δ-Selective Functionalization of Alkanols Enabled by Visible-Light-Induced Ligand-to-Metal Charge Transfer

Cerium photocatalyzed radical smiles rearrangement of 2-aryloxybenzoic acids

We report herein a cerium photocatalyzed aryl migration from an aryl ether to a carboxylic acid group through radical-Smiles rearrangement. This operationally simple protocol utilizes inexpensive CeCl₃ as a photocatalyst and converted a variety of 2-aryloxybenzoic acids into aryl-2-hydroxybenzoates in good yields.

Visible light-driven CeCl₃-mediated aryl migration from an aryl ether to a carboxylic acid group through radical-Smiles rearrangement is reported.

Visible light photoredox-catalysed remote C–H functionalisation enabled by 1,5-hydrogen atom transfer (1,5-HAT)

The generation of heteroatom-centred radicals followed by intramolecular 1,5-HAT and functionalisation of the translocated carbon-centred radical is an efficient way to functionalize chemo- and regio-selectively the remote unactivated C(sp³)–H bond.

δ-Regioselective heteroarylation of free alcohols through 1,5-hydrogen-atom transfer

Silver-catalyzed δ-regioselective heteroarylation of C(sp³)–H bonds (1°, 2°, and 3°) in alcohols with divergent N-heteroarenes.

Construction of polysubstituted spiro[2.3] or [3.3] cyclic frameworks fused with a tosylated pyrrolidine promoted by visible-light-induced photosensitization

A novel visible-light-induced intramolecular [2 + 2] cycloaddition of methylenecyclopropanes (MCPs) for the rapid construction of polysubstituted spiro[2.3] or [3.3] cyclic frameworks fused with a tosylated pyrrolidine.

Ligand-accelerated site-selective Csp2–H and Csp3–H alkynylations of alcohols via Pd(ii) catalysis

A ligand accelerated site-selective C–H alkynylation, including secondary and tertiary Csp3–H alkynylation of weakly coordinated yet synthetically promising alcohols, via putative 6, 7 and 8-membered palladacycle intermediates, was developed. 

Metallaphotoredox catalysis with organic dyes

Here…comes the fun…Combination of metals and organic photocatalysts allows the practical invention of new methodologies!

Visible light photocatalysis in the late-stage functionalization of pharmaceutically relevant compounds

The late stage functionalization (LSF) of complex biorelevant compounds is a powerful tool to speed up the identification of structure-activity relationships (SARs) and to optimize ADME profiles. To this end, visible-light photocatalysis offers unique opportunities to achieve smooth and clean functionalization of drugs by unlocking site-specific reactivities under generally mild reaction conditions. This review offers a critical assessment of current literature, pointing out the recent developments in the field while emphasizing the expected future progress and potential applications. Along with paragraphs discussing the visible-light photocatalytic synthetic protocols so far available for LSF of drugs and drug candidates, useful and readily accessible synoptic tables of such transformations, divided by functional groups, will be provided, thus enabling a useful, fast, and easy reference to them.

Catalytic generation of alkoxy radicals from unfunctionalized alcohols

Alkoxy radicals have long been recognized as powerful synthetic intermediates with well-established reactivity patterns. Due to the high bond dissociation free energy of aliphatic alcohol O-H bonds, these radicals are difficult to access through direct homolysis, and conventional methods have instead relied on activation of O-functionalized precursors. Over the past decade, however, numerous catalytic methods for the direct generation of alkoxy radicals from simple alcohol starting materials have emerged and created opportunities for the development of new transformations. This minireview discusses recent advances in catalytic alkoxy radical generation, with particular emphasis on progress toward the direct activation of unfunctionalized alcohols enabled by transition metal and photoredox catalysis.

Silver-Catalyzed para-Selective Amination and Aminative Dearomatization of Phenols with Azodicarboxylates in Water

An efficient silver-catalyzed para-selective amination and aminative dearomatization of phenols with azodicarboxylates was developed. It afforded the para-aminophenols or amino cyclohexadieneones from free phenols depending on whether it has a para-substituent. The reaction proceeded smoothly in water under simple and mild conditions, giving the highly selective products in good yields within a short reaction time. It also showed broad substrate scope and good functional group compatibility.

