On the behaviour of the (Z )-phenylhydrazones of some 5-alkyl-3-benzoyl-1,2,4-oxadiazoles in solution and in the gas phase: kinetic and spectrometric evidence in favour of self-assembly

Rate constants, k(A,R), for the rearrangement of the (Z)-phenylhydrazones (1a-e) of a series of 5-alkyl-3-benzoyl-1,2,4-oxadiazoles substituted at C(5) with linear alkyl chains of different length (from C-4 up to C-12) into the relevant 4-acylamino-2,5-diphenyl-1,2,3-triazoles (2a-e) have been measured in dioxan/water in the base-catalyzed region (pS+ 10.5-12.6). For each substrate log k(A,R) are linearly related to pS+. The significant decrease of the slopes of these straight lines (from 0.96 down to 0.78) upon increasing the length of the linear alkyl chain at C5 and that of the reactivity (down to 14-26%) upon increasing the substrate concentration suggest a decrease of the polarity of the 'actual' reaction medium and provide indirect evidence of the tendency of the substrates (Z)-1a-e to self-assemble. To confirm the above outcome direct evidence of the formation of self-assemblies in solution were obtained from 1H NMR and spectrofluorimetry measurements while ESI-MS experiments point out the presence of aggregated substrates also in the gas phase.