Mechanism of Cu/Pd-Catalyzed Decarboxylative Cross-Couplings: A DFT Investigation

Cationic Iridium-Catalyzed Decarboxylation of Aromatic Carboxylic Acids

Practical synthetic applications of catalytic decarboxylation in producing useful molecules are limited. We report herein the cationic Ir-catalyzed decarboxylations of various electron-rich and -poor aromatic carboxylic acids to produce hydrocarbons in good yield (up to >99%). Additionally, this reaction is applicable in decarboxylative hydroarylation of bicyclic alkenes and decarboxylative fluorination, indicating the potential utility of this catalytic decarboxylation in synthetic chemistry.

Skeletal Editing of Dibenzolactones to Fluorenes via Ni- or Pd-Catalyzed Decarboxylation

The skeletal editing of dibenzolactones to fluorenes by Ni- or Pd-catalyzed decarboxylation is reported. In contrast to previously reported intramolecular decarboxylative couplings, inductively electron-withdrawing ortho substituents on the aryl carboxylate moiety and metal additives are not required. The decarboxylation reaction proceeds cleanly and can be applied to the skeletal editing of a natural product analogue. Mechanistic observations are consistent with stabilization of the carboxylate-ligated Ni complex over the Ni-carboxylate ion pair, which is the key factor in promoting the challenging decarboxylation step in the catalytic cycle.

Decarboxylative Amination of Benzoic Acids Bearing Electron-Donating Substituents and Non-Activated Amines

Efficient methods for decarboxylative activation of benzoic acids into great valuable products are highly sought after. Here we report a highly desirable and straightforward decarboxylative amination of readily available benzoic...

Combined Experimental and Computational Mechanistic Investigation of the Palladium-Catalyzed Decarboxylative Cross-Coupling of Sodium Benzoates with Chloroarenes

Reported herein is a mechanistic investigation into the palladium-catalyzed decarboxylative cross-coupling of sodium benzoates and chloroarenes. The reaction was found to be first-order in Pd. A minimal substituent effect was observed with respect to chloroarene, and the reaction was zero-order with respect to chloroarene. Palladium-mediated decarboxylation was assigned as the turnover-limiting step based on an Eyring plot and density functional theory computations. Catalyst performance was found to vary based on the electrophile, which is best explained by catalyst decomposition at Pd(0). The 1,5-cyclooctadiene (COD) ligand contained in the precatalyst CODPd(CH₂TMS)₂ (Pd1) was shown to be a beneficial additive. The bench-stable Buchwald complex XPhosPdG2 could be used with exogenous COD and 2-dicyclohexylphosphino-2',4',6'-triisopropylbiphenyl (XPhos) instead of complex Pd1. Adding exogenous XPhos significantly increased the catalyst turnover number and enhanced reproducibility.

Recent developments in decarboxylative C(aryl)-X bond formation from (hetero)aryl carboxylic acids

Decarboxylative coupling reactions using readily available (hetero)aryl carboxylic acids are a highly efficient approach for the formation of new C-C and C-X bonds. These decarboxylative coupling reactions eliminate CO2 as a by-product, resulting in a greener and environmentally more benign approach than conventional coupling reactions. In this review, we summarize the recent developments in ipso-decarboxylative C-X (X = O/N/halo/S/Se/P/CN) bond formations using (hetero)aryl carboxylic acids. Furthermore, we highlight the current limitations and future research opportunities of aryl-decarboxylative coupling reactions.

Palladium-Catalyzed Decarboxylative ortho-C(sp2 )-H Aroylation of N-Sulfoximine Benzamides at Room Temperature

A palladium-catalyzed method for the decarboxylative ortho C-H acylation of N-sulfoximine benzamides is developed at room temperature. The catalytic method enables easy access to various functionalized 2-aroylaromatic carboxylic acid derivatives in good isolated yields. Based on our mechanistic studies, a Pd(II)/Pd(IV) catalytic cycle that involves aroyl radical intermediate is proposed for the reaction.

Is Cu(iii) a necessary intermediate in Cu-mediated coupling reactions? A mechanistic point of view

The different pathways have been summarized to disclose the key intermediate in copper-mediated coupling reactions.

Redox-Neutral Decarbonylative Cross-Couplings Coming of Age

Major progress has recently been made in the challenging redox-neutral decarbonylative cross-coupling of carboxylic acids. For example, the use of acid fluorides as effective cross-coupling partners has been found to enable control of the decarbonylation selectivity and facilitates challenging Pd⁰ -catalyzed nucleophilic trifluoromethylation and exogenous base-free Suzuki cross-coupling reactions. In another recent advance, the use of acid chlorides in room temperature difluoromethylation and direct decarbonylative cross-coupling of carboxylic acids allows these classical substrates to be used as aryl electrophiles in cross-coupling reactions. Further challenges that are yet to be addressed in redox-neutral decarbonylative cross-couplings are also briefly summarized.

