Application of high-melting pyridinium salts as ionic liquid catalysts and media for fischer esterification

Pyridinium methanesulfonate (m.p. 185°C) and pyridinium p-toluenesulfonate (m.p. 121°C) were used as catalysts and media for the esterification of carboxylic acids with primary alcohols to give the corresponding esters selectively in high yields. The high melting points of these pyridinium salts did not prevent their application as ionic liquid medium for these reactions which were performed at 90°C.     

AlCl3 · 6H2O/KI/CH3CN/H2O: An Efficient and Versatile System for Chemoselective C–O Bond Cleavage and Formation of Halides and Carbonyl Compounds from Alcohols in Hydrated Media

AlCl₃ · 6H₂O/KI/CH₃CN/H₂O, an efficient and versatile system, cleaves the C–O bonds of esters, acetals, ethers, and oxathiolanes to the corresponding acids, alcohols, and carbonyl compounds chemoselectively at 80 °C in hydrated media with good yields. This system also converts the alcohols (primary/secondary) to halides and oxidizes the alcohols (primary/secondary) to the corresponding carbonyl compounds in the presence of DMSO.     

Highly Efficient Direct Aerobic Oxidative Esterification of Cinnamyl Alcohol with Alkyl Alcohols Catalysed by Gold Nanoparticles Incarcerated in a Nanoporous Polymer Matrix: A Tool for Investigating the Role of the Polymer Host

The selective aerobic oxidation of cinnamyl alcohol to cinnamaldehyde, as well as direct oxidative esterification of this alcohol with primary and secondary aliphatic alcohols, were achieved with high chemoselectivity by using gold nanoparticles supported in a nanoporous semicrystalline multi‐block copolymer matrix, which consisted of syndiotactic polystyrene‐co‐cis‐1,4‐polybutadiene. The cascade reaction that leads to the alkyl cinnamates occurs through two oxidation steps: the selective oxidation of cinnamyl alcohol to cinnamaldehyde, followed by oxidation of the hemiacetal that results from the base‐catalysed reaction of cinnamaldehyde with an aliphatic alcohol. The rate constants for the two steps were evaluated in the temperature range 10–45 °C. The cinnamyl alcohol oxidation is faster than the oxidative esterification of cinnamaldehyde with methanol, ethanol, 2‐propanol, 1‐butanol, 1‐hexanol or 1‐octanol. The rate constants of the latter reaction are pseudo‐zero order with respect to the aliphatic alcohol and decrease as the bulkiness of the alcohol is increased. The activation energy (E_a) for the two oxidation steps was calculated for esterification of cinnamyl alcohol with 1‐butanol (E_a=57.8±11.5 and 62.7±16.7 kJ mol^?1 for the first and second step, respectively). The oxidative esterification of cinnamyl alcohol with 2‐phenylethanol follows pseudo‐first‐order kinetics with respect to 2‐phenylethanol and is faster than observed for other alcohols because of fast diffusion of the aromatic alcohol in the crystalline phase of the support. The kinetic investigation allowed us to assess the role of the polymer support in the determination of both high activity and selectivity in the title reaction.     

Esterification activity and stability of Candida rugosa lipase immobilized into chitosan

Microbial lipase from Candida rugosa immobilized into porous chitosan beads was tested for esterification selectivity with butanol and different organic acids (C2–C12), and butyric acid and different aliphatic alcohols (C2–C10). After 24 h, the acids tested achieved conversions of about 40–45%. Acetic acid was the only exception, and in this case butanol was not consumed. Different alcohols led to butyric acid conversions >40%, except for ethanol, in which case butyric acid was converted only 26%. The system’s butanol and butyric acid were selected for a detailed study by employing an experimental design. The influence of temperature, initial catalyst concentration, and acid:alcohol molar ratio on the formation of butyl butyrate was simultaneously investigated, employing a 2³ full factorial design. The range studied was 37–50°C for temperature (X₁), 1.25–2.5% (w/v) for the catalyst concentration (X₂), and 1 and 2 for the acid:alcohol molar ratio (X₃). Catalyst concentration (X₂) was found to be the most significant factor and its influence was positive. Maximum ester yield (83%) could be obtained when working at the lowest level for temperature (37°C), highest level for lipase concentration (2.5% [w/v]), and center level of acid:alcohol molar ratio (1.5). The immobilized lipase was also used repeatedly in batch esterification reactions of butanol with butyric acid, revealing a half-life of 86 h.     

