Visible light photoredox-catalyzed deoxygenation of alcohols

Unified Approach to Deamination and Deoxygenation Through Isonitrile Hydrodecyanation: A Combined Experimental and Computational Investigation

Herein, we describe a general hydrodefunctionalization protocol of alcohols and amines through a common isonitrile intermediate. To cleave the relatively inert C-NC bond, we leveraged dual hydrogen atom transfer (HAT) and photoredox catalysis to generate a nucleophilic boryl radical, which readily forms an imidoyl radical intermediate from the isonitrile. Rapid β-scission then accomplishes defunctionalization. This method has been applied to the hydrodefunctionalization of both amine and alcohol-containing pharmaceuticals, natural products, and biomolecules. We extended this approach to the reduction of carbonyls and olefins to their saturated counterparts, as well as the hydrodecyanation of alkyl nitriles. Both experimental and computational studies demonstrate a facile β-scission of the imidoyl radical, and reconcile differences in reactivity between nitriles and isonitriles within our protocol.

Engaging Alkenes in Metallaphotoredox: A Triple Catalytic, Radical Sorting Approach to Olefin-Alcohol Cross-Coupling

Metallaphotoredox cross-coupling is a well-established strategy for generating clinically privileged aliphatic scaffolds via single-electron reactivity. Correspondingly, expanding metallaphotoredox to encompass new C(sp3)-coupling partners could provide entry to a novel, medicinally relevant chemical space. In particular, alkenes are abundant, bench-stable, and capable of versatile C(sp3)-radical reactivity via metal-hydride hydrogen atom transfer (MHAT), although metallaphotoredox methodologies invoking this strategy remain underdeveloped. Importantly, merging MHAT activation with metallaphotoredox could enable the cross-coupling of olefins with feedstock partners such as alcohols, which undergo facile open-shell activation via photocatalysis. Herein, we report the first C(sp3)-C(sp3) coupling of MHAT-activated alkenes with alcohols by performing deoxygenative hydroalkylation via triple cocatalysis. Through synergistic Ir photoredox, Mn MHAT, and Ni radical sorting pathways, this branch-selective protocol pairs diverse olefins and methanol or primary alcohols with remarkable functional group tolerance to enable the rapid construction of complex aliphatic frameworks.

Photo- and electro-chemical strategies for the activations of strong chemical bonds

The employment of light and/or electricity - alternatively to conventional thermal energy - unlocks new reactivity paradigms as tools for chemical substrate activations. This leads to the development of new synthetic reactions and a vast expansion of chemical spaces. This review summarizes recent developments in photo- and/or electrochemical activation strategies for the functionalization of strong bonds - particularly carbon-heteroatom (C-X) bonds - via: (1) direct photoexcitation by high energy UV light; (2) activation via photoredox catalysis under irradiation with relatively lower energy UVA or blue light; (3) electrochemical reduction; (4) combination of photocatalysis and electrochemistry. Based on the types of the targeted C-X bonds, various transformations ranging from hydrodefunctionalization to cross-coupling are covered with detailed discussions of their reaction mechanisms.

Practical and General Alcohol Deoxygenation Protocol

Herein, we describe a practical protocol for the removal of alcohol functional groups through reductive cleavage of their benzoate ester analogs. This transformation requires a strong single electron transfer (SET) reductant and a means to accelerate slow fragmentation following substrate reduction. To accomplish this, we developed a photocatalytic system that generates a potent reductant from formate salts alongside Brønsted or Lewis acids that promote fragmentation of the reduced intermediate. This deoxygenation procedure is effective across structurally and electronically diverse alcohols and enables a variety of difficult net transformations. This protocol requires no precautions to exclude air or moisture and remains efficient on multigram scale. Finally, the system can be adapted to a one-pot benzoylation-deoxygenation sequence to enable direct alcohol deletion. Mechanistic studies validate that the role of acidic additives is to promote the key C(sp3 )-O bond fragmentation step.

Visible Light-Induced Deoxygenation and Allylation/Vinylation of Pyridyl Ethers

The generation of alkyl radicals by deoxygenation of unactivated ethers under visible light catalysis is a hitherto unmet challenge. Herein, we report a visible light-induced deoxygenation of pyridyl ethers via formation of their pyridinium salts. The generated benzylic radicals further react with allyl/alkenyl sulfones to provide a series of coupling products in good to moderate yields. This process is proposed to undergo a reductive quenching cycle, which was elucidated by chemical, optical, and electrical experiments.

