Palladium-Catalyzed β-C–H Arylation of Aliphatic Aldehydes and Ketones Using Amino Amide as a Transient Directing Group

Iodoarene-directed photoredox β-C(sp3)-H arylation of 1-(o-iodoaryl)alkan-1-ones with cyanoarenes via halogen atom transfer and hydrogen atom transfer

Site selective functionalization of inert remote C(sp3)-H bonds to increase molecular complexity offers vital potential for chemical synthesis and new drug development, thus it has been attracting ongoing research interest. In particular, typical β-C(sp3)-H arylation methods using chelation-assisted metal catalysis or metal-catalyzed oxidative/photochemical in situ generated allyl C(sp3)-H bond processes have been well developed. However, radical-mediated direct β-C(sp3)-H arylation of carbonyls remains elusive. Herein, we describe an iodoarene-directed photoredox β-C(sp3)-H arylation of 1-(o-iodoaryl)alkan-1-ones with cyanoarenes via halogen atom transfer (XAT) and hydrogen atom transfer (HAT). The method involves diethylaminoethyl radical-mediated generation of an aryl radical intermediate via XAT, then directed 1,5-HAT to form the remote alkyl radical intermediate and radical-radical coupling with cyanoarenes, and is applicable to a broad scope of unactivated remote C(sp3)-H bonds like β-C(sp3)-H bonds of o-iodoaryl-substituted alkanones and α-C(sp3)-H bonds of o-iodoarylamides. Experimental findings are supported by computational studies (DFT calculations), revealing that this method operates via a radical-relay stepwise mechanism involving multiple SET, XAT, 1,5-HAT and radical-radical coupling processes.

Iridium(III)-catalyzed photoredox cross-coupling of alkyl bromides with trialkyl amines: access to α-alkylated aldehydes

A C(sp3)-C(sp3) cross coupling approach based on an iridium-photocatalytic radical process has been developed enabling the synthesis of various α-alkylated aldehydes from easily available/synthesized alkyl bromides and trialkyl amines under mild photocatalytic conditions. The synthesized aldehydes are also explored as a functional handle for various useful products such as carboxylic acid, alcohol and N-heterocycle synthesis.

Palladium-Catalyzed β-C(sp3)-H Arylation of Silyl Prop-1-en-1-ol Ethers with Aryl Halides: Entry to α,β-Unsaturated Ketones

A palladium(0)-catalyzed β-C(sp3)-H arylation of silyl prop-1-en-1-ol ethers with aryl halides for the synthesis of α,β-unsaturated ketones is presented. In contrast to the reported β-C(sp3)-H arylation of ketones, the chemoselectivity of this current method relies on the Pd(0) catalytic systems and reaction temperatures: While using the Pd(dba)2/DavePhos/KF system at 80 °C resulted in β-C(sp3)-H monoarylation to produce β-monoarylated α,β-unsaturated ketones, harnessing the Pd(OAc)2/t-Bu XPhos/K2HPO4 system at 110 °C induced β-C(sp3)-H diarylation to afford β,β-diarylated α,β-unsaturated ketones. The method provides a versatile route that uses readily available ketone-derivatized α-nonsubstituted silyl prop-1-en-1-ol ethers as the alkene sources and is characterized by a good functional group compatibility, a broad substrate scope, and an excellent selectivity.

Formal γ-C-H Functionalization of Cyclobutyl Ketones: Synthesis of cis-1,3-Difunctionalized Cyclobutanes

1,3-Difunctionalized cyclobutanes are an emerging scaffold in medicinal chemistry that can confer beneficial pharmacological properties to small-molecule drug candidates. However, the diastereocontrolled synthesis of these compounds typically requires complicated synthetic routes, indicating a need for novel methods. Here, we report a sequential C-H/C-C functionalization strategy for the stereospecific synthesis of cis-γ-functionalized cyclobutyl ketones from readily available cyclobutyl aryl ketones. Specifically, a bicyclo[1.1.1]pentan-2-ol intermediate is generated from the parent cyclobutyl ketone via an optimized Norrish-Yang procedure. This intermediate then undergoes a ligand-enabled, palladium-catalyzed C-C cleavage/functionalization to produce valuable cis-γ-(hetero)arylated, alkenylated, and alkynylated cyclobutyl aryl ketones, the benzoyl moiety of which can subsequently be converted to a wide range of functional groups including amides and esters.

