Diversified Fluoroalkylation of Alkenes Using Quaternary Fluoroalkyl Alcohols as the Fluoroalkylating Reagents.

Given the widespread presence of fluoroalkyl functionalities in bioactive molecules, the development of fluoroalkylation reactions with bench-stable and easy-to-use fluoroalkylating reagents is highly desirable. In addition, realization of mono-, di-, tri-, or polyfluoroalkyation usually requires distinct types of fluoroalkylating reagents under different or even harsh reaction conditions, and a universal method to accomplish different hydrofluoroalkylation of alkenes is lacking. Herein, the use of quaternary fluoroalkyl alcohols is reported as the universal fluoroalkylating reagents to readily facilitate mono-, di-, tri-, or polyfluoroalkylation of a wide range of alkene substrates in high yields.

Palladium (II)-Catalyzed C-H Activation with Bifunctional Ligands: From Curiosity to Industrialization.

In 2001, our curiosity to understand the stereochemistry of C-H metalation with Pd prompted our first studies in Pd(II)-catalyzed asymmetric C-H activation (RSC Research appointment: 020 7451 2545, Grant: RG 36873, Dec. 2002). We identified four central challenges: 1.

Macrocyclization remote -selective C-H olefination using a practical indolyl template.

The synthesis of macrocyclic compounds with different sizes and linkages remains a great challenge transition metal-catalysed intramolecular C-H activation. Herein, we disclose an efficient macrocyclization strategy Pd-catalysed remote -C-H olefination using a practical indolyl template. This approach was successfully employed to access macrolides and coumarins.

Ruthenium-Catalyzed Ligand-Enabled Regiodivergent Difluoroallylation of Aryl C-H Bonds.

The -difluoroallyl group is a sought-after structural motif commonly found in pharmaceutical compounds. Despite its appeal, achieving a controlled synthesis of both α,α- and γ,γ-difluoroallylated compounds has proven to be a challenging task. This study presents a new approach to difluoroallylation, which utilizes a regiodivergent C-H bond reaction catalyzed by ruthenium catalysis.

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.

Empirical Guidelines for the Development of Remote Directing Templates through Quantitative and Experimental Analyses.

The ability to differentiate and selectively activate remote C-H bonds represents a perennial challenge in the field of C-H activation. Since its first report in 2012, a now-established "directing template" (DT) approach remains demonstrably effective for the functionalization of remote C-H bonds. As selectivity is hypothesized to be principally determined by the optimal positioning of the reactive catalyst to a target C-H bond, a DT's spatial factors are particularly important toward achieving high selectivity, though a systematic study on its requisite factors remain unelucidated.

Merging C(sp)-H activation with DNA-encoding.

DNA-encoded library (DEL) technology has the potential to dramatically expedite hit identification in drug discovery owing to its ability to perform protein affinity selection with millions or billions of molecules in a few experiments. To expand the molecular diversity of DEL, it is critical to develop different types of DNA-encoded transformations that produce billions of molecules with distinct molecular scaffolds. Sequential functionalization of multiple C-H bonds provides a unique avenue for creating diversity and complexity from simple starting materials.

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

Reported herein is the distal γ-C(sp )-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(sp )-H olefination of ketone substrates. In addition, MPAAs enabled the γ-C(sp )-H olefination of free carboxylic acids to form diverse six-membered lactones.

Ligand-Enabled Monoselective β-C(sp)-H Acyloxylation of Free Carboxylic Acids Using a Practical Oxidant.

The development of C-H activation reactions that use inexpensive and practical oxidants remains a significant challenge. Until our recent disclosure of the β-lactonization of free aliphatic acids, the use of peroxides in C-H activation reactions directed by weakly coordinating native functional groups was unreported. Herein, we report C(sp)-H β-acetoxylation and γ-, δ-, and ε-lactonization reactions of free carboxylic acids enabled by a novel cyclopentane-based mono--protected β-amino acid ligand.

Rational Development of Remote C-H Functionalization of Biphenyl: Experimental and Computational Studies.

A simple and efficient nitrile-directed meta-C-H olefination, acetoxylation, and iodination of biaryl compounds is reported. Compared to the previous approach of installing a complex U-shaped template to achieve a molecular U-turn and assemble the large-sized cyclophane transition state for the remote C-H activation, a synthetically useful phenyl nitrile functional group could also direct remote meta-C-H activation. This reaction provides a useful method for the modification of biaryl compounds because the nitrile group can be readily converted to amines, acids, amides, or other heterocycles.

Recent advances in transition metal-catalyzed C(sp)-H nitration.

Nitroaromatic compounds play an important role in organic synthesis, medicinal chemistry and chemical industry. Among the recently reported synthetic methods to access nitroarenes, transition metal-catalyzed C(sp2)-H activation/nitration has become one of the most attractive protocols, showing high regioselectivity, excellent functional group tolerance and step-economy. In this review, we discuss advances in direct C(sp2)-H nitration for the synthesis of nitroaromatic compounds and mechanistic insights over the past decade.

Ligand-promoted ruthenium-catalyzed meta C-H chlorination of arenes using N-chloro-2,10-camphorsultam.

A practical meta C-H chlorination protocol is established via a Ru(0)-catalyzed ortho-metalation strategy. The use of N-chloro-2,10-camphorsultam as a new chlorinating agent is crucial for the success of the current reaction and an N-heterocyclic carbene (NHC) ligand could significantly enhance the reactivity of the catalytic transformation. The mechanistic studies reveal that an unusual ortho C-H ruthenation relay process with ortho chlorination of the C-Ru bond is probably involved.