Facile esterification promoted by the triphenylstibine oxide-phosphorus sulfide (Ph3SbO/P4S10) system

Various esters are conveniently prepared by direct esterification of the corresponding carboxylic acids with alcohols catalyzed by a triphenylstibine oxide–phosphorus(V) sulfide combined system (Ph₃SbO/P₄S₁₀) under mild conditions (25–80 °C).     

Solvation of ethylmaltol and of its iron(III) complex

Summary Solubilities of tris(ethylmaltolato)iron(III) (ethylmaltol = 3-hydroxy-2-ethyl-4-pyrone) were measured in MeOH-H₂O, t-BuOH-H₂O and diol-H₂O mixtures, and in several primary alcohols. Solvation of the ethylmaltol ligand and of two 4-pyridinone analogues has been investigated through solubility measurements in MeOH- H₂O and in t-BuOH-H₂O mixtures, and in a series of primary alcohols. The solvation characteristics of these compounds are compared with those of the parent maltol, its iron(III) complex and a number of other nonelectrolytes.     

Transition metal free oxidative esterification of alcohols with toluene

Using Bu₄NI as the catalyst and tert-butyl hydroperoxide as the oxidant, direct esterification of alcohols with toluene derivatives was achieved. Mechanistic investigations indicate that the alcohols are sequentially oxidized to aldehydes, carboxylic acids, and then to benzyl esters. Bu₄N⁺ functions as a phasetransfer reagent and iodide catalyzes the reaction.     

A facile and efficient [bmim]N3 catalyzed direct oxidative esterification of arylaldehydes with alcohols

Task-specific ionic liquid, [bmim]N₃ was used as an effective catalyst and reaction medium for the direct oxidative esterification of arylaldehydes with alcohols. The oxidative esterification reaction of a variety of arylaldehydes took place smoothly with some primary and secondary alcohols in [bmim]N₃. Satisfactory results were obtained with arylaldehydes containing electron withdrawing groups. Tertiary alcohols did not react under these conditions.     

Synthesis of bicyclic diesters of malonic and succinic acids

Monoesters of malonic and succinic acids have been prepared in high yields by the BF₃·OEt₂-catalyzed addition of the acids to bicyclo[2.2.1]hept-2-ene. The synthesized monoesters were introduced in the reaction of esterification with saturated monoatomic alcohols С₂–С₇ in the presence of the KU-2-8 catalyst in the H-form, and the corresponding mixed diesters of dicarboxylic acids were obtained in high yields.     

Iron Perchlorate on Silica Gel as Multi-purpose Reagent for Catalysis of Closure and Rupture of Carbon-Oxygen Bond in Epoxides,Alcohols, and Esters

Aliphatic alcohols and water in the presence of catalytic amounts of Fe³⁺ ion introduced as acids esterification with various type alcohols was carried out using the system Fe(ClO₄)₃-silica gel in dichloromethane under conditions excluding solvolysis. Acetylation and formylation of alcohols was performed by efficient transesterification with ethyl acetate and ethyl formate.     

Towards Carotenoid Dendrimers: Carotenoid Diesters and Triesters with Aromatic Cores

For the evaluation of the synthesis of dendritic esters from carotenoids, various carotenols or their succinates were reacted with bi‐ and trifunctional aromatic acids and alcohols. Several methods were tested, from which the Steglich esterification was found to give the best yields. Apocarotenoids showed higher reactivity in most of the reactions and served as model compounds for C₄₀ carotenoids and their derivatives. The synthesized triesters can be regarded as first generation dendrimers.     

