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

Visible Light-Promoted Oxidative Cross-Coupling of Alcohols to Esters

Ester is one of the most significant functional groups in organic chemistry and is enclosed in several valued molecules. Usually, esters are prepared through the acid-catalyzed esterification reaction of carboxylic acids with alcohols, transesterification of esters with alcohols, or via activation of carboxylic acids followed by the addition of alcohols. However, these procedures typically imply the excess use of reactants and harsh reaction conditions. Visible light-mediated photoreactions have been disclosed to display a safe, sustainable, and accessible alternative to traditional methods, and to lead new reactivity modes in organic procedures. In this context, we propose a transition metal-based and organic-based photocatalyst-free synthesis of esters from alcohols induced by visible light. The methodology can be carried out using sunlight or artificial visible light as a solar simulator or a blue LED source.

Direct Synthesis of Esters from Alkylarenes and Carboxylic Acids: The C-H Bond Dehydroesterification

Herein, a reaction in which the benzyl C-H bonds of alkylarenes are directly esterified by carboxylic acids to produce benzyl esters in high yields is reported. This reaction is catalyzed by Pd nanoparticles (NPs) on N-doped carbon (CN) composites based on a carbonizing Al-MIL-101(NH₂) material, and no oxidants or hydrogen acceptors are required. Use of o-alkylbenzoic acids as substrates leads to phthalides, whereas with carboxylic acids and alkylarenes as the feedstock, the reaction produces the benzyl esters. These reactions that use readily available alkylarenes instead of benzyl halides or benzyl alcohols as raw materials for one-step synthesis of benzyl esters without oxidants are inherently atom- and step-efficient. The CN composites and the CN-supported Pd NP catalysts were prepared and are well characterized. The proposed mechanism involves dehydrogenation of both the carboxylic groups and the benzylic groups and the transformation of benzylic C-H bond into the C-O bond via hydrogen abstraction from the benzylic group through an organopalladium intermediate. The kinetic isotope effect (k_H/k_D = 2.77) indicated that C(sp³)-H bond cleavage of the alkane aromatics is the rate-determining step.

LiHMDS-Mediated Deprotonative Coupling of Toluenes with Ketones

We demonstrate that lithium hexamethyldisilazide (LiHMDS) acts as an effective base for deprotonative coupling reactions of toluenes with ketones to afford stilbenes. Various functionalities (halogen, OCF₃ , amide, Me, aryl, alkenyl, alkynyl, SMe, and SPh) are allowed on the toluenes. Notably, this system proved successful with low-reactive toluenes bearing a large pKa value compared to that of the conjugate acid of LiHMDS (hexamethyldisilazane, 25.8, THF), as demonstrated by 4-phenyltoluene (38.57, THF) and toluene itself (∼43, DMSO).

Protocol for chemo- and regioselective C(sp3)-H activation using a heterogeneous copper powder-catalyzed reaction

Here, we present a protocol for the synthesis of dibenzo[c,e]oxepin-5(7H)-ones starting from 2'-alkyl-[1,1'-biphenyl]-2-carboxylic acids. This technique uses two copper(0)-catalyzed benzylic C(sp³)-H activation strategies taking either di-tertbutyl peroxide or gaseous oxygen as an oxidant. We detail a photocatalytic thermal approach for copper powder-catalyzed reaction with oxygen. We also describe a procedure for catalyst recycling in both the strategies. The product has been successfully synthesized both in mmol and gram scale. For complete details on the use and execution of this protocol, please refer to Nandi et al. (2022).

Oxidative Trifluoroacetoxylation of 1°, 2°, and 3° Benzylic C(sp3)-H Bond Donors Using N-Trifluoroacetoxyquinuclidinium Salts under Photoredox Catalysis

N-Trifluoroacetoxyquinuclidinium trifluoroacetate was prepared in situ from quinuclidine N-oxide and (CF₃CO)₂O. Except for some electron-poor substrates, this reagent allows for the high-yielding oxidative trifluoroacetoxylation of 1°, 2°, and 3° benzylic C-H bonds under photocatalytic conditions. The trifluoroacetoxylation of an ibuprofen methyl ester allowed the selective functionalization of a 2° benzylic C-H bond. For alkylbenzenes, hydrogen-atom transfer from a benzylic C-H bond to a quinuclidine cation radical was proposed to be the reaction-product-determining step.