Highly Stereoselective Assembly of Polycyclic Molecules from 1,6-Enynes Triggered by Rhodium(III)-Catalyzed C-H Activation.

An Rh(III)-catalyzed C-H activation of pyrazolones with 1,6-enynes was investigated. The regioselectivity of the C-H activation/alkyne insertion is readily solved by using symmetric enyne coupling partners, and a C-H activation-triggered cascade reaction is realized, which involves alkyne insertion, tautomerization, and double cyclization to offer a class of structurally complex polycyclic architectures. This cascade reaction tolerates a broad substrate scope in high regioselectivity and stereospecificity and furnishes three new chemical bonds and four chiral centers in a single operation.

Total synthesis of (±)-tanshinol B, tanshinone I, and (±)-tanshindiol B and C.

A concise and efficient approach was established for the divergent total synthesis of (±)-tanshindiol B and C and tanshinone I from a ubiquitous ene intermediate in 1-2 steps. This critical intermediate was derived from (±)-tanshinol B, which was synthesized in 50% overall yield over 3 steps using an ultrasound-promoted cycloaddition as a key step. Compared to a previously reported strategy, our approach is more step-economic, thus greatly improving the synthetic efficiency.

PMes-Promoted Ruthenium-Catalyzed Meta C-H Nitration of 6-Arylpurines.

To address the challenge of meta nitration of 6-arylpurine substrates, a versatile ruthenium catalyzed meta C-H nitration is developed. The use of a sterically hindered phosphine ligand is crucial for the catalytic efficiency, and a broad class of 6-arylpurines and nucleosides were found suitable for this process providing corresponding exclusively meta nitrated products in good yields.

Difluorination of Furonaphthoquinones.

An unprecedented difluorination reaction was developed based on the furonaphthoquinone skeleton of natural products tanshinones and their analogues. By using Selectfluor as the fluorinating source and HO as the hydroxyl source, a wide range of unique polycyclic α,α-difluoro β,β-dihydroxyl para-quinone products were achieved with yields up to 90%. The mechanistic studies revealed that the reaction might undergo tandem multiple electrophilic and nucleophilic substitutions, as well as cleavages of C-O and C-C bonds.

Rhodium-catalyzed redox-neutral coupling of phenidones with alkynes.

A switchable synthesis of N-substituted indole derivatives from phenidones via rhodium-catalyzed redox-neutral C-H activation has been achieved. In this protocol, we firstly disclosed that the reactivity of Rh(iii) catalysis could be enhanced through employing palladium acetate as an additive. Some representative features include external oxidant-free, applicable to terminal alkynes, short reaction time and operational simplicity.

Monomeric Octahedral Ruthenium(II) Complex Enabled meta-C-H Nitration of Arenes with Removable Auxiliaries.

A removable oxime-assisted meta-C-H nitration of arenes is reported. Mechanistic investigations and DFT calculations reveal a new monomeric octahedral ruthenium(II) complex is responsible for the meta-selective nitration. Dioxygen as a cooxidant is crucial for achieving high conversion and good yields.

Cobalt-Catalyzed Carbonylation of C(sp)-H Bonds with Azodicarboxylate as the Carbonyl Source.

A novel and efficient approach for the C(sp)-H bond carbonylation of benzamides has been developed using stable and inexpensive Co(OAc)·4HO as the catalyst and the commercially available and easily handling azodicarboxylates as the nontoxic carbonyl source. A broad range of substrates bearing diverse functional groups were tolerated. This is the first example where cobalt-catalyzed C(sp)-H bond carbonylation occurs with azodicarboxylate as the carbonyl source.

Meta-Selective CAr-H Nitration of Arenes through a Ru3(CO)12-Catalyzed Ortho-Metalation Strategy.

The first example of transition metal-catalyzed meta-selective CAr-H nitration of arenes is described. With the use of Ru3(CO)12 as the catalyst and Cu(NO3)2·3H2O as the nitro source, a wide spectrum of arenes bearing diversified N-heterocycles or oximido as the directing groups were nitrated with meta-selectivity exclusively. Mechanism studies have demonstrated the formation of a new 18e-octahedral ruthenium species as a key ortho-CAr-H metalated intermediate, which may be responsible for the subsequent meta-selective electrophilic aromatic substitution (SEAr).

Ru(II)-Catalyzed Direct C(sp(2))-H Activation/Selenylation of Arenes with Selenyl Chlorides.

A new ruthenium catalytic system was developed for the construction of a C(sp(2))-Se bond with the assistance of directing groups. This protocol features mild reaction conditions, wider substrate scope, and convenient late-stage selenylation of bioactive molecules.

Pd-catalyzed α-selective C(sp3)-H acetoxylation of amides through an unusual cyclopalladation mechanism.

We report the first example of Pd-catalyzed site-selective α-C(sp(3))-H oxidation/acetoxylation of amides through an unusual [4,6]-bicyclic metallacycle intermediate with 1-aminoanthraquinone as a new bidentate directing group. In addition to the distinct mechanism and high efficiency, the reaction is highly appealing due to the ample commercial source, low-cost, as well as easy removal and recycling of the auxiliary group.

Rh(III)-catalyzed redox-neutral C-H activation of pyrazolones: an economical approach for the synthesis of N-substituted indoles.

A new strategy is reported for the economical synthesis of indoles bearing an N-(3-aminobut-2-enoyl) substituent through Rh(III)-catalyzed redox-neutral C-H activation of pyrazolones and alkynes. This approach utilizes cheap substrates and mild reaction conditions to access a unique class of indoles via a N-N bond oxidative cleavage without loss of the N-terminus, therefore meeting all the atom/step/redox economy principles.