Photoredox Catalysis by 21-Thiaporphyrins: A Green and Efficient Approach for C-N Borylation and C-H Arylation

Photoredox catalysis provides a green and sustainable alternative for C-H activation of organic molecules that eludes harsh conditions and use of transition metals. The photocatalytic C-N borylation and C-H arylation mostly depend on the ruthenium and iridium complexes or eosin Y and the use of porphyrin catalysts is still in infancy. A series of novel 21-thiaporphyrins (A2B2 and A3B type) were synthesized having carbazole/phenothiazine moieties at their meso-positions and screened as catalysts for C-N borylation and C-H arylation. This paper demonstrates the 21-thiaporphyrin catalyzed C-N borylation and het-arylation of anilines under visible light. The method utilizes only 0.1 mol % of 21-thiaporphyrin catalyst under blue light for the direct C-N borylation and het-arylation reactions. A variety of substituted anilines were used as source for expensive and unstable aryl diazonium salts in the reactions. The heterobiaryls and aryl boronic esters were obtained in decent yields (up to 88 %). Versatility of the 21-thiaporphyrin catalyst was tested by thiolation and selenylation of anilines under similar conditions. Mechanistic insight was obtained from DFT studies, suggesting that 21-thiaporphyrin undergo an oxidative quenching pathway. The photoredox process catalyzed by 21-thiaporphyrins offers a mild, efficient and metal-free alternative for the formation of C-C, C-S, and C-Se bonds in aryl compounds; it can also be extended to borylation reaction.

3-Chloropropylbis(catecholato)silicate as a Bifunctional Reagent for the One-Pot Synthesis of Tetrahydroquinolines from o-Bromosulfonamides

Bis(catecholato)silicate salts are easily accessible reagents that can be used to install alkyl fragments through photoredox-enabled cross-coupling. These reagents can incorporate various functional groups including pendant alkyl halides. A halogenated organosilicate reagent was leveraged to develop a one-pot synthesis of tetrahydroquinolines from o-bromosulfonamides, where the bifunctional reagent participates in a nickel/photoredox cross-coupling followed by intramolecular nucleophilic substitution. The functional group tolerance of this cross-coupling strategy allowed for the preparation of a series of substituted tetrahydroquinolines.

Regime Switch in the Dual-Catalyzed Coupling of Alkyl Silicates with Aryl Bromides

Metallaphotoredox catalyzed cross-coupling of an arylbromide (Ar-Br) with an alkyl bis(catecholato)silicate (R-Si⊖ ) has been analyzed in depth using a continuum of analytical techniques (EPR, fluorine NMR, electrochemistry, photophysics) and modeling (micro-kinetics and DFT calculations). These studies converged on the impact of four control parameters consisting in the initial concentrations of the iridium photocatalyst ([Ir]0 ), nickel precatalyst ([Ni]0 ) and silicate ([R-Si⊖ ]0 ) as well as light intensity I0 for an efficient reaction between Ar-Br and R-Si⊖ . More precisely, two regimes were found to be possibly at play. The first one relies on an equimolar consumption of Ar-Br with R-Si⊖ smoothly leading to Ar-R, with no side-product from R-Si⊖ and a second one in which R-Si⊖ is simultaneously coupled to Ar-Br and degraded to R-H. This integrative approach could serve as a case study for the investigation of other metallaphotoredox catalysis manifolds of synthetic significance.

Reversible C-H bond silylation with a neutral silicon Lewis acid

The silicon-carbon bond is a valuable linchpin for synthetic transformations. However, installing Si-C functionalities requires metalated C-nucleophiles, activated silicon reagents (silylium ions, silyl radicals, and silyl anions), or transition metal catalysis, and it occurs irreversibly. In contrast, spontaneous C-H silylations with neutral silanes leading to anionic silicates, and their reversible deconstruction, are elusive. Herein, the CH-bond silylation of heterocycles or a terminal alkyne is achieved by reaction with bis(perfluoro(N-phenyl-ortho-amidophenolato))silane and 1,2,2,6,6-pentamethylpiperidine. Computational and experimental insights reveal a frustrated Lewis pair (FLP) mechanism. Adding a silaphilic donor to the ammonium silicate products induces the reformation of the C-H bond, thus complementing previously known irreversible C-H bond silylation protocols. Interestingly, the FLP "activated" N-methylpyrrole exhibits "deactivated" features against electrophiles, while a catalytic functionalization is found to be effective only in the absence of a base.

