Mechanism of Mechanochemical C-H Bond Activation in an Azobenzene Substrate by PdII Catalysts

Mechanism of C-H bond activation by various Pd-II catalysts under milling conditions has been studied by insitu Raman spectroscopy. Common Pd-II precursors, that is PdCl2, [Pd(OAc)(2)](3), PdCl2(MeCN)(2) and [Pd(MeCN)(4)][BF4](2), have been employed for the activation of one or two C-H bonds in an unsymmetrical azobenzene substrate. The C-H activation was achieved by all used Pd-II precursors and their reactivity increases in the order [Pd(OAc)(2)](3)<[Pd(MeCN)(4)][BF4](2). Insitu Raman monitoring in combination with stepwise exsitu NMR, IR and PXRD experiments has provided direct probing of the reaction mechanism and kinetics, and revealed how liquids of different acid-base properties and proticity as well as selected solids used as additives modify precursors or intermediates and their reactivity. Reaction intermediates that were isolated and structurally characterized agree with the observed species during reaction. Insitu Raman spectroscopy has also enabled the derivation of reaction profiles suggesting an electrophilic process which proceeds via a coordination complex (adduct) undergoing deprotonation by a bound or an external base depending on the used Pd-II precursor. Slow step of the first palladation for two chloride precursors and [Pd(MeCN)(4)][BF4](2) is the C-H bond cleavage whereas palladation using [Pd(OAc)(2)](3) depends primarily on breaking of its trimeric structure by the azobenzene substrate and/or liquid additives.