Visible-Light-Driven Decarboxylative Alkylation of C-H Bond Catalyzed by Dye-Sensitized Semiconductor

Titanium in photocatalytic organic transformations: current applications and future developments

Titanium, as an important transition metal, has garnered extensive attention in both industry and academia due to its excellent mechanical properties, corrosion resistance, and unique reactivity in organic synthesis. In the field of organic photocatalysis, titanium-based compounds such as titanium dioxide (TiO2), titanocenes (Cp2TiCl2, CpTiCl3), titanium tetrachloride (TiCl4), tetrakis(isopropoxy)titanium (Ti(OiPr)4), and chiral titanium complexes have demonstrated distinct reactivity and selectivity. This review focuses on the roles of these titanium compounds in photocatalytic organic reactions, and highlights the reaction pathways such as photo-induced single-electron transfer (SET) and ligand-to-metal charge transfer (LMCT). By systematically surveying the latest advancements in titanium-involved organic photocatalysis, this review aims to provide references for further research and technological innovation within this fast-developing field.

Direct C-H alkylation of 3,4-dihydroquinoxaline-2-ones with N-(acyloxy)phthalimides via radical-radical cross coupling

We present an organophotoredox-catalyzed direct Csp3-H alkylation of 3,4-dihydroquinoxalin-2-ones employing N-(acyloxy)pthalimides to provide corresponding products in good yields. A broad spectrum of NHPI esters (1°, 2°, 3°, and sterically encumbered) participates in the photoinduced alkylation of a variety of 3,4-dihydroquinoxalin-2-ones. In general, mild conditions, broad scope with good functional group tolerance, and scalability are the salient features of this direct alkylation process.

Visible-light-induced decarboxylative cascade cyclization of acryloylbenzamides with N-hydroxyphthalimide esters via EDA complexes

A visible-light-induced decarboxylative cascade reaction of acryloylbenzamides with alkyl N-hydroxyphthalimide (NHP) esters for the synthesis of various 4-alkyl isoquinolinediones mediated by triphenylphosphine (PPh3) and sodium iodide (NaI) was developed. This operationally simple protocol proceeded via the photoactivation of electron donor-acceptor (EDA) complexes between N-hydroxyphthalimide esters and NaI/PPh3, resulting in multiple carbon-carbon bond formations without the use of precious metal complexes or synthetically elaborate organic dyes, which provided an alternative practical approach to synthesize diverse isoquinoline-1,3(2H,4H)-dione derivatives.

Visible light as a sole requirement for alkylation of α-C(sp3)-H of N-aryltetrahydroisoquinolines with alkylboronic acids

An alkylation of α-C(sp3)-H at N-aryltetrahydroisoquinolines with alkylboronic acids was developed under visible-light irradiation in the absence of additional photocatalyst. The reaction proceeded well, tolerating a variety of functional groups, and featured low-cost and mild reaction conditions. A preliminary mechanistic study indicated that an electron donor-acceptor (EDA) complex between an electron-rich N-aryltetrahydroisoquinoline and an electron-poor alkylboronic acid was involved in the reaction.

Visible-light promoted photoredox catalysis in flow: addition of biologically important α‑amino radicals to michael acceptors

Visible light promoted photoredox catalyzed formation of α-amino radicals from cyclic tertiary amine compounds and their subsequent addition to Michael acceptors performed in flow conditions allowed access to a wide range of functionalized N-aryl-substituted tetrahydroisoquinolines (THIQs) and N-aryl-substituted tetrahydro-β-carbolines (THBCs). Visible light in conjunction with Ru(bpy)3Cl2 photocatalyst allowed the formation and high reactivities of α-amino radicals in flow conditions at room temperature. These reactions gave valuable products with high efficiencies; some previously unavailable reaction pathways photo or thermal reaction conditions; i.e. direct synthesis of 1-substituted (THBCs) via α-amino radical path were successfully realized in flow. The use of custom-made FEP tube microreactor proved to be the key to succesfull α-amino-radical formation and overall reaction performance in flow. Three types of light transparent custom-made microfluidic devices were tested, among them glass/silicon and FEP type reactor showed very good results in the conversion of tested compounds. Plausible reaction mechanism is proposed in accordance with known principles of photo activation of tertiary amines. Visible light promoted C(sp3)-H functionalization of N-aryl-protected tetrahydroisoquinolines and N-aryl-protected tetrahydro-β-carbolines in microflow conditions via a-amino radical pathway with various coupling partners in excellent yields and efficiencies.

