Phase-transfer catalyzed asymmetric arylacetate alkylation

Phenethyl arylacetates are alkylated under phase-transfer conditions with cinchona catalysts with alkyl halides in high yield with excellent enantioselectivity (84-99% ee) following recrystallization. Cinchonidine (CD) derived catalyst gave the (R)-product and cinchonine (CN) catalyst produced the (S)-product. The phenethyl (PE) ester group is removed, using ammonium formate and catalytic Pd/C, to give alkylated carboxylic acid products in high selectivity. The utility of the approach is demonstrated by a direct synthesis of (S)-naproxen^™.     

Phase-transfer catalysis in analytical chemistry

Phase-transfer catalysis (PTC) has been a well-established technique on the synthesis of organic chemicals for more than three decades. Its scope and underlying mechanistic features have been the subject of numerous studies and appear to be recognized and understood.

This review is intended to approach the subject by focusing on the extraction–preconcentration–derivatization/reaction prior to analysis and to chronicle recent progress made. We present the salient aspects of PTC modes followed by a brief review of mechanistic considerations including reaction mechanisms, selectivity, rates and kinetics pointing out to the potency of PTC in analytical chemistry. Specific guidelines are given on how to optimize a PTC-based analysis with respect to catalyst, solvent, reaction conditions and more, based on reaction characteristics.

Finally, using the PTC principles as a framework, selected real-life applications are provided, the capabilities and limitations of PTC are addressed for the purpose of direct analysis of organic analytes and certain advantages are highlighted.     

Fe(III)-catalyzed oxidative coupling of alkylnitriles with aromatic carboxylic acids: Facile access to cyanomethyl esters

The first oxidative coupling of alkylnitriles with aromatic carboxylic acids using di-tert-butyl peroxide (DTBP) as oxidant was achieved under the catalysis of ionic Fe(III) complexes bearing an imidazolinium cation. This protocol features nontoxic iron catalysis, direct α-C(sp³)–H bond oxidative esterification of alkylnitriles, non-prefunctionalized starting materials, and a broad substrate scope with outstanding steric hindrance tolerance, providing a novel, straightforward, and green approach toward cyanomethyl ester synthesis.     

Gas chromatographic analysis of aromatic carboxylic acids in the presence of C1−C22 fatty acids and C2−C16 dicarboxylic acids esterified in aqueous solutions as the n-propyl esters

Summary The direct esterification and gas chromatographic analysis of aromatic carboxylic acids as n-propyl esters is described.Derivatization is performed in aqueous solution with n-propanol in the presence of sulfuric acid. The n-propyl esters of benzoic, phthalic, trimellitic and pyromellitic acids permit their gas chromatographic separation from each other and from fatty acids and aliphatic dicarboxylic acids. At mode ratios of [H₂O]/[n-PrOH]0.03 the water present does not interfere with the esterification reaction. At mole ratios above 0.03 anhydrous sodium sulfate is used for binding the water, at mole ratios of [Na₂SO₄anh.]/[H₂O]=0.51–1.52 and of [H₂SO₄]/[Na₂SO₄anh.]=0.43–1.28, respectively.     

Gas chromatographic analysis of aromatic,carboxylic acids in the presence of C1−C5 fatty acids and C2−C7 dicarboxylic acids esterified in aqueous solutions as the n-butyl esters

Summary The direct esterification and gas chromatographic analysis of aromatic carboxylic acids as n-butyl esters is described.Derivatization is performed in aqueous solution with n-butanol in the presence of sulfuric acid. The butyl esters of benzoic, phthalic, hemimellitic, trimellitic, trimesic and pyromellitic acids permit their gas chromatographic separation from each other and from fatty acids and alipatic dicarboxylic acids. At mole ratios of [H₂O]/ [n-BuOH]0.04 the water present does not interfere with the esterification reaction. At mole ratios above 0.04 anhydrous sodium sulfate is used for binding the water, at mole ratios of [Na₂SO₄ anh.]/[H₂O]=0.25–0.75.     

