Transition-Metal-Free Poly(thiazolium) Iodide/1,8-Diazabicyclo[5.4.0]undec-7-ene/Phenazine-Catalyzed Esterification of Aldehydes with Alcohols

Ni(II)-Mediated Photochemical Oxidative Esterification of Aldehydes with Phenols

The photopromoted, Ni-catalyzed acceptorless dehydrogenation esterification of phenols and aromatic aldehydes has been achieved in an oxidant- and external photosensitizer-free manner. This reliable and atom-economical transformation was tolerant to a wide range of functional groups and proceeded efficiently to give various aryl benzoates in moderate to high yields. Additionally, this photocatalytic system displayed high activity for the hydrogen-evolution cross coupling of aliphatic aldehydes and phenols employing dual nickel and aromatic aldehyde catalysis.

Synthesis of N-Heteroarenemethyl Esters via C–C Bond Cleavage of Acyl Cyanides Under Transition Metal-Free Conditions

A practical method to synthesize N-heteroaryl esters from N-heteroaryl methanols with acyl cyanides via C–C bond cleavage without using any transition metal is demonstrated here. The use of Na₂CO₃/15-crown-5 couple enables access to a series of N-heteroaryl esters in high efficiency. This protocol is operationally simple and highly environmentally benign producing only cyanides as byproducts.

Aromatization as the driving force for single electron transfer towards C–C cross-coupling reactions

There is a strong current interest in C–H functionalization reactions under metal-free conditions. We report herein that the deprotonated form of dihydrophenazine (DPh) as a potent initiator under photochemical conditions...

N-Heterocyclic Carbene/Carboxylic Acid Co-Catalysis Enables Oxidative Esterification of Demanding Aldehydes/Enals, at Low Catalyst Loading

We report the discovery that simple carboxylic acids, such as benzoic acid, boost the activity of N-heterocyclic carbene (NHC) catalysts in the oxidative esterification of aldehydes. A simple and efficient protocol for the transformation of a wide range of sterically hindered α- and β-substituted aliphatic aldehydes/enals, catalyzed by a novel and readily accessible N-Mes-/N-2,4,6-trichlorophenyl 1,2,4-triazolium salt, and benzoic acid as co-catalyst, was developed. A whole series of α/β-substituted aliphatic aldehydes/enals hitherto not amenable to NHC-catalyzed esterification could be reacted at typical catalyst loadings of 0.02-1.0 mol %. For benzaldehyde, even 0.005 mol % of NHC catalyst proved sufficient: the lowest value ever achieved in NHC catalysis. Preliminary studies point to carboxylic acid-induced acceleration of acyl transfer from azolium enolate intermediates as the mechanistic basis of the observed effect.

Highly Regioselective and Chemoselective [3 + 3] Annulation of Enaminones with ortho-Fluoronitrobenzenenes: Divergent Synthesis of Aposafranones and Their N-Oxides

A base-promoted unprecedented strategy for the regioselective and chemoselective divergent synthesis of highly functionalized aposafranones and their N-oxides has been developed from the [3 + 3] annulation of enaminones with o-fluoronitrobenzenenes. This novel synthetic strategy offers an alternative method for the construction of aposafranones and their N-oxides are meaningful in the fields of both biology and organic synthesis. The established protocol explores the annulation scope of enaminones, and it expands the application of nitro-based cyclization.

Photoinduced Specific Acylation of Phenolic Hydroxy Groups with Aldehydes

A convenient method is reported to specifically acylate phenolic hydroxyl groups through a radical pathway. When a mixture of an aldehyde and a phenol in ethyl acetate is irradiated with blue light in the presence of iridium and nickel bromide catalysts at ambient temperature, phenoxyl and acyl radicals are transiently generated in situ and cross-couple to furnish an ester. Aliphatic hydroxy groups remain untouched under the reaction conditions.

Acyl Cyanides as Bifunctional Reagent: Application in Copper-Catalyzed Cyanoamidation and Cyanoesterification Reaction

Cu-catalyzed domino decyanation and cyanation reaction of acyl cyanides with amines or alcohols have been developed. The cyano sources were generated in situ via C-CN cleavage yielding the corresponding cyano substituted amides or esters in moderate to excellent yields. This approach features a cheap copper catalyst, domino decyanation and cyanation reaction, readily available starting materials, broad substrate scope, operational simplicity, and the potential for further transformation of the cyano group.

Pd-Colloids-Catalyzed/Ag2 O-Oxidized General and Selective Esterification of Benzylic Alcohols

Palladium colloids obtained from the degradation of Hermann-Beller palladacycle proved to be an efficient catalytic system in combination with silver oxide as a selective oxidant for the oxidative esterification of differently substituted benzyl alcohols in MeOH as solvent. Excellent reactivity exhibited by the catalytic system also allowed the alcoholic coupling partner to be changed from MeOH to a wide range of alcohols having diverse functionalities. The mildness of the developed protocol also made it possible to employ propargyl alcohol as the coupling partner without any observation of any interference of the terminal alkyne. Selective oxidative coupling of a primary alcoholic functional group over secondary in the case of glycols and glycerols was also made possible using the developed catalyst system. To test the relevancy of Pd/Ag combined catalysis mixed Pd/Ag colloids were synthesized, characterized by TEM, XRD and XPS and applied to oxidative-esterification successfully.

Efficient synthesis of esters through oxone-catalyzed dehydrogenation of carboxylic acids and alcohols

An environmentally friendly oxone (20 mol%) catalyzed esterification of carboxylic acids with alcohols has been developed, providing an attractive alternative to the construction of valuable carbonyl esters.