Photochemical Ring Contraction of 5,5-Dialkylcyclopent-2-enones and in situ Trapping by Primary Amines

If substituted in the 5,5-position, cyclopent-2-enones undergo a smooth photochemical rearrangement to ketenes. A concomitant cyclopropane formation occurs due to a 1,3-shift of the C5 carbon atom from the carbonyl carbon atom (C1) to carbon atom C3. In this study, the cyclopropyl-substituted ketene intermediates were trapped in situ by primary amines providing an efficient entry into 2,2-disubstituted cyclopropaneacetic amides (24 examples, 49-95% yield). A remarkable feature of the reaction is the fact that the photochemical rearrangement can occur from either the first excited singlet (S₁) or the respective triplet state (T₁). In line with experimental results (triplet quenching, sensitization), XMS-CASPT2 calculations support the existence of efficient reaction pathways to the intermediate ketene both on the singlet and on the triplet hypersurface.

Dehydroxylative Alkylation of α-Hydroxy Carboxylic Acid Derivatives via a Spin-Center Shift

A strategically distinct dehydroxylative alkylation reaction of α-hydroxy carboxylic acid derivatives with alkenes is developed. The reaction starts with the attack of a 4-dimethylaminopyridine (DMAP)-boryl radical to the carbonyl oxygen atom, followed by a spin-center shift (SCS) to trigger the C-O bond scission. The resulting α-carbonyl radicals couple with a wide range of alkenes to furnish various alkylated products. This strategy allows for the efficient conversion of a wide array of α-hydroxy amides and esters derived from several biomass molecules and natural products to value-added compounds. Experimental and computational studies verified the reaction mechanism.

Photoinduced C–O bond cleavage for copper-catalyzed allenyl radical cyanation

The synthesis of allenyl nitriles was realized through the combination of photoinduced C–O bond cleavage and copper catalysis. Both 1,3-disubstituted and trisubstituted allenyl nitriles were accessed with good yields and high regioselectivity.

Metal-free deoxygenative coupling of alcohol-derived benzoates and pyridines for small molecules and DNA-encoded libraries synthesis

A visible-light-mediated metal-free method for the deoxygenative coupling of alcohol-derived benzoates and pyridines. Given the mild and water-compatible conditions, small molecules and DNA headpieces are functionalized with a wide range of alcohols.

Electro-mediated PhotoRedox Catalysis for Selective C(sp3 )-O Cleavages of Phosphinated Alcohols to Carbanions

We report a novel example of electro-mediated photoredox catalysis (e-PRC) in the reductive cleavage of C(sp3 )-O bonds of phosphinated alcohols to alkyl carbanions. As well as deoxygenations, olefinations are reported which are E-selective and can be made Z-selective in a tandem reduction/photosensitization process where both steps are photoelectrochemically promoted. Spectroscopy, computation, and catalyst structural variations reveal that our new naphthalene monoimide-type catalyst allows for an intimate dispersive precomplexation of its radical anion form with the phosphinate substrate, facilitating a reactivity-determining C(sp3 )-O cleavage. Surprisingly and in contrast to previously reported photoexcited radical anion chemistries, our conditions tolerate aryl chlorides/bromides and do not give rise to Birch-type reductions.

Selective deoxygenative alkylation of alcohols via photocatalytic domino radical fragmentations

The delivery of alkyl radicals through photocatalytic deoxygenation of primary alcohols under mild conditions is a so far unmet challenge. In this report, we present a one-pot strategy for deoxygenative Giese reaction of alcohols with electron-deficient alkenes, by using xanthate salts as alcohol-activating groups for radical generation under visible-light photoredox conditions in the presence of triphenylphosphine. The convenient generation of xanthate salts and high reactivity of sequential C–S/C–O bond homolytic cleavage enable efficient deoxygenation of primary, secondary and tertiary alcohols with diverse functionality and structure to generate the corresponding alkyl radicals, including methyl radical. Moreover, chemoselective radical monodeoxygenation of diols is achieved via selective formation of xanthate salts.

The generation of alkyl radicals through deoxygenation of abundant alcohols via photoredox catalysis is of interest. In this study, the authors report a one-pot strategy for visible-light-promoted photoredox coupling of alcohols with electron-deficient alkenes, assisted by carbon disulfide and triphenylphosphine.

Defluorodearomatization: A Photocatalytic Birch-Like Reduction That Enables C-C Bond Formation and Provides Access to Unnatural Cannabinoids

Within the framework of discovery chemistry, polyfluorination remains a synthetic challenge despite its ability to provide useful characteristics, such as a reduction in the number of hydrogen bond donors and metabolic stability. Coupling a reversal of this methodology with photocatalysis has been demonstrated to allow the rapid synthesis of previously difficult or impossible targets by starting with fluorines everywhere and selectively removing or functionalizing them. Herein, we demonstrate a novel method to synthesize 1,4-cyclohexadienes through a dearomative photocatalytic C-C coupling reaction. This allows for access to materials that are orthogonal to the selectivity of the Birch reaction and are more functional-group-tolerant. The reaction also allows the efficient synthesis of polyfluorinated cannabinoids. While the yields are modest, the access to the new chemical space provided by the reaction is unprecedented by any means. The trifluorinated analog of THC, 1-deoxy-1,2,4-trifluoro-THC, is synthesized, demonstrating the importance of discovery chemistry and the ability to explore otherwise unknown structure-activity relationships.

