Microwave Encounters Ionic Liquid: Synergistic Mechanism, Synthesis and Emerging Applications

Progress in microwave (MW) energy application technology has stimulated remarkable advances in manufacturing and high-quality applications of ionic liquids (ILs) that are generally used as novel media in chemical engineering. This Review focuses on an emerging technology via the combination of MW energy and the usage of ILs, termed microwave-assisted ionic liquid (MAIL) technology. In comparison to conventional routes that rely on heat transfer through media, the contactless and unique MW heating exploits the electromagnetic wave-ions interactions to deliver energy to IL molecules, accelerating the process of material synthesis, catalytic reactions, and so on. In addition to the inherent advantages of ILs, including outstanding solubility, and well-tuned thermophysical properties, MAIL technology has exhibited great potential in process intensification to meet the requirement of efficient, economic chemical production. Here we start with an introduction to principles of MW heating, highlighting fundamental mechanisms of MW induced process intensification based on ILs. Next, the synergies of MW energy and ILs employed in materials synthesis, as well as their merits, are documented. The emerging applications of MAIL technologies are summarized in the next sections, involving tumor therapy, organic catalysis, separations, and bioconversions. Finally, the current challenges and future opportunities of this emerging technology are discussed.

Mechanism of Friedel-Crafts Acylation Using Metal Triflate in Deep Eutectic Solvents: An Experimental and Computational Study

In this paper, we develop a method for Friedel-Crafts acylation using metal triflate in deep eutectic solvents. Various metal triflates were tested and provided good to excellent yields of corresponding ketone products. The density functional theory calculation revealed the metal effects on the formation of active intermediate acylium triflate as well as the acidic condition. The metal triflate in the deep eutectic solvent can be recovered and reused with a little loss in the catalytic activity.

Palladium Metallaphotoredox-Catalyzed 3-Acylation of Indole Derivatives

Aldehydes and indoles generally undergo nucleophilic addition reactions to generate alcohols rather than ketones. Here, we report a method for synthesis of 3-acyl indoles from aldehydes by a combination of...

DFT Insights into the Hydrodenitrogenation and Ring-Opening of Indole on an M (M = Ni, Pt, Ni–Pt) Slab Model

Density functional theory (DFT) calculation has been used to study the hydrodenitrogenation (HDN) and ring-opening of indole on an M (M = Ni, Pt, Ni–Pt) slab surface. The possible reaction pathway of indole hydrogenation has been investigated in order to reveal the bimetallic synergistic effects of an M slab surface. Compared to the mechanism of indole hydrogenation on an M slab surface, it was found that a PtNi(111) surface favors indole hydrogenation. According to the results of DFT calculation, it suggests that the bimetallic effect of the M surface plays an important role in indole hydrogenation.

Copper(II)-Catalyzed Direct C-H (Hetero)arylation at the C3 Position of Indoles Assisted by a Removable N,N-Bidentate Auxiliary Moiety

The regioselective arylation of inert C3-H bonds in indoles reacting with arylboronates via effective copper-mediated catalysis with the aid of a facile and removable 2-pyridinylisopropyl (PIP) group without ligand participation is reported. This newly established method features high compatibility with diverse functional groups between coupling partners, including both indole substrates and arylboron reagents, consequentially leading to operational simplicity and providing access to generate the desired arylated products in good to excellent yields of up to 97%. Synthetically, the PIP-derived amide moiety could subsequently be readily removed under mild reaction conditions to produce useful indole carboxylic acids for further transformation.

A copper-promoted synthesis of epoxy-bridged [60]fullerene-fused lactones and further derivatization

A facile copper-promoted decarboxylative annulation of [60]fullerene (C60) with two identical α-oxocarboxylic acids via an unprecedented cascade addition pathway has been exploited to synthesize the unique epoxy-bridged C60-fused lactones for the first time. Further transformations into the rare epoxy-bridged C60-fused hemiacetals and bicyclic-fused 1,2,3,4-adducts as well as application in a perovskite solar cell device of the obtained products have also been demonstrated. The structures of the epoxy-bridged C60-fused lactones and derived reductive products have been unequivocally established by single-crystal X-ray crystallography. Plausible reaction mechanisms leading to the observed products are proposed.

Decarboxylative C–H alkylation of heteroarenes by copper catalysis

A copper-catalyzed decarboxylative C–H alkylation of heteroarenes with alkyl carboxylic acids has been realized.

Transition metal-catalyzed C–H functionalizations of indoles

This review summarises a wide range of transformations on the indole skeleton, including arylation, alkenylation, alkynylation, acylation, nitration, borylation, and amidation, using transition-metal catalyzed C–H functionalization as the key step.

Synthesis of 3-acylindoles via copper-mediated oxidative decarbethoxylation of ethyl arylacetates

An efficient regioselective C-3 acylation of free indoles (N-H) has been accomplished via oxidative decarbethoxylation of easily available ethyl arylacetates using Cu(OAc)2 and KOtBu in DMSO.