ortho-C(sp3)-H arylation of aromatic aldehydes using 2-amino-N-methyl-acetamide as a L,L-type transient directing group

Pd-catalyzed ortho-C(sp³)-H arylation of aromatic aldehydes using 2-amino-N-methyl-acetamide as a simple, efficient and commercially available L,L-type transient directing group (TDG) is reported. The reaction exhibited excellent substrate compatibility and generated the desired products in moderate-to-high yields up to 78%. Further acid-catalyzed cyclization and dehydrative aromatization were also tested, and furnished some polycyclic aromatic hydrocarbons with excellent yields up to 96%. The X-ray crystal structure of a 2-methylbenzaldehyde ortho-C(sp³)-H palladation intermediate was obtained. Then, a plausible reaction mechanism involving the formation of a [5,6]-fused palladacycle was proposed. This approach offers valuable insights for exploiting novel L,L-type TDGs.

PdII-Catalyzed γ-C(sp3)-H (Hetero)Arylation of Ketones Enabled by Transient Directing Groups

Pd(II)-catalyzed γ-C(sp3)-H (hetero)arylation of aliphatic ketones is developed using α-amino acid as transient directing groups (TDG). A variety of aliphatic ketones were (hetero)arylated at the γ-position via a 5,6-membered fused cyclopalladation intermediate to afford the remotely arylated products in up to 88% yield. The crucial ligand effect of 2-pyridone is further enhanced by reducing the loading of acid additives. Consequentially, the improved reactivity of this catalytic system has also made possible the cyclic γ-methylene C(sp3)-H arylation of ketones. Mechanistic investigtigation and comparison to the γ-C-H arylation of aldehydes revealed a structural insight for designing site selective TDG.

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

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.

PdII-Catalyzed Site-selective β- and γ-C(sp3)-H Arylation of Primary Aldehydes Controlled by Transient Directing Groups

Pd(II)-catalyzed site-selective β- and γ-C(sp³)-H arylation of primary aldehydes is developed by rational design of L,X-type transient directing groups (TDG). External 2-pyridone ligands are identified to be crucial for the observed reactivity. By minimizing the loading of acid additives, the ligand effect is enhanced to achieve high reactivities of the challenging primary aldehyde substrates. Site selectivity can be switched from the proximate to the relatively remote position by changing the bite angle of TDG to match the desired palladacycle size. Experimental and computational investigations support this rationale for designing TDG to potentially achieve remote site-selective C(sp³)-H functionalizations.

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.

Pd-Catalyzed β-C-H Arylation of Aldehydes and Ketones Based on a Transient Directing Group

The direct Pd-catalyzed β-C-H arylation of aldehydes and ketones was developed by using 2-amino-N,N'-diisopropylsuccinamide as a novel transient directing group (TDG). The TDG showed good versatility in functionalizing unactivated β-C-H bonds of aldehydes and ketones. It was effective not only for aliphatic aldehydes and ketones but also for aromatic aldehydes and ketones. Besides, it was applicable to o-methylbenzaldehydes.

Palladium-Catalyzed C-H Silylation of Aliphatic Ketones Using an Aminooxyamide Auxiliary

A palladium-catalyzed β-C(sp³)-H silylation of aliphatic ketones with disilanes to afford β-silyl ketones is reported. The aminooxyamide auxiliary is critical for the C-H activation and silylation. The reaction tolerates a number of functional groups and shows good selectivity in silylating β-C(sp³)-H bonds in the company of C(sp²)-H bonds and acidic α-C(sp³)-H bonds. The reaction is scalable, and the aminooxyamide auxiliary is readily removed to give β-silyl ketones, which could serve as useful building blocks for organic synthesis. Late-stage diversification using this protocol is demonstrated in the silylation of santonin with good yield.

Palladium-Catalyzed β-C(sp3)-H Arylation of Aliphatic Ketones Enabled by a Transient Directing Group

The direct arylation of aliphatic ketones has been developed via Pd-catalyzed β-C(sp³)-H bond functionalization with 2-(aminooxy)-N,N-dimethylacetamide as a novel transient directing group (TDG), which showed remarkable directing ability to generate arylated products in moderate to good yields. Furthermore, the reaction can tolerate abundant substrate of ketones and aryl iodides. This study expands the scope of applications for TDGs.

