Synthesis of esters via sp(3) C-H functionalisation.

Oxidative C-O bond formation via sp(3) C-H functionalisation represents an attractive approach for the synthesis of esters. This review focuses on the recent advances of sp(3) C-H bond functionalisation strategies for ester synthesis from unconventional precursors using transition metal/metal free catalysts in combination with various oxidants. Various classes of esters and the mechanisms of their formation are discussed with numerous examples.

Bu4NI Catalyzed C-N Bond Formation via Cross-Dehydrogenative Coupling of Aryl Ethers (Csp3-H) and Tetrazoles (N-H).

Intermolecular C-N bond formations via cross-dehydrogenative coupling (CDC) of aryl ethers and tetrazoles have been developed under a metal-free condition. In the presence of catalytic amount of tetrabutylammonium iodide (TBAI) and aqueous TBHP, aryl ethers coupled efficiently with tetrazoles to afford hemiaminal ethers. This strategy showed high level of regioselectivity for substrates possessing multiple sp(3) C-H bonds adjacent to the ethereal oxygen.

Generation of bis-acyl ketals from esters and benzyl amines under oxidative conditions.

Treatment of benzylamines with esters at an elevated temperature are expected to give amides. However, in the presence of TBAI/TBHP, esters possessing a methylene carbon α-to oxygen with benzylamines provide bis-esters rather than the expected amides. Benzylamines under oxidative conditions generate less nucleophilic carboxylates, which couples at the sp(3) C-H bonds of esters and cyclic ethers to give bis-acyl ketals and α-acyloxy ethers, respectively.

Cyclic ethers to esters and monoesters to bis-esters with unconventional coupling partners under metal free conditions via sp(3) C-H functionalisation.

An efficient metal free oxidative esterification of sp(3) C-H bonds (adjacent to an oxygen atom) in simple solvents like 1,4-dioxane, tetrahydropyran, tetrahydrofuran and ethyl acetate has been achieved using terminal aryl alkenes and alkynes as the ArCOO(-) sources.

Terminal aryl alkenes and alkynes as arylcarboxy surrogates toward o-benzoxylation of 2-phenylpyridine catalyzed by copper.

A variety of styrenes and phenylacetylenes serve as excellent arylcarboxy sources in bringing about substrate directed o-benzoxylation of 2-phenylpyridine derivatives catalyzed by Cu(II) in the presence of TBHP. This reaction proceeds via formation of phenylglyoxal followed by decarbonylation to benzoyl radical/benzaldehyde which acts as the arylcarboxy source.

Easy access to benzylic esters directly from alkyl benzenes under metal-free conditions.

An efficient metal free protocol has been developed for the synthesis of benzylic esters via a cross dehydrogenative coupling (CDC) involving alkylbenzene(s) as a self- or as a cross-coupling partner(s) via the intermediacy of Ar-COOH and the benzylic carbocation obtained from the other half of the alkylbenzene; both symmetrical as well as unsymmetrical esters can be prepared using Bu4NI and TBHP.