Efficient and Simple Approaches Towards Direct Oxidative Esterification of Alcohols

The present article describes novel oxidative protocols for direct esterification of alcohols. The protocols involve successful demonstrations of both “cross” and “self” esterification of a wide variety of alcohols. The cross‐esterification proceeds under a simple transition‐metal‐free condition, containing catalytic amounts of TEMPO (2,2,6,6‐tetramethyl‐1‐piperidinyloxy)/TBAB (tetra‐n‐butylammonium bromide) in combination with oxone (potassium peroxo monosulfate) as the oxidant, whereas the self‐esterification is achieved through simple induction of Fe(OAc)₂/dipic (dipic=2,6‐pyridinedicarboxylic acid) as the active catalyst under an identical oxidizing environment.     

Esterifications of carboxylic acids and alcohols catalyzed by Al<Subscript>2</Subscript>(SO<Subscript>4</Subscript>)<Subscript>3</Subscript> · 18H<Subscript>2</Subscript>O under solvent-free condition

Esterifications of equimolar mixture of carboxylic acids and alcohols can be effectively catalyzed by Al₂(SO₄)₃ · 18H₂O under solvent-free condition. The esterification catalyzed by Al₂(SO₄)₃ · 18H₂O is a promising green method thanks to no need of organic solvent, no pollution, no causticity and ease to handle after reaction. This catalyst is of a low toxicity (usually it is used as purifying agent for drinking water), low-cost compound and is easily separated from the reaction mixture by simple filtration. Moreover, the catalyst can be recycled for the further esterification and the conversion does not evidently decrease.     

A model study for coatings containing hexamethoxymethylmelamine

Using a model reaction we have studied the crosslinking chemistry of hydroxy-functional polymers and hexamethoxymethylmelamine. The transetherification of optically active monofunctional alcohols and hexamethoxymethylmelamine was monitored with polarimetry and ¹H-NMR. The reaction rate constants for both the forward (k₁) and the backward (k_?1) reaction of the sulphonic-acid-catalyzed alcoholysis were determined. Primary and secondary alcohols showed the same reaction rate and activation energy (E_a = 96 kJ/mol) for the forward reaction. However, the backward reaction in the equilibrium is considerably slower for primary alcohols than for secondary alcohols, with activation energies of E_a = 96 and 79 kJ/mol, respectively. When amine salts of sulphonic acids are used as catalysts, the E_a is increased from 97 to 116 kJ/mol in the case of primary alcohols. In concentrated aprotic solutions the reaction order in acid is 2.5. The same order in acid is found for the alcoholysis of acetaldehyde diethyl acetal. All the results strongly support the statement that the crosslinking reaction proceeds by an Sn-1 mechanism. The results of this model study are compared with results obtained in network-forming reactions. The important role of the evaporation of the condensation product methanol is discussed.     

CARBODIIMIDE-MEDIATED ESTERIFICATIONS AND CONDENSATIONS OF PHOSPHORIC ACIDS DISSOLVED IN VARIOUS ALCOHOLS

Abstract

Carbodiimide-mediated phosphorylations carried out with ortho-, pyro-, trimeta-, tetrameta-, and long-chain polyphosphoric acids dissolved in various alcohols, with and without an excess of tri-n-butylamine at 27° were found to consist of complicated reaction sequences involving various phosphoric acids, then esters and complexes formed between the carbodiimide and these acids or esters. In the case of the condensation of orthophosphoric acid, the process of stepwise esterification is seen to compete with the process of condensation to form condensed phosphoric acids and their esters. In general, esterification as opposed to condensation is promoted by (a) increased acidity, (b) smaller size, and (c) increased concentration of the alcohol, as well as by (d) increased concentration of dissociable protons in the solution. ³¹P chemical-shift data are given for ortho, chain, and ring phosphoric acids and their esters dissolved in the corresponding alcohol. Typical kinetic curves and the distribution of products obtained upon the cessation of condensation are also presented.     

