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Strong evidence of solution-phase catalysis associated with palladium leaching from immobilized thiols during Heck and Suzuki coupling of aryl iodides, bromides, and chlorides

“…These low levels of palladium leaching suggest that either the leached species are very active (i.e., if all conversions are catalyzed by the amount of palladium in solution after the reaction), or that a redeposition process is at play, whereby larger amounts of leached palladium are active during reaction, with some or most of the palladium redepositing on the SH-SBA-15 surface after reaction. After recovery, this redeposited palladium may be available for further reaction, as was similarly found for other immobilized palladium precatalysts [16,52,61,62]. However, if the S:Pd ratio is >2:1, then palladium capture and overcoordination by excess surface thiols may result in a partial or complete deactivation of catalysts, as was observed for partially metalated SH-SBA-15 surfaces (vide supra).…”

Concept of ?magic? number clusters as a new approach to the interpretation of unusual kinetics of the Heck reaction with aryl bromides

“…Bromobenzene (10 mmol), 10 mmol styrene, 13 mmol sodium acetate (or 11.2 mmol sodium acetate and 1.8 mmol NBu 3 as reducing agent [1,2]), 2 mmol naphthalene (as internal standard for GC analysis), 10 mL DMF (as solvent) and 0.16 mmol PdCl 2 were introduced in glass reactor. The preparations and manipulations were performed under argon atmosphere when PPh 3 has added to catalytic system.…”

“…It leads to withdrawal of catalyst from catalytic cycle due to undesirable competitive Pd(0) aggregation [1][2][3]. Taking into account a higher order of aggregation with respect to Pd(0) concentration [1][2][3], one might anticipate that a decrease of catalyst concentration favours at least a raise of catalytic cycle turnover frequency (i.e. order in palladium concentration from 0 up to 1).…”

Effect of macrokinetic factors on the ligand-free Heck reaction with non-activated bromoarenes

“…The Pd(II) reduction to Pd(0) (Scheme 1, A) proceeding as an autocatalytic reaction was found to be necessary not only at the reaction starting but throughout the overall process due to side reductive conversions of aryl halides into Ar-Ar (Scheme 1, B) [18,19]. However, that problem might be solved by the addition of reducing agent small amounts (for example, sodium formate) [15,20].…”