Generation of Functionalized Alkyl Radicals via the Direct Photoexcitation of 2,2'-(Pyridine-2,6-diyl)diphenol-Based Borates

Photochemical 1,3-boronate rearrangement enables three-component N-alkylation for α-tertiary hydroxybenzylamine synthesis

Hydroxybenzylamines are prevalent in drugs and bioactive molecules, including various antimalarial and anticancer drugs. α-tertiary alkylation of amines impacts drug-target interactions significantly through their influence on basicity and lipophilicity. Traditional N-alkylation methods, especially for α-tertiary amines, suffer from limitations due to high energy barriers from steric hindrance. In this work, we leverage visible light irradiation to enable the organoboronic acid addition to sterically hindered ketimines in the excited state. Notably, it overcomes the limitations of the well-explored Petasis reaction, which is restricted to aldimines due to the high energy barrier associated with ketimines (51.3 kcal/mol). This three-component coupling of aliphatic amines, o-phenolic ketones, and organoboronic acids delivers diverse α-tertiary o-hydroxybenzylamines (77 examples, yields up to 82%) with broad functional group tolerance. The light-driven 1,3-boronate rearrangement introduces quaternary carbon centers adjacent to the amine moiety to enable late-stage functionalization of complex bioactive molecules. This versatile tool for complex amine synthesis holds significant potential for accelerating advancements in drug discovery, chemical biology, and materials science research.

Direct Excitation Strategy for Deacylative Couplings of Ketones

The homolysis of chemical bonds represents one of the most fundamental reactivities of excited molecules. Historically, it has been exploited to generate radicals under ultraviolet (UV) light irradiation. However, unlike most contemporary radical-generating mechanisms, the direct excitation to homolyze chemical bonds and produce aliphatic carbon-centered radicals under visible light remains rare, especially in metallaphotoredox cross couplings. Herein, we present our design of the dihydropyrimidoquinolinone (DHPQ) reagents derived from ketones, which can undergo formal deacylation and homolytic C-C bond cleavage to release alkyl radicals without external photocatalysts. Spectroscopic and computational analysis reveal unique optical and structural features of DHPQs, rationalizing their faster kinetics in alkyl radical generation than a structurally similar but visible-light transparent radical precursor. Such a capability allows DHPQ to facilitate a wide range of Ni-metallaphotoredox cross couplings with aryl, alkynyl and acyl halides. Other catalytic and non-catalyzed alkylative transformations of DHPQs are also feasible with various radical acceptors. We believe this work would be of broad interest, aiding the synthetic planning with simplified operation and expanding the synthetic reach of photocatalyst-free approaches in cutting-edge research.

Radical C-Glycosylation Using Photoexcitable Unprotected Glycosyl Borate

We have developed radical C-glycosylation using photoexcitable unprotected glycosyl borate. The direct excitation of glycosyl borate under visible light irradiation enabled the generation of anomeric radical without any photoredox catalysts. The in situ generated anomeric radical was applicable to the radical addition such as Giese-type addition and Minisci-type reaction to introduce alkyl and heteroaryl groups at the anomeric position. In addition, the radical-radical coupling between the glycosyl borate and acyl imidazolide provided unprotected acyl C-glycosides.

Visible photons as ideal reagents for the activation of coloured organic compounds

In recent decades, the traceless nature of visible photons has been exploited for the development of efficient synthetic strategies for the photoconversion of colourless compounds, namely, photocatalysis, chromophore activation, and the formation of an electron donor/acceptor (EDA) complex. However, the use of photoreactive coloured organic compounds is the optimal strategy to boost visible photons as ideal reagents in synthetic protocols. In view of such premises, the present review aims to provide its readership with a collection of recent photochemical strategies facilitated via direct light absorption by coloured molecules. The protocols have been classified and presented according to the nature of the intermediate/excited state achieved during the transformation.

