Highly Efficient Solvent-Free Condensation of Carboxylic Acids with Alcohols Catalysed by Zinc Perchlorate Hexahydrate, Zn(ClO4)2?6?H2O

Titanium Tetrachloride-Assisted Direct Esterification of Carboxylic Acids

Ester compounds, widely found in pharmaceutical and natural products, play a crucial role in organic synthesis, prompting the development of numerous methods for their synthesis. An important chemical approach in synthesizing esters from carboxylic acids involves the activation of the carboxyl function, requiring the conversion of the hydroxyl group into a suitable leaving group. This paper presents the findings of our investigations into an efficient method for producing esters from carboxylic acids and alcohols, using the Lewis acid titanium tetrachloride. Titanium tetrachloride has proven highly effective as a coupling reagent for the one-pot formation of esters from carboxylic acids and alcohols operating under mild and neutral conditions. Notably, the reaction eliminates the need for bases, yielding carboxylic esters in high purity and yields. The method is efficient, even with long-chain carboxylic acids, and operates well with primary alcohols in dichloromethane. Steric hindrance, potentially present in carboxylic acids, has a moderate effect on the reaction. Alcohol substrates that easily form stable carbocations require, instead, the use of non-polar solvents like hexane for the reaction.

Homogeneous Catalysis and Heterogeneous Recycling: A Simple Zn(II) Catalyst for Green Fatty Acid Esterification

This work describes the use of simple zinc(II) salts (ZnCl₂, ZnCO₃, Zn(OAc)₂, ZnO, Zn(ClO₄)₂, Zn(TfO)₂, and Zn(BF₄)₂) as effective catalysts for the esterification of fatty acids with long-chain alcohols and simple polyols through a homogeneous system that allows the gradual and selective removal of water. The results show that the catalytic activity depends on the nature of the counterion: the most effective are the salts with poorly coordinating anions (perchlorate and triflate) or containing basic Brønsted anions (oxide, acetate, and carbonate). However, only with the latter is it possible to fully recover the catalyst at the end of each run, which is easily filtered in the form of zinc carboxylate, given its insolubility in the ester produced. In this way, it is possible to recycle the catalyst numerous times, without any loss of activity. This beneficial prerogative couples the efficiency of the homogeneous catalysis with the advantage of the heterogeneous catalysis. The process is, therefore, truly sustainable, given its high efficiency, low energy consumption, ease of purification, and the absence of auxiliary substances and byproducts.

Rh(iii)-Catalyzed Csp2–Csp3 bond alkoxylation of α-indolyl alcohols via C–C σ bond cleavage

A Rh(iii)-catalyzed direct Csp²–Csp³ bond alkoxylation of α-(2-indolyl)alcohols with alcohols has been achieved via C–C sigma bond/C–O single bond switch.

An Efficient and Practical Procedure for Preparation of Gem-Diacetates from Aldehydes Catalysed by Magnesium Perchlorate

A mild and efficient method for the preparation of gem-diacetates from aldehydes with acetic anhydride is described using a catalytic amount of magnesium perchlorate giving excellent yields under solvent-free conditions.

Synthesis and characterization of phosphine-(thio)phenolate-based half-zirconocenes and their application in ethylene (co-)polymerization

A series of novel half-zirconocenes containing phosphine-(thio)phenolate chelating ligands of the type Cp'Zr(thf)Cl2[X-2-R(1)-4-R(2)-6-(PPh2)C6H2] (Cp' = C5Me5, 2a: X = O, R(1) = Ph, R(2) = H; 2b: X = O, R(1) = (t)Bu, R(2) = H; 2c: X = O, R(1) = R(2) = (t)Bu; 2d: X = O, R(1) = SiMe3, R(2) = H; 2e: X = S, R(1) = SiMe3, R(2) = H; Cp' = C5H5, 3b: X = O, R(1) = (t)Bu, R(2) = H; 3c: X = O, R(1) = R(2) = (t)Bu; 3d: X = O, R(1) = SiMe3, R(2) = H; 3e: X = S, R(1) = SiMe3, R(2) = H) have been synthesized in high yields (74-85%). These complexes were identified by (1)H, (13)C and (31)P NMR as well as elemental analyses. Structural analysis for 2a-b and 2d revealed that these complexes adopt six-coordinate, distorted octahedral geometry around the zirconium center. These novel half-zirconocenes possessed high catalytic performance for ethylene polymerization at high temperature in the presence of MMAO. The phosphine-phenolate-based Cp* complexes produced high molecular weight polymers (M(W) > 400,000), while the Cp analogues displayed much higher activities at high temperature. Complex 3c with MMAO showed a maximum ethylene polymerization activity of 17,580 kg mol(Zr)(-1) h(-1) at 75 °C. In addition, the Cp based complexes 3b-e could promote ethylene and 1-hexene copolymerization with high activities.

Direct dehydrative esterification of alcohols and carboxylic acids with a macroporous polymeric acid catalyst

A macroporous polymeric acid catalyst was prepared for the direct esterification of carboxylic acids and alcohols that proceeded at 50-80 °C without removal of water to give the corresponding esters with high yield. Flow esterification for the synthesis of biodiesel fuel was also achieved by using a column-packed macroporous acid catalyst under mild conditions without removal of water.

Insight into the isoelectronic character of azomethines and vinylenes using representative models: a spectroscopic and electrochemical study

A series of azomethine and vinylene dyad and triad analogues were prepared. Their absorbance, fluorescence, and redox properties were examined experimentally and theoretically using density functional theory (DFT) calculations. These measurements were done to determine the effect of the heteroatom of the azomethine relative to its all-carbon counterpart and to assess the isoelectronic character of the two bonds. The orientation of the azomethine was found to have little effect on the absorbance, fluorescence, and electrochemical properties. In contrast, the spectral and electrochemical properties were highly contingent on the electronic groups and degree of conjugation. The spectral properties could be tuned 200 nm across the visible region. More importantly, the heteroatom in the conjugated bond was found to give rise to only a 20 nm bathochromic shift in the absorbance and fluorescence spectra. The fluorescence quantum yield (ΦFl) of the vinylene derivatives varied between 5% and 20% with fluorescence quenching occurring by photoisomerization from the E to Z isomers. In contrast, the fluorescence of the analogous azomethine derivatives was completely quenched. The collective spectroscopic and electrochemical ab initio DFT data additionally confirmed that the azomethine and its analogous vinylene are isoelectronic. It was also found that a conjugated thiophene vinylene dyad with primary amines in the α,α'-positions could be prepared and isolated. The compound was stable under aerobic conditions providing electron withdrawing (either ester or nitrile) groups were located in the adjacent positions.

Gemini imidazolium surfactants: synthesis and their biophysiochemical study

New gemini imidazolium surfactants 9-13 have been synthesized by a regioselective epoxy ring-opening reaction under solvent-free conditions. The surface properties of these new gemini surfactants were evaluated by surface tension and conductivity measurements. These surfactants have been found to have low critical micelle concentration (cmc) values as compared to other categories of gemini cationic surfactants and also showed the tendency to form premicellar aggregates in solution at sufficiently low concentration below their cmc values. The thermal degradation of these surfactants was determined by thermograviometry analysis (TGA). These new cationic surfactants have a good DNA binding capability as determined by agarose gel electrophoresis and ethidium bromide exclusion experiments. They have also been found to have low cytotoxicity by MTT (3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) assay on the C6 glioma cell line.