Intensification and Selectivities in Complex Multiphase Reactions: Insight into the Selectivity of Liquid−Liquid Phase-Transfer-Catalyzed O-Alkylation of p -Methoxyphenol with Allyl Bromide (original) (raw)
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Industrial & Engineering Chemistry Research, 2007
In the current work, the merits of the creation of a third phase in a typical biphasic reaction have been illustrated. The advantages of liquid-liquid-liquid phase-transfer catalysis (L-L-L PTC) have been brought out over liquid-liquid phase-transfer catalysis (L-L PTC) by considering the etherification of phenol by benzyl chloride to benzyl phenyl ether. L-L-L PTC is a novel strategy for waste reduction and improving profitability, in which a catalyst-rich middle phase is formed between the other two phases, wherein the main reaction takes place and intensifies the rates of reaction as well as offers better selectivity including catalyst reusability, unlike in the L-L PTC. The etherification of phenol with benzyl chloride under L-L PTC is accompanied by side reactions that lower the selectivity, and the catalyst cannot be recovered but wasted as an effluent, causing a load on the environment. However, the transformation of L-L PTC into L-L-L PTC leads to 100% conversion of the limiting reactant benzyl chloride with 100% selectivity to benzyl phenyl ether. The catalyst-rich phase is recovered and reused to some extent. This also helps in waste minimization, which is a major theme of green chemistry. The current work deals with the effects of different kinetic and processes parameters on enhancement in rates and selectivities in L-L-L PTC over L-L PTC. A mathematical model is also developed.
Alkylation of Phenol: A Mechanistic View
The Journal of Physical Chemistry A, 2006
The current work utilizes the ab initio density functional theory (DFT) to develop a molecular level of the mechanistic understanding on the phenol alkylation in the presence of a cation-exchange resin catalyst, Amberlyst-15. The catalyst is modeled with the benzene sulfonic acid, and the effect of this acid on olefins such as isopropene (i-Pr) and tributene (t-Bu) in a phenol solution mimics the experimental condition. A neutral-pathway mechanism is established to account for early-stage high concentration of the phenolic ether observed in experiments. The mechanism involves an exothermic reaction between olefin and the benzene sulfonic acid to form ester followed by three reaction pathways leading to direct O-alkylation, o-C-alkylation, and p-C-alkylation. Our calculations conclude that O-alkylation to form the phenolic ether is the most energetically favorable in the neutral condition. An ionic rearrangement mechanism describes intramolecular migrations of the alkyl group from the phenolic ether to form C-alkylphenols, while the positively charged protonation significantly lowers transition barriers for these migrations. The ionic rearrangement mechanism accounts for high yields of o-C-alkylphenol and p-C-alkylphenol. Competition between the H atom and the alkyl R group at the substitutive site of the protonated ortho configuration is found to be the determining factor to the ortho/para ratio of C-alkylation products.
Phenoxide Allylation in a Phase-Transfer Catalytic Extraction System
Industrial & Engineering Chemistry Research, 1995
Under two-phase conditions, the extractiodremoval of phenol from a n aqueous alkaline medium via reaction with allyl bromide dissolved in dichloromethane was investigated in this study using tetra-n-butylammonium bromide as a phase-transfer catalyst. The concentrations of base, reactant and catalyst, and solvents were evaluated to find the optimum condition in this reaction. In examining 10 kinds of phase-transfer catalysts, aliquat 336, tetra-n-butylammonium iodide, tetra-n-butylphosphonium bromide, and benzyltributylammonium bromide were found to be the best for increasing the volumetric rate of extraction. The extractive efficiency of phenol increased with increasing basic concentration and was significantly influenced by the activity of solvent. The conversion of phenol in a batch-agitated reactor was found to be more than 99% under suitable conditions of catalyst, solvent, reactant, and basic concentration.
Journal of Molecular Catalysis A-chemical, 2008
The kinetics of dichlorocarbene addition to allyl phenyl ether have been studied under phase-transfer catalytic conditions using aqueous sodium hydroxide as the base and benzyltriethylammonium bromide as a phase-transfer catalyst. The reaction was carried out at 35 • C under pseudo-first-order conditions by keeping aqueous sodium hydroxide and chloroform in excess and was monitored by GC. The effect of various experimental parameters on the rate of the reaction has been studied and based on the results obtained, a suitable mechanism is proposed.