Visible-Light Activation of Diorganyl Bis(pyridylimino) Isoindolide Aluminum(III) Complexes and Their Organometallic Radical Reactivity

We report on the synthesis and characterization of a series of (mostly) air-stable diorganyl bis(pyridylimino) isoindolide (BPI) aluminum complexes and their chemistry upon visible-light excitation. The redox non-innocent BPI pincer ligand allows for efficient charge transfer homolytic processes of the title compounds. This makes them a universal platform for the generation of carbon-centered radicals. The photo-induced homolytic cleavage of the Al-C bonds was investigated by means of stationary and transient UV/Vis spectroscopy, spin trapping experiments, as well as EPR and NMR spectroscopy. The experimental findings were supported by quantum chemical calculations. Reactivity studies enabled the utilization of the aluminum complexes as reactants in tin-free Giese-type reactions and carbonyl alkylations under ambient conditions, which both indicated radical-polar crossover behavior. A deeper understanding of the physical fundamentals and photochemical process was provided, furnishing in turn a new strategy to control the reactivity of bench-stable aluminum organometallics.

Deoxygenative coupling of alcohols with aromatic nitriles enabled by direct visible light excitation

A general and practical protocol is presented for visible-light-driven deoxygenative coupling of alcohols with aromatic nitriles in the absence of external photocatalysts. Utilizing a hydroxyl activation strategy with carbon disulfide, this C(sp3)-C(sp2) constructing platform accommodates a broad scope of alcohols and aryl nitriles to deliver various alkyl-substituted arenes. Mechanism studies show that a single electron transfer event between a photoexcited aryl nitrile and a xanthate anion is key to the transformation.

Bis(neopentylglycolato)diboron (B2nep2) as a bidentate ligand and a reducing agent for early transition metal chlorides giving MCl4(B2nep2) complexes

We found that bis(neopentylglycolato)diboron (B2nep2) served as a bidentate ligand and a one-electron reducing agent for early transition metal chlorides to afford MCl4(B2nep2). Treatment of B2nep2 with MCl5 (M = Nb and Mo) produced MCl4(B2nep2) via two successive reactions, coordination of B2nep2 to the metal center and one-electron reduction from M(V) to M(IV), while coordination of B2nep2 to MCl4 (M = Zr, Ti) was observed without reduction of the central metals. DFT studies for the reduction of NbCl5 by B2nep2 clarified the initial formation of seven-coordinated and B2nep2-ligated Nb(V) species, NbCl5(B2nep2), and one chloride on niobium(V) moves to the Lewis acidic boron center to generate NbCl4[(B2nep2)Cl]. The chloride on the boron atom of NbCl4[(B2nep2)Cl] is trapped by the second B2nep2 to give [NbCl4(B2nep2)][ClB2nep2]. After the formation of [ClB2nep2]- as an anionic sp2-sp3 diboron adduct, one-electron reduction of the niobium(V) center produces NbCl4(B2nep2) along with [ClB2nep2]˙ as a plausible diboron species, whose decomposition affords ClBnep and B2nep2. The reduction of metal halides in the presence of B2nep2 was exemplified by green LED irradiation of TiCl4(B2nep2), producing chloride-bridged titanium(III) species, (B2nep2)TiCl2(μ-Cl)2TiCl2(B2nep2).

