An Electrochemical Design for Catalytic Dehydration: Direct, Room-Temperature Esterification without Acid or Base Additives

An electrochemical approach has been leveraged to underpin a new conceptual platform for dehydration reactions, which has been demonstrated in the context of esterification. Esters were prepared from the corresponding acid and alcohol partners at room temperature without acid or base additives and without consuming stoichiometric reagents. This methodology therefore addresses key complications that plague esterification and dehydration reactions more broadly and that represent a leading challenge in synthetic chemistry.

4-(Nitro)-diphenylammonium triflate(NDPAT) catalysed esterification of carboxylic acids with alcohols

4-(Nitro)-diphenylammonium triflate(NDPAT) efficiently catalysed the esterification between equimolar amounts of carboxylic acids and alcohols in good yield under mild reaction conditions.

In-Water and Neat Batch and Continuous-Flow Direct Esterification and Transesterification by a Porous Polymeric Acid Catalyst

A porous phenolsulphonic acid—formaldehyde resin (PAFR) was developed. The heterogeneous catalyst PAFR was applied to the esterification of carboxylic acids and alcohols, affording the carboxylic acid esters in a yield of up to 95% where water was not removed from the reaction mixture. Surprisingly, the esterification in water as a solvent proceeded to afford the desired esters in high yield. PAFR provided the corresponding esters in higher yield than other homogeneous and heterogeneous catalysts. The transesterification of alcohols and esters was also investigated by using PAFR, giving the corresponding esters. PAFR was applied to the batch-wise and continuous-flow production of biodiesel fuel FAME. The PAFR-packed flow reactor that was developed for the synthesis of carboxylic acids and FAME worked for four days without loss of its catalytic activity.

Dehydration reactions in water. Brønsted Acid-surfactant-combined catalyst for ester, ether, thioether, and dithioacetal formation in water

Dehydration reactions in water have been realized by a surfactant-type catalyst, dodecylbenzenesulfonic acid (DBSA). These reactions include dehydrative esterification, etherification, thioetherification, and dithioacetalization. In these reactions, DBSA and substrates form emulsion droplets whose interior is hydrophobic enough to exclude water molecules generated during the reactions. Detailed studies on the esterification revealed that the yields of esters were affected by temperature, amounts of DBSA used, and the substrates. Esters were obtained in high yields for highly hydrophobic substrates. On the basis of the difference in hydrophobicity of the substrates, unique selective esterification and etherification in water were attained. Furthermore, chemospecific, three-component reactions under DBSA-catalyzed conditions were also found to proceed smoothly. This work not only may lead to environmentally benign systems but also will provide a new aspect of organic chemistry in water.

Asymmetric intramolecular [2 + 2] photocycloadditions: alpha- and beta-hydroxy acids as chiral tether groups

Chiral alpha- and beta-hydroxy acids such as (S)-lactic acid, (S)-phenyllactic acid, (S)-mandelic acid, or (3R)-3-hydroxybutyric acid have been used as tether groups for intramolecular and diastereoselective [2 + 2] photocycloaddition of 3-oxocyclohexene carboxylic acid derivatives. Total regiocontrol toward the straight adduct and high diastereoselectivities (up to 94%) were observed in the case of butenyl lactate 11. After separation of the two diastereoisomers, cleavage of the chiral tether under basic conditions afforded cyclobutane lactones in good yield and enantiomeric pure form. An X-ray structure has been recorded that confirmed the relative and absolute configuration of the three contiguous stereogenic centers assigned according to CD spectra.

Direct condensation of carboxylic acids with alcohols catalyzed by Hafnium(IV) salts

In order to promote atom efficiency in synthesis and to avoid the generation of environmental waste, the use of stoichiometric amounts of condensing reagents or excess substrates should be avoided. In esterification, excesses of either carboxylic acids or alcohols are normally needed. We show that the direct condensation of equimolar amounts of carboxylic acids and alcohols can be achieved with the use of hafnium(IV) salts, such as commercially available hafnium(IV) chloride and hafnium(IV) tert-butoxide. The present method can be applied to direct polyesterification and may be suitable for large-scale operations.