Palladium/NHC-catalyzed tandem benzylic oxidation/oxidative esterification of benzylic alcohols with phenols

Relay Catalysis for Selective Aerobic Oxidative Esterification of Primary Alcohols with Methanol

Esters are bulk and fine chemicals and ubiquitous in polymers, bioactive compounds, and natural products. Their traditional synthetic approach is the esterification of carboxylic acids or their activated derivatives with alcohols. Herein, a bimetallic relay catalytic protocol was developed for the aerobic esterification of one alcohol in the presence of a slowly oxidizing alcohol, which has been identified as methanol. A concise synthesis of phlomic acid was executed to demonstrate the practicality and potential of this reaction.

Synthesis of phenol esters by direct C–H activation of aldehydes using a highly efficient and reusable copper-immobilized polyimide covalent organic framework (Cu@PI-COF)

In the present study, we report the design and fabrication of a thermally and chemically stable copper-based polyimide covalent organic framework (Cu@PI-COF) via a facile and straightforward synthetic approach for the oxidative esterification of aldehydes.

Palladium-Catalyzed Suzuki-Miyaura Coupling of Aryl Esters

The Suzuki-Miyaura coupling is among the most important C-C bond-forming reactions available due to its reliability, chemoselectivity, and diversity. Aryl halides and pseudohalides such as iodides, bromides, and triflates are traditionally used as the electrophilic coupling partner. The expansion of the reaction scope to nontraditional electrophiles is an ongoing challenge to enable an even greater number of useful products to be made from simple starting materials. Herein, we present how an NHC-based Pd catalyst can enable Suzuki-Miyaura coupling where the C(acyl)-O bond of aryl esters takes on the role of electrophile, allowing the synthesis of various ketone-containing products. This contrasts known reactions of similar esters that provide biaryls via nickel catalysis. The underlying cause of this mechanistic divergence is investigated by DFT calculations, and the robustness of esters compared to more electrophilic acylative coupling partners is analyzed.

Cross-dehydrogenative coupling for the intermolecular C-O bond formation

The present review summarizes primary publications on the cross-dehydrogenative C-O coupling, with special emphasis on the studies published after 2000. The starting compound, which donates a carbon atom for the formation of a new C-O bond, is called the CH-reagent or the C-reagent, and the compound, an oxygen atom of which is involved in the new bond, is called the OH-reagent or the O-reagent. Alcohols and carboxylic acids are most commonly used as O-reagents; hydroxylamine derivatives, hydroperoxides, and sulfonic acids are employed less often. The cross-dehydrogenative C-O coupling reactions are carried out using different C-reagents, such as compounds containing directing functional groups (amide, heteroaromatic, oxime, and so on) and compounds with activated C-H bonds (aldehydes, alcohols, ketones, ethers, amines, amides, compounds containing the benzyl, allyl, or propargyl moiety). An analysis of the published data showed that the principles at the basis of a particular cross-dehydrogenative C-O coupling reaction are dictated mainly by the nature of the C-reagent. Hence, in the present review the data are classified according to the structures of C-reagents, and, in the second place, according to the type of oxidative systems. Besides the typical cross-dehydrogenative coupling reactions of CH- and OH-reagents, closely related C-H activation processes involving intermolecular C-O bond formation are discussed: acyloxylation reactions with ArI(O2CR)2 reagents and generation of O-reagents in situ from C-reagents (methylarenes, aldehydes, etc.).

Palladium-catalysed oxidative cross-esterification between two alcohols

Oxidative cross-esterification between two alcohols was achieved by using PdCl₂(PPh₃)₂ as the sole catalyst and benzyl chloride as the oxidant.

Directing-group-assisted copper-catalyzed oxidative esterification of phenols with aldehydes

A directing-group-assisted copper-catalyzed oxidative esterification of phenols with aldehydes using TBHP as an oxidant was described.

Pd/C-catalyzed aerobic oxidative esterification of alcohols and aldehydes: a highly efficient microwave-assisted green protocol

We herein describe an environmentally friendly microwave-assisted oxidative esterification of alcohols and aldehydes in the presence of molecular oxygen and a heterogeneous catalysis (Pd/C, 5 mol %). This efficient and ligandless conversion procedure does not require the addition of an organic hydrogen acceptor. The reaction rate is strongly enhanced by mild dielectric heating. Furthermore, it is a versatile green procedure which generally enables the isolation of esters to be carried out by simple filtration in almost quantitative yields.

Metal-free arylation of oxygen nucleophiles with diaryliodonium salts

Phenols and carboxylic acids are efficiently arylated with diaryliodonium salts. The reaction conditions are mild, metal free, and avoid the use of halogenated solvents, additives, and excess reagents. The products are obtained in good-to-excellent yields after short reaction times. Steric hindrance is very well tolerated, both in the nucleophile and diaryliodonium salt. The scope includes ortho- and halo-substituted products, which are difficult to obtain by metal-catalyzed protocols. Many functional groups are tolerated, including carbonyl groups, heteroatoms, and alkenes. Unsymmetric salts can be chemoselectively utilized to obtain products with hitherto unreported levels of steric congestion. The arylation has been extended to sulfonic acids, which can be converted to sulfonate esters by two different approaches. With recent advances in efficient synthetic procedures for diaryliodonium salts the reagents are now inexpensive and readily available. The iodoarene byproduct formed from the iodonium reagent can be recovered quantitatively and used to regenerate the diaryliodonium salt, which improves the atom economy.