Emerging developments in dye-sensitized metal oxide photocatalysis: exploring the design, mechanisms, and organic synthesis applications

In the present day, the incorporation of environmentally conscious practices in the realm of photocatalysis holds a prominent position within the domain of organic synthesis. The imperative to tackle environmental issues linked to catalysts that cannot be recycled, generation of waste, byproducts, and challenges in achieving reaction selectivity during organic synthesis are more crucial than ever. One potential solution involves the integration of recyclable nanomaterials with light as a catalyst, offering the possibility of achieving sustainable and atom-efficient transformations in organic synthesis. Metal oxide nanoparticles exhibit activation capabilities under UV light, constituting a small percentage (4-8%) of sunlight. However, this method lacks sufficient environmental friendliness, and the issue of electron-hole recombination poses a significant hurdle. To tackle these challenges, multiple approaches have been proposed. This comprehensive review article focuses on the efficacy of dyes in enhancing the capabilities of heterogeneous photocatalysts, offering a promising avenue to overcome the constraints associated with metal oxides in their role as photocatalysts. The article delves into the intricate design aspects of dye-sensitized photocatalysts and sheds light on their mechanisms in facilitating organic transformations.

Electrochemically enabled decyanative C(sp3)-H oxygenation of N-cyanomethylamines to formamides

Selective oxygenation of C(sp³)-H bonds adjacent to nitrogen atoms is a highly attractive strategy for synthesizing various formamide derivatives while preserving the substrate skeletons. Herein, an environmentally benign electrochemically enabled decyanative C(sp³)-H oxygenation of N-cyanomethylamines using H₂O as a carbonyl oxygen atom source is described, leading to the synthesis of a large class of formamides in good to excellent yields with a broad substrate scope under metal- and oxidant-free conditions. This electrochemical technology highlights the facile incorporation of N-formyl into some important bioactive molecules.

The effect of N-vacancy on the photocatalytic activity of graphitic carbon nitride in the oxidative Mannich reaction

N-vacancy g-CN was used in Mannich oxidative reaction as a photocatalyst, having mid-gap states that enhance reaction kinetics. This facile photocatalyst enabled successful formation of challenging THIQ with EWG and chemo-selectivity on C–C bond.

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.

Photoredox-Catalysed Regioselective Synthesis of C-4-Alkylated Pyridines with N -(Acyloxy)phthalimides

A method of direct C-4 selective alkylation of pyridine under visible light irradiation at room temperature was reported, using simple maleate-derived pyridinium salts as pyridine precursors, and the readily available...

Visible-Light-Driven Photoredox-Catalyzed C(sp3)-C(sp3) Cross-Coupling of N-arylamines with Cycloketone Oxime Esters

A novel photoredox-catalyzed C(sp3)-C(sp3) cross-coupling between N-arylamines and cycloketone oxime esters under mild conditions has been accomplished. The redox-neutral reaction proceeds good functional group tolerance and excellent regioselectivity without any...

Photocatalytic cross-couplings via the cleavage of N-O bonds

N-(Acyloxy)phthalimide and oxime derivatives containing N-O bonds are important chemicals and synthetic intermediates, and visible light photoredox reductions of the N-O bonds provide carbon- or nitrogen-centered radicals for N-(acyloxy)phthalimide derivatives and iminyl radicals for oxime derivatives. This feature article summarises the recent progress in the visible light photoredox organic reactions, including decarboxylative addition reactions, alkylation, allylation, alkenylation, alkynylation, arylation, heteroarylation and cascade annulation of N-(acyloxy)phthalimide derivatives through the formation of carbon-carbon bonds, decarboxylative borylation, amination, oxygenation, sulfuration, selenylation, fluorination and iodination of N-(acyloxy)phthalimide derivatives through the formation of carbon-heteroatom bonds, and additions to arenes and alkenes, hydrogen atom transfer and the cleavage of α-carbon-carbon bonds via the iminyl radical intermediates for oxime derivatives.

Photoinduced Decarboxylative Phosphorothiolation of N-Hydroxyphthalimide Esters

A visible-light-induced protocol for the synthesis of phosphorothioates is developed by employing the Ir-catalyzed decarboxylative phosphorothiolation of N-hydroxyphthalimide esters. This novel synthesis method utilizes carboxylic acids as raw material, which is stable, cheap, and commercially available. Scope studies show that this reaction has good compatibility of functional groups. Notably, both the synthesis of steric hindrance phosphorothioates and the later modification of some bioactive compounds are successfully achieved.

Visible light promoted photoredox C(sp3)-H bond functionalization of tetrahydroisoquinolines in flow

A merger of organocatalysis and visible light photoredox catalysis performed in flow allowed access to a wide range of functionalized N-aryl-substituted tetrahydroisoquinolines (THIQs) in a formal C-H oxidation/Mannich reaction. Strecker type functionalization and copper-catalyzed alkynylation of several N-aryl-substituted THIQs were also successfully performed in flow, giving valuable products with high efficiencies. The use of custom-made porous polymeric type microreactors proved to be crucial regarding the C-H oxidation step and overall reaction performance.

