The Versatile and Strategic O-Carbamate Directed Metalation Group in the Synthesis of Aromatic Molecules: An Update

The aryl O-carbamate (ArOAm) group is among the strongest of the directed metalation groups (DMGs) in directed ortho metalation (DoM) chemistry, especially in the form Ar-OCONEt2. Since the last comprehensive review of metalation chemistry involving ArOAms (published more than 30 years ago), the field has expanded significantly. For example, it now encompasses new substrates, solvent systems, and metalating agents, while conditions have been developed enabling metalation of ArOAm to be conducted in a green and sustainable manner. The ArOAm group has also proven to be effective in the anionic ortho-Fries (AoF) rearrangement, Directed remote metalation (DreM), iterative DoM sequences, and DoM-halogen dance (HalD) synthetic strategies and has been transformed into a diverse range of functionalities and coupled with various groups through a range of cross-coupling (CC) strategies. Of ultimate value, the ArOAm group has demonstrated utility in the synthesis of a diverse range of bioactive and polycyclic aromatic compounds for various applications.

Rh(III)-Catalyzed Controlled Ortho-Amidation of Arylamides with Dioxazolones Using Weakly Coordinating Native Primary Amide as the Directing Group

Herein, we report a controlled introduction of an amide unit at the ortho-position of an electron-deficient arylamide system without affording any cyclized products using user-friendly dioxazolone as an amidating reagent in the presence of a Rh(III)-catalyst. This is the first report where native primary amide has been utilized as a weakly coordinating group for site-selective C-N bond formation reaction. The developed protocol works under external auxiliary-free conditions with a wide substrate scope.

Selective C-H Iodination of Weinreb Amides and Benzamides through Iridium Catalysis in Solution and under Mechanochemical Conditions

The acid mediated ortho-iodination of Weinreb amides using a readily available catalyst is described. The selective ortho-iodination of Weinreb amides, challenging substrates in directed C-H activations, and also of benzamides is achieved. The process works under mild conditions and tolerates air and moisture, having a great potential for industrial applications. The methodology can be applied under mechanochemical conditions maintaining the reaction outcome and selectivity.

2-Hydroxypyridine-based Ligands as Promoter in Ruthenium(II) Catalyzed C-H Bond Activation/Arylation Reactions

A class of 2-hydroxypyridine based ligands are explored to achieve enhanced catalytic activity for ortho-C-H bond activation/arylation reaction over [(η6 -p-cymene)RuCl2 ]2 catalyst in water. Extensive studies using a series of substituted 2-hydroxypyridine based ligands (L1-L6) inferred that 5-trifluoromethyl-2-hydroxypyridine (L6) exhibited favorable effects to enhance the catalytic activity of Ru(II) catalyst for ortho C-H bond arylation of 2-phenylpyridine by 8 folds compared to those performed without ligands. The (η6 -p-cymene)Ru - L6 system also exhibited enhanced catalytic activity for ortho C-H bond arylation of 2-phenylpyridine using a variety of aryl halides. NMR and mass investigations inferred the presence of several ligand coordinated Ru(II) species, suggesting the involvement of these species in C-H bond activation reaction. Further in concurrence with the experimental findings, the density functional theory (DFT) calculations also evidenced the prominent role of 2-hydroxypyridine based ligands in Ru(II) catalyzed C-H bond arylation of 2-phenylpyridine with lower energy barrier for the C-H activation step.

Weakly coordinating tert-amide assisted Rh(III)-catalyzed C4-cyanation of indoles: application in photophysical studies

A rhodium(III)-catalyzed indole C4-selective cyanation is described using the bench-stable, user-friendly electrophilic cyanation agent N-cyano-N-phenyl-p-toluenesulfonamide (NCTS) as a coupling partner. A suitably positioned weakly coordinating tert-amide group was utilized for this site selectivity. The developed protocol proceeded with a broad scope. [Cp*Rh(MeCN)3][SbF6]2 was found to be an effective Rh(III) catalyst for this transformation. An initial study was carried out to know the photophysical properties of the C4-cyanated indole frameworks.

Synthesis of Selenoflavones via Ruthenium-Catalyzed Selenylation of Unsaturated Acids

An efficient method for pharmaceutically useful selenoflavones via a ruthenium-catalyzed selenylation reaction is demonstrated. The ruthenium-catalyzed selenylation was applied to synthesize diverse alkenyl selenides from simple unsaturated acids/amides and diaryl diselenides. A wide range of differently substituted diaryl diselenides can be applied in this protocol with a good functional group with excellent stereo- and regioselectivity.