Applied Catalysis A: General, 2005
Alkylation of phenol with cyclohexene with acid catalysts leads to the formation of both O-and C-alkylated products, which are all useful in a variety of industries. The O-alkylated product cyclohexyl phenyl ether is a valuable perfume and can also serve as a precursor to diphenyl ether, a very important bulk chemical. The efficacy of various acid catalysts such as sulphated zirconia, sulphonic acid treated hexagonal mesoporous silica (SO 3 -HMS), 20% (w/w) dodecatungstophospheric acid (DTP) supported on K-10 clay, 20% (w/w) cesium salt of DTP (Cs 2.5 H 0.5 PW 12 O 40 ) supported on K-10 clay (Cs-DTP/K-10) and 20% (w/w) DTP/HMS was studied to improve the selectivity to cyclohexyl phenyl ether. A mixture of 2-cyclohexylphenol, 4-cyclohexylphenol and cyclohexyl phenyl ether was obtained with different selectivities. However, 20% (w/w) DTP/K-10 clay was the most active and selective catalyst for O-alkylation in the range of 45-70 8C at atmospheric pressure. The selectivity to O-versus C-alkylation is strongly dependent on temperature, and at lower temperatures, the selectivity to cyclohexyl phenyl ether increases. The best operating temperature is 60 8C. A mathematical model is built to interpret the kinetic data and develop a mechanism. #
Efficient palladium-catalyzed coupling reactions of aryl bromides and chlorides with phenols
Chemical Communications, 2009
Experimental section: Reagents: All reagents were purchased from Aladdin Reagent Company and Alfa-Aesar Company without further purification. Toluene and dioxane were purified by distillation though a standard procedure. Analytical Methods: 1 H-NMR and 13 C-NMR spectra were recorded on a Bruker 300 MHz instrument with chemical shifts reported in ppm relative to the residual deuterated solvent or the internal standard tetramethylsilane. Gas chromatography analyses were performed on a Hewlett Packard 5890 instrument with a FID detector and Hewlett Packard 24 m x 0.2 mm i.d. HP-5 capillary column. Yield refers to isolated yields of compounds greater than 95% purity as determined by capillary gas chromatography (GC) and proton Nuclear Magnetic Resonance spectroscopy (1 H-NMR) analysis. General procedure for the Pd-catalyzed synthesis of diaryl ethers from aryl halides with phenols. All reactions were carried out under an argon atmosphere in schlenk tubes (containing a stir bar). A schlenk tube containing a stir bar was charged with Pd(OAc) 2 (2.0 mol% Pd), ligand (6.0 mol%) and K 3 PO 4 (2.0 equiv.). If the aryl halides (1.0 mmol) and/or phenols (1.2 equiv.) were solids, they were also added at this time. The tube was evacuated and backfilled with argon (this sequence was repeated three times). If the aryl halides and/or phenols were liquids, they were added to the tube at this time along with the solvents. The mixture was stirred in a preheated oil bath (80-100°C) until the aryl halide was consumed as judged by TLC or GC analysis (10-24 h, reaction times were not optimized). The crude material was purified by column chromatography on silica gel (eluting with ethyl acetate/hexane or diethyl ether/hexane mixtures). 1-(4-Phenoxyphenyl)ethanone 1 : The crude material was purified by flash chromatography column on silica gel (10:1 ethyl acetate/hexane) to afford the title compound as a white solid. M.p.
Applied Catalysis A: General, 2005
Reactions in three immiscible liquid phases (L-L-L) are attractive, and one of the phases can be the locale of reaction which will have a dramatic effect on product distribution in complex reactions. Thus, converting a bi-liquid system into a tri-liquid phase is of considerable scientific and commercial interest. 4-Benzyloxy-vanillin is used as a perfume and also as a starting material for synthesis of thalifoline and ephedradine as alkaloids and in synthesis of flavonoid compounds. Etherification of vanillin with benzyl chloride under biphasic phase transfer catalysis leads to formation of 4-benzyloxy-vanillin, but selectively suffers due to side reactions. Waste minimization is a major theme of green chemistry. In the traditional liquid-liquid phase transfer catalysis, the catalyst is not recovered but disposed off causing load on environment. However, the transformation of two-liquid phases into three-liquid phases L-L-L, PTC leads to 100% conversion of the limiting reactant benzyl chloride with 100% selectivity to 4-benzyloxy-vanillin, using TBAB as a catalyst. The rates of reactions are very high under L-L-L PTC, and reaction can be completed within 1 h as against 8 h required in L-L PTC. The catalyst-rich middle phase is recycled many times, thereby leading to profitability. The current work deals with effect of different kinetics and process parameters leading to enhancement in rates and selectivities and greener aspects of phase transfer catalysis. #
Selectivity engineering in multiphase transfer catalysis in the preparation of aromatic ethers
Journal of Molecular Catalysis A: Chemical, 2004
Phase transfer catalysis (PTC) is a very mature discipline now. However, there are hardly any studies on the theoretical and experimental analysis of the effect of nature and number of phases on enhancement of rates and selectivities. PTC reactions can be carried out in liquid-liquid (L-L), solid-liquid (S-L), and liquid-liquid-liquid (L-L-L) and solid-liquid-liquid (S-L-L) conditions bringing into picture dominance of mass transfer effects. The current work addresses these issues in the alkylation reaction of -naphthol with benzyl chloride with the C-and O-alkylation being the competing reactions. The role of various phases in enhancing the selectivity towards benzyl-2-naphthyl ether has been extensively investigated. The L-L-L PTC process has been found to be the most effective and economical route giving only the desired ether within a short reaction times and high catalyst reusability, unlike the L-L PTC process. A mathematical model is developed to establish the rate constant.