Alcohol Etherification via Alkoxy Radicals Generated by Visible-Light Photoredox Catalysis

A mechanistically divergent method is described that, employing a commercially available hypervalent iodine(III) reagent, generates alkoxy radicals from 1°, 2°, and 3° alcohols and allows their use in the functionalization of C(sp³)–H and C(sp²)–H bonds. This visible-light photoredox catalysis produces alkyl ethers via 1,5/6-hydrogen atom transfer or aryl ethers via 1,5-addition. This mild methodology provides a practical strategy for the synthesis of acetals, orthoesters, tetrahydrofurans, and chromanes.

DMSO-Enabled Selective Radical O-H Activation of 1,3(4)-Diols

Control of selectivity is one of the central topics in organic chemistry. Although unprecedented alkoxyl-radical-induced transformations have drawn a lot of attention, compared to selective C-H activation, selective radical O-H activation remains less explored. Herein, we report a novel selective radical O-H activation strategy of diols by combining spatial effects with proton-coupled electron transfer (PCET). It was found that DMSO is an essential reagent that enables the regioselective transformation of diols. Mechanistic studies indicated the existence of the alkoxyl radical and the selective interaction between DMSO and hydroxyl groups. Moreover, the distal C-C cleavage was realized by this selective alkoxyl-radical-initiation protocol.

Luminescent tungsten(vi) complexes as photocatalysts for light-driven C-C and C-B bond formation reactions

The realization of photocatalysis for practical synthetic application hinges on the development of inexpensive photocatalysts which can be prepared on a large scale. Herein an air-stable, visible-light-absorbing photoluminescent tungsten(vi) complex which can be conveniently prepared at the gram-scale is described. This complex could catalyse photochemical organic transformation reactions including borylation of aryl halides, such as aryl chloride, reductive coupling of benzyl bromides for C-C bond formation, reductive coupling of phenacyl bromides, and decarboxylative coupling of redox-active esters of alkyl carboxylic acid with high product yields and broad functional group tolerance.

Ligand-Enabled β-Methylene C(sp3 )-H Arylation of Masked Aliphatic Alcohols

Despite recent advances, reactivity and site-selectivity remain significant obstacles for the practical application of C(sp3 )-H bond functionalization methods. Here, we describe a system that combines a salicylic-aldehyde-derived L,X-type directing group with an electron-deficient 2-pyridone ligand to enable the β-methylene C(sp3 )-H arylation of aliphatic alcohols, which has not been possible previously. Notably, this protocol is compatible with heterocycles embedded in both alcohol substrates and aryl coupling partners. A site- and stereo-specific annulation of dihydrocholesterol and the synthesis of a key intermediate of englitazone illustrate the practicality of this method.

PPh3/NaI driven photocatalytic decarboxylative radical cascade alkylarylation reaction of 2-isocyanobiaryls

The first triphenylphosphine/sodium iodide driven photocatalytic decarboxylative cascade cyclization of 2-isocyano-biaryls with alkyl N-hydroxyphthalimide (NHP) esters was developed. This operationally simple protocol results in multiple carbon-carbon bond formation under transition metal free conditions, affording a novel and environmentally benign entry to producing 6-alkyl phenanthridines with moderate to good yields.

Sequential C-O decarboxylative vinylation/C-H arylation of cyclic oxalates via a nickel-catalyzed multicomponent radical cascade

A selective, sequential C-O decarboxylative vinylation/C-H arylation of cyclic alcohol derivatives enabled by visible-light photoredox/nickel dual catalysis is described. This protocol utilizes a multicomponent radical cascade process, i.e. decarboxylative vinylation/1,5-HAT/aryl cross-coupling, to achieve efficient, site-selective dual-functionalization of saturated cyclic hydrocarbons in one single operation. This synergistic protocol provides straightforward access to sp3-enriched scaffolds and an alternative retrosynthetic disconnection to diversely functionalized saturated ring systems from the simple starting materials.