1,3-Diynes: A Versatile Precursor in Transition-Metal Catalyzed (Mediated) C-H Functionalizations

Transition metal-catalyzed C-H functionalization of diverse arenes with alkyne units has attracted enormous attention for decades since they provide straightforward access to various functionalization/annulations, which are commonly present in bioactive compounds and natural products. Recently, conjugated alkynes (1,3-diynes) have been utilized as key coupling partner in many C-H activation reactions due to their versatile characteristic properties. The presence of two C≡C bonds in conjugated 1,3-diyne brings the new diversity in synthetic transformations, such as chemo-, regioselective pathways, mono-bis functionalizations, cascade annulations, etc. Herein, we summarized the latest developments in the realm of transition-metal-catalyzed C-H functionalizations of diverse arenes with 1,3-diynes. Moreover, we highlighted the diverse transformations, conditions, mechanisms and applications of the corresponding reaction in detail.

Base-catalyzed Effective C2-Amidation of Azolium Salts Using Isocyanates under Mild Conditions

An unprecedented base-catalyzed hydroarylation of isocyanates with azolium salts was developed, which follows a simple reaction pathway and provided facile access to diverse C2-amidated azolium salts under mild conditions. Importantly, this methodology can also be applied for the successive C2-amidation of a bisimidazolium salt with two different isocyanates to provide the corresponding unsymmetrically substituted bisamide derivatives. Notably, the obtained amidated salts can also serve as a prominent carbene surrogate for the synthesis of metal-NHC complexes.

Pd(II)-catalyzed coupling of C-H bonds of carboxamides with iodoazobenzenes toward modified azobenzenes

In this paper, we report a synthetic protocol for the construction of biaryl motif-based or π-extended azobenzene and alkylated azobenzene derivatives via the Pd(II)-catalyzed bidentate directing group (DG)-aided C-H activation and functionalization strategy. In the past, the synthesis of biaryl motif-based azobenzenes was accomplished through the traditional cross-coupling reaction involving organometallic reagents and aryl halides or equivalent coupling partners. We have shown the direct coupling of C-H bonds of aromatic/aliphatic carboxamides (possessing a DG) with iodoazobenzenes as the coupling partners through the Pd(II)-catalyzed bidentate DG-aided, site-selective C-H functionalization method. Azobenzene-containing compounds are a versatile class of photo-responsive molecules that have found applications across branches of chemical, biological and materials sciences and are prevalent in medicinally relevant molecules. Accordingly, the synthesis of new and functionalized azobenzene-based scaffolds has been an attractive topic of research. Although the classical methods are efficient, they need pre-functionalized starting materials. This protocol involving the Pd(II)-catalyzed, directing group-aided site-selective C-H arylation of aromatic and aliphatic carboxamides using iodoazobenzene as the coupling partner affording azobenzene-based carboxamides is an additional route and also a contribution towards enriching the library of modified azobenzenes. We have also shown the photoswitching properties of representative compounds synthesized via the Pd(II)-catalyzed directing group-aided site-selective C-H functionalization method.

Pd(II)-catalyzed selective β-C-H functionalization of azobenzene carboxamides

We report a Pd(II)-catalyzed bidentate directing group 8-aminoquinoline-aided, site-selective β-C-H functionalization protocol for assembling modified azobenzene carboxamides. Considering the importance of azobenzenes in chemical sciences, this paper reports a new route for enriching the library of modified azobenzene motifs.

Redox-Neutral Ruthenium(II)-Catalyzed Enol-Directed Arene C-H Alkylation with Maleimides

An enol-assisted regioselective arene C-H alkylation with maleimides is developed under redox-neutral ruthenium(II) catalysis, offering a wide variety of valuable 3-aryl succinimides including amino acid embedded frameworks in good to excellent yields. The products were also aromatized to produce synthetically useful resorcinol-based biaryls. Mechanistic studies support an organometallic pathway with a reversible C-H metalation step for this reaction.

Cobalt(III)-catalyzed weakly coordinating arylurea-directed regioselective mono-olefination

Here, we developed an air-stable, earth-abundant cobalt(III)-catalyzed regioselective mono-olefination of arenes directed by urea under mild conditions through a cross-dehydrogenative coupling (CDC) process. Under the optimized conditions, a high regioselectivity of mono-olefination was achieved with various electron-rich and electron-deficient arenes, which afforded E-alkenylated products (with yields of up to 90%). In contrast to the conditions used for noble-metal-catalyzed olefination directed by weakly coordinating groups, our reaction was operated under mild conditions, including mild temperature (40 °C) and non-metallic oxidant.