Sodium borohydride reduction of aromatic carboxylic acids via methyl esters

A number of important aromatic carboxylic acids precursors, or intermediates in the syntheses of natural products, are converted into methyl esters and reduced to the corresponding primary alcohols using a sodium borohydride-THF-methanol system. The alcohols are obtained in 70–92% yields in 2–5 hours, in a pure state. This two-step procedure not only provides a better alternative to aluminum hydride reduction of acids but also allows the selective reduction of esters in presence of acids, amides, nitriles or nitro functions which are not affected under these conditions.     

Esterification of C1−C8 carboxylic acids to n-butyl esters in aqueous solutions for their gas chromatographic analysis

Summary The esterification of C₁–C₈ fatty acid in aqueous solutions and in the presence of sulfuric or hydrochloric acid and with n-butanol is described. It has been established that the esterification can be used for the quantitative determination of these fatty acids in the concentration ratio range of [H₂O]/[n-BuOH]=0.01–5.3. In the concentration ratio range of 0.01–0.8 the water present does not interfere. In the concentration range of above 0.8 anhydrous sodium sulfate is used for binding the water, in the amount of [Na₂SO₄.anh.]/[H₂O]0.2.     

Efficient condensation of carboxylic acids with alcohols catalyzed by fluorous ammonium triflates

The use of fluorous ammonium salts as metal-free catalysts for the direct condensation of equimolar amounts of carboxylic acids and aliphatic alcohols has been investigated. Esterification reactions were thus conveniently carried out under mild fluorous biphasic conditions, in the presence of 1 mol % of fluorous ammonium triflate and without recourse to any additional water removal technique. Good to excellent ester yields were obtained in the case of primary and secondary aliphatic alcohols. The fluorous salt was easily recovered by simple phase separation and reused at least three times without considerable loss of activity.     

A simple and convenient method for preparation of carboxylic acid alkyl esters, phenolic and thioesters using chlorodiphenylphosphine/I2 and imidazole reagent system

Condensation of carboxylic acids with alcohols, phenols and thiols proceeded smoothly to afford carboxylic acid alkyl esters, phenolic esters and thioesters by using the combination of chlorodiphenylphosphine, imidazole and molecular iodine in refluxing acetonitrile. Esterification with this mixed reagent system gave the corresponding products in excellent yields. The phosphorus-containing byproduct was simply removed from the organic phase by basic aqueous workup without the need for chromatography purification.     

Selective esterification of non-conjugated carboxylic acids in the presence of conjugated or aromatic carboxylic acids over active carbon supported methanesulfonic acid

Non-conjugated carboxylic acids are selectively esterified in good yields in the presence of conjugated or aromatic carboxylic acids by stirring over active carbon supported methanesulfonic acid in di-chloromethane at room temperature.     

Facile synthesis of methylthiomethyl esters through Pummerer-type rearrangement of carboxylic acids and DMSO under metal-free conditions

A green and cost-effective Pummerer-type rearrangement between readily accessible carboxylic acids and DMSO has been achieved under metal-free conditions in satisfactory to excellent yields. The transformation for the synthesis of valuable methylthiomethyl esters is shown to be a convenient strategy for various (hetero)aromatic acids, α,β-unsaturated carboxylic acids and saturated alkyl carboxylic acids with good functional group tolerance.     

The effect of cyclodextrins on the hydrolysis of carboxylic acid esters

Cyclodextrins and their derivatives act as inverse phase-transfer catalysts in the hydrolysis of carboxylic acid esters. The reaction is affected by temperature, choice of cyclodextrin and also by the stirring rate. Lipophilic esters are better hydrolyzed under inverse phase-transfer catalysis conditions than under classic phase-transfer catalysis (PTC) conditions. Some typical features of the reaction are emphasized and compared with classical PTC reactions.     

光学活性四面体过渡金属簇合物的诱导合成

报导了用手性季铵盐诱导合成SFeCoMo等手性四面体过渡金属簇合物的新方法。发现,苄基辛可宁对四面体簇合成有手性诱导作用;在不同溶剂中,手性簇产物的绝对构型不同,而且含钌簇的含光度比含铁簇的旋光度小。     

Palladium-catalysed arylation of acetoacetate esters to yield 2-arylacetic acid esters

The coupling reaction between ethyl acetoacetate and a number of aryl halides in the presence of palladium acetate, a bulky and electron rich phosphine and K₃PO₄ is described. The arylated acetoacetate ester is de-acylated under the reaction conditions resulting in the generation of 2-arylacetic acid esters, constituting a mild alternative to direct arylation of carboxylate esters.     