Organocopper cross-coupling reaction for C-C bond formation on highly sterically hindered structures

We describe a powerful, broadly applicable cross-coupling protocol that enables carbon-carbon bond formation at highly sterically hindered carbon centers (both sp2 and sp3) by employing organocopper reagents under palladium catalysis. Experimental studies and theoretical calculations indicated that the key to the unique reactivity of copper is the relatively low activation energy of the compact transmetalation transition state, due to Cu(i)-Pd(ii) interaction, which is associated with small values of deformation energy of the reactants. This reaction is applicable to a variety of bulky substrates, including compounds inert to previous cross-coupling chemistry and has high functional group tolerance.

Highly-chemoselective step-down reduction of carboxylic acids to aromatic hydrocarbons via palladium catalysis

Herein, we describe a highly selective method for the direct decarbonylative step-down reduction of carboxylic acids to arenes, proceeding via well defined Pd(0)/(II) catalytic cycle.

Aryl carboxylic acids are among the most abundant substrates in chemical synthesis and represent a perfect example of a traceless directing group that is central to many processes in the preparation of pharmaceuticals, natural products and polymers. Herein, we describe a highly selective method for the direct step-down reduction of carboxylic acids to arenes, proceeding via well-defined Pd(0)/(ii) catalytic cycle. The method shows a remarkably broad substrate scope, enabling to direct the classical acyl reduction towards selective decarbonylation by a redox-neutral mechanism. The utility of this reaction is highlighted in the direct defunctionalization of pharmaceuticals and natural products, and further emphasized in a range of traceless processes using removable carboxylic acids under mild, redox-neutral conditions orthogonal to protodecarboxylation. Extensive DFT computations were conducted to demonstrate preferred selectivity for the reversible oxidative addition and indicated that a versatile hydrogen atom transfer (HAT) pathway is operable.

Pd-Catalyzed Intramolecular Chemoselective C(sp2)-H and C(sp3)-H Activation of N-Alkyl- N-arylanthranilic Acids

A controllable palladium-catalyzed intramolecular C-H activation of N-alkyl- N-arylanthranilic acids has been developed. The methodology allows selective synthesis of 1,2-dihydro-(4 H)-3,1-benzoxazin-4-ones and carbazoles from the same starting materials and palladium catalyst. The selectivity is controlled by the oxidant. Silver oxide promotes C(sp3)-H activation/C-O cyclization to provide 1,2-dihydro-(4 H)-3,1-benzoxazin-4-ones, while copper acetate contributes to C(sp2)-H activation/decarboxylative arylation to afford carbazoles. This protocol is demonstrated by its wide substrate scope and good functional group tolerance.

Acyl Radical Chemistry via Visible-Light Photoredox Catalysis

Visible light photoredox catalysis has enabled easy access to acyl radicals under mild reaction conditions. Reactive acyl radicals, generated from various acyl precursors such as aldehydes, α-ketoacids, carboxylic acids, anhydrides, acyl thioesters, acyl chlorides, or acyl silanes, can undergo a diverse range of synthetically useful transformations, which were previously difficult or inaccessible. This review summarizes such recent progress of visible-light-driven acyl radical generation using transition metal photoredox catalysts, metallaphotocatalysts, hypervalent iodine catalysts or organic photocatalysts.

Palladium-Catalyzed Chemoselective Protodecarboxylation of Polyenoic Acids

Conditions for the first palladium-catalyzed chemoselective protodecarboxylation of polyenoic acids to give the desired polyenes in good yields are presented. The reactions proceed under mild conditions using either a Pd(0) or Pd(II) catalyst and tolerate a variety of aryl and aliphatic substitutions. Unique aspects of the reaction include the requirement of phosphines, water, and a polyene adjacent to the carboxylic acid.

Cyclometallation of a germylene ligand by concerted metalation-deprotonation of a methyl group

The reaction of [CH{(CMe)(2,6-iPr2C6H3N)}2]GeCl with LiN(SiMe3)2 was previously reported, which led to the formation of a hetero-fulvene type germylene, [CH{(CMe)(C[double bond, length as m-dash]CH2)(2,6-iPr2C6H3N)}2]Ge through the deprotonation of the C-H bond from the methyl substituents. In this paper, we attempted the analogous reaction with (Dipp)NCMeCHCOMeGeCl using LiN(SiMe3)2 which gave rise to a metathesis product, (Dipp)NCMeCHCOMeGeN(SiMe3)2 (2). However, the reactions of 2 with [M2Cl2(μ-Cl)2(η5-Cp*)2] (M = Rh and Ir) resulted in cyclometallated Rh and Ir complexes through the activation of the C-H bond from the germylene ligand. The complexes were characterized by single crystal X-ray analysis, which authenticated the presence of Ge-Rh and Ge-Ir bonds. DFT studies have been performed to understand the mechanism.