Recent Advances in Alkenyl sp2 C-H and C-F Bond Functionalizations: Scope, Mechanism, and Applications

Alkenes and their derivatives are featured widely in a variety of natural products, pharmaceuticals, and advanced materials. Significant efforts have been made toward the development of new and practical methods to access this important class of compounds by selectively activating the alkenyl C(sp²)-H bonds in recent years. In this comprehensive review, we describe the state-of-the-art strategies for the direct functionalization of alkenyl sp² C-H and C-F bonds until June 2022. Moreover, metal-free, photoredox, and electrochemical strategies are also covered. For clarity, this review has been divided into two parts; the first part focuses on currently available alkenyl sp² C-H functionalization methods using different alkene derivatives as the starting materials, and the second part describes the alkenyl sp² C-F bond functionalization using easily accessible gem-difluoroalkenes as the starting material. This review includes the scope, limitations, mechanistic studies, stereoselective control (using directing groups as well as metal-migration strategies), and their applications to complex molecule synthesis where appropriate. Overall, this comprehensive review aims to document the considerable advancements, current status, and emerging work by critically summarizing the contributions of researchers working in this fascinating area and is expected to stimulate novel, innovative, and broadly applicable strategies for alkenyl sp² C-H and C-F bond functionalizations in the coming years.

Total Synthesis of the Antimycobacterial Natural Product Chlorflavonin and Analogs via a Late-Stage Ruthenium(II)-Catalyzed ortho-C(sp2)-H-Hydroxylation

The continuous, worldwide spread of multidrug-resistant (MDR) and extensively drug-resistant (XDR) tuberculosis (TB) endanger the World Health Organization’s (WHO) goal to end the global TB pandemic by the year 2035. During the past 50 years, very few new drugs have been approved by medical agencies to treat drug-resistant TB. Therefore, the development of novel antimycobacterial drug candidates to combat the threat of drug-resistant TB is urgent. In this work, we developed and optimized a total synthesis of the antimycobacterial natural flavonoid chlorflavonin by selective ruthenium(II)-catalyzed ortho-C(sp²)-H-hydroxylation of a substituted 3′-methoxyflavonoid skeleton. We extended our methodology to synthesize a small compound library of 14 structural analogs. The new analogs were tested for their antimycobacterial in vitro activity against Mycobacterium tuberculosis (Mtb) and their cytotoxicity against various human cell lines. The most promising new analog bromflavonin exhibited improved antimycobacterial in vitro activity against the virulent H37Rv strain of Mtb (Minimal Inhibitory Concentrations (MIC₉₀) = 0.78 μm). In addition, we determined the chemical and metabolic stability as well as the pK_a values of chlorflavonin and bromflavonin. Furthermore, we established a quantitative structure–activity relationship model using a thermodynamic integration approach. Our computations may be used for suggesting further structural changes to develop improved derivatives.

A Ruthenium-Catalyzed Cyclization to Dihydrobenzo[c]phenanthridinone from 7-Azabenzonorbornadienes with Aryl Amides

An efficient ruthenium(II)-catalyzed tandem C-C/C-N bond formation with aryl amides and 7-azabenzonorbornadienes has been developed to synthesize cis-fused dihydrobenzo[c]phenanthridinones. The amide group functions as a directing group as well as a leaving group and provides an easy access to the pharmaceutically useful benzo[c]phenanthridine alkaloids such as nitidine and fagaronine analogues. The present methodology is compatible with various functional groups with respect to azabicyclic alkenes and aromatic amides. The reaction mechanism involving directing-group-assisted C-H activation was proposed and supported by the deuterium labeling studies.

Weakly Coordinating tert-Amide-Assisted Ru(II)-Catalyzed Synthesis of Azacoumestans via Migratory Insertion of Quinoid Carbene: Application in the Total Synthesis of Isolamellarins

A weakly coordinating tert-amide-directed straightforward method was developed for the synthesis of azacoumestans using the corresponding azaheterocycle derivatives and diazonaphthoquinones under cheap Ru(II)-catalyzed conditions. The reaction proceeds via migratory insertion of quinoid carbene and subsequent Brønstead acid-mediated cyclization. The optimized C2-selective method offered a wide scope of important azaheterocycles. Bioactive natural products like isolamellarins A and B were synthesized via the developed protocol. Preliminary mechanistic studies highlighted the probable mechanistic pathway.