Transition metal-free approach for late-stage benzylic C(sp3)-H etherifications and esterifications

We report a transition metal-free approach for the regioselective functionalization of benzylic C(sp³)-H bonds using alcohols and carboxylic acids as the nucleophiles. This straightforward and general route has provided various benzylic ethers and esters, including twelve pharmaceutically relevant compounds.

A novel quinoline-based NNN-pincer Cu(II) complex as a superior catalyst for oxidative esterification of allylic C(sp3)-H bonds

We report for the first time that the quinoline-based NNN-pincer Cu(II) complex acts as an air stable superior catalyst for the oxidative cross-coupling of the allyl sp³ C-H bond with an acid for the synthesis of allyl esters in a homogeneous system at ambient temperature. The synthesized catalyst, 1, has been well characterized by various analytical techniques (HRMS, single crystal X-ray diffraction, CV, EPR, UV-vis spectroscopy) and showed excellent catalytic activity for the oxidative esterification of allylic C(sp³)-H bonds at 40 °C within a very short period of time (1 h) using only 1 mol% of the catalyst. A wide variety of aromatic allylic esters were synthesized in moderate to good yields, which could be extended to aliphatic allyl esters as well.

From methylarenes to Esters: Efficient oxidative Csp3-H activation promoted by CuO decorated magnetic reduced graphene oxide

Magnetic reduced graphene oxide supported CuO (rGO/Fe3O4-CuO) as the heterogeneous catalyst in cross dehydrogenative coupling (CDC) reactions has been demonstrated for the synthesis of esters using methyl aromatics, aldehydes/benzyl alcohols...

Alkaline-Metal-Catalyzed One-Pot Aminobenzylation of Aldehydes with Toluenes

A novel and easily accessible MN(SiMe₃)₂ (M = Li or Na)/Cs₂CO₃ co-catalyzed benzylation of in situ generated N-(trimethylsilyl) aldimines with toluene derivatives has been successfully developed. The catalyst exhibits high chemoselectivity for deprotonation of toluenes at the benzylic position. The utility of this system is exemplified by the one-pot synthesis of a diverse array of bioactive 1,2-diarylethylamines with excellent efficiency and broad functional group tolerance.

One-pot aminobenzylation of aldehydes with toluenes

Amines are fundamental motifs in bioactive natural products and pharmaceuticals. Using simple toluene derivatives, a one-pot aminobenzylation of aldehydes is introduced that provides rapid access to amines. Simply combining benzaldehydes, toluenes, NaN(SiMe3)2, and additive Cs(O2CCF3) (0.35 equiv.) generates a diverse array of 1,2-diarylethylamine derivatives (36 examples, 56-98% yield). Furthermore, suitably functionalized 1,2-diarylethylamines were transformed into 2-aryl-substituted indoline derivatives via Buchwald-Hartwig amination. It is proposed that the successful deprotonation of toluene by MN(SiMe3)2 is facilitated by cation-π interactions between the arene and the group(I) cation that acidify the benzylic C-Hs.

Selective carboxylation of reactive benzylic C-H bonds by a hypervalent iodine(III)/inorganic bromide oxidation system

An oxidation system comprising phenyliodine(III) diacetate (PIDA) and iodosobenzene with inorganic bromide, i.e., sodium bromide, in an organic solvent led to the direct introduction of carboxylic acids into benzylic C–H bonds under mild conditions. The unique radical species, generated by the homolytic cleavage of the labile I(III)–Br bond of the in situ-formed bromo-λ³-iodane, initiated benzylic carboxylation with a high degree of selectivity for the secondary benzylic position.

C-H functionalisation of cycloalkanes

Among organic compounds hydrocarbons are inexpensive and possibly the most abundant among natural resources. Developing strategies for selective functionalisation of inert hydrocarbon C-H bonds is one of the most ideal synthetic paths that a synthetic chemist could think of. This critical review focuses on the recent development of various directed and non-directed cycloalkylations leading to the formation of carbon-carbon (C-C) and carbon-heteroatom (C-X) bonds. Apart from various transition metal catalysed cycloalkylations, this review also covers various metal-free cycloalkylation processes.