Photocatalytic decarboxylations

During the last years, the field of photocatalytic decarboxylations has emerged rapidly. Carboxylic acids are inexpensive, non-toxic and renewable starting materials for the synthesis of pharmaceuticals or platform chemicals. The traceless extrusion of CO2 gives radical intermediates, which react in diverse cross-coupling reactions. Merging photocatalysis with metal catalysis enables even broader substrate scopes or enantioselective reactions. An overview of photocatalytic decarboxylative reactions of different classes of carboxylic acids is given within this chapter.

Theoretical studies on CuCl-catalyzed C-H activation/C-O coupling reactions: oxidant and catalyst effects

Copper-complex catalyzed coupling reactions have been widely applied in the production of many important organic moieties from a synthetic perspective. In this work, a series of density functional theory (DFT) calculations, employing the B3LYP + IDSCRF/DZVP method, have been performed for a typical CuCl-catalyzed C-O cross-coupling reaction. The novel reaction mechanism was reported as four successive processes: oxidative radical generation (ORG) or oxidative addition (OA), hydrogen abstraction (HA), C-H activation/reductive elimination, and separation of product and recycling of catalyst (SP & RC). Our calculations provided a deep understanding on the dissimilar chemical activities associated with varying the oxidants used; detailed energy profile analyses suggested that the first oxidation process could proceed via either of the two competing channels (ORG and OA mechanisms) which is the basis to explain the different experimental yields. In addition, our molecular modelling gave theoretical evidence that Cu(ii) → Cu(i) reduction by solvent DMF (and a water molecule) might serve as a preliminary step to produce some more active Cu(i) species that could subsequently be oxidized into Cu(iii) favorably. In contrast, the Cu(ii) → Cu(iii) direct pathway was estimated to be prohibited from thermodynamics. All the calculation results in this work are parallel with the experimental observations.

Copper-catalyzed decarboxylative regioselective synthesis of 1,5-disubstituted 1,2,3-triazoles

A copper-catalyzed decarboxylative regioselective protocol for the synthesis of 1,5-disubstituted 1,2,3-triazoles via direct annulation of cinnamic acids with aryl azides has been developed. This is the first example of 1,5-disubstituted 1,2,3-triazoles using Cu(ii) as the catalyst, which were generally synthesized using a ruthenium(ii) catalyst.

Transition-Metal-Free Decarboxylative Iodination: New Routes for Decarboxylative Oxidative Cross-Couplings

Constructing products of high synthetic value from inexpensive and abundant starting materials is of great importance. Aryl iodides are essential building blocks for the synthesis of functional molecules, and efficient methods for their synthesis from chemical feedstocks are highly sought after. Here we report a low-cost decarboxylative iodination that occurs simply from readily available benzoic acids and I₂. The reaction is scalable and the scope and robustness of the reaction is thoroughly examined. Mechanistic studies suggest that this reaction does not proceed via a radical mechanism, which is in contrast to classical Hunsdiecker-type decarboxylative halogenations. In addition, DFT studies allow comparisons to be made between our procedure and current transition-metal-catalyzed decarboxylations. The utility of this procedure is demonstrated in its application to oxidative cross-couplings of aromatics via decarboxylative/C–H or double decarboxylative activations that use I₂ as the terminal oxidant. This strategy allows the preparation of biaryls previously inaccessible via decarboxylative methods and holds other advantages over existing decarboxylative oxidative couplings, as stoichiometric transition metals are avoided.

Synthesis of 3-Substituted 2-Arylpyridines via Cu/Pd-Catalyzed Decarboxylative Cross-Coupling of Picolinic Acids with (Hetero)Aryl Halides

A decarboxylative cross-coupling of 3-substituted picolinic acids with (hetero)aryl halides is presented. In the presence of catalytic Cu₂O and Pd(1,5-cyclooctadiene)Cl₂ with 2-dicyclohexylphosphino-2'-(N,N-dimethylamino)biphenyl as the ligand, both electron-rich and electron-deficient aryl bromides and chlorides as well as heteroaryl bromides were successfully coupled with various picolinate salts under mild conditions in yields up to 96%. This protocol provides an efficient entry to 2-(hetero)arylpyridines, an attractive substance class in drug discovery.

The effect of hydroxyl on the solution behavior of a quaternary ammonium gemini surfactant

The influence of the introduction of a hydroxyl group into the system is studied at the macro and micro levels.

Mechanistic study of graphitic carbon layer and nanosphere formation on the surface of T-ZnO

New vapour-deposition-solid (VDS) growth mechanism of graphitic carbon has been proposed and supported by both theoretical calculations and experimental evidence.

Rhodium-catalyzed regiospecific C–H ortho-phenylation of benzoic acids with Cu/air as an oxidant

An efficient and practical synthetic approach for the regiospecific C–H ortho-phenylation of aromatic carboxylic acids in the absence of silver.