Cp*Co(III)-Catalyzed Selective C8-Olefination and Oxyarylation of Quinoline N-Oxides with Terminal Alkynes

Herein we report Cp*Co(III)-catalyzed site-selective (C8)-H olefination and oxyarylation of quinoline N-oxides with terminal alkynes. The selectivity for C8-olefination and oxyarylation is sterically and electronically controlled. In the case of quinoline N-oxides (unsubstituted at the C2 position), only the olefination product was obtained irrespective of the nature of the alkynes. In contrast, oxyarylation was observed exclusively when 2-substituted quinoline N-oxides were reacted with 9-ethynylphenanthrene. However, alkynes with electron-withdrawing groups provided only olefination products with 2-substituted quinoline N-oxides. The developed strategy allowed a facile functionalization of quinoline N-oxides bearing natural molecules and an estrone-derived terminal alkyne to deliver the corresponding olefinated and oxyarylated products. To understand the reaction mechanism, control experiments, deuterium-labeling experiments, and kinetic isotope effect (KIE) studies were performed.

Electrophile-Dependent Reactivity of Lithiated N-Benzylpyrene-1-Carboxamide

In this paper, we describe the lithiation of N-benzylpyrene-1-carboxamide with RLi-TMEDA. We found that the reaction outcome strongly depends on the electrophile used in the quenching step. The electrophile can be introduced at either the benzylic position or at the C-2 position in the pyrene nucleus. Furthermore, when H⁺ was used as the quencher, the product of the intramolecular carbolithiation of the pyrene K-region was formed. Dehydrogenation of the obtained compound with DDQ allowed the synthesis of a novel nitrogen polycyclic compound with an aza-benzo[c,d]pyrene (azaolympicene) skeleton. Attempts to extend the reaction scope to the amides substituted in the phenyl ring 8a and 8b gave an unexpected result. The reaction of both compounds with BuLi gave 1-valerylpyrene (9) in good yield. Photophysical properties, including absorption spectra, emission spectra and quantum yields of the emission of selected products, were studied and discussed.

Ruthenium-catalyzed (spiro)annulation of N-aryl-2,3-dihydrophthalazine-1,4-diones with quinones to access pentacyclic spiro-indazolones and fused-cinnolines

Ru(II)-catalyzed strategies were developed for the [4 + 1] and [4 + 2] oxidative coupling between N-aryl-2,3-dihydrophthalazine-1,4-diones and 1,4-benzoquinones, achieving spiro-indazolones and fused-cinnolines, respectively. Mild, aerobic and external oxidant-free conditions, as well as the use of a ruthenium catalyst for such (spiro)annulative strategies with quinones over reported Rh/Ir-catalyts, underline the rewards of the disclosed protocols.

Ru-Catalyzed C-H alkenylation on the arene ring of pirfenidone using pyridone as a directing group

A method to functionalize the arene ring of pirfenidone has been demonstrated using pyridone as a directing group. Unlike the functionalization of the pyridone nucleus, the method demonstrated here is the alkenylation of the N-aryl ring of pirfenidone with internal alkynes using ruthenium catalyst. High functional group tolerance, simple reaction conditions and site-selective functionalization permit the synthesis of new analogues of drugs in a step-economical manner. The data of the control experiments suggest the possibilities of a base-assisted internal electrophilic substitution (BIES) pathway.

Controlled Reduction of Activated Primary and Secondary Amides into Aldehydes with Diisobutylaluminum Hydride

A practical method is disclosed for the reduction of activated primary and secondary amides into aldehydes using diisobutylaluminum hydride (DIBAL-H) in toluene. A wide range of aryl and alkyl N-Boc,...

Cobalt-catalysed acyl silane directed ortho C–H functionalisation of benzoyl silanes

Acyl silanes can be engaged as weakly coordinating directing groups in cobalt catalysed C–H functionalisation reactions to prepare benzoyl silanes that are highly amenable to subsequent synthetic manipulations yet inaccessible via existing methods.

Iridium(III)-Catalyzed C(3)-H Alkylation of Isoquinolines via Metal Carbene Migratory Insertion

An Ir(III)-catalyzed C(3)-H alkylation of N-acetyl-1,2-dihydroisoquinolines with diverse acceptor-acceptor diazo compounds has been achieved under a single catalytic system via metal carbene migratory insertion. Moreover, further synthetic transformations of the alkylated products such as aromatization, selective decarboxylation, and decarbonylation lead to the formation of several synthetically viable isoquinoline derivatives having immense potentials.