Radical Caging Strategy for Cholinergic Optopharmacology

Photo-caged methodologies have been indispensable for elucidating the functional mechanisms of pharmacologically active molecules at the cellular level. A photo-triggered removable unit enables control of the photo-induced expression of pharmacologically active molecular function, resulting in a rapid increase in the concentration of the bioactive compound near the target cell. However, caging the target bioactive compound generally requires specific heteroatom-based functional groups, limiting the types of molecular structures that can be caged. We have developed an unprecedented methodology for caging/uncaging on carbon atoms using a unit with a photo-cleavable carbon-boron bond. The caging/uncaging process requires installation of the CH₂-B group on the nitrogen atom that formally assembles an N-methyl group protected with a photoremovable unit. N-Methylation proceeds by photoirradiation via carbon-centered radical generation. Using this radical caging strategy to cage previously uncageable bioactive molecules, we have photocaged molecules with no general labeling sites, including acetylcholine, an endogenous neurotransmitter. Caged acetylcholine provides an unconventional tool for optopharmacology to clarify neuronal mechanisms on the basis of photo-regulating acetylcholine localization. We demonstrated the utility of this probe by monitoring uncaging in HEK cells expressing a biosensor to detect ACh on the cell surface, as well as Ca²⁺ imaging in Drosophila brain cells (ex vivo).

Radical generation enabled by photoinduced N-O bond fragmentation

Recent advances in synthetic chemistry have seen a resurgence in the development of methods for visible light-mediated radical generation. Herein, we report the development of a photoactive ester based on a quinoline N-oxide core structure, that provides a strong oxidant in its excited state. The heteroaromatic N-oxide provides access to primary, secondary, and tertiary radical intermediates, and its application toward the development of a photochemical Minisci alkylation is reported.

The Impact of Boron Hybridisation on Photocatalytic Processes

Recently the fruitful merger of organoboron chemistry and photocatalysis has come to the forefront of organic synthesis, resulting in the development of new technologies to access complex (non)borylated frameworks. Central to the success of this combination is control of boron hybridisation. Contingent on the photoactivation mode, boron as its neutral planar form or tetrahedral boronate can be used to regulate reactivity. This Minireview highlights the current state of the art in photocatalytic processes utilising organoboron compounds, paying particular attention to the role of boron hybridisation for the target transformation.

Ni/Photoredox-Catalyzed C(sp2)-C(sp3) Cross-Coupling of Alkyl Pinacolboronates and (Hetero)Aryl Bromides

Utilizing quinoline as a mild, catalytic additive, broadly applicable conditions for the Ni/photoredox-catalyzed C(sp²)-C(sp³) cross-coupling of (hetero)aryl bromides and alkyl pinacolboronate esters were developed, which can be applied to both batch and flow reactions. In addition to primary benzylic nucleophiles, both stabilized and nonstabilized secondary alkyl boronic esters are effective coupling partners. Density functional theory calculations suggest that alkyl radical generation occurs from an alkyl-B(pin)-quinoline complex, which may proceed via an energy transfer process.

Direct Photoexcitation of Xanthate Anions for Deoxygenative Alkenylation of Alcohols

In this report, we identify xanthate salts as a unique class of visible-light-excitable alkyl radical precursors that act simultaneously as strong photoreductants and alkyl radical sources. Upon direct photoexcitation of xanthate anions, efficient deoxygenative alkenylation and alkylation of a wide range of primary, secondary, and tertiary alcohols have been achieved via a one-pot protocol, avoiding any photocatalysts. This method exhibits a broad substrate scope and good functional group tolerance, enabling late-stage functionalization of complex molecules.

Selective Coupling of 1,2-Bis-Boronic Esters at the more Substituted Site through Visible-Light Activation of Electron Donor-Acceptor Complexes

1,2-Bis-boronic esters are useful synthetic intermediates particularly as the two boronic esters can be selectively functionalized. Usually, the less hindered primary boronic ester reacts, but herein, we report a coupling reaction that enables the reversal of this selectivity. This is achieved through the formation of a boronate complex with an electron-rich aryllithium which, in the presence of an electron-deficient aryl nitrile, leads to the formation of an electron donor-acceptor complex. Following visible-light photoinduced electron transfer, a primary radical is generated which isomerizes to the more stable secondary radical before radical-radical coupling with the arene radical-anion, giving β-aryl primary boronic ester products. The reactions proceed under catalyst-free conditions. This method also allows stereodivergent coupling of cyclic cis-1,2-bis-boronic esters to provide trans-substituted products, complementing the selectivity observed in the Suzuki-Miyaura reaction.