Transient Ligand-Enabled Transition Metal-Catalyzed C-H Functionalization

Recent Advances in the Synthesis of Benzo[d]isothiazol-3(2H)-One and Benzo[e][1,3]Thiazin-4-One Derivatives

Benzo[d]isothiazol-3(2H)-one and benzo[e][1,3]thiazin-4-one derivatives, as significant sulfur- and nitrogen-containing heterocyclic compounds, are prevalent in natural products, pharmaceuticals, and food items. In recent years, a variety of innovative synthetic methodologies for these compounds have been developed. In this review, we will comprehensively introduce the major advances in the synthesis of benzo[d]isothiazol-3(2H)-one and benzo[e][1,3]thiazin-4-one derivatives via both intramolecular and intermolecular pathways from 2012 to the present.

Three-component Minisci reaction involving trifluoromethyl radicals promoted by TBHP

A three-component Minisci reaction based on radical polarity inversion and mediated by a trifluoromethyl radical is reported in this work. Under transition metal-free conditions, the electrophilic trifluoromethyl radical was added to an electron-rich alkene, generating a nucleophilic alkyl radical, which underwent a Minisci reaction with various electron-deficient N-heteroarenes. This protocol is easily executed using CF3SO2Na as the trifluoromethyl radical source under air- and moisture-free conditions, providing a versatile way to obtain trifluoromethyl-containing N-heteroarenes. Pyridines, quinolines, isoquinoline, phenanthridine and acridine were tolerated under the reaction conditions and reacted with both acyclic and cyclic alkenes. The potential of this reaction was demonstrated through a gram-scale preparation of trifluoromethyl-containing N-heteroarene and their various synthetic applications in synthesizing divergent and structurally complex molecules.

Ru(II)-catalyzed oxidative (4+3) C-H/C-H annulation of 2-aryl-4H-pyrido[1,2-a]pyrimidin-4-ones with allyl alcohol

A ruthenium(II)-catalyzed direct oxidative (4+3) C-H/C-H annulation of 2-aryl-4H-pyrido[1,2-a]pyrimidin-4-ones with allyl alcohol has been described. The developed method yielded the corresponding annulated products in moderate to good yields. In addition, based on the control experimental results, a possible reaction mechanism has been proposed for the (4+3) C-H/C-H annulation reaction.

Rhodium-Catalyzed Meta-C-H Arylation of Arenes with Varied Linker Lengths: Bridging Catalytic Selectivity with Structural Diversity

The directing group (DG)-assisted approach has so far been the major route to achieve selective C-H activation at both proximal and distal positions. While rhodium catalysts are highly effective in DG-assisted ortho-C-H arylation, meta-C-H arylation with rhodium has not yet been reported. In this study, we present the first example of Rh-catalyzed meta-C-H arylation of arenes. We found that the 2-cyanophenyl-based directing group, in conjunction with arylboronic acids, selectively promotes meta-arylation with complete mono-selectivity. Despite significant advancements in meta-C-H activation for substrates with shorter linkers, such as hydrocinnamic acids, benzyl alcohols/amines, etc., meta-C-H activation of substrates with longer alkyl chains remains challenging with limited literature examples. We demonstrated that arenes with varying chain lengths, including conformationally flexible and less rigid ones such as 4-phenylbutanoic acid, 5-phenylvaleric acid, 6-phenylcaproic acid, 3-phenylpropanol, and 4-phenylbutanol underwent meta-arylation with high levels of regiocontrol. From a synthetic perspective, this approach could be valuable as it allows to produce biaryl derivatives of flexible arenes with native functional groups at the meta-position. The synthetic utility of this strategy is demonstrated through the total synthesis of CNBCA, a bioactive compound possessing promising potency against the SHP2 enzyme activity in vitro.

Pd-catalyzed regioselective activation of C(sp2)-H and C(sp3)-H bonds

Differentiating between two highly similar C-H bonds in a given molecule remains a fundamental challenge in synthetic organic chemistry. Directing group assisted strategies for the functionalisation of proximal C-H bonds has been known for the last few decades. However, distal C-H bond functionalisation is strenuous and requires distinctly specialised techniques. In this review, we summarise the advancement in Pd-catalysed distal C(sp2)-H and C(sp3)-H bond activation through various redox manifolds including Pd(0)/Pd(II), Pd(II)/Pd(IV) and Pd(II)/Pd(0). Distal C-H functionalisation, where a Pd-catalyst is directly involved in the C-H activation step, either through assistance of an external directing group or directed by an inherent functionality or functional group incorporated at the site of the Pd-C bond is covered. The purpose of this review is to portray the current state of art in Pd-catalysed distal C(sp2)-H and C(sp3)-H functionalisation reactions, their mechanism and application in the late-stage functionalisation of medicinal compounds along with highlighting its limitations, thus leaving the field open for further synthetic adjustment.