Hexafluoroisopropanol: the magical solvent for Pd-catalyzed C-H activation

Among numerous solvents available for chemical transformations, 1,1,1,3,3,3-hexafluoro-2-propanol (popularly known as HFIP) has attracted enough attention of the scientific community in recent years. Several unique features of HFIP compared to its non-fluoro analogue isopropanol have helped this solvent to make a difference in various subdomains of organic chemistry. One such area is transition metal-catalyzed C-H bond functionalization reactions. While, on one side, HFIP is emerging as a green and sustainable deep eutectic solvent (DES), on the other side, a major proportion of Pd-catalyzed C-H functionalization is heavily relying on this solvent. In particular, for distal aromatic C-H functionalizations, the exceptional impact of HFIP to elevate the yield and selectivity has made this solvent irreplaceable. Recent research studies have also highlighted the H-bond-donating ability of HFIP to enhance the chiral induction in Pd-catalyzed atroposelective C-H activation. This perspective aims to portray different shades of HFIP as a magical solvent in Pd-catalyzed C-H functionalization reactions.

Harnessing the Efficacy of 2-Pyridone Ligands for Pd-Catalyzed (β/γ)-C(sp3)-H Activations

Mechanisms of palladium-aminooxyacetic acid and 2-pyridone-enabled cooperative catalysis for the β- and γ-C(sp³)-H functionalizations of ketones are investigated with density functional theory. 2-Pyridone-assisted dissociation of the trimeric palladium acetate [Pd₃(OAc)₆] is found to be crucial for these catalytic pathways. The evolution of the [6,6]-membered palladacycles (Int-4) are elucidated and are active complexes in Pd(II/IV) catalytic cycles. Nevertheless, 2-pyridone acts as an external ligand, which accelerates β-C(sp³)-H activation. Computational investigations suggest that the C(sp³)-H bond activation is the rate-limiting step for both the catalytic processes. To overcome the kinetic inertness, an unsubstituted aminooxyacetic acid auxiliary is used for the β-C(sp³)-H activation pathway to favor the formation of the [5,6]-membered palladacycle intermediate, Int-IV. Among the several modeled ligands, 3-nitro-5-((trifluoromethyl)sulfonyl)pyridine-2(1H)-one (L8) is found to be highly valuable for both the (β/γ)-C(sp³)-H functionalization catalytic cycles. A favorable free energy pathway of late-stage functionalization of (R)-muscone paves the path to design other bioactive molecules.

Transient imine directing groups for the C-H functionalisation of aldehydes, ketones and amines: an update 2018-2020

The use of pre-installed directing groups has become a popular and powerful strategy to control site selectivity in transition metal catalysed C-H functionalisation reactions. However, the necessity for directing group installation and removal reduces the efficiency of a directed C-H functionalisation method. To overcome this limitation, taking inspiration from organocatalytic methodologies, the use of transient directing groups has arisen. These methods allow for a transient ligand to be used, potentially in catalytic quantities, without the need for discrete installation or removal steps, enabling the discovery of more efficient, and mechanistically intriguing, dual catalytic methods. This review summarises recent developments in this fast moving field covering >70 new methodologies, highlighting new directing group designs and advances in mechanistic understanding. It covers progress since 2018, providing an update to our previous review of the field.

Distal γ-C(sp3 )-H Olefination of Ketone Derivatives and Free Carboxylic Acids

Reported herein is the distal γ-C(sp3 )-H olefination of ketone derivatives and free carboxylic acids. Fine tuning of a previously reported imino-acid directing group and using the ligand combination of a mono-N-protected amino acid (MPAA) and an electron-deficient 2-pyridone were critical for the γ-C(sp3 )-H olefination of ketone substrates. In addition, MPAAs enabled the γ-C(sp3 )-H olefination of free carboxylic acids to form diverse six-membered lactones. Besides alkyl carboxylic acids, benzylic C(sp3 )-H bonds also could be functionalized to form 3,4-dihydroisocoumarin structures in a single step from 2-methyl benzoic acid derivatives. The utility of these protocols was demonstrated in large scale reactions and diversification of the γ-C(sp3 )-H olefinated products.

