Heteropolyacid-Catalyzed Phosphorylation of Secondary Aromatic Alcohols with H-Phosphine Oxides in DMC: A Simple Protocol for C-P Bond Formation.

We successfully achieved the phosphorylation of secondary aromatic alcohols with H-phosphine oxides (less developed system) using phosphotungstic acid as a catalyst in dimethyl carbonate. The system was simple and environmentally friendly and showed better activity than traditional Lewis or Brønsted acids such as FeCl3, p-TsOH·H2O, etc., generating up to a 97% isolated yield. Control experiments indicated that the reaction did not occur through the radical pathway, and ethers and carbocation were the key intermediates in the pathway.

Metal-Free, Adjustable, and Recyclable Catalytic Systems for the Construction of C-C Bonds by Activating Propargylic Alcohols.

Pyrano[3,2-c]coumarin derivatives and C3-substituted 4-hydroxycoumarins as important skeletal structures of active natural products and pharmaceutically relevant molecules have received increasing attention. However, developing an adjustable system for selectively synthesizing them is still a challenging task. Herein, sulfonic acid-functionalized ionic liquid was successfully used as the catalyst for the alkylation of 4-hydroxycoumarin derivatives with secondary aromatic propargylic alcohols using dimethyl carbonate as the green solvent, giving up to 98% yield. On the other hand, protonated imidazole-based ionic liquid-catalyzed cyclization was also selectively achieved with a nearly quantitative yield. Developed metal-free catalytic systems exhibited well adjustable and recyclable properties, avoiding the contamination of metal and halogen, reducing the neutralization after the reaction, and benefiting the separation between the catalyst and the product. New strategies were applied for performing the gram-scale reaction smoothly. The adjustable systems might occur through two different mechanisms involving propargylic or allenic carbocation and hydrogen bonding effects between the catalysts and the substrates.

Direct deoxygenation of active allylic alcohols via metal-free catalysis.

Direct metal-free deoxygenation of highly active allylic alcohols catalyzed by a Brønsted acid was achieved, which avoids tedious reaction steps and eliminates metal contamination. By examining a series of Brønsted acids, alcohols, reaction temperatures and so on, up to 94% yield was obtained with 10 mol% TsOH·H2O as the catalyst and 2 equiv. of p-methylbenzyl alcohol as the reductant at 80 °C for 2 h. The system was mainly suitable for aromatic allylic alcohols, and the yield was excellent as determined via gram-scale synthesis. The main product was double bond near the side of a more electron-rich aryl group when allylic alcohols featuring different substituents at the 1 and 3 positions were used as the substrates. Deuterium-labelled experiments clearly demonstrated that the hydrogen source was the methylene of p-methylbenzyl alcohol and other control experiments indicated the existence of two ether intermediates. Interestingly, in situ hydrogen transfer of allylic benzyl ether is a key process, but kinetic isotopic effect studies (kH/kD = 1.28) showed that the C-H bond cleavage was not the rate-determining step. A possible mechanism involving carbocations, ether intermediates and hydrogen transfer is proposed.

Proton-Promoted and Anion-Enhanced Epoxidation of Olefins by Hydrogen Peroxide in the Presence of Nonheme Manganese Catalysts.

We report a remarkable Brønsted acid effect in the epoxidation of olefins by nonheme manganese catalysts and aqueous hydrogen peroxide. More specifically, a mononuclear nonheme manganese complex bearing a tetradentate N4 ligand, Mn(II)(Dbp-MCP)(OTf)2 (Dbp-MCP = (1R,2R)-N,N'-dimethyl-N,N'-bis((R)-(3,5-di-tert-butyl-phenyl)-2-pyridinylmethyl)cyclohexane-1,2-diamine; OTf(-) = CF3SO3(-)), is a highly efficient catalyst in the epoxidation of olefins by aqueous H2O2 in the presence of H2SO4 (1-3 mol %). The yields of epoxide products as well as the chemo- and enantioselectivities increase dramatically in the presence of H2SO4; no formation of epoxides is observed in the absence of H2SO4. In addition, the product yields and enantioselectivities are dependent significantly on the manganese catalysts and Brønsted acids. The catalytic epoxidation of olefins by other oxidants, such as peracids, alkyl hydroperoxides, and iodosylbenzene, is also affected by the presence of H2SO4; product yields and enantioselectivities are high and similar irrespective of the oxidants in the presence of H2SO4, suggesting that a common epoxidizing intermediate is generated in the reactions of [Mn(II)(Dbp-MCP)](2+) and the oxidants. Mechanistic studies, performed with (18)O-labeled water (H2(18)O) and cumyl hydroperoxide, reveal that a high-valent manganese-oxo species is formed as an epoxidizing intermediate via O-O bond heterolysis of Mn-OOH(R) species. The role of H2SO4 is proposed to facilitate the formation of a high-valent Mn-oxo species and to increase the oxidizing power and enantioselectivity of the Mn-oxo oxidant in olefin epoxidation reactions. Density functional theory (DFT) calculations support experimental results such as the formation of a Mn(V)-oxo species as an epoxidizing intermediate.

