Catalytic Oxidative Cyclization of 2'-Arylbenzaldehyde Oxime Ethers under Photoinduced Electron Transfer Conditions

Visible light-induced reductive aza-6π electrocyclization access to phenanthridines

Visible light-induced aza-6π electrocyclization was developed for the synthesis of aza-arenes from nitroarenes with diverse aldehydes. This protocol allows the reduction of nitroarenes by B2nep2 and subsequent 6π-electrocyclization of the in situ formed imine under visible light. An array of 6- and multi-substituted phenanthridines were constructed in moderate to good yields under purple LEDs at room temperature. A wide scope of substrates with diverse functional groups were well tolerated. In addition, the synthetic utility of this methodology was further demonstrated in the late-stage functionalization of celecoxib.

The antimicrobial potential of adarotene derivatives against Staphylococcus aureus strains

Multidrug-resistant (MDR) pathogens are severely impacting our ability to successfully treat common infections. Here we report the synthesis of a panel of adarotene-related retinoids showing potent antimicrobial activity on Staphylococcus aureus strains (including multidrug-resistant ones). Fluorescence and molecular dynamic studies confirmed that the adarotene analogues were able to induce conformational changes and disfunctions to the cell membrane, perturbing the permeability of the phospholipid bilayer. Since the major obstacle for developing retinoids is their potential cytotoxicity, a selected candidate was further investigated to evaluate its activity on a panel of human cell lines. The compound was found to be well tolerated, with IC50 5-15-fold higher than the MIC on S. aureus strains. Furthermore, the adarotene analogue had a good pharmacokinetic profile, reaching a plasma concentration of about 6 μM after 0.5 h after administration (150 mg/kg), at least twice the MIC observed against various bacterial strains. Moreover, it was demonstrated that the compound potentiated the growth-inhibitory effect of the poorly bioavailable rifaximin, when used in combination. Overall, the collected data pave the way for the development of synthetic retinoids as potential therapeutics for hard-to-treat infectious diseases caused by antibiotic-resistant Gram-positive pathogens.

A unified approach to benzo[c]phenanthridines via the cascade dual-annulation/formylation of 2-alkynyl/alkenylbenzonitriles

A base-mediated versatile cascade dual-annulation and formylation of 2-alkenyl/alkynylbenzonitriles with 2-methylbenzonitriles has been established for the construction of four different classes of amino and amido substituted benzo[c]phenanthridines and benzo[c]phenanthrolines. The synthesized molecules could be of utmost relevance in pharmaceuticals. The transformation uses the solvent DMF as the formyl source for synthesis of the amido-substituted scaffolds. This transition-metal-free unique strategy enables the formation of multiple C-C and C-N bonds in one pot at room temperature.

Enroute sustainability: metal free C-H bond functionalisation

The term "C-H functionalisation" incorporates C-H activation followed by its transformation. In a single line, this can be defined as the conversion of carbon-hydrogen bonds into carbon-carbon or carbon-heteroatom bonds. The catalytic functionalisation of C-H bonds using transition metals has emerged as an atom-economical technique to engender new bonds without activated precursors which can be considered as a major drawback while attempting large-scale synthesis. Replacing the transition-metal-catalysed approach with a metal-free strategy significantly offers an alternative route that is not only inexpensive but also environmentally benign to functionalize C-H bonds. Recently metal free synthetic approaches have been flourishing to functionalize C-H bonds, motivated by the search for greener, cost-effective, and non-toxic catalysts. In this review, we will highlight the comprehensive and up-to-date discussion on recent examples of ground-breaking research on green and sustainable metal-free C-H bond functionalisation.

o-Nitrobenzyl Oxime Ethers Enable Photoinduced Cyclization Reaction to Provide Phenanthridines under Aqueous Conditions

In this paper, we describe a novel N-O photolysis of o-nitrobenzyl oxime ethers that enables the synthesis of phenanthridines via intramolecular cyclization reactions. Without the use of additional photocatalysts or photosensitizers, the process proceeds with an efficiency of ≤96% upon exposure of the sample to near-visible light (405 nm) under aqueous conditions. Through the photoinduced production of a fluorescent phenanthridine derivative in HeLa cells, the progress of the reaction under biological conditions was demonstrated. This photoinduced cyclization reaction could be used as a different photochemical instrument to control biological processes by inducing the production of bioactive molecules.

Computational Investigation of the Formation of Substituted Isoindole N-Oxides through the Photo-oxidative Cyclization of 2'-Alkynylacetophenone Oximes

Our recently published joint experiment-theory study of the photo-oxidative intramolecular