An experimental mechanistic study of the aerobic copper-catalyzed decarboxylative thiolation of benzoic acids with aryl thiols is reported. For the model reaction, the findings support the corresponding disulfide (PhSSPh) of the aryl thiol (PhSH) as the active thiolating source under reaction conditions. Synthesis and reactivity studies along with kinetic measurements support the chemical and kinetic competence of catalytically active welldefined Cu-complexes; (phen)Cu I (O2CC6H4-o-NO2) (2), (phen)Cu I (C6H4-o-NO2) (4), [(phen)Cu I (µ-SC6H5)]2 (3) and (phen)Cu II (O2CC6H4-o-NO2)2 (5), (phen = 1,10-phenanthroline). The presence of an induction period in the stoichiometric reaction of copper(II) complex (phen)Cu II (O2CC6H4-o-NO2)2 (5) with PhSSPh, and the absence of an induction period in the analogous stoichiometric reaction of copper(I) complex (phen)Cu I (O2CC6H4-o-NO2) (2), suggests a copper(I) carboxylate is a more likely intermediate than a copper(II) carboxylate. The observation of in situ reduction of Cu II to Cu I further supports Cu I as the primary active catalytic species and spectroscopic studies also indicate the catalyst resting state to be a Cu I species. The catalytic reaction exhibits a first order dependence on [Cu I ] and [2nitrobenzoic acid] and a zero order dependence on [PhSSPh] and pO2, suggestive of turnover limiting decarboxylation of a copper(I) carboxylate. Oxygen was found to promote the essential oxidative cleavage of the copper(I)thiolate intermediate [(phen)Cu(µ-SC6H5)]2 (3) to regenerate a catalytically active [(phen)Cu II ] (Cuox) species with concomitant formation of PhSSPh. On the basis of these findings, a reaction pathway is proposed for the C-S coupling reaction that includes the key Cu I-based intermediates, (phen)Cu(O2CC6H4-o-NO2) (2), and (phen)Cu(C6H4o-NO2) (4). The pathway accounts for the role of O2 in generating the active thiolating source, PhSSPh, as well as enabling catalytic turnover of in situ generated [(phen)Cu(µ-SC6H5)]2 (3).
Alternative procedures are reported for the preparation of PEPPSI-type palladium complexes (PEPPSI = pyridine-enhanced precatalyst preparation stabilization and initiation) in good to excellent yields. One method involves the reaction of [(NHC)Pd(acac)Cl] complexes (NHC = N-heterocyclic carbene, acac = acetylacetonate) with hydrohalides (HX; X = Cl, Br, I) of pyridine or 2,6-lutidine. Two other one-pot syntheses involve Pd(acac) 2 , the azolium salt and pyridine [a]
Ni-catalyzed C−H functionalization reactions are becoming efficient routes to access a variety of functionalized arenes, yet the mechanisms of these catalytic C−C coupling reactions are not well understood. Here, we report the catalytic and stoichiometric arylation reactions of a nickel(II) metallacycle. Treatment of this species with silver(I)−aryl complexes results in facile arylation, consistent with a redox transmetalation step. Additionally, treatment with electrophilic coupling partners generates C−C and C−S bonds. We anticipate that this redox transmetalation step may be relevant to other coupling reactions that employ silver salts as additives.
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