N-Bromosuccinimide as an oxidant for the transition-metal-free synthesis of 2-aminobenzoxazoles from benzoxazoles and secondary amines.

A facile and transition-metal-free method was developed through merging the ring opening of benzoxazoles with secondary amines and N-bromosuccinimide (NBS) mediated oxidative cyclization toward the synthesis of 2-aminobenzoxazoles. NBS was selected as a powerful oxidant in the oxidative cyclization of ring-opening amidines to provide the desirable 2-aminobenzoxazoles in excellent yields (up to 94%).

Effective adsorption and catalytic reduction of nitrophenols by amino-rich Cu(I)-I coordination polymer.

Nitrophenols are identified as the priority organic pollutants due to the chemical stability, water solubility, persistence, and toxicity to human health and the environment. Hence, removal of nitrophenols from waste water is vitally essential. In this study, amino-rich coordination polymer Cu2I2(MA)2 (MA = melamine) has been applied for efficient adsorption and catalytic reduction of nitrophenols, like 4-nitrophenol (4-NP), 2, 4-dinitrophenol (DNP) and 2, 4, 6-trinitrophenol (TNP). The effect of various parameters like contact time, initial concentrations, pH, and temperature on adsorption were investigated. The adsorption of nitrophenols fitted the pseudo-second-order kinetic model and Langmuir isotherms model well. The maximum adsorption capacities were 285.71, 232.02, and 131.57 mg g-1 for 4-NP, DNP, and TNP when initial concentrations were 50 mg L-1 at 293.15 K, respectively. The adsorption of nitrophenols is a spontaneous, endothermic, and entropy-driven process. The reduction reaction followed the pseudo-first-order kinetics, and the kinetic rate constants were 0.4413, 0.3167, and 0.17538 min-1 for 4-NP, DNP, and TNP, respectively. The effect of initial nitrophenols concentration, anions, and temperature on reduction process was investigated. The mechanism of adsorption and catalytic reduction of Cu2I2(MA)2 was studied. The results demonstrated that Cu2I2(MA)2 exhibits excellent adsorption and catalytic activity to remove nitrophenols.

Self-neutralizing in situ acidic CO2/H2O system for aerobic oxidation of alcohols catalyzed by TEMPO functionalized imidazolium salt/NaNO2.

A reversible in situ acidic catalytic system comprising recyclable TEMPO functionalized imidazolium salt ([Imim-TEMPO][Cl])/NaNO(2)/CO(2)/H(2)O was developed for selective transformation of a series of aliphatic, allylic, heterocyclic, and benzylic alcohols to the respective carbonyl compounds. Notably, the system avoids any conventional acid and can eliminate unwanted byproducts, facilitate reaction, ease separation of the catalyst and product, and also provide a safe environment for oxidation involving oxygen gas.

Manganese complex-catalyzed oxidation and oxidative kinetic resolution of secondary alcohols by hydrogen peroxide.

The highly efficient catalytic oxidation and oxidative kinetic resolution (OKR) of secondary alcohols has been achieved using a synthetic manganese catalyst with low loading and hydrogen peroxide as an environmentally benign oxidant in the presence of a small amount of sulfuric acid as an additive. The product yields were high (up to 93%) for alcohol oxidation and the enantioselectivity was excellent (>90% ee) for the OKR of secondary alcohols. Mechanistic studies revealed that alcohol oxidation occurs via hydrogen atom (H-atom) abstraction from an α-CH bond of the alcohol substrate and a two-electron process by an electrophilic Mn-oxo species. Density functional theory calculations revealed the difference in reaction energy barriers for H-atom abstraction from the α-CH bonds of R- and S-enantiomers by a chiral high-valent manganese-oxo complex, supporting the experimental result from the OKR of secondary alcohols.

Synergistic Acid-Catalyzed Synthesis of N-Aryl-Substituted Azacycles from Anilines and Cyclic Ethers.

A metal-free and efficient approach to N-aryl-substituted azacycles from arylamines and cyclic ethers is described. In this synthesis, the synergistic effect between Lewis and Brønsted acids is crucial to the ring-opening of cyclic ethers and the subsequent cyclization. The use of B(C6F5)3 enabled the formation of frustrated Lewis pairs (FLPs) from the reactants, and the resulting FLPs allowed ready access to the N-arylazacycles in moderate to good yields via further cyclization. Water is the sole waste resulting from the reaction, thereby making it an environmentally benign process.