Visible-Light Photoredox-Catalyzed Giese Reaction of α-Silyl Ethers with Various Michael Acceptors

We developed a photocatalyzed Giese reaction of various weakly activated Michael acceptors with a neutral silicon-based radical precursor and applied it at large-scale using a continuous flow reactor. The developed method successfully overcomes the substrate scope limitations of previous studies, shows good functional groups tolerance, and affords good to excellent yields. On the basis of mechanistic studies, we propose a reaction mechanism that involves an in situ generated alkoxymethyl radical via single-electron oxidation of α-trimethylsilyl-substituted ethers.

Photocatalytic C(sp3) radical generation via C-H, C-C, and C-X bond cleavage

C(sp³) radicals (R˙) are of broad research interest and synthetic utility. This review collects some of the most recent advancements in photocatalytic R˙ generation and highlights representative examples in this field. Based on the key bond cleavages that generate R˙, these contributions are divided into C–H, C–C, and C–X bond cleavages. A general mechanistic scenario and key R˙-forming steps are presented and discussed in each section.

C(sp³) radicals (R˙) are of broad research interest and synthetic utility.

Mesoporous Graphitic Carbon Nitride as a Heterogeneous Organic Photocatalyst in the Dual Catalytic Arylation of Alkyl Bis(catecholato)silicates

Mesoporous graphitic carbon nitride (mpg-CN) is introduced as a heterogeneous photocatalyst to perform dual photoredox- and nickel-catalyzed cross-coupling reactions between alkyl bis(catecholato)silicates as radical precursors and aryl or alkenyl bromides. The synergy between this recyclable photocatalyst and the broadly applied homogeneous nickel complex [Ni(dtbbpy)Br₂] gives access to C(sp²)-C(sp³) cross-coupling products in a sustainable fashion. The recycled mpg-CN photocatalyst was analyzed by time-resolved emission spectroscopy and EPR spectroscopy.

Boron, silicon, nitrogen and sulfur-based contemporary precursors for the generation of alkyl radicals by single electron transfer and their synthetic utilization

Recent developments in the use of boron, silicon, nitrogen and sulfur derivatives in single-electron transfer reactions for the generation of alkyl radicals are described. Photoredox catalyzed, electrochemistry promoted or thermally-induced oxidative and reductive processes are discussed highlighting their synthetic scope and discussing their mechanistic pathways.

Alkyl-GeMe3: Neutral Metalloid Radical Precursors upon Visible-Light Photocatalysis

Single-electron transfer (SET) oxidation of ionic hypervalent complexes, representatively alkyltrifluoroborates (Alkyl-BF3-) and alkylbis(catecholato)silicates (Alkyl-Si(cat)2-), have contributed substantially to alkyl radical generation compared to alkali or alkaline earth organometallics because of their excellent activity-stability balance. Herein, we report another proposal using neutral metalloid compounds, Alkyl-GeMe3, as radical precursors. Compared to Alkyl-BF3- and Alkyl-Si(cat)2-, Alkyl-GeMe3 show comparable activity in radical addition reactions. Moreover, Alkyl-GeMe3 gives the first success of group 14 tetraalkyl nucleophiles in nickel catalyzed cross-coupling. Meanwhile, the neutral nature of these organogermanes supplemented the limination of ionic precursors in purification and derivatization. A preliminary mechanism study corresponds to the procedure that alkyl radical generates from tetraalkylgermane radical cation with the assistance of a nucleophile, which may also enlighten the development of more non-ionic alkyl radical precursors with metalloid center.

Metallaphotoredox: The Merger of Photoredox and Transition Metal Catalysis

The merger of photoredox catalysis with transition metal catalysis, termed metallaphotoredox catalysis, has become a mainstay in synthetic methodology over the past decade. Metallaphotoredox catalysis has combined the unparalleled capacity of transition metal catalysis for bond formation with the broad utility of photoinduced electron- and energy-transfer processes. Photocatalytic substrate activation has allowed the engagement of simple starting materials in metal-mediated bond-forming processes. Moreover, electron or energy transfer directly with key organometallic intermediates has provided novel activation modes entirely complementary to traditional catalytic platforms. This Review details and contextualizes the advancements in molecule construction brought forth by metallaphotocatalysis.

