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Mechanistic Study of the Mechanochemical PdII- Catalyzed Bromination of Aromatic C–H bonds by Experimental and Computational methods

The mechanism of mechanochemical PdII-catalyzed selective bromination of the carbon–hydrogen bond in azobenzene by N-bromosuccinimide (NBS) was investigated using in situ time-resolved Raman spectroscopy and quantum-chemical (DFT) calculations. Raman monitoring of the reactions in the presence of different amounts of Pd(OAc)2, p- toluenesulfonic acid (TsOH), and acetonitrile (MeCN) as solid and liquid additives, respectively, provided direct evidence that the formation of the carbon–halogen bond in the solid state proceeds from catalytically active cyclopalladated intermediates that are monomeric in the presence of MeCN or dimeric without MeCN. The reaction pathway via the monomeric palladacycle is more efficient than the pathway via the dimeric palladacycle for the bromination of azobenzene, offering better yields and faster reactions. Both reaction routes require the presence of TsOH, which is involved in the formation of the active PdII catalysts and palladacyclic intermediates, as well as in the activation of NBS. Four possible reaction mechanisms for the bromination of cyclopalladated azobenzene were investigated by DFT modeling. Three mechanistic pathways include (i) oxidative addition of Br to Pd atom and (ii) reductive elimination by 1, 2-displacement of Br to the carbon atom. In one pathway, transfer of Br to Pd occurs only after the initial displacement of the neutral ligand by NBS. In another, Pd atom is inserted directly into the N–Br bond of NBS, and in the last one, Br+ migrates spontaneously from the protonated NBS to Pd. In all three cases, the subsequent elimination step is remarkably lower in energy. In the fourth mechanism, Br+ migrates from free NBS directly to the activated carbon, simultaneously with the Pd–C bond breaking. Besides NBS, the hydrogen bond complex NBS∙∙∙TsOH was also considered as the bromine source. None of the considered mechanisms can be definitely rejected on the basis of experimental findings and the current modeling level and more than one could be operative depending on the reaction conditions.

halogenation ; DFT ; mechanochemistry ; mechanism ; Raman spectroscopy

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