Palladium-Catalyzed Benzylation of Carboxylic Acids with Toluene via Benzylic C–H Activation

Palladium-catalyzed aerobic oxidative dehydrogenation of cyclohexenes to substituted arene derivatives

A palladium(II) catalyst system has been identified for aerobic dehydrogenation of substituted cyclohexenes to the corresponding arene derivatives. Use of sodium anthraquinone-2-sulfonate (AMS) as a cocatalyst enhances the product yields. A wide range of functional groups are tolerated in the reactions, and the scope and limitations of the method are described. The catalytic dehydrogenation of cyclohexenes is showcased in an efficient route to a phthalimide-based TRPA1 activity modulator.

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.).

Titanium superoxide – a stable recyclable heterogeneous catalyst for oxidative esterification of aldehydes with alkylarenes or alcohols using TBHP as an oxidant

A single step catalytic approach for the synthesis of esters from aldehydes using a recyclable heterogeneous catalyst is described.

Copper(i)-promoted cycloalkylation–peroxidation of unactivated alkenes via sp3C–H functionalisation

A copper-promoted cycloalkylation–peroxidation strategy has been developedviaa three-component reaction involving cycloalkanes,tert-butyl hydroperoxide and internal conjugated alkene.

Pd-catalyzed cross-coupling of aromatic compounds with carboxylic acids via C–H bond activation

Ester bonds were readily achieved by directly coupling aromatic compounds and carboxylic acids via Pd-catalyzed C–H bond activation.

Copper(ii)-promoted direct conversion of methylarenes into aromatic oximes

A simple and efficient catalytic system for direct conversion of methylarenes into aromatic oximes has been developed, with Cu(OAc)₂ as catalyst, NHPI (N-hydroxyphthalimide) as additive, tert-butyl nitrite (TBN) as both the nitrogen source and the oxidant. The process proceeds under mild conditions, tolerates a wide range of substrates, affording the corresponding aromatic oximes in 63–86% yields.

Copper-catalyzed oxidative esterification of unactivated C(sp3)–H bonds with carboxylic acids via cross dehydrogenative coupling

An effective copper-catalyzed esterification of unactivated (non-benzylic and allylic) C(sp³)–H bonds of hydrocarbons with Selectfluor as an oxidant has been developed.

Sulphur promoted C(sp3)–C(sp2) cross dehydrogenative cyclisation of acetophenone hydrazones with aldehydes: efficient synthesis of 3,4,5-trisubstituted 1H-pyrazoles

The first-time sulfur-promoted CDC approach involving acetophenone hydrazones and aldehydes has been developed for the synthesis of substituted pyrazoles.

Pd-catalyzed benzylic C–H oxidation of cyclotriveratrylene – product diversity

The inner-rim oxidation of CTV has been explored with palladium and an interesting array of CTV derivatives has been obtained by simple changes in the solvent/conditions employed.

Utilization of methylarenes as versatile building blocks in organic synthesis

This review attempts to describe the latest developments in the utilisation of methylarenes adopting C–H functionalization strategies and covers all the developments until March 2015.

C1-Benzyl and benzoyl isoquinoline synthesis through direct oxidative cross-dehydrogenative coupling with methyl arenes

An oxidative cross-dehydrogenative coupling of isoquinolines with methyl arenes has been developed, yielding structurally diverse C₁-benzyl and -benzoyl isoquinolines selectively. The direct use of readily available methyl arenes as coupling partners avoids unproductive steps for preactivating functional group installation, and is thereby attractive.

Copper-catalyzed oxidative coupling of acids with alkanes involving dehydrogenation: facile access to allylic esters and alkylalkenes

A new copper-catalyzed oxidative coupling of acids with alkanes for selective synthesis of allylic esters and alkylalkenes is described.

TBHP/TFA mediated oxidative cross-dehydrogenative coupling of N-heterocycles with aldehydes

A metal-free oxidative cross-dehydrogenative coupling of N-heterocycles with diverse aldehydes has been established in the presence of TBHP/TFA.

Chemoselective activation of sp(3) vs sp(2) C-H bonds with Pd(II)

The first selective coupling of a carbon nucleophile with methyl, ethyl, propyl, and butyl arenes in the absence of a directing group is described. Pd(OAc)₂ double C–H activation displays remarkable selectivity for the terminal methyl sites in alkyl arenes, rather than the more commonly observed arene sp² C–H activation. Mechanistic studies indicate the intermediacy of an azlactone dimer, obtained from oxidation with Pd(OAc)₂, and are consistent with a Pd-catalyzed C–H activation vs a radical process. The observed reactivity establishes that typical reaction solvents (e.g., toluene) can readily participate in C–H activation chemistry.

Selective approach to thioesters and thioethers via sp3 C–H activation of methylarenes

Novel CDC approaches for the synthesis of thioesters and thioethers was developed via sp³ C–H activation of methylarenes and subsequent functionalization.

Palladium-catalyzed synthesis of 2-aminobenzoxazinones by aerobic oxidative coupling of anthranilic acids and isocyanides

Isocyanides have emerged as valuable C1 building blocks in palladium catalysis. Their potential has, however, mainly been exploited for the synthesis of amidines and amidine-containing heterocycles. To illustrate the broader applicability of isocyanides, we have recently developed a novel oxidative coupling of diamines and isocyanides furnishing valuable guanidine-containing heterocycles. We here report the extension of this protocol to the coupling of anthranilic acids and isocyanides leading to medicinally relevant 2-aminobenzoxazinones. This is a particularly challenging substrate class for this reaction due to the possibility of undesired decarboxylative pathways and the susceptibility of the products to nucleophilic attack. Therefore, this work underlines the generality and broad potential of the oxidative coupling of bisnucleophiles and isocyanides, facilitating the further implementation of this chemistry in library design.