Stimuli-Responsive Supramolecular Coaggregation and Disaggregation of Host-Guest Conjugates Having a Disulfide Linkage

Water-soluble cationic and anionic cyclophanes (1a and 2a, respectively) having a dabsyl group with a cleavable disulfide linkage were synthesized as a host-guest conjugate covalently bound with both host and guest components. Self-inclusion phenomena but not self-aggregation behaviors were observed for each cyclophane in aqueous media. Each cyclophane includes its own dabsyl moiety (guest component) in its macrocyclic cavity (host component) through hydrophobic interaction. When 1 equiv. of cationic 1a was added to an aqueous solution of anionic 2a, however, supramolecular coaggregates formed spontaneously through host-guest complexation. As regard the supramolecular coaggregates, the existence of larger particles was confirmed by DLS measurements and TEM observation. The hydrophobic interaction between the dabsyl moiety and macrocyclic cavity and electrostatic interactions between 1a and 2a play important roles in the supramolecular coaggregate formation. Each cyclophane having a cleavable disulfide linkage was easily transformed to the corresponding thiols by reducing reagents such as DTT, which was confirmed by MALDI-TOF MS. Disaggregation of the supramolecular coaggregates composed of 1a and 2a was successfully performed upon addition of DTT, with release of the thiol derivative of dabsyl. Such disaggregation of the coaggregates was also conducted by other external stimuli such as salts and competitive guests.

Cerium-Catalyzed C-H Functionalizations of Alkanes Utilizing Alcohols as Hydrogen Atom Transfer Agents

Modern photoredox catalysis has traditionally relied upon metal-to-ligand charge-transfer (MLCT) excitation of metal polypyridyl complexes for the utilization of light energy for the activation of organic substrates. Here, we demonstrate the catalytic application of ligand-to-metal charge-transfer (LMCT) excitation of cerium alkoxide complexes for the facile activation of alkanes utilizing abundant and inexpensive cerium trichloride as the catalyst. As demonstrated by cerium-catalyzed C-H amination and the alkylation of hydrocarbons, this reaction manifold has enabled the facile use of abundant alcohols as practical and selective hydrogen atom transfer (HAT) agents via the direct access of energetically challenging alkoxy radicals. Furthermore, the LMCT excitation event has been investigated through a series of spectroscopic experiments, revealing a rapid bond homolysis process and an effective production of alkoxy radicals, collectively ruling out the LMCT/homolysis event as the rate-determining step of this C-H functionalization.

Cerium(IV) Carboxylate Photocatalyst for Catalytic Radical Formation from Carboxylic Acids: Decarboxylative Oxygenation of Aliphatic Carboxylic Acids and Lactonization of Aromatic Carboxylic Acids

We found that in situ generated cerium(IV) carboxylate generated by mixing the precursor Ce(O^tBu)₄ with the corresponding carboxylic acids served as efficient photocatalysts for the direct formation of carboxyl radicals from carboxylic acids under blue light-emitting diodes (blue LEDs) irradiation and air, resulting in catalytic decarboxylative oxygenation of aliphatic carboxylic acids to give C-O bond-forming products such as aldehydes and ketones. Control experiments revealed that hexanuclear Ce(IV) carboxylate clusters initially formed in the reaction mixture and the ligand-to-metal charge transfer nature of the Ce(IV) carboxylate clusters was responsible for the high catalytic performance to transform the carboxylate ligands to the carboxyl radical. In addition, the Ce(IV) carboxylate cluster catalyzed direct lactonization of 2-isopropylbenzoic acid to produce the corresponding peroxy lactone and γ-lactone via intramolecular 1,5-hydrogen atom transfer (1,5-HAT).

Pushing the boundaries of C–H bond functionalization chemistry using flow technology

C–H functionalization chemistry is one of the most vibrant research areas within synthetic organic chemistry. While most researchers focus on the development of small-scale batch-type transformations, more recently such transformations have been carried out in flow reactors to explore new chemical space, to boost reactivity or to enable scalability of this important reaction class. Herein, an up-to-date overview of C–H bond functionalization reactions carried out in continuous-flow microreactors is presented. A comprehensive overview of reactions which establish the formal conversion of a C–H bond into carbon–carbon or carbon–heteroatom bonds is provided; this includes metal-assisted C–H bond cleavages, hydrogen atom transfer reactions and C–H bond functionalizations which involve an SE-type process to aromatic or olefinic systems. Particular focus is devoted to showcase the advantages of flow processing to enhance C–H bond functionalization chemistry. Consequently, it is our hope that this review will serve as a guide to inspire researchers to push the boundaries of C–H functionalization chemistry using flow technology.

