Experimental evidence for the direct involvement of Pd(0) and Pd(II) anionic phosphine complexes in the Mizoroki–Heck coupling reaction

Time-Resolved 3D Hammett Correlation to Monitor Catalyst Behavior with No Differential Data in Hand

“…The results obtained in the Mizoroki–Heck reaction with aryl halides (Figure ) or aromatic carboxylic anhydrides (Figure S11), and in the direct C–H arylation of indole (Figure S12) are in good agreement with the data of numerous previous studies aimed at the establishment of active Pd species in these processes. The ligation of truly active Pd species by endogenous and/or exogenous halide anions, revealed in this work using the analysis of 3D Hammett correlations, and variable compositions of such complexes depending on specific reaction conditions were also proposed earlier by different research methods. ,,,− ,,− ,, Therefore, the results presented here confirm the prospects of 3D Hammett plots for analyzing ρ­( t,X ) as a valuable diagnostic tool in complex catalytic reactions, facilitating the study of dynamic behaviors of active species and providing insights into catalyst transformations under real turnover conditions.…”

“…The ligation of truly active Pd species by endogenous and/or exogenous halide anions, revealed in this work using the analysis of 3D Hammett correlations, and variable compositions of such complexes depending on specific reaction conditions were also proposed earlier by different research methods. 4,15,17,[26][27][28]46,[49][50][51][52]54,56 Therefore, the results presented here confirm the prospects of 3D Hammett plots for analyzing ρ(t,X) as a valuable diagnostic tool in complex catalytic reactions, facilitating the study of dynamic behaviors of active species and providing insights into catalyst transformations under real turnover conditions. ■ EXPERIMENTAL SECTION General Information.…”

“…The Pd-catalyzed Mizoroki–Heck reaction of aryl halide with alkene was chosen as a reference to test the construction of the 3D Hammett plot (Scheme ). The commonly accepted mechanistic scheme of Mizoroki–Heck reaction fundamentally differs from that of Suzuki–Miyaura one due to another set of elementary steps affording the reaction product. ,,, At the same time, the dynamics of Pd transformations outside the main catalytic cycle in these two chemically different processes has a lot of similar tendencies. ,,,, It should be noted here that for this reaction a true homogeneous catalysis mechanism became commonly accepted. ,,,, Meanwhile, there is plenty of evidence for the participation of active Pd species ligated by anions present in the reaction mixture in Mizoroki–Heck catalysis, including endogenous and exogenous halide anions. ,,− As follows from the data obtained under competition of aryl bromides in the Mizoroki–Heck reaction, ρ­( t,X ) patterns in the experiments without salt additives and with NBu 4 Br were somewhat different at the very beginning of the reactions but became close to each other at quite low conversion degrees (Figure a). At the same time, ρ­( t,X ) patterns demonstrated a similar tendency to achieve nearly constant values of ρ for short reaction times.…”

“…In our opinion, the results obtained unambiguously indicate the coordination of active Pd species by halide anions proposed earlier. 28,46,[49][50][51][52]54 We believe that a moderate decrease of ρ(t,X) in the beginning of halide-containing experiments with carboxylic anhydrides can be due to a change in relative degrees of Pd coordination by strongly coordinating exogenous halide anions and lowcoordinating endogenous benzoate ones. The considerably lower coordination ability of the latter gave higher ρ(t,X) in the experiments with exogenous halide salt additives in comparison with the reaction without halide salt when considerable conversion of the substrates was achieved.…”