When is an Imine Directing Group a Transient Imine Directing Group in C-H Functionalization?

'Transient' C-H functionalization has emerged in recent years to describe the use of a dynamic linkage, often an imine, to direct cyclometallation and subsequent functionalization. As the field continues to grow in popularity, we consider the features that make an imine directing group transient. A transient imine should be i) formed dynamically in situ, ii) avoid discrete introduction or cleavage steps, and iii) offer the potential for catalysis in both the directing group and metal. This concept article contrasts transient imines with pioneering early studies of imines as directing groups for the formation of metallacycles and the use of preformed imines in C-H functionalization. Leading developments in the use of catalytic additives to form transient directing groups (as aldehyde or amine) are covered including selected highlights of the most recent examples of catalytic imine directed C-H functionalization with transition metals.

Native Amino Group Directed Meta-Selective C-H Arylation of Primary Amines Via Pd/Norbornene Catalysis

The selective functionalization of remote C-H bonds in free primary amines holds significant promise for the late-stage diversification of pharmaceuticals. However, to date, the direct functionalization of the meta position of amine substrates lacking additional directing groups remains underexplored. In this Letter, we present a successful meta-C-H arylation of free primary amine derivatives using aryl iodides, resulting in synthetically valuable yields. This meta-selective C-H functionalization is achieved through a sequence involving native amino-directed Pd-catalyzed seven-membered cyclometalation, followed by the utilization of a norbornene-type transient mediator.

Ring Walking Mediated by Ni-Ni Species as a Vehicle for Enabling Distal C(sp2)-H Functionalization of Aryl Pivalates

Herein, we report the utilization of Ni-Ni species as a manifold for enabling a "ring-walking" event by dynamic translocation of the metal center over the arene backbone. Experimental and computational studies support a translocation occurring via a 1,2-hydride shift. The synthetic applicability of the method is illustrated in a series of C-C bond formations that occur at distal C(sp2)-H sites of simple aryl pivalates.

Palladium-Catalyzed β-C(sp3)-H Bond Arylation of Tertiary Aldehydes Facilitated by 2-Pyridone Ligands

2-Pyridone ligand-facilitated palladium-catalyzed direct C-H bond functionalization via the transient directing group strategy has become an attractive topic. Here, we report a Pd-catalyzed direct β-C(sp3)-H arylation reaction of tertiary aliphatic aldehydes by using an α-amino acid as a transient directing group in combination with a 2-pyridone ligand.

C-H Bond Functionalization of N-Heteroarenes Mediated by Selectfluor

Herein, recent developments for Selectfluor-mediated C-H functionalization of N-heteroarenes are described. This type of C-H bond activation is an attractive and competitive alternative to traditional methodologies, allowing the functionalization of a variety of chemical functions. In addition, Selectfluor is a more sustainable and economically accessible oxidant compared with expensive/toxic metals or hazardous peroxides. For a practical understanding, the current review classified systematically the reported strategies in four subsections as follows: (1) carbon-carbon formation, (2) carbon-nitrogen bond formation, (3) carbon-chalcogen bond, and (4) carbon-halogen bond formation. Mechanistic aspects and reaction conditions are fully discussed to provide an understanding of the aspects that govern C-H functionalization in N-heteroarenes mediated by Selectfluor.

Transition-metal-catalyzed C-H bond activation as a sustainable strategy for the synthesis of fluorinated molecules: an overview

The last decade has witnessed the emergence of innovative synthetic tools for the synthesis of fluorinated molecules. Among these approaches, the transition-metal-catalyzed functionalization of various scaffolds with a panel of fluorinated groups (XRF, X = S, Se, O) offered straightforward access to high value-added compounds. This review will highlight the main advances made in the field with the transition-metal-catalyzed functionalization of C(sp2) and C(sp3) centers with SCF3, SeCF3, or OCH2CF3 groups among others, by C-H bond activation. The scope and limitations of these transformations are discussed in this review.