δ-C-H Mono- and Dihalogenation of Alcohols

Alkoxy radicals have long been known to enable remote C-H functionalization via 1,5-hydrogen atom abstraction. However, methods for their generation traditionally have relied upon highly oxidizing metals, ultraviolet radiation, or preformed peroxide intermediates, which has prevented the development of many desirable transformations. Herein we report a new bench-stable precursor that decomposes to free alkoxy radicals via a previously unreported single-electron oxidation pathway. This new precursor enables the fluorination and chlorination of remote C-H bonds under exceptionally mild conditions with exceedingly high monoselectivity. Iterative use of this precursor enables the introduction of a second halogen atom, granting access to remote dihalide motifs, including CF₂ and CFCl.

Radical Dehydroxymethylative Fluorination of Carbohydrates and Divergent Transformations of the Resulting Reverse Glycosyl Fluorides

A mild and convenient method for the synthesis of reverse glycosyl fluorides (RGFs) has been developed that is based on the silver-promoted radical dehydroxymethylative fluorination of carbohydrates. A salient feature of the reaction is that furanoid and pyranoid carbohydrates furnish structurally diverse RGFs bearing a wide variety of functional groups in good to excellent yields. Intramolecular hydrogen atom transfer experiments revealed that the reaction involves an underexploited radical fluorination that proceeds via β-fragmentation of sugar-derived primary alkoxyl radicals. Structurally divergent RGFs were obtained by catalytic C-F bond activation, and our method thus offers a concise and efficient strategy for the synthesis of reverse glycosides by late-stage diversification of RGFs. The potential of this method is showcased by the preparation and diversification of sotagliflozin, leading to the discovery of a promising SGLT2 inhibitor candidate.

Cerium photocatalyzed dehydrogenative lactonization of 2-arylbenzoic acids

The first cerium photocatalyzed dehydrogenative lactonization of 2-arylbenzoic acids has been developed. This operationally simple protocol allows rapid access to synthetically useful coumarins on gram scale by employing CeCl₃ as a photocatalyst and O₂ as a terminal oxidant. Overall, this delivers an economical and environmentally amiable entry to diversely substituted coumarins, important structural motifs in bioactive molecules.

Transition-metal-free radical relay cyclization of vinyl azides with 1,4-dihydropyridines involving a 1,5-hydrogen-atom transfer: access to α-tetralone scaffolds

The remote C(sp³)–H functionalization enabled by a radical-mediated 1,5-hydrogen-atom transfer (HAT) process using vinyl azides and 1,4-dihydropyridines as precursors has been described.

Remote C(sp3)–H vinylation via radical-mediated consecutive fission of C–H and C–C bonds

Described herein is a radical-mediated vinylation of the remote C(sp³)–H bonds of propargylic alcohols.

Cross coupling of sulfonyl radicals with silver-based carbenes: a simple approach to β-carbonyl arylsulfones

A radical–carbene coupling reaction of sulfonyl radicals and silver-based carbenes has been well established, which provides an efficient approach to various β-carbonyl arylsulfones.

Ring-opening iodination and bromination of unstrained cycloalkanols through β-scission of alkoxy radicals

Ring-opening iodination or bromination of unstrained cycloalkanols with NaI or NaBr and PhI(OAc)₂ under visible light irradiation is developed.

Remote azidation of C(sp3)–H bonds to synthesize δ-azido sulfonamidesviairon-catalyzed radical relay

A selective remote C–H azidation was developed with an iron catalyst playing dual roles as a radical initiator and terminator.

Isolation of heterometallic cerium(iii) complexes with a multidentate nitrogen–phosphorus ligand

Heterobimetallic clusters containing a rare-earth metal and transition metals were constructed by a multidentate nitrogen–phosphorus ligand.

Benzylic C–H heteroarylation of N-(benzyloxy)phthalimides with cyanopyridines enabled by photoredox 1,2-hydrogen atom transfer

Visible light initiated α-C(sp³)–H hetroarylation of N-(benzyloxy)phthalimides with cyanopyridines via 1,2-hydrogen atom transfer is depicted.