Porphyrins. 39. Synthesis and Chemical Transformations of Porphyrins and Chlorins with 2-Acetyl-1-methyl-3-oxobutyl Substituents

Porphyrins and chlorins containing acetylacetone residues in the peripheral substituents [-CH(Me)CHAc₂] are converted under alkaline conditions into the corresponding deacetylated compounds with [-CH(Me)CH₂Ac] residues. Reduction of the latter as acids using sodium borohydride gives the corresponding alcohols and, after esterification, their acetates with branched peripheral [-CH(Me)CH₂CH(OAc)Me] substituents. Porphyrins and chlorins with such substituents in water-soluble form may hold interest as new photosensitizers in photodynamic cancer therapy.     

Selective Acetylation of Primary Alcohols: Acetyl and Formyl Transfer Reactions with Copper(II) Salts

The efficient esterification of primary and secondary alcohols in acetic acid was achieved in the presence of Cu(NO₃)₂.3H₂O in high yields. Selective acetylation of primary in the presence of secondary hydroxyl groups in excellent yields were performed in EtOAc. Formylation of primary and secondary alcohols was also achieved easily in ethyl formate. High retention of configuration was observed in the acetylation and formylation of (-) menthol in the presence of Cu(NO₃)₂. 3H₂O and Cu(OAc)₂.H₂O.     

Substituent effects in 13C NMR spectroscopy: Methyl,ethyl, 2-propyl and 2-methyl-2-propyl carboxylates

The ¹³C chemical shifts of the 28 carboxylic esters have been determined by high-resolution NMR spectroscopy with the aid of proton decoupling. A linear relationship is shown to exist between the ¹³C chemical shifts of the carbinyl carbon (C-1) of the esters and the pK_a values of the acids from which they are derived. This is a consequent of the polar character of the

bond. Similarly, if the carboxyl group is kept constant, but the alcoholic part of the ester is varied from primary to secondary and tertiary alcohols, the esterification effect on C-1 can be correlated with the increasing stability of the +δ charge on the carbinyl carbon. The smallest esterification effect at C-1 (1.3 ppm, relative to the parent alcohol) is observed for methyl pivalate (pK_a 5.03 for the parent acid), and the highest effect (17.7 ppm) for 2-methyl-2-propyl trichloroacetate (pK_a 0.70). In contrast, the C-2 esterification effect has been found to be essentially constant (?3.8±0.7 ppm), which is in agreement only with a conformation of the ester group in which the carbinyl carbon is cis with respect to the CO group.     

Selective Esterification of Aliphatic Carboxylic Acids in the Presence of Aromatic Carboxylic Acids Over Monoammonium Salt of 12‐Tungstophosphoric Acid

Monoammonium salt of 12‐tungstophosphoric acid [(NH₄)H₂PW₁₂O₄₀] was found to be a practical and useful heterogeneous catalyst for an efficient and selective esterification of aliphatic carboxylic acids with alcohols in the presence of aromatic carboxylic acids. The heteropoly acid–based heterogeneous catalyst has the advantages of a simple workup procedure, water insolubility, and good activity.     

One-Pot Esterification and Amide Formation via Acid-Catalyzed Dehydration and Ritter Reactions

Esterification of carboxylic acid is achieved using acetonitrile as a water trap. Water liberated during esterification is consumed in cyanide hydrolysis, thereby driving the esterification to completion. Substrates having carboxylic acid and nitrile groups undergo intramolecular dehydration and rehydration to amido esters in the absence of acetonitrile. Cyano acids also undergo esterification and Ritter reaction in one pot when excess alcohol is used. For the first time, we have observed an interesting Ritter reaction of primary alcohols, leading to ester amide product in one pot.

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