Substituted Hantzsch esters as radical reservoirs with the insertion of sulfur dioxide under photoredox catalysis

A three-component reaction between 4-substituted Hantzsch esters, DABCO·(SO₂)₂, and vinyl azides in the presence of photoredox catalysts under visible light irradiation is developed. A range of (Z)-2-(alkylsulfonyl)-1-arylethen-1-amines is obtained in moderate to good yields with good regioselectivity and stereoselectivity.

Redox and photocatalytic properties of a NiII complex with a macrocyclic biquinazoline (Mabiq) ligand

We present a late, first row transition metal photosensitizer that promotes photocatalytic C-C bond formation. The title compound, [Ni(Mabiq)]OTf, as well as its one-electron reduced form, Ni(Mabiq), were synthesized and molecular structures of both were obtained. The electronic structure of the reduced complex additionally was characterized by spectroscopic and DFT computational methods. Notably, [NiII(Mabiq)]OTf is photoactive: reduction of the compound was achieved photochemically upon irradiation at λ = 457 nm and reductive quenching by NEt3. The performance of [Ni(Mabiq)]OTf as a photoredox catalyst was examined in the cyclization of a bromoalkyl-substituted indole. In this reaction, the first-row transition metal compound is comparable if not superior to [Ru(bpy)3]2+ in terms of efficiency (turnover number) and chemoselectivity. Studies using a series of sacrificial donor amines indicate that the excited state redox potential of [Ni(Mabiq)]+* is ≥1.25 V vs. SCE. This value is similar to the excited state potential of commonly employed noble metal based photocatalysts. The Ni-Mabiq compound thus provides a rare example of an earth-abundant photoredox catalyst.

Evidence for Triplet Sensitization in the Visible-Light-Induced [2+2] Photocycloaddition of Eniminium Ions

Εniminium ions were prepared from the corresponding α,β-unsaturated carbonyl compounds (enones and enals), and were found to be promoted to their respective triplet states by energy transfer. The photoexcited intermediates underwent intra- or intermolecular [2+2] photocycloaddition in good yields (50-78 %) upon irradiation at λ=433 nm or λ=457 nm. Iridium or ruthenium complexes with a sufficiently high triplet energy were identified as efficient catalysts (2.5 mol % catalyst loading) for the reaction. The intermolecular [2+2] photocycloaddition of an eniminium ion derived from a chiral secondary amine proceeded with high enantioselectivity (88 % ee).

Synthesis of Chiral Tetrahydrofurans and Pyrrolidines by Visible-Light-Mediated Deoxygenation

The synthesis of chiral tetrahydrofurans and pyrrolidines starting from 1,2-diols or β-amino alcohols, respectively, by visible-light-mediated deoxygenation is described. Easily accessible monoallylated/propargylated substrates were activated either as inexpensive ethyl oxalates or as recyclable 3,5-bis(trifluoromethyl)benzoates to generate alkyl radicals suitable for 5-exo-trig/5-exo-dig cyclizations under visible-light irradiation.

Brønsted Acid Catalysis in Visible-Light-Induced [2+2] Photocycloaddition Reactions of Enone Dithianes

1,3-Dithiane-protected enones (enone dithianes) were found to undergo an intramolecular [2+2] photocycloaddition under visible-light irradiation (λ=405 nm) in the presence of a Brønsted acid (7.5-10 mol %). Key to the success of the reaction is presumably the formation of colored thionium ions, which are intermediates of the catalytic cycle. Cyclobutanes were thus obtained in very good yields (78-90 %). It is also shown that the dithiane moiety can be reductively or oxidatively removed without affecting the photochemically constructed ring skeleton.

Selective photocatalysis of lignin-inspired chemicals by integrating hybrid nanocatalysis in microfluidic reactors

In an ideal symbiosis of nanophotocatalysis and microflow chemistry, this tutorial review delineates a micro-highway decorated with semiconductor-based photocatalytic thin layers for the selective transformations of lignin-based chemicals to high-value organic products.