Photoredox Heterobimetallic Dual Catalysis Using Engineered Covalent Organic Frameworks

The functionalization of an imine-based layered covalent organic framework (COF), containing phenanthroline units as ligands, has allowed the obtention of a heterobimetallated material. Photoactive Ir and Ni fragments were immobilized within the porous structure of the COF, enabling heterogeneous light-mediated Csp3-Csp2 cross-couplings. As radical precursors, potassium benzyl- and alkoxy-trifluoroborates, organic silicates, and proline derivatives were employed, which brings out the good versatility of Ir,Ni@Phen-COF. Moreover, in all the studied cases, an enhanced activity and stability have been observed in comparison with analogous homogenous systems.

Visible-Light-Mediated Z-Stereoselective Monoalkylation of β,β-Dichlorostyrenes by Photoredox/Nickel Dual Catalysis

Metal-catalyzed alkylation of 1,1-dihalovinyl moiety commonly suffers from both a lack of stereoselectivity and the overreaction leading to the dialkylation product. The methodology described herein features a new pathway to alkylate stereoselectively β,β-dichlorostyryl substrates to provide the Z-trisubstituted olefin only with fair to good yields. This cross-coupling reaction bears on the smooth and photoinduced formation of a C-centered radical that engages in a nickel-catalyzed organometallic cycle to form the key Csp2–Csp3 bond.

Phenyl Silicates with Substituted Catecholate Ligands: Synthesis, Structural Studies and Reactivity

While the generation of aryl radicals by photoredox catalysis under reductive conditions is well documented, it has remained challenging under an oxidative pathway. Because of the easy photo-oxidation of alkyl bis-catecholato silicates, a general study of phenyl silicates bearing substituted catecholate ligands has been achieved. The newly synthesized phenyl silicates have been fully characterized, and their reactivity has been explored. It was found that, thanks to the substitution of the catecholate moiety, notably with the 4-cyanocatecholato ligand, the phenyl radical could be generated and trapped. Computational studies provided a rationale for these findings.

A Parisian Vision of the Chemistry of Hypercoordinated Silicon Derivatives

Less than ten years of acquaintance with hypercoordinated silicon derivatives in our lab is described in this account. Martin's spirosilane derivatives open new opportunities as ligands and as agents for the activation of small molecules and bis-catecholato silicates have proven to be exquisite radical precursors in photoredox conditions for broad synthetic applications.

Metallaphotoredox catalysis with organic dyes

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

Visible-light-promoted cross-coupling reaction of hypervalent bis-catecholato silicon compounds with selenosulfonates or thiosulfonates

A visible-light-promoted free radical cross coupling of hypervalent bis-catecholato silicon compounds with selenosulfonates or thiosulfonates is developed.

Generation of Alkyl Radicals: From the Tyranny of Tin to the Photon Democracy

Alkyl radicals are key intermediates in organic synthesis. Their classic generation from alkyl halides has a severe drawback due to the employment of toxic tin hydrides to the point that "flight from the tyranny of tin" in radical processes was considered for a long time an unavoidable issue. This review summarizes the main alternative approaches for the generation of unstabilized alkyl radicals, using photons as traceless promoters. The recent development in photochemical and photocatalyzed processes enabled the discovery of a plethora of new alkyl radical precursors, opening the world of radical chemistry to a broader community, thus allowing a new era of photon democracy.

Stereochemistry of Transition Metal Complexes Controlled by the Metallo-Anomeric Effect