PdII -Catalyzed Enantioselective C(sp3 )-H Arylation of Cyclobutyl Ketones Using a Chiral Transient Directing Group

The use of chiral transient directing groups (TDGs) is a promising approach for developing PdII -catalyzed enantioselective C(sp3 )-H activation reactions. However, this strategy is challenging because the stereogenic center on the TDG is often far from the C-H bond, and both TDG covalently attached to the substrate and free TDG are capable of coordinating to PdII centers, which can result in a mixture of reactive complexes. We report a PdII -catalyzed enantioselective β-C(sp3 )-H arylation reaction of aliphatic ketones using a chiral TDG. A chiral trisubstituted cyclobutane was efficiently synthesized from a mono-substituted cyclobutane through sequential C-H arylation reactions, thus demonstrating the utility of this method for accessing structurally complex products from simple starting materials. The use of an electron-deficient pyridone ligand is crucial for the observed enantioselectivity. Interestingly, employing different silver salts can reverse the enantioselectivity.

Transient Directing Group Enabled Pd-catalyzed γ-C(sp3)-H Oxygenation of Alkyl Amines

We report a general protocol for γ-C(sp³)-H acyloxylation and alkoxylation of free amines using 2-hydroxynicotinaldehyde as the transient directing group. In the presence of an electrophilic fluorinating bystanding oxidant and acetic acid, a wide range of aliphatic amines could be oxygenated selectively at the γ-methyl positions. A vast variety of aryl, heteroaryl, and aliphatic acids could also be successfully coupled under this C-O bond formation reaction to afford amine containing esters. Switching the nucleophile from acids to alcohols enables alkoxylation of free amines. Importantly, natural products and drug molecules such as ibuprofen, isozepac, fenbufen, and lithocholic acid are all compatible coupling partners. Notably, synthesis of these mono-protected amino alcohols from free amino alcohols using conventional selective protection are not always feasible.

Intramolecular β-Alkenylation of Cyclohexanones via Pd-Catalyzed Desaturation-Mediated C(sp3)-H/Alkyne Coupling

Site-selective C-C bond formation through the direct coupling of C(sp³)-H bonds with unsaturated hydrocarbons represents an atom-economical and redox-neutral way to functionalize chemically inert positions, such as those β to a carbonyl group. While most existing β-functionalization methods utilize a directing group (DG) strategy, here we report a Pd-catalyzed intramolecular β-alkenylation of ketones using alkynes as the coupling partner without the aid of DGs. Mediated by a ketone desaturation process, the reaction is redox-neutral and avoids using strong acids or bases. The resulting cis-5,6-fused bicycles can be diversely derivatized with excellent selectivity. Mechanistic studies imply an unusual "hydride-transfer" chain-like pathway, which involves the cyclometalation of an enyne intermediate and protonation of the resulting Pd enolate followed by an intermolecular hydride transfer through the desaturation of another substrate.

Expeditious synthesis of phenanthridines through a Pd/MnO2-mediated C–H arylation/oxidative annulation cascade from aldehydes, aryl iodides and amino acids

The expeditious access to phenanthridines through a Pd/MnO₂-mediated arylation/oxidative annulation cascade from aldehydes, aryl iodides and amino acids is described.

A dual role for acetohydrazide in Pd-catalyzed controlled C(sp3)–H acetoxylation of aldehydes

The palladium catalyzed aldehyde directed acetoxylation of C(sp³)–H bonds was realized by a transient directing group approach for the first time.

Palladium-Catalyzed Site-Selective C(sp3)-H Arylation of Phenylacetaldehydes

We describe a Pd-catalyzed selective C-H arylation reaction of phenylacetaldehydes using l-valine as the transient directing group. This process showed a broad substrate scope and excellent selectivity in which a ligand-controlled functionalization of the unactivated β-C(sp³)-H bond. In addition, enantioselective arylation of phenylacetaldehydes was preliminarily explored by utilizing a bulky chiral transient directing group.

Rhodium(III)-Catalyzed Regioselective C(sp2)-H Functionalization of 7-Arylpyrazolo[1,5-a]pyrimidines with Dioxazolones as Amidating Agents

Rh(III)-catalyzed C-H functionalization of 7-arylpyrazolo[1,5-a]pyrimidines was developed wherein the pyrazolo[1,5-a]pyrimidine moiety is reported for the first time to direct the C-H bond activation. Various 7-arylpyrazolo[1,5-a]pyrimidines underwent smooth C-H amidation with alkyl-, aryl-, and heteroaryl-substituted dioxazolones to afford the products in moderate to good yields. Mechanistic studies suggest that a six-membered rhodacycle intermediate involving N1 might play a key role in the regioselective catalytic cycle.