Visible-Light Photoredox-Catalyzed Decarboxylative α-tert-Butylation of C(sp3)–H Bonds of N-Aryltetrahydroisoquinolines with Pivalic Acid under Transition-Metal-Free Conditions

A transition-metal-free visible-light photoredox-catalyzed decarboxylative alkylation of the benzylic C(sp3)–H bonds of N-aryltetrahydroisoquinolines is reported. The method tolerates various functional groups and proceeds smoothly without requiring a stoichiometric oxidant. A preliminary mechanistic study indicated that a radical process is involved in the reaction.

Photocatalyst- and additive-free decarboxylative alkylation of N-aryl tetrahydroisoquinolines induced by visible light

A photocatalyst- and additive-free visible light induced decarboxylative alkylation of N-aryl tetrahydroisoquinolines has been developed using tetrachloro-N-hydroxyphthalimide esters as alkylation agents.

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.

Photoredox-catalyzed stereoselective alkylation of enamides with N-hydroxyphthalimide esters via decarboxylative cross-coupling reactions

Stereoselective β-C(sp2)-H alkylation of enamides with redox-active N-hydroxyphthalimide esters via a photoredox-catalyzed decarboxylative cross-coupling reaction is demonstrated. This methodology features operational simplicity, broad substrate scopes, and excellent stereoselectivities and functional group tolerance, affording a diverse array of geometrically defined and synthetically valuable enamides bearing primary, secondary or tertiary alkyl groups in satisfactory yields.

Visible-light-mediated de-aminative alkylation of N-arylamines with alkyl Katritzky salts

A visible-light-mediated de-aminative alkylation of N-arylamines has been developed, providing direct access to α-amino C–H functionalization from readily available Katritzky salts under mild conditions.

Application of metal oxide semiconductors in light-driven organic transformations

Herein, we provide an up-to-date overview of metal oxide semiconductors (MOS) as versatile and inexpensive photocatalysts to enable light-driven organic transformations.

Radical alkylation of C(sp3)–H bonds with diacyl peroxides under catalyst-free conditions

Herein, we describe a protocol for alkylation reactions of C(sp³)–H bonds with diacyl peroxides by means of a process involving cross-coupling between an alkyl radical and an α-aminoalkyl radical.

Recent advances in modified TiO2 for photo-induced organic synthesis

The recent advancements of modified TiO₂ materials as photocatalysts for organic synthesis are summarized.

Dehydrogenative C(sp3)–H bond functionalization of tetrahydroisoquinolines mediated by organic oxidants under mild conditions

DDQ and IBX are effective SET oxidants of N-aryl-protected and unprotected THIQs. Mannich and Strecker type functionalizations ensued in an overall CDC reaction.

TiO2 Photocatalyzed C–H Bond Transformation for C–C Coupling Reactions

Fulfilling the direct inert C–H bond functionalization of raw materials that are earth-abundant and commercially available for the synthesis of diverse targeted organic compounds is very desirable and its implementation would mean a great reduction of the synthetic steps required for substrate prefunctionalization such as halogenation, borylation, and metalation. Successful C–H bond functionalization mainly resorts to homogeneous transition-metal catalysis, albeit sometimes suffering from poor catalyst reusability, nontrivial separation, and severe biotoxicity. TiO2 photocatalysis displays multifaceted advantages, such as strong oxidizing ability, high chemical stability and photostability, excellent reusability, and low biotoxicity. The chemical reactions started and delivered by TiO2 photocatalysts are well known to be widely used in photocatalytic water-splitting, organic pollutant degradation, and dye-sensitized solar cells. Recently, TiO2 photocatalysis has been demonstrated to possess the unanticipated ability to trigger the transformation of inert C–H bonds for C–C, C–N, C–O, and C–X bond formation under ultraviolet light, sunlight, and even visible-light irradiation at room temperature. A few important organic products, traditionally synthesized in harsh reaction conditions and with specially functionalized group substrates, are continuously reported to be realized by TiO2 photocatalysis with simple starting materials under very mild conditions. This prominent advantage—the capability of utilizing cheap and readily available compounds for highly selective synthesis without prefunctionalized reactants such as organic halides, boronates, silanes, etc.—is attributed to the overwhelmingly powerful photo-induced hole reactivity of TiO2 photocatalysis, which does not require an elevated reaction temperature as in conventional transition-metal catalysis. Such a reaction mechanism, under typically mild conditions, is apparently different from traditional transition-metal catalysis and beyond our insights into the driving forces that transform the C–H bond for C–C bond coupling reactions. This review gives a summary of the recent progress of TiO2 photocatalytic C–H bond activation for C–C coupling reactions and discusses some model examples, especially under visible-light irradiation.

Organocatalytic decarboxylative alkylation of N-hydroxy-phthalimide esters enabled by pyridine-boryl radicals

The decarboxylative alkylation of N-hydroxyphthalimide (NHPI) based reactive esters with olefins has been achieved via an organocatalytic strategy.