The use of stereoelectronic interactions to control reactivity and selectivity has a long history in chemistry. The anomeric effect, one of the fundamental concepts in organic chemistry, describes the preferences of a substituent at the anomeric carbon in glycosides to adopt axial configuration when the anomeric group is an electronegative element such as oxygen or a halogen. The origin of the anomeric effect has been the subject of intense debate. Explanations capitalizing on either the delocalization of the endocyclic oxygen lone pair into the antibonding σ*_(C-X) orbital or the minimization of the dipole-dipole interactions are currently the two leading theoretical models. Although the majority of experimental and theoretical studies have focused on the elements from groups 6 and 7, little is known about conformational preferences of tetrahydropyran rings substituted with a transition metal at the anomeric carbon and the role of these interactions in stereoselective synthesis. Here, we report studies on conformational and configurational preferences of organometallic complexes stabilized by vicinal heteroatoms. We provide computational evidence that late transition metals adopt the axial position in heterocycles or synclinal geometry in acyclic systems. Furthermore, the anomeric preferences of late transition metals correlate with the oxidation state of the metal and can be explained by hyperconjugative interactions between endocyclic heteroatom and the σ* acceptor orbitals of the C-M bond. In a broader context, this discovery provides insight into the role of previously unanticipated stereoelectronic effects that can be harnessed in the design of stereoselective reactions, including chemical glycosylation and enantioselective catalysis.

Alkaline Earth Metal Template (Cross-)Coupling Reactions with Hybrid Disila-Crown Ether Analogues

Alkaline earth metal iodides were used as templates for the synthesis of novel silicon-based ligands. Siloxane moieties were (cross-)coupled and ion-specific, silicon-rich crown ether analogues were obtained. The reaction of 1,2,7,8-tetrasila[12]crown-4 (I) and 1,2-disila[9]crown-3 (II) with MgI2 yielded exclusively [Mg(1,2,7,8-tetrasila[12]crown-4)I2 ] (1). The larger Ca2+ ion was then employed for cross-coupling of I and II and yielded the complex [Ca(1,2,7,8-tetrasila[15]crown-5)I2 ] (2). Cross-coupling of I and 1,2,4,5-tetrasila[9]crown-3 (III) with SrI2 enables the synthesis of the silicon-dominant 1,2,4,5,10,11-hexasila[15]crown-5 ether complex of SrI2 (3). Further, the compounds [Sr(1,2,10,11-tetrasila[18]crown-6)I2 ] (4), [Sr(1,2,13,14-tetrasila[24]crown-8)I2 ] (5), and [Sr(1,2,13,14-tetrasila-dibenzo[24]crown-8)I2 ] (6) were obtained by coupling I, 1,2-disila[12]crown-4 (IV) or 1,2-disila-benzo[12]crown-4 (V), respectively. Using various anions, the (cross-)coupled ligands were also observed in an X-ray structure within the mentioned complexes. These template-assisted (cross-)couplings of various ligands are the first of their kind and a novel method to obtain macrocycles and/or their metal complexes to be established. Further, the Si-O bond activations presented herein might be of importance for silane or even organic functionalization.

Alkyl Carbon-Carbon Bond Formation by Nickel/Photoredox Cross-Coupling

The union of photoredox and nickel catalysis has resulted in a renaissance in radical chemistry as well as in the use of nickel-catalyzed transformations, specifically for carbon-carbon bond formation. Collectively, these advances address the longstanding challenge of late-stage cross-coupling of functionalized alkyl fragments. Empowered by the notion that photocatalytically generated alkyl radicals readily undergo capture by Ni complexes, wholly new feedstocks for cross-coupling have been realized. Herein, we highlight recent developments in several types of alkyl cross-couplings that are accessible exclusively through this approach.

Cross coupling of alkylsilicates with acyl chlorides via photoredox/nickel dual catalysis: a new synthesis method for ketones

Photoredox/nickel dual catalysis using silicates and acyl chlorides allows a new method of formation of ketones. Flow chemistry can be applied.

Metal-free desilylative C–C bond formation by visible-light photoredox catalysis

A novel metal-free desilylative C–C bond formation from simple organosilanes by visible-light acridinium photoredox catalysis is presented.

Metal-, photocatalyst-, and light-free late-stage C–H alkylation of N-heteroarenes with organotrimethylsilanes using persulfate as a stoichiometric oxidant

Minisci C–H alkylation of N-heteroarenes with readily available benzylsilanes and heteroatom substituted silanes was developed.