Stereoselective synthesis of esters with vinylsilicon functionality via ruthenium carbene catalyzed cross-metathesis

A new catalytic route for the synthesis of unsaturated organosilicon compounds with vinylic functionality based on cross-metathesis of vinyltrialkoxy- and vinyltris(trimethylsiloxy)-silanes with allyl esters of carboxylic acids catalyzed by ruthenium carbenes (Grubbs catalysts): [Cl₂(PCy₃)₂Ru(=CHPh)] (I), [Cl₂(PCy)₃(1,3-dimesityl-4,5-dihydroimidazol-2-ylidene)Ru(=CHPh)] (II) is reported. The efficient reaction can be extended to alkyl esters of unsaturated acids, e.g., of methyl 3-butenoate.     

Regio- and stereoselective addition of carboxylic acids to phenylacetylene catalyzed by cyclopentadienyl ruthenium complexes

The direct addition of carboxylic acids to terminal alkynes such as phenylacetylene in the presence of catalytic amount of [CpRu(CO)₂Cl] (1) or [{CpRu(CO)₂}₂] (2) affords the anti-Markovnikov adducts with high selectivity. In most instances, the E-enol esters are the major products.     

Copper-catalyzed annulation of 2-bromobenzoic esters with terminal alkynes towards 3-substituted isocoumarins

An efficient method for the synthesis of 3-substituted isocoumarins that are an important class of biologically active scaffolds via annulation of 2-bromobenzoic esters with terminal alkynes by copper catalyzed is described. The advantages of this method include mild reaction conditions, high yield and regioselectivity, and wide tolerance toward functional groups.     

Positive ion chemistry of esters of carboxylic acids in air plasma at atmospheric pressure

Ionization of esters of carboxylic acids RCOOR' (R = H, alkyl; R' = alkyl) within the air plasma of the Atmospheric Pressure Chemical Ionization (APCI) source occurs largely via H(+)-transfer and, to a minor extent, via NO(+) association. The protonated ester MH(+) is normally observed as M(2)H(+) and as higher aggregates (M(3)H(+), M(3)H(+)(H(2)O)) also at high source temperature. The behavior of M(2)H(+) upon collisional activation is consistent with the reported dissociation of proton-bound dimers to MH(+) species that, in turn, fragment according to the known paths of lowest energy. In addition, other important product ions form within the plasma, some in very high relative abundance, which are attributed to ion-molecule condensation reactions between neutral M and either MH(+) or M(2)H(+) resulting in the elimination of CO, R'OH, alkene from the alkoxy moiety of the ester and HCOOH. A general scheme is proposed to account for the experimental observations, which suggest that the encounter complex formed between MH(+) and M or between M(2)H(+) and M may either collisionally relax to the protonated dimer or trimer, respectively, or react via covalent bond forming and cleaving steps to eliminate stable neutral molecules. The proposed scheme is supported by both the observed concentration dependence and the temperature dependence of the products relative abundances within the plasma. Such reactions can be the dominant process, as in the case of formate esters. A second significant ionization route involves addition of NO(+) to form M(n)NO(+) (n = 1, 2, 3). An additional product corresponding to [M(2)NO(+) - CO(2)] is also observed with iso- and n-butyl formate esters.     

Ruthenium complex catalyzed selective addition of carboxylic acids to acetylenes giving enol esters

Carboxylic acids react with acetylenes in the presence of a catalytic amount of bis(η⁵-cyclooctadienyl)ruthenium-PR₃-maleic anhydride in toluene to give enol esters in good to excellent yields with high regioselectivity.     

The efficient one-pot synthesis of tetraalkyl substituted furans from symmetrical acetylenes,EtAlCl2, and carboxylic esters catalyzed by Cp2TiCl2

A new one-pot method for the synthesis of tetraalkyl substituted furans in 74–80% yields has been implemented via the reaction between symmetrical acetylenes, ethylaluminum dichloride, and carboxylic esters in the presence of metallic Mg as an acceptor of halide ions, catalyzed by bis(cyclopentadienyl)titanium dichloride. A mechanism for the reaction is proposed and discussed.