Photoredox Catalysis for the Generation of Carbon Centered Radicals

Radical chemistry has witnessed over the last decades important advances that have positioned it as a methodology of choice in synthetic chemistry. A number of great attributes such as specific reactivities, the knowledge of the kinetics of most elementary processes, the functional group tolerance, and the possibility to operate cascade sequences are clearly responsible for this craze. Nevertheless, at the end of the last century, radical chemistry appeared plagued by several hurdles to overcome such as the use of environmentally problematic mediators or the impossibility of scale up. While the concept of photocatalysis was firmly established in the coordination chemistry community, its diffusion in organic synthetic chemistry remained sporadic for decades until the end of the 2000s with the breakthrough merging of organocatalysis and photocatalysis by the MacMillan group and contemporary reports by the groups of Yoon and Stephenson. Since then, photoredox catalysis has enjoyed particularly active and intense developments. It is now the topic of a still increasing number of publications featuring various applications from asymmetric synthesis, total synthesis of natural products, and polymerization to process (flow) chemistry. In this Account, we survey our own efforts in this domain, focusing on the elaboration of new photocatalytic pathways that could lead to the efficient generation of C-centered functionalized alkyl and aryl radicals. Both reductive and oxidative manifolds are accessible through photoredox catalysis, which has guided us along these lines in our projects. Thus, we studied the photocatalytic reduction of onium salts such as sulfoniums and iodoniums for the production of the elusive aryl radical intermediates. Progressing to more relevant chemistry for synthesis, we examined the cleavage of C-O and the C-Br bonds for the generation of alkyl C-centered radicals. Activated epoxides could serve as valuable substrates of a photocatalyzed variant of the Nugent-RajanBabu-Gansäuer homolytic cleavage of epoxides. Using imidazole based carbamates, we could also devise the first photocatalyzed Barton-McCombie deoxygenation reaction. Finally, bromophenylacetate can be reduced using the [Au2(μ-dppm)2]Cl2 photocatalyst under UVA or visible-light. This was used for the initiation of the controlled atom transfer radical polymerization of methacrylates and acrylates in solution or laminate. Our next endeavors concerned the photocatalyzed oxidation of stabilized carbanions such as enolates of 1,3-dicarbonyl substrates, trifluoroborates, and more extensively bis-catecholato silicates. Because of their low oxidation potentials, the later have proved to be exquisite sources of radical entities, which can be engaged in diverse intermolecular reactions such as vinylation, alkynylation, and conjugate additions. The bis-catecholato silicates were also shown to behave as excellent partners of dual photoredox-nickel catalysis leading in an expeditious manner to libraries of cross coupling products.

Photoredox-Catalyzed Three-Component Tandem Process: An Assembly of Complex Trifluoromethylated Phthalans and Isoindolines

A novel photoredox-mediated tandem three-component process afforded a wide variety of CF3-containing phthalans and isoindolines in respectable yields and with moderate to excellent diastereoselectivity.

Hantzsch Ester as a Photosensitizer for the Visible-Light-Induced Debromination of Vicinal Dibromo Compounds

The debromination of vicinal dibromo compounds to generate alkenes usually requires harsh reaction conditions and the addition of catalysts. Just recently the visible-light-induced debromination of vicinal dibromo compounds emerged as a possible alternative to commonly used methods, but the substrate scope of this reaction is limited and a photocatalyst is necessary for the successful conversion of the starting compounds. A catalyst-free visible-light-induced debromination of vicinal dibromo compounds with a base-activated Hantzsch ester as photosensitizer is reported. The method has a wide substrate scope and a broad functional-group compatibility.

Enantioselective Visible-Light-Induced Radical-Addition Reactions to 3-Alkylidene Indolin-2-ones

The title compounds underwent a facile and high-yielding addition reaction (19 examples, 66-99% yield) with various N-(trimethylsilyl)methyl-substituted amines upon irradiation with visible light and catalysis by a metal complex. If the alkylidene substituent is non-symmetric and if the reaction is performed in the presence of a chiral hydrogen-bonding template, products are obtained with significant enantioselectivity (58-72% ee) as a mixture of diastereoisomers. Mechanistic studies suggest a closed catalytic cycle for the photoactive metal complex. However, the silyl transfer from the amine occurs not only to the product, but also to the substrate, and interferes with the desired chirality transfer.

Recent synthetic additions to the visible light photoredox catalysis toolbox

The boom in visible light photoredox catalysis (VLPC) research has demonstrated that this novel synthetic approach is here to stay. VLPC enables reactive radical intermediates to be catalytically generated at ambient temperature, a feat not generally allowed through traditional pyrolysis- or radical initiator-based methodologies. VLPC has vastly extended the range of substrates and reaction schemes that have been traditionally the domain of radical reactions. In this review the photophysics background of VLPC will be briefly discussed, followed by a report on recent inroads of VLPC into decarboxylative couplings and radical C-H functionalization of aromatic compounds. The bulk of the review will be dedicated to advances in synergistic catalysis involving VLPC, namely the combination of photoredox catalysis with organocatalysis, including β-functionalization of carbonyl groups, functionalization of weak aliphatic C-H bonds, and anti-Markovnikov hydrofunctionalization of alkenes; dual catalysis with gold or with nickel, photoredox catalysis as an oxidation promoter in transition metal catalysis, and acid-catalyzed enantioselective radical addition to π systems.