Reactivity of the diphosphinodithio ligated nickel(0) complex toward alkyl halides and resultant nickel(i) and nickel(ii)-alkyl complexes

Diphosphinodithio ligated complexes of nickel(0), nickel(i) and nickel(ii)-alkyl with a reactivity relevant to the C-C bond formation were described. Stoichiometric reactions of the nickel(0) complex, [(P2S2)Ni] ([1]0, P2S2 = (Ph2PC6H4CH2S)2(C2H4)), with alkyl halides (RX) such as C6H5CH2Br, C2H3CH2Br, C2H5I and (CH3)2CHI were investigated, from which the products were found to be highly dependent on the nature of RX used. Oxidative addition of C2H3CH2Br to [1]0 provides the stable Ni(ii)-alkyl complexes [1-allyl]+. The reaction of [1]0 with C6H5CH2Br proceeds through a radical pathway resulting in the formation of the nickel(i) complex [1]+ and an organic homo-coupled product 1,2-diphenylethane. Oxidative addition of C2H5I or (CH3)2CHI to [1]0 can be achieved but it competes with the halogen atom abstraction reaction as found for C6H5CH2Br. [1]0 was shown to be an active catalyst for the coupling reactions of primary halides and alkyl Grignard reagents.

Visible-Light Photocatalysis: Does It Make a Difference in Organic Synthesis?

Visible-light photocatalysis has evolved over the last decade into a widely used method in organic synthesis. Photocatalytic variants have been reported for many important transformations, such as cross-coupling reactions, α-amino functionalizations, cycloadditions, ATRA reactions, or fluorinations. To help chemists select photocatalytic methods for their synthesis, we compare in this Review classical and photocatalytic procedures for selected classes of reactions and highlight their advantages and limitations. In many cases, the photocatalytic reactions proceed under milder reaction conditions, typically at room temperature, and stoichiometric reagents are replaced by simple oxidants or reductants, such as air, oxygen, or amines. Does visible-light photocatalysis make a difference in organic synthesis? The prospect of shuttling electrons back and forth to substrates and intermediates or to selectively transfer energy through a visible-light-absorbing photocatalyst holds the promise to improve current procedures in radical chemistry and to open up new avenues by accessing reactive species hitherto unknown, especially by merging photocatalysis with organo- or metal catalysis.

Rhodium/Silver Synergistic Catalysis in Highly Enantioselective Cycloisomerization/Cross Coupling of Keto-Vinylidenecyclopropanes with Terminal Alkynes

A rhodium/silver synergistic catalysis has been established, enabling cycloisomerization/cross coupling of keto-vinylidenecyclopropanes (VDCPs) with terminal alkynes toward the regio- and enantioselective formation of diversified tetrahydropyridin-3-ol tethered 1,4-enynes in good yields and high ee values. In this synergistic catalysis, Rh(I) and Ag(I) catalysts selectively activate keto-VDCP substrates and terminal alkynes to generate the π-allyl Rh(III) complex of oxa-rhodacyclic intermediate and Ag alkynyl intermediate, respectively. The rapid transmetalation of alkynyl groups from Ag to Rh is proposed to play a key role in realizing the regioselective cleavage of the distal bond of the three-membered ring in this transformation.

Photoredox-Mediated Routes to Radicals: The Value of Catalytic Radical Generation in Synthetic Methods Development

Photoredox catalysis has experienced a revitalized interest from the synthesis community during the past decade. For example, photoredox/Ni dual catalysis protocols have been developed to overcome several inherent limitations of palladium-catalyzed cross-couplings by invoking a single-electron transmetalation pathway. This Perspective highlights advances made by our laboratory since the inception of the photoredox/Ni cross-coupling of benzyltrifluoroborates with aryl bromides. In addition to broadening the scope of trifluoroborate coupling partners, research using readily oxidized hypervalent silicates as radical precursors that demonstrate functional group compatibility is highlighted. The pursuit of electrophilic coupling partners beyond (hetero)aryl bromides has also led to the incorporation of several new classes of C(sp²)-hybridized substrates into light-mediated cross-coupling. Advances to expand the radical toolbox by utilizing feedstock chemicals (e.g., aldehydes) to access radicals that were previously inaccessible from trifluoroborates and silicates are also emphasized. Additionally, several organic photocatalysts have been investigated as replacements for their expensive iridium- and ruthenium-based counterparts. Lastly, the net C-H functionalization of the radical partner in an effort to improve atom economy is presented. An underlying theme in all of these studies is the value of generating radicals in a catalytic manner, rather than stoichiometrically.

