Isolated Neutral [4]Helicene Radical Provides Insight into Consecutive Two-Photon Excitation Photocatalysis

Direct activation of strong bonds in readily available, benchtop substrates offer a straightforward simplification, albeit in most cases existing catalytic systems are limited to unlock such activation. In recent years, a surge of in-situ generated organic radicals that can act as potent photoinduced electron transfer (PET) agents have proved to be a powerful manifold for the activation of remarkably stable bonds. Herein we document the use of N,N′-di-n-propyl-1,13-dimethoxyquinacridine (nPr-DMQA•), an isolated and stable neutral helicene radical, as a highly photoreducing species. This isolable doublet state open shell radical offers a unique opportunity to shed light on the mechanism behind PET reactions of organic radicals. Experimental and spectroscopic studies revealed that this doublet radical has a long lifetime of 4.6 ± 0.2 ns, an estimated excited state oxidation potential of -3.31 V vs SCE, and can undergoes PET with organic substrates. The strongly photoreducing nature of the nPr-DMQA• was experimentally confirmed by the demonstration of photo activation of electron rich aryl bromides and chlorides. We further demonstrated that nPr-DMQA• can be photochemically generated from its cation analog (nPr-DMQA+) allowing catalytic functionalization of aryl halide via a consecutive photoexcitation mechanism (ConPET). Dehalogenation, photo-Arbuzov, photo-borylation and C-C bond formation reactions with aryl chlorides and bromides are reported herein, as well as the α-arylation of carbonyl using cyclic ketones. The latter transformation exhibits the facile synthesis of α-arylated cyclic ketones as critical feedstock chemical for diverse useful molecules, especially in the biomedical enterprises.

Halogen-bonding-promoted Photo-induced C–X Borylation of Aryl Halide using Phenol Derivatives

This study investigates the photo-induced C–X borylation reaction of aryl halides by forming a halogen-bonding complex. The method employs 2-naphthol as a halogen-bonding acceptor and proceeds under mild conditions without a photoredox catalyst under 420 nm blue light irradiation. The method is highly chemoselective, broadly functional group tolerant, and provides concise access to corresponding boronate esters. Mechanistic studies reveal that forming the halogen-bonding complex between aryl halide and naphthol acts as an electron donor-acceptor complex to furnish aryl radicals through photo-induced electron transfer.

A general arene C–H functionalization strategy via electron donor-acceptor complex photoactivation

The photoactivation of electron donor-acceptor (EDA) complexes has emerged as a sustainable, selective and versa-tile strategy for the generation of radical species. However, when it comes to aryl radical formation, this strategy remains hamstrung by the electronic properties of the aromatic radical precursors and electron-deficient aryl halide acceptors are required. This has prevented the implementation of a general synthetic platform for aryl radical for-mation. Our study introduces triarylsulfonium salts as acceptors in photoactive EDA-complexes, used in combina-tion with catalytic amounts of newly-designed amine donors. The sulfonium salt label renders inconsequential the electronic features of the aryl radical precursor and, more importantly, it is installed regioselectively in native aro-matic compounds by C–H sulfenylation. Using this general, site-selective aromatic C–H functionalization approach, we have developed metal-free protocols for the alkylation and cyanation of arenes, and showcased their application in both the synthesis and the late-stage modification of pharmaceuticals and agrochemicals.

Mechanistic insight into deep holes from interband transitions in Palladium nanoparticle photocatalysts

Photocatalysis of metallic nanoparticles, especially utilizing hot electrons generated from localized surface plasmon resonance, is of widespread interest. However, the role of hot holes, especially generated from interband transitions, has not been emphasized in exploring the photocatalytic mechanism yet. In this study, a photocatalyzed Suzuki-Miyaura reaction using mesoporous Pd nanoparticle photocatalyst served as a model reaction to study the role of hot holes by accurately measuring the quantum yields of the photocatalyst. The quantum yields increase under shorter wavelength excitations and correlate to the “deeper” energy of the holes from the Fermi level. Our mechanistic study suggests that deeper holes in the d-band can catalyze the oxidative addition of aryl halide R-X onto Pd0 at the surface of nanoparticles to form the R-PdII-X complex, the rate-determining step of the established catalytic cycle. We pointed out that this deep hole mechanism should deserve as much attention as the well-known hot electron transfer mechanism in previous studies.

One-pot synthesis of N-benzyl-2-aminobenzoic acid, via ring opening of isatoic anhydride derivative, and its antibacterial screening

2-(N-Benzyl) amino benzoic acid is a bifunctional molecule that could be produced from the reaction between isatoic anhydride and aryl halide. Analogues and derivatives of isatoic anhydride have wide application in pharmaceuticalsincluding antibacterial activity. The aim of this study is to synthesize, characterize and screen N-benzyl isatoic anhydride and 2-(N-benzyl) amino benzoic acid for antibacterial activity. The reaction of isatoic anhydride and benzyl bromide in the presence of potassium carbonate in DMSO at room temperature yielded N-benzyl isatoic anhydride, which under hydrolysis yielded 2-(N-benzyl) amino benzoic acid in which the anhydride ring is opened up. This compound was screened against Gram positive and negative bacteria. Moderate yield of 2-(N-benzyl) amino benzoic acid, a yellow crystal (melting point of 160-162oC, percentage yield 65 %, Rf 0.19) formed by ring opening of N-benzyl isatoic anhydride. The compound showed no antibacterial activity against Escherichia coli, Staphylococcus aureus, Pseudomonas pyocyanea, Salmonella typhimurium, Klebsiella aeruginosa and Bacillus subtilis. 2-(N-benzyl)amino benzoic was synthesized, characterized and showed no activity against bacteria.

Elucidating the Mechanism of Excited State Bond Homolysis in Nickel–Bipyridine Photoredox Catalysts

Ni 2,2’–bipyridine (bpy) complexes are commonly employed photoredox catalysts of bond-forming reactions in organic chemistry. However, the mechanisms by which they operate are still under investigation. One potential mode of catalysis is via entry into Ni(I)/Ni(III) cycles, which can be made possible by light-induced, excited state Ni(II)–C bond homolysis. Here we report experimental and computational analyses of a library of Ni(II)-bpy aryl halide complexes, Ni(Rbpy)(R′Ph)Cl (R = MeO, t-Bu, H, MeOOC; R′ = CH3, H, OMe, F, CF3), to illuminate the mechanism of excited state bond homolysis. At given excitation wavelengths, photochemical homolysis rates span two orders of magnitude across these structures and correlate linearly with Hammett parameters of both bpy and aryl ligands, reflecting structural control over key metal-to-ligand charge transfer (MLCT) and ligand-to-metal charge transfer (LMCT) excited state potential energy surfaces (PESs). Temperature- and wavelength-dependent investigations reveal moderate excited state barriers (ΔH‡ ~4 kcal mol-1) and a minimum energy excitation threshold (~55 kcal mol-1, 525 nm), respectively. Correlations to electronic structure calculations further support a mechanism in which repulsive triplet excited state PESs featuring a critical aryl-to-Ni LMCT lead to bond rupture. Structural control over excited state PESs provides a rational approach to utilize photonic energy and leverage excited state bond homolysis processes in synthetic chemistry.