Copper-catalyzed α-benzylation of BODIPYs via radical-triggered oxidative cross-coupling of two C–H bonds

An oxidative cross-dehydrogenative coupling of BODIPYs with toluene and it derivatives has been developed, allowing for the facile synthesis of a broad range of structurally diverse α-benzylated BODIPYs with high solid-state fluorescence.

Direct functionalization of benzylic and non-benzylic C(sp3)–H bonds via keteniminium ion initiated cascade [1,5]-hydrogen transfer/cyclization

Keteniminium intermediates generated in situ initiate the cascade [1,5]-hydride transfer/cyclization for the direct functionalization of benzylic and non-benzylic C(sp³)–H bonds.

Reusable ionic liquid-catalyzed oxidative esterification of carboxylic acids with benzylic hydrocarbons via benzylic Csp3–H bond activation under metal-free conditions

A protocol for oxidation of the Csp³–H bond in benzylic hydrocarbons with carboxylic acids using ionic liquid as reusable catalyst has been reported.

PdII-Catalyzed Oxidative Aldehyde-sp2C-H Functionalization and Cyclization Using NHC with Mild Oxidant DMSO for the Selective Synthesis of Esters, Sugar-Based Analogues, and β-Hydroxy Chromanones: An 18O-Labeling Study

We assume formation of acyl-Pd^II-N-heterocyclic-carbene (NHC) organometalics for diverse C-O/O-C and C-C/C-O coupling catalysis of direct functionalization and cyclization reactions. We report the first use of dimethyl sulfoxide (DMSO) as an oxidant under an inert atmosphere to O₂-sensitive NHC for oxidative transformations. In situ generated imidazolium halides are utilized as a precursor of NHC and as a source of alkyl group for the sp²C-H functionalization of aldehydes to esters under mild conditions. In contrast to the reported NHC-catalyzed esterification strategies, the outstanding substrate scope of this mild catalysis approach is established through synthesis of thermally labile sugar-based chiral esters. Our competition experiments using various unsymmetrical imidazolium halides revealed an ascending rate of migratory aptitude among methyl ≪ allyl < crotyl < cinnamyl < benzyl moiety. DMSO is used as an oxidant for both esterification and cyclization reactions, and the transfer of the DMSO-oxygen to ester is confirmed using an ¹⁸O-labeling experiment. The diverse activity using DMSO-oxygen to acyl-Pd^II-NHC is verified by developing a unique C-C-coupled cyclization with side-chain hydroxylation of olefin to achieve valuable β-hydroxy chromanones.

Direct difunctionalization of activated alkynes via domino oxidative benzylation/1,4-aryl migration/decarboxylation reactions under metal-free conditions

The oxidative difunctionalization of aryl alkynoatesviaa tandem oxidative benzylation/1,4-aryl migration/decarboxylation to trisubstituted alkenes was developed under metal-free conditions.

Copper-catalyzed oxidative cyclization of vinyl azides with benzylic Csp3–H bonds for the synthesis of substituted phenanthridines

A copper-catalyzed oxidative cyclization of vinyl azides with benzylic C_sp3–H bondsviaa tandem dual C–H functionalization process has been developed. This reaction allows access to substituted phenanthridines containing a variety of functional groups.

Synthesis of esters via sp3 C–H functionalisation

This review describes various methods for the synthesis of eleven classes of esters involving sp³ C–H functionalisation.

Traceless directing group mediated branched selective alkenylation of unbiased arenes

–COOH group assisted branched selective olefination of simple arenes.

Palladium-Catalyzed Synthesis of Δ(2)-Isoxazoline from Toluene Derivatives Enabled by the Triple Role of Silver Nitrate

A palladium-catalyzed direct synthesis of Δ(2)-isoxazoline from toluene derivatives has been established. The present reaction proceeds through nondirected Csp(3)-H activation, benzylic nitration, dehydration, and cycloaddition. This protocol also features the unusual triple role of silver nitrate in a one-pot reaction.