Mild, Redox-Neutral Alkylation of Imines Enabled by an Organic Photocatalyst

An operationally simple, mild, redox-neutral method for the photoredox alkylation of imines is reported. Utilizing an inexpensive organic photoredox catalyst, alkyl radicals are readily generated from the single-electron oxidation of ammonium alkyl bis(catecholato)silicates and are subsequently engaged in a C–C bond-forming reaction with imines. The process is highly selective, metal-free, and does not require a large excess of the alkylating reagent or the use of acidic additives.

Preparation of visible-light-activated metal complexes and their use in photoredox/nickel dual catalysis

Visible-light-activated photoredox catalysts provide synthetic chemists with the unprecedented capability to harness reactive radicals through discrete, single-electron transfer (SET) events. This protocol describes the synthesis of two transition metal complexes, [Ir{dF(CF₃)₂ppy}₂(bpy)]PF₆ (1a) and [Ru(bpy)₃](PF₆)₂ (2a), that are activated by visible light. These photoredox catalysts are SET agents that can be used to facilitate transformations ranging from proton-coupled electron-transfer-mediated cyclizations to C-C bond constructions, dehalogenations, and H-atom abstractions. These photocatalysts have been used in the synthesis of medicinally relevant compounds for drug discovery, as well as the degradation of biological polymers to access fine chemicals. These catalysts are prepared from IrCl₃ and RuCl₃, respectively, in three chemical steps. These steps can be described as a series of two ligand modifications followed by an anion metathesis. Using the cost-effective, scalable procedures described here, the ruthenium-based photocatalyst 2a can be synthesized in a 78% overall yield (∼8.1 g), and the iridium-based photocatalyst 1a can be prepared in a 56% overall yield (∼4.4 g). The total time necessary for the complete protocols ranges from ∼2 d for 2a to 5-7 d for 1a. Procedures for applying each catalyst in representative photoredox/Ni cross-coupling to form C_sp3-C_sp2 bonds using the appropriate radical precursor-organotrifluoroborates with 1a and bis(catecholato)alkylsilicates with 2a-are described. In addition, more traditional photoredox-mediated transformations are included as diagnostic tests for catalytic activity.

C(sp3)-C(sp2) cross-coupling of alkylsilicates with borylated aryl bromides - an iterative platform to alkylated aryl- and heteroaryl boronates

The attractive field of iterative cross-coupling has seen numerous advances, although almost exclusively in the union of sp2-hybridized partners. Conspicuously absent from this useful synthetic manifold is the inclusion of sp3-hybridized pronucleophiles that can undergo transmetalation under mild conditions. Described here is the use of primary and secondary ammonium alkylsilicates, which undergo facile C(sp3)-C(sp2) cross-coupling with borylated aryl bromide partners under photoredox/nickel dual catalysis conditions. This operationally simple procedure allows the production of alkylated small molecules possessing boronate ester (BPin, Bneopentyl, BMIDA) functional handles. Because of the extremely mild reaction conditions and the innocuous byproduct generated upon fragmentative oxidation of silicates, the corresponding borylated compounds were isolated in good to excellent yields. Aryl bromides bearing unprotected boronic acids are also generally tolerated for the first time and prove useful in multistep syntheses. Unlike many previously reported photoredox/Ni dual cross-couplings, the C(sp3)-C(sp2) bonds were forged using a transition metal-free photocatalyst, allowing a substantial increase in sustainability as well as a cost reduction. Because the developed Ni-catalyzed cross-coupling does not require discrete boron speciation control, as in many popular orthogonal Pd-based methods, this protocol represents a significant advance in atom- and step-economy.

Heteromultimetallic catalysis for sustainable organic syntheses

Fully complementary bimetallic catalysis has been identified as an increasingly powerful tool for molecular transformations, which was largely inspired by early examples of sequential catalytic transformations.

1,4-Dihydropyridines as Alkyl Radical Precursors: Introducing the Aldehyde Feedstock to Nickel/Photoredox Dual Catalysis

A Ni/photoredox dual catalytic cross-coupling is disclosed in which a diverse range of (hetero)aryl bromides are used as electrophiles, with 1,4-dihydropyridines serving as precursors to C_sp³-centered alkyl radical coupling partners. The reported method is characterized by its extremely mild reaction conditions, enabling access to underexplored cores.