-Semidine and p-Semidine Rearrangements and Disproportionation of 4,4'-Dichlorohydrazobenzene

Article abstract of DOI:10.1021/ja00265a026

In acidic 60percent aqueous dioxane solution at 0 deg C, 4,4'-dichlorohydrazobenzene (18) undergoes concurrent disproportionation, to p-chloroaniline (19) and 4,4'-dichloroazobenzene (20), and o- (21) and p-semidine (22) rearrangement.In the p-semidine rearrangement one of the chlorine atoms of 18 is displaced, in essence, as Cl⁺.This requires participation of a second molecule of 18 in a redox reaction.The overall fate of 18, therefore, is to give 11percent o- and 12percent p-semidine rearrangement (along with 12percent of 20) and 60percent disproportionation, accounting for 95percent of the 18.Nitrogen and carbon kinetic isotope effects (KIE) have been determined for each of these reactions, using <15N,15N'>18, <2-14C>18, <4-14C>18, and <4,4'-13C2>18.Isotope ratios were obtained, measured on the trifluoroacetyl derivatives of 19, 21, and 22, with a combination of scintillation counting, whole-molecule-ion mass spectrometry (WMIMS), and isotope-ratio mass spectrometry (IRMS).Nitrogen KIE were obtained by WMIMS for two 15N atoms in disproportionation (1.0260) and p-semidine rearrangement (1.0282) and by IRMS for one (naturally abundant) 15N atom in disproportionation (1.0141) and o-(1.0155) and p-semide (1.0162) rearrangement. 13C (IRMS) and 14C KIE were measured for all reactions, but in no case was a KIE other than, effectively, unity obtained.These results show that o-semidine formation from 18 complies with exceptations of sigmatropic shifts; that is, that this 1,3-sigmatropic shift is not a concerted process.The results suggest that, although a concerted 1,5-sigmatropic shift is possible, the p-semidine rearrangement of 18 is not characterized by one.It this case, however, a firm decision is not possible.Finally, the results indicate that disproportionation involves one (or both) of the two semidine rearrangement intermediates.The most likely one is that of the p-semidine.Rapid redox reaction of this intermediate (26), formed in the rate-determining step, with a second molecule of 18 can then lead to the p-semidine by removal of Cl⁺ and to disproportionation by scission of the central C-C bond of the intermediate.These paths account for the distribution of the products formed and the KIE of their formation.