The emergence of the C-H functionalization strategy in medicinal chemistry and drug discovery

Owing to the market competitiveness and urgent societal need, an optimum speed of drug discovery is an important criterion for successful implementation. Despite the rapid ascent of artificial intelligence and computational and bioanalytical techniques to accelerate drug discovery in big pharma, organic synthesis of privileged scaffolds predicted in silico for in vitro and in vivo studies is still considered as the rate-limiting step. C-H activation is the latest technology added into an organic chemist's toolbox for the rapid construction and late-stage modification of functional molecules to achieve the desired chemical and physical properties. Particularly, elimination of prefunctionalization steps, exceptional functional group tolerance, complexity-to-diversity oriented synthesis, and late-stage functionalization of privileged medicinal scaffolds expand the chemical space. It has immense potential for the rapid synthesis of a library of molecules, structural modification to achieve the required pharmacological properties such as absorption, distribution, metabolism, excretion, toxicology (ADMET) and attachment of chemical reporters for proteome profiling, metabolite synthesis, etc. for preclinical studies. Although heterocycle synthesis, late-stage drug modification, ¹⁸F labelling, methylation, etc. via C-H functionalization have been reviewed from the synthetic standpoint, a general overview of these protocols from medicinal and drug discovery aspects has not been reviewed. In this feature article, we will discuss the recent trends of C-H activation methodologies such as synthesis of medicinal scaffolds through C-H activation/annulation cascade; C-H arylation for sp²-sp² and sp²-sp³ cross-coupling; C-H borylation/silylation to introduce a functional linchpin for further manipulation; C-H amination for N-heterocycles and hydrogen bond acceptors; C-H fluorination/fluoroalkylation to tune polarity and lipophilicity; C-H methylation: methyl magic in drug discovery; peptide modification and macrocyclization for therapeutics and biologics; fluorescent labelling and radiolabelling for bioimaging; bioconjugation for chemical biology studies; drug-metabolite synthesis for biodistribution and excretion studies; late-stage diversification of drug-molecules to increase efficacy and safety; cutting-edge DNA encoded library synthesis and improved synthesis of drug molecules via C-H activation in medicinal chemistry and drug discovery.

Ru(II)-Catalyzed Controlled Cross-Dehydrogenative Coupling of Benzamides with Activated Olefins via Weakly Coordinating Primary Amides

Ru(II)-catalyzed regioselective ortho-alkenylation of primary benzamides with activated olefins has been realized over the competitive cyclized products. This reaction overall proceeds via a cross-dehydrogenative coupling (CDC) reaction using a simple and weakly coordinating primary amide group in the presence of an inexpensive Ru(II) salt and allows the controlled introduction of olefin motifs at the ortho-position of benzamides. The key to the success of this strategy depends on fine-tuning the reaction conditions. The developed protocol has demonstrated excellent regio/diastereoselectivity and a good functional group tolerance with wide substrate scope and obviates the requirement of external auxiliaries as well as the costly metal catalyst. Detailed mechanistic studies indicate the involvement of the base-assisted internal electrophilic-type substitution (BIES) step in the reaction mechanism.

Palladium-Catalyzed Weakly Coordinating Lactone-Directed C-H Bond Functionalization of 3-Arylcoumarins: Synthesis of Bioactive Coumestan Derivatives

A palladium-catalyzed highly regioselective ortho-selective C-H functionalization of 3-arylcoumarins has been developed. The method utilizes the weakly coordinating lactone as a directing group. The versatility of the strategy is highlighted by developing methodologies for alkenylation, halogenation, fluoroalkoxylation, and hydroxylation. Different functional groups were well tolerated, and functionalized coumarins were obtained in moderate to high yields. The method also showed good selectivity for monofunctionalization versus difunctionalization. The generated ortho-hydroxy derivatives were cyclized in the presence of DDQ, thus developing a simple and fast method for the synthesis of bioactive coumestan from 3-arylcoumarins.

