Diazonium Exchange and Migration of Pivaloyl Group upon Azo Coupling of β-Enaminones

4,4-Dimethyl-1-methylamino-1-phenyl-2-(substituted phen- yldiazenyl)pent-1-en-3-ones (prepared upon azo coupling of 4,4-dimethyl-1-methylamino-1-phenylpent-1-en-3-one 6a with the corresponding benzenediazonium tetrafluorobor- ates) react with another molecule of substituted benzenedi- azonium tetrafluoroborate (in dichloromethane in the pres- ence of anhydrous sodium acetate) to form substituted 4,4- bis(substituted phenyldiazenyl) derivatives 8. The second azo coupling is reversible. Derivatives 8 undergo either re- verse cleavage of a diazonium ion or [1,3] sigmatropic re- arrangement forming substituted formazane 9. In the case of the sequential use of two different diazonium tetrafluoro- borates, the less electrophilic group splits off more easily. The structures of the products 9 were studied by means of X-ray, 1H, 13C, 19Fand 15N NMR and MALDI HRMS analyses. The formazans 9 exhibit a reduced mobility of the phenyl group adjacent to the pivaloyl group, giving rise to anisochronism of proton and carbon atoms, to eventually form conformers. The reduced mobility was observed by means of NMR spec- troscopy. A temperature dependence of the spectral behav- iour was also studied.