Iodine mediated oxidative cross-coupling of unprotected anilines and heteroarylation of benzothiazoles with 2-methylquinoline

Iodine-promoted oxidative C-H/C-H cross-coupling of unprotected anilines and 2-methylquinoline to furnish C4-carbonylated aniline (4-aminophenyl)(quinoline-2-yl) methanones in moderate to good yields has been demonstrated. This work provides the first site-selective approach for the synthesis of free amino groups containing methanones including unprecedented C-H functionalization rather than the N-H functionalization of unprotected anilines via the Kornblum oxidation of 2-methylquinoline. Furthermore, we noticed that the incorporation of KOH under standard conditions provides 2-heteroarylbenzothiazoles from benzothiazoles and 2-methylquinoline in good to excellent yields. These transformations do not require any transition metals or peroxides and tolerate various functional groups such as methoxy, hydroxy, bromo, chloro and nitro groups. Moreover, a plausible mechanistic pathway is proposed.

Decarboxylative acylation of N-free indoles enabled by a catalytic amount of copper catalyst and liquid-assisted grinding

A mechanochemically Cu(ii)-catalyzed decarboxylative acylation of N-free indoles with O₂ as a terminal oxidant was developed for the mild synthesis of 3-acylindoles.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

Oxidative Decarboxylation Enables Chemoselective, Racemization-Free Esterification: Coupling of α-Ketoacids and Alcohols Mediated by Hypervalent Iodine(III)

An α-ketoacid could be converted into a reactive acylating agent by treatment with hypervalent iodine(III) species, and in so doing, we discovered a novel decarboxylative acylation of alcohols that affords a variety of esters in excellent yields. The esterification has been applied to a sterol bearing a free carboxylic acid and shows unique chemoselectivity. The procedure is racemization-free and operates under mild conditions.

Copper-catalyzed oxidative decarboxylative coupling of α-keto acids and sulfoximines

The formation of N-aroylsulfoximines via a copper-catalyzed oxidative decarboxylative coupling of α-keto acids with NH-sulfoximines is reported.

Copper-catalyzed synthesis of indolyl diketones via C–H oxidation/diacylation of indoles with arylglyoxal hydrates

Copper-catalyzed synthesis of indolyl diketonesviaC–H oxidation/diacylation of indoles with arylglyoxal hydrates is described.

Direct C-2 acylation of indoles with toluene derivatives via Pd(ii)-catalyzed C–H activation

A simple and efficient Pd-catalyzed method for the C2-acylation of indoles is described using toluene derivatives.

Brønsted acidic ionic liquid-promoted direct C3-acylation of N-unsubstituted indoles with acid anhydrides under microwave irradiation

A green and efficient method for the synthesis of 3-acylindoles using a Brønsted acidic ionic liquid under microwave irradiation has been developed.

Silver-catalyzed decarboxylative acylation of quinoxalin-2(1H)-ones with α-oxo-carboxylic acids

A novel silver-catalyzed decarboxylative acylation of α-oxo-carboxylic acids was developed, by which various 3-acyl quinoxalin-2(1H)-ones were synthesized by direct C–H bond acylation of quinoxalin-2(1H)-ones.

Synthesis of 3-acylindoles by oxidative rearrangement of 2-aminochalcones using a hypervalent iodine reagent and cyclization sequence

An efficient one-pot 3-acylindole synthesis by oxidative rearrangement of 2-aminochalcones and sequential cyclization has been developed.

Silver-promoted decarboxylative amidation of α-keto acids with amines

A general and effective method for the synthesis of amides through decarboxylative amidation of α-keto acids with amines has been developed.

Transition metal-free oxidative ortho-acylation of phenols with N-heteroarylmethanes via double C–H activation

The direct oxidative acylation of phenols with N-heteroarylmethanes via sp³C–H and sp²C–H double activation was achieved under metal-free conditions. The reaction proceeds in a facile I₂/DMSO/O₂ system regio-selectively to produce valuable (2-hydroxyphenyl)arylmethanones. A plausible mechanism was proposed.

An efficient and green method for regio- and chemo-selective Friedel–Crafts acylations using a deep eutectic solvent ([CholineCl][ZnCl2]3)

An efficient and environmentally benign method for regio- and chemo-selective Friedel–Crafts acylation using deep eutectic solvent ([CholineCl][ZnCl₂]₃) under solvent-free microwave irradiation.

β-Carboline-directed decarboxylative acylation of ortho-C(sp2)–H of the aryl ring of aryl(β-carbolin-1-yl)methanones with α-ketoacids under palladium catalysis

A Pd-catalysed β-carboline assisted decarboxylative acylation of ortho-C(sp²)–H of aryl ring of aryl(β-carbolin-1-yl)methanones using α-oxocarboxylic acid as the acyl ion source to form (2-aroylaryl)(β-carbolin-2-yl)methanones is demonstrated.

A Simple, Effective, Green Method for the Regioselective 3-Acylation of Unprotected Indoles

A fast and green method is developed for regioselective acylation of indoles in the 3-position without the need for protection of the NH position. The method is based on Friedel-Crafts acylation using acid anhydrides. The method has been optimized, and Y(OTf)₃ in catalytic amounts is found to be the best catalyst together with the commercially available ionic liquid [BMI]BF₄ (1-butyl-3-methylimidazolium tetrafluoro-borate) as solvent. The reaction is completed in a very short time using monomode microwave irradiation. The catalyst can be reused up to four times without significant loss of activity. A range of substituted indoles are investigated as substrates, and thirteen new compounds have been synthesized.

A metal free domino synthesis of 3-aroylindoles via two sp³ C-H activation

A metal free synthesis of 3-aroylindole involving two sp(3) C-H activation has been achieved starting from o-alkynyl-N,N-dialkylamines using catalyst TBAI and oxidant TBHP.