Ruthenium-Catalyzed Regioselective C(sp2)-H Activation/Annulation of N-(7-Azaindole)amides with 1,3-Diynes Using N-Amino-7-azaindole as the N,N-Bidentate Directing Group

The ruthenium(II)-catalyzed regioselective annulation of N-(7-azaindole)amides with 1,3-diynes has been demonstrated. Bioactive N-amino-7-azaindole has been used as a new bidentate directing group to furnish an array of 3-alkynylated isoquinolones. Furthermore, the developed protocol works efficiently for both aryl- and heteroaryl-substituted amides producing a range of pharmacologically useful 7-azaindole-based isoquinolones with a wide range of functionality.

Dual Role of the Rhodium(III) Catalyst in C-H Activation: [4 + 3] Annulation of Amide with Allylic Alcohols to 7-Membered Lactams

[4 + 3] annulation of primary and secondary benzamide and cinnamamide derivatives using allyl alcohol as a coupling partner catalyzed by Rh(III) is reported, where Rh(III) is playing a dual role of an oxidant and a catalyst for C-H activation. The Rh-catalyst oxidizes allyl alcohol to its carbonyl derivative, and the in situ-generated carbonyl compound reacts with benzamide in the presence of the Rh-catalyst, forming the corresponding alkylated products. Mechanistic studies show that AgSbF₆ is also playing a dual role. Apart from being a halide scavenger, AgSbF₆ catalyzes the cyclization of the alkylated product, forming the desired lactam. The current method has good synthetic application and is useful for synthesizing a few biologically active compounds that can act as the dopamine D₃ receptor ligand, including berberine-like analogues. The deuteration study and control experiments helped us to propose the mechanism.

Rhodium(III)-Catalyzed Aerobic Oxidative C-H Olefination of Unsaturated Acrylamides with Unactivated Olefins

A rhodium(III)-catalyzed aerobic oxidative cross-coupling of acrylamides with unactivated alkenes via vinylic C-H activation has been developed. The present cross-coupling reaction was examined with a variety of differently functionalized acrylamides and unactivated olefins. In these reactions, highly valuable amide-functionalized butadienes were prepared in good to excellent yields. This protocol was also compatible with Weinreb amides. A possible reaction mechanism involving the chelation-assisted vinylic C-H activation via a carboxylate-assisted deprotonation pathway is proposed.

Ruthenium(ii)-catalyzed reductive N–O bond cleavage of N-OR (R = H, alkyl, or acyl) substituted amides and sulfonamides

A convenient method for the reductive cleavage of the N–O bonds of amide derivatives was developed using ruthenium(ii)-catalyzed transfer hydrogenation reaction.

Reagent-free aerobic oxidative synthesis of amides from aldehydes and isothiocyanates

A reagent-free autoxidative reaction of aldehydes with isothiocyanates is developed to readily access amides, involving capture of carboxylic acids in situ generated from aldehydes by isothiocyanates as both coupling mediators and amine surrogates.

Chiral Transient Directing Group Strategies in Asymmetric Synthesis

The development of novel methodologies for catalytic enantioselective functionalization reactions enabled by chiral transient directing groups is accompanying in a paradigm shift in the field of asymmetric synthesis. In particular, these highly atom- and step-economic enantioinduction processes commonly proceed either via enantioselective C-H functionalization, or via enantioselective hydroarylation of the pro-chiral substrates generating point, axial or planar chirality. The use of the transient directing group strategy in C-H functionalizations precludes the stoichiometric installations and removal of directing groups and enables efficient, more compatible and economical chemical routes. This minireview highlights asymmetric transition-metal-catalyzed methodologies involving chiral transient directing groups together with the scope, utility and future perspective of the field.

Esterification of Tertiary Amides: Remarkable Additive Effects of Potassium Alkoxides for Generating Hetero Manganese-Potassium Dinuclear Active Species

A catalyst system of mononuclear manganese precursor 3 combined with potassium alkoxide served as a superior catalyst compared with our previously reported manganese homodinuclear catalyst 2 a for esterification of not only tertiary aryl amides, but also tertiary aliphatic amides. On the basis of stoichiometric reactions of 3 and potassium alkoxide salt, kinetic studies, and density functional theory (DFT) calculations, we clarified a plausible reaction mechanism in which in situ generated manganese-potassium heterodinuclear species cooperatively activates the carbonyl moiety of the amide and the OH moiety of the alcohols. We also revealed details of the reaction mechanism of our previous manganese homodinuclear system 2 a, and we found that the activation free energy (ΔG≠ ) for the manganese-potassium heterodinuclear complex catalyzed esterification of amides is lower than that for the manganese homodinuclear system, which was consistent with the experimental results. We further applied our catalyst system to deprotect the acetyl moiety of primary and secondary amines.