Native Amino Group Directed Site-Selective ε-C(sp2)-H Iodination of Primary Amines

Selective remote C-H activating amines using unmodified NH₂ as a native directing group demonstrate compelling synthetic utilities. The 3-arylpropan-1-amine moiety is present in many drugs and candidates in clinical trials. Selective iodination of 3-arylpropan-1-amines on remote aryl rings gives valuable intermediates for modifying bioactive molecules and synthesizing quinolones. Here we report the first palladium-catalyzed selective ε-C(sp²)-H iodination of free 3-arylpropan-1-amines via a seven-membered palladacycle.

Palladium-Catalyzed ortho-C-H Alkoxycarbonylation of Aromatic Aldehydes via a Transient Directing Group Strategy

Transient directing groups (TDGs) can be a powerful strategy for directly functionalizing C-H bonds of aldehydes. We report a palladium-catalyzed o-C-H alkoxycarbonylation of benzaldehydes using a catalytic amount of aromatic amine to form a transient imine that plays the role of a monodentate TDG. The reaction conditions were applied to a broad range of aldehydes, and the corresponding 2-formyl benzoates were used as direct precursors for the synthesis of phthalides and 1-isoindolinones.

Ligand-assisted olefin-switched divergent oxidative Heck cascade with molecular oxygen enabled by self-assembled imines

Divergent oxidative Heck reaction has proven to be reliable for the rapid construction of molecular complexity, while olefins switched the outcome that remained underexplored.

Nickel-Catalyzed Decarbonylative Reductive Alkylation of Aroyl Fluorides with Alkyl Bromides

This paper describes the nickel-catalyzed reductive alkylation of aroyl fluorides with alkyl bromides in a decarbonylative manner. In this reaction, various functional groups are well tolerated and the C(sp²)-C(sp³) bond can be constructed directly without the use of organometallic reagents. The present reaction is a cross-electrophile coupling via the radical pathway, affording the corresponding alkylarenes in moderate to good yields.

Recent Strategies in Nickel-Catalyzed C–H Bond Functionalization for Nitrogen-Containing Heterocycles

N-heterocycles are ubiquitous in natural products, pharmaceuticals, organic materials, and numerous functional molecules. Among the current synthetic approaches, transition metal-catalyzed C–H functionalization has gained considerable attention in recent years due to its advantages of simplicity, high atomic economy, and the ready availability of starting materials. In the field of N-heterocycle synthesis via C–H functionalization, nickel has been recognized as one of the most important catalysts. In this review, we will introduce nickel-catalyzed intramolecular and intermolecular pathways for N-heterocycle synthesis from 2008 to 2021.

Amine-Catalyzed Copper-Mediated C-H Sulfonylation of Benzaldehydes via a Transient Imine Directing Group

Transient directing groups (TDGs) can provide a powerful means for C-H functionalization without requiring additional steps for directing group introduction and removal. We report the first use of a TDG in combination with copper to effect C-H functionalization. The regioselective copper mediated β-C(sp2 )-H sulfonylation of aldehydes with sulfinate salts is accomplished using catalytic β-alanine to form a transient imine. A broad range of sulfonylated benzaldehydes are prepared using copper fluoride as both copper source and oxidant, involving a [5,6] cupracyclic intermediate. γ-(peri)-Sulfonylation of napthyl and phenanthrenyl carboxaldehydes is achieved through [6,6] cupracyclic intermediates. Further derivatisation of the aldehyde products is demonstrated. Kinetic experiments and Hammett analysis suggest the turnover limiting step to be a concerted asynchronous C-H cleavage via a dearomative Wheland-type transition state.

Transient directing group enabled Pd-catalyzed C-H oxygenation of benzaldehydes and benzylic amines

We report a general protocol for ortho-C-H fluoroalkoxylation of benzaldehydes and benzylic amines utilizing an inexpensive amino amide as a transient directing group. In the presence of an electrophilic fluorinating bystanding oxidant and fluorinated alcohols, a wide range of benzaldehydes and benzylic amines could be oxygenated selectively at the ortho positions to afford fluoroalkyl aryl ethers. This elegant approach would provide appealing strategies for synthesis of drug molecules and natural products.

Ligand-promoted palladium-catalyzed β-methylene C-H arylation of primary aldehydes

The transient directing group (TDG) strategy allowed long awaited access to the direct β-C(sp³)-H functionalization of unmasked aliphatic aldehydes via palladium catalysis. However, the current techniques are restricted to terminal methyl functionalization, limiting their structural scopes and applicability. Herein, we report the development of a direct Pd-catalyzed methylene β-C-H arylation of linear unmasked aldehydes by using 3-amino-3-methylbutanoic acid as a TDG and 2-pyridone as an external ligand. Density functional theory calculations provided insights into the reaction mechanism and shed light on the roles of the external and transient directing ligands in the catalytic transformation.

Directing group assisted rhodium catalyzed meta-C-H alkynylation of arenes

Site-selective C-H alkynylation of arenes to produce aryl alkynes is a highly desirable transformation due to the prevalence of aryl alkynes in various natural products, drug molecules and in materials. To ensure site-selective C-H functionalization, directing group (DG) assisted C-H activation has been evolved as a useful synthetic tool. In contrast to DG-assisted ortho-C-H activation, distal meta-C-H activation is highly challenging and has attracted significant attention in recent years. However, developments are majorly focused on Pd-based catalytic systems. In order to diversify the scope of distal meta-C-H functionalization, herein we disclosed the first Rh(i) catalyzed meta-C-H alkynylation protocol through the inverse Sonogashira coupling reaction. The protocol is compatible with various substrate classes which include phenylacetic acids, hydrocinnamic acids, 2-phenyl benzoic acids, 2-phenyl phenols, benzyl sulfonates and ether-based scaffolds. The post-synthetic modification of meta-alkynylated arenes is also demonstrated through DG-removal as well as functional group interconversion.

Transition-metal-catalyzed remote C-H functionalization of thioethers

In the last decade, transition-metal-catalyzed direct C-H bond functionalization has been recognized as one of most efficient approaches for the derivatization of thioethers. Within this category, both mono- and bidentate-directing group strategies achieved the remote C(sp2)-H and C(sp3)-H functionalization of thioethers, respectively. This review systematically introduces the major advances and their mechanisms in the field of transition-metal-catalyzed remote C-H functionalization of thioethers from 2010 to 2021.

Metal-free and Selectfluor-mediated diverse transformations of 2-alkylthiobenzamides to access 2,3-dihydrobenzothiazin-4-ones, benzoisothiazol-3-ones and 2-alkylthiobenzonitriles

Diverse transformations of 2-alkylthiobenzamides have been established to synthesize 2,3-dihydrobenzothiazin-4-ones, benzoisothiazol-3-ones and 2-alkylthiobenzonitriles in the presence of Selectfluor.

Application of a transient directing strategy in cyclization reactions via C–H activation

This review introduces seven types of cyclization reactions via C–H activation using a transient directing strategy.

Amine-Directed Mizoroki-Heck Arylation of Free Allylamines

The transition metal-catalyzed Mizoroki−Heck reaction is a powerful method to synthesize C–C bonds, allowing access to several important pharmaceuticals. Traditionally free amines have not been compatible with these approaches due...

Synthesis of Indole-Fused Six-, Seven-, or Eight-Membered N,O-Heterocycles via Rhodium-Catalyzed NH-Indole-Directed C-H Acetoxylation/Hydrolysis/Annulation

We report herein the facile synthesis of indole-fused six-, seven-, or eight-membered N,O-heterocycles through rhodium-catalyzed C-H acetoxylation/hydrolysis/annulation. The notable features of this method include C-H acetoxylation using NH-indole as the intrinsic directing group, high functional group compatibility, and construction of indole-fused medium-sized rings.

Facile Synthesis of Alkylidene Phthalides by Rhodium-Catalyzed Domino C-H Acylation/Annulation of Benzamides with Aliphatic Carboxylic Acids

The Rh-catalyzed ortho-C(sp² )-H functionalization of 8-aminoquinoline-derived benzamides with aliphatic acyl fluorides generated in situ from the corresponding acids has been developed. This reaction initiated with 8-aminoquinoline-directed ortho-C(sp² )-H acylation, which was accompanied by subsequent intramolecular nucleophilic acyl substitution of amide group to produce alkylidene phthalides This approach exhibits high stereo-selectivity for Z-isomer products, and tolerates a variety of functional groups as well as aliphatic carboxylic acids with diverse structural scaffolds.

Transient Directing Groups in Metal-Organic Cooperative Catalysis

The direct functionalization of C-H bonds is among the most fundamental chemical transformations in organic synthesis. However, when the innate reactivity of the substrate cannot be utilized for the functionalization of a given single C-H bond, this selective C-H bond functionalization mostly relies on the use of directing groups that allow bringing the catalyst in close proximity to the C-H bond to be activated and these directing groups need to be installed before and cleaved after the transformation, which involves two additional undesired synthetic operations. These additional steps dramatically reduce the overall impact and the attractiveness of C-H bond functionalization techniques since classical approaches based on substrate pre-functionalization are sometimes still more straightforward and appealing. During the past decade, a different approach involving both the in situ installation and removal of the directing group, which can then often be used in a catalytic manner, has emerged: the transient directing group strategy. In addition to its innovative character, this strategy has brought C-H bond functionalization to an unprecedented level of usefulness and has enabled the development of remarkably efficient processes for the direct and selective introduction of functional groups onto both aromatic and aliphatic substrates. The processes unlocked by the development of these transient directing groups will be comprehensively overviewed in this review article.

Tuning Ligand-Coordinated Single Metal Atoms on TiO2 and their Dynamic Response during Hydrogenation Catalysis

Ligand-coordinated supported catalysts (LCSCs) are of growing interest for heterogeneous single-atom catalysis. Here, the effect of the choice of organic ligand on the activity and stability of TiO₂ -supported single-atom Pt-ligand catalysts was investigated for ethylene hydrogenation. The activity of these catalysts showed a significant dependence on the choice of ligand and also correlated with coordination number for Pt-ligand and Pt-Cl^- . Of the three ligands examined in this study, the one with the lowest Pt coordination number, 1,10-phenanthroline-5,6-dione (PDO), showed the lowest reaction temperature and highest reaction rate, likely due to those metal sites being more accessible to reactant adsorption. In-situ X-ray absorption spectroscopy (XAS) experiments showed that the activity also correlated with good heterolytic dissociation of hydrogen, which was supported by OH/OD exchange experiments and was the rate-determining step of the hydrogenation reaction. In these in-situ XAS experiments up to 190 °C, the supported Pt-ligand catalyst showed excellent stability against structural and chemical change. Instead of Pt, the PDO ligand could be coordinated with Ir on TiO₂ to form Ir LCSCs that showed slow activation by loss of Ir-Cl bonds, then excellent stability in the hydrogenation of ethylene. These results provide the chance to engineer ligand-coordinated supported catalysts at the single-atom catalyst level by the choice of ligand and enable new applications at relatively high temperature.

Rhodium-Catalyzed Oxidative Annulation of 2- or 7-Arylindoles with Alkenes/Alkynes Using Molecular Oxygen as the Sole Oxidant Enabled by Quaternary Ammonium Salt

Developing an efficient catalytic system using molecular oxygen as the oxidant for rhodium-catalyzed cross-dehydrogenative coupling remains highly desirable. Herein, rhodium-catalyzed oxidative annulation of 2- or 7-phenyl-1H-indoles with alkenes or alkynes to assemble valuable 6H-isoindolo[2,1-a]indoles, pyrrolo[3,2,1-de]phenanthridines, or indolo[2,1-a]isoquinolines using the atmospheric pressure of air as the sole oxidant enabled by quaternary ammonium salt has been accomplished. Mechanistic studies provided evidence for the fast intramolecular aza-Michael reaction and aerobic reoxidation of Rh(I)/Rh(III), facilitated by the addition of quaternary ammonium salt.

Transient directing ligands for selective metal-catalysed C–H activation

C-H activation is a 'simple-to-complex' transformation that nature has perfected over millions of years of evolution. Transition-metal-catalysed C-H activation has emerged as an expeditious means to expand the chemical space by introducing diverse functionalities. Notably, among the strategies to selectively cleave a particular C-H bond, the catalytic use of a small molecule as co-catalyst to generate a transient directing group, which provides a balance between step economy and chemical productivity, has gained immense attention in recent years. This allows one to convert a desired C-H bond irrespective of its geometrical or stereochemical configuration. This Review describes the various transient directing groups used in C-H activation and explains their mechanistic significance.

Ruthenium-Catalyzed Deuteration of Aromatic Carbonyl Compounds with a Catalytic Transient Directing Group

A novel ruthenium-catalyzed C-H activation methodology for hydrogen isotope exchange of aromatic carbonyl compounds is presented. In the presence of catalytic amounts of specific amine additives, a transient directing group is formed in situ, which directs selective deuteration. A high degree of deuteration is achieved for α-carbonyl and aromatic ortho-positions. In addition, appropriate choice of conditions allows for exclusive labeling of the α-carbonyl position while a procedure for the preparation of merely ortho-deuterated compounds is also reported. This methodology proceeds with good functional group tolerance and can be also applied for deuteration of pharmaceutical drugs. Mechanistic studies reveal a kinetic isotope effect of 2.2, showing that the C-H activation is likely the rate-determining step of the catalytic cycle. Using deuterium oxide as a cheap and convenient source of deuterium, the methodology presents a cost-efficient alternative to state-of-the-art iridium-catalyzed procedures.

Biaryl Formation via Base-Promoted Direct Coupling Reactions of Arenes with Aryl Halides

In the absence of ligand, Cs2CO3-promoted cross-coupling reaction of arenes with cyano-/nitro-substituted aryl halides in DMSO affording biaryls is reported. The cyano/nitro group in biaryls is useful and convenient for further transformation. The formation of dibenzofurans resulting from the reactions between arenes and 1-bromo-2-iodobenzene is also reported. On the basis of control experiments and theoretical studies, a radical mechanism is proposed for the formation of biaryls.

The Emergence of Palladium-Catalyzed C(sp3 )-H Functionalization of Free Carboxylic Acids

Palladium-catalyzed directing group assisted C-H bond activation has emerged as a powerful tool in synthetic organic chemistry. However, only recently, among various directing groups, widely available carboxylate moiety is recognized as a versatile candidate for the regioselective transformations. Notably, palladium-catalyzed carboxylate directed C(sp³ )-H bond activation and diverse functionalization is highly challenging and has gained huge attention for its versatile applications. Mono- and bidentate ligands have proven to be useful for accelerating the C(sp³ )-H bond activation step, which helps to control reactivity and selectivity (including enantioselectivity). In this Minireview, we discuss the recent progress made in palladium-catalyzed C(sp³ )-H bond functionalization reactions for the construction of C-C and C-Heteroatom bonds with the direction of free carboxylic acid.

Recent development in transition metal-catalysed C-H olefination

Transition metal-catalysed functionalizations of inert C-H bonds to construct C-C bonds represent an ideal route in the synthesis of valuable organic molecules. Fine tuning of directing groups, catalysts and ligands has played a crucial role in selective C-H bond (sp2 or sp3) activation. Recent developments in these areas have assured a high level of regioselectivity in C-H olefination reactions. In this review, we have summarized the recent progress in the oxidative olefination of sp2 and sp3 C-H bonds with special emphasis on distal, atroposelective, non-directed sp2 and directed sp3 C-H olefination. The scope, limitation, and mechanism of various transition metal-catalysed olefination reactions have been described briefly.

Palladium-Catalyzed Regioselective Arylation of Unprotected Allylamines

Palladium-catalyzed organometallic transformations of free amines are often unsuccessful due to side reactions, such as oxidation, that can occur. However, the ability to furnish the free amine products from these reactions is important for improving the utility and sustainability of these processes, especially for accessing their potential as medicinal and agrochemical agents. Notably, the 3,3-diarylallylamine motif is prevalent in a variety of biologically relevant structures, yet there are few catalytic approaches to their synthesis, and none involving the free amine. Herein, we describe a simple protocol for the arylation of cinnamylamines and the diarylation of terminal allylamines to generate a diverse group of 3,3-diarylallylamine products using a PdII precatalyst. Key features of the method are the ability to access relatively mild conditions that facilitate a broad substrate scope as well as direct diarylation of terminal allylamine substrates. In addition, several complex and therapeutically relevant molecules are included to demonstrate the utility of the transformation.

Manganese-catalyzed selective C-H activation and deuteration by means of a catalytic transient directing group strategy

A novel manganese-catalyzed C-H activation methodology for selective hydrogen isotope exchange of benzaldehydes is presented. Using D₂O as a cheap and convenient source of deuterium, the reaction proceeds with excellent functional group tolerance. High ortho-selectivity is achieved in the presence of catalytic amounts of specific amines, which in situ form a transient directing group.

Mechanistic understanding enables chemoselective sp3 over sp2 C–H activation in Pd-catalyzed carbonylative cyclization of amino acids

An understanding on the factors controlling C(sp²)–H vs, C(sp³)–H selectivity in Pd-catalyzed carbonylative cyclization of γ-arylated valine derivatives has allowed to reverse the remarkable selectivity of Pd for aryl C(sp²)–H over C(sp³)–H cleavage.