Zwitterion-Induced Organic–Metal Hybrid Catalysis in Aerobic Oxidation

Abstract

In many metal catalyses, the traditional strategy of removing chloride ions is to add silver salts via anion exchange to obtain highly active catalysts. Herein, we reported an alternative strategy of removing chloride anions from ruthenium trichloride using an organic [P + -N − ] zwitterionic compound via multiple hydrogen bond interactions. The resultant organic−metal hybrid catalytic system has successfully been applied to the aerobic oxidation of alcohols, tetrahydroquinolines, and indolines under mild conditions. The performance of zwitterion is far superior to that of many other common Lewis bases or Brønsted bases. Mechanistic studies revealed that the zwitterion triggers the dissociation of chloride from ruthenium trichloride via nonclassical hydrogen bond interaction. Preliminary studies show that the zwitterion is applicable to catalytic transfer semi-hydrogenation.

Zwitterion‐Catalyzed Amino‐Dibromination of Nitroalkenes: Scope, Mechanism, and Application to The Synthesis of Glycinamides

“…By replacing DMAP with trialkylphosphines, the phosphonium zwitterions 1 d and 1 e were prepared [22] and applied in the catalytic intermolecular haloesterification and aerobic oxidation. [23] At the outset of the investigation, the catalytic effect on amino dibromination of β-nitrostyrene (2 a) was examined using NBS as the source of imide and bromine in CD 3 CN (Table 1). In the absence of catalyst, no desired product 3 a was detected and substrate 2 a was recovered quantitatively (entry 1).…”

“…A possible catalytic cycle is proposed based on the experimental observations (Scheme 5). Based on our previous studies, [21,23] zwitterions 1 readily interact with acidic protons and electrophilic Br, and the counteranions (e. g. succinimide of NBS) could be stabilized by the cationic moieties of 1 by nonclassical hydrogen bond (NCHB). [26] Thus, we believe that equilibrium among zwitterion 1 d, succinimide, and NBS might exist during the reaction.…”

“…[21,23] Based on the kinetics data, subsequent nucleophilic addition of succinimide anion to 2 a could possibly be the rate-determining step of the reaction, yielding the nitronate anion in species B. We rationalized that the acidic NÀ H proton in species B might exchange with NBS to give species C. [23,27] The close proximity of the Br and nitronate in species C could facilitate the bromination (against the polymerization via the intermolecular reaction between species B and nitroalkene 2 a) to give species D. Species D, which possesses an acidic α-proton to the nitro group, might be deprotonated by zwitterion 1 d followed by another α-bromination through species (D!E!F) to yield the desired dibromide product 3 a. However, the possibility of phosphonium unit in zwitterion 1 d being a Lewis acid in catalyzing the reaction could not be completely ruled out.…”

Improved catalytic performance by changing surface and textural properties of Ru supported bifunctional periodic mesoporous organosilicas in aerobic oxidation of alcohols

“…[1][2][3] To avoid the shortcomings of traditional-to-recent applied methods for this key transformation, including the use of toxic organic solvents and stoichiometric amounts of hazardous oxidants and reagents, which resulted in production of a large volume of wastes, as well as the problems associated with the use of homogeneous catalysts, a lot of attention has been given to perform this reaction in the presence of heterogeneous catalysts and green oxidants, especially molecular oxygen. [4][5][6][7][8] During the past decade, numerous metal-based heterogeneous catalysts, such as palladium, 9-19 ruthenium, [20][21][22][23][24][25][26][27][28] cobalt, [29][30][31][32][33][34][35][36][37] platinum, [38][39][40][41][42][43][44][45] gold, 46-57 copper, [58][59][60][61][62][63][64][65][66]…”

“…During the past decade, numerous metal-based heterogeneous catalysts, such as palladium, 9–19 ruthenium, 20–28 cobalt, 29–37 platinum, 38–45 gold, 46–57 copper, 58–69 vanadium, 70–72 and nickel, 73,74 have successfully been employed in the aerobic oxidation of alcohols to their corresponding carbonyl compounds. Among them, ruthenium-based catalysts, in either homogeneous or heterogeneous form, are the most reliable catalytic systems reported in this research area because of possessing wide range of oxidation states and displaying a high degree of selectivity and efficiency in the oxidation of a variety of structurally diverse alcohols.…”