213540-61-5Relevant academic research and scientific papers
Factors controlling the reactivity of heteroarenes in direct arylation with arylpalladium acetate complexes
Wakioka, Masayuki,Nakamura, Yuki,Hihara, Yoshihiro,Ozawa, Fumiyuki,Sakaki, Shigeyoshi
supporting information, p. 4423 - 4430 (2013/09/02)
The palladium-catalyzed direct arylation of heteroarenes with aryl halides has emerged as a viable alternative to conventional cross-coupling reactions. This paper reports a detailed mechanistic study on factors controlling the reactivity of heteroarenes in direct arylation with well-defined models of the presumed intermediate [PdAr(O2CMe-κ2O)L] (1). Although recent theoretical studies have provided a reasonable description of the mechanism of C-H bond cleavage by 1, its model compounds so far tested have been evidently less reactive than that expected. We found that [PdPh(O 2CMe-κ2O)(PPh3)] (1a) and [Pd(2,6-Me 2C6H3)(O2CMe-κ2O) (PPh3)] (1c), generated in situ from isolated [PdPh(μ-O 2CMe)(PPh3)]2 (4a) and [Pd(2,6-Me 2C6H3)(μ-O2CMe)(PPh 3)]4 (4c), respectively, react with a variety of heteroarenes in almost quantitative yields. The reactivity order of heteroarenes was evaluated by competitive reactions, showing that benzothiazole (8) is significantly less reactive than 2-methylthiophene (6), despite the acidity of 8 (pKa = 27) being much higher than that of 6 (pKa = 42). This reason was examined by kinetic experiments using 1c as well as DFT calculations using the model compound [PdPh(O2CMe- κ2O)(PH3)] (1d). Both heteroarenes reacted with 1 via a sequence of three elementary processes (i.e., substrate coordination, C-H bond cleavage, and C-C reductive elimination), but their energy profiles were significantly different from each other. The reaction of 6 obeyed simple second-order kinetics, and the deuterium-labeling experiments and DFT calculations indicated the occurrence of rate-determining reductive elimination. On the other hand, the reaction of 8 displayed saturation kinetics due to the occurrence of relatively stable coordination of 8 prior to C-H bond cleavage. This coordination stability enhances the activation barrier for C-H bond cleavage, thereby causing the modest reactivity of 8. Thus, although the previous mechanistic studies on direct arylation have been focused largely on the C-H bond cleavage process, not only the C-H bond cleavage but also the substrate coordination and C-C reductive elimination must be considered.
Carbon-sulfur bond-forming reductive elimination involving sp-, sp2-, and sp3-hybridized carbon. Mechanism, steric effects, and electronic effects on sulfide formation
Mann, Grace,Baranano, David,Hartwig, John F.,Rheingold, Arnold L.,Guzei, Ilia A.
, p. 9205 - 9219 (2007/10/03)
Palladium thiolato complexes [(L)Pd(R)(SR')], within which L is a chelating ligand such as DPPE, DPPP, DPPBz, DPPF, or TRANSPHOS, R is a methyl, alkenyl, aryl, or alkynyl ligand, and R' is an aryl or alkyl group, were synthesized by substitution or proton-transfer reactions. All of these thiolato complexes were found to undergo carbon-sulfur bond-forming inductive elimination in high yields to form dialkyl sulfides, diaryl sulfides, alkyl aryl sulfides, alkyl alkenyl sulfides, and alkyl alkynyl sulfides. Reductive eliminations forming alkenyl alkyl sulfides and aryl alkyl sulfides were the fastest. Eliminations of alkynyl alkyl sulfides were slower, and elimination of dialkyl sulfide was the slowest. Thus the relative rates for sulfide elimination as a function of the hybridization of the palladium-bound carbon follow the trend sp2 > sp >> sp3. Rates of reductive elimination were faster for cis-chelating phosphine ligands with larger bite angles. Kinetic studies, along with results from radical trapping reactions, analysis of solvent effects; and analysis of complexes with chelating phosphines of varying rigidity, were conducted with [Pd(L)(S-tert-butyl)(Ar)] and [Pd(L)(S- tert-butyl)(Me)]. Carbon-sulfur bond-forming reductive eliminations involving both saturated and unsaturated hydrocarbyl groups proceed by an intramolecular, concerted mechanism. Systematic changes in the electronic properties of the thiolate and aryl groups showed that reductive elimination is the fastest for electron deficient aryl groups and electron rich arenethiolates, suggesting that the reaction follows a mechanism in which the thiolate acts as a nucleophile and the aryl group an electrophile. Studies with thiolate ligands and hydrocarbyl ligands of varying steric demands favor a migration mechanism involving coordination of the hydrocarbyl ligand in the transition state.