Enantioselective Epoxidation of Olefins with H2O2 Catalyzed by Bioinspired Aminopyridine Manganese Complexes.

A novel family of bioinspired manganese(II) complexes bearing chiral aminopyridine ligands that possessed additional aromatic groups and strong donating dimethylamino groups were synthesized and characterized. These manganese complexes exhibited efficient and improved activities in the asymmetric epoxidation of various olefins, such as styrene derivatives (up to 93% ee) with H2O2 as the oxidant, even with a catalytic amount of carboxylic acid as the additive.

Highly efficient oxidation of secondary alcohols to ketones catalyzed by manganese complexes of N4 ligands with H2O2.

The manganese complex Mn(S-PMB)(CF3SO3)2 was proven to be highly efficient in the catalytic oxidation of several benzylic and aliphatic secondary alcohols with H2O2 as the oxidant and acetic acid as the additive. A maximum turnover number of 4700 was achieved in the alcohol oxidation. In addition, the Hammett analysis unveiled the electrophilic nature of this manganese catalyst with N4 ligand.

Efficient benzylic and aliphatic C-H oxidation with selectivity for methylenic sites catalyzed by a bioinspired manganese complex.

A benzimidazole-based nonheme manganese complex efficiently catalyzes benzylic, aliphatic C-H as well as tertiary C-H oxidation with hydrogen peroxide as the oxidant in the presence of acetic acid as additive. (18)O labeling experiments suggest the reaction may proceed via a high-valent manganese-oxo intermediate.

Asymmetric 5-endo chloroetherification of homoallylic alcohols toward the synthesis of chiral ß-chlorotetrahydrofurans.

An asymmetric 5-endo chloroetherification of homoallylic alcohols is successfully developed that employs an easily available quaternary ammonium salt derived from cinchonine as a conventional organocatalyst. This approach provides ready access to ß-chlorotetrahydrofurans in high enantioselectivities.

Synthesis of bimagnetic ionic liquid and application for selective aerobic oxidation of aromatic alcohols under mild conditions.

The first bimagnetic ionic liquid based on Fe and TEMPO with cooperative functionalities not only exhibited strong paramagnetic behaviour at room temperature under an applied magnetic field of 5000 Oe but also proved to be an effective catalyst for selective aerobic oxidation of aromatic alcohols under mild and clean conditions.

The free-radical chemistry of polyethylene glycol: organic reactions in compressed carbon dioxide.

The thermal oxidative degradation of polyethylene glycol (PEG) is known to occur in an oxygen atmosphere at elevated temperatures. In this study, PEG radicals assumed to result from thermal oxidative degradation are successfully applied, in combination with compressed CO(2), to initiate a range of free-radical reactions, such as selective formylation of primary and secondary aliphatic alcohols, oxidation of benzylic alcohols, benzylic C==C bond cleavage, and benzylic sp(3) C--H oxidation, demonstrating enormous synthetic potential in a cost-efficient, practically useful, and environmentally friendly manner; not requiring any catalyst or additional free-radical initiator. We find that both PEG and molecular oxygen are prerequisites in order to perform these reactions smoothly. Given that dense CO(2) is immune to free-radical chemistry; it is an ideal solvent for such reactions. As a result, compressed CO(2) allows reactions initiated by PEG radicals to be tuned by subtly adjusting reaction parameters such as the CO(2) pressure, thereby enhancing the product selectivity. By attaining a high selectivity towards the desired products this methodology is practical for organic syntheses.

Bifunctional metal-salen complexes as efficient catalysts for the fixation of CO2 with epoxides under solvent-free conditions.

A bifunctional cobalt-salen complex containing a Lewis acidic metal center and a quaternary phosphonium salt unit anchored on the ligand effectively catalyzes the synthesis of cyclic carbonates from CO2 and epoxides under mild conditions without the utilization of additional organic solvents or co-catalysts. The effects of various reaction variables on the catalytic performance were studied in detail and indicate an optimized reaction temperature of about I00 degrees C and CO2 pressure of around 4 MPa, although the reaction proceeds smoothly even at pressures as low as 2 MPa. The catalyst is applicable to a variety of epoxides, producing the corresponding cyclic carbonates in good yields in most cases. Furthermore, the catalyst can be easily recovered and reused several times without significant loss of its catalytic activity. This process thus represents a greener pathway for the environmentally benign chemical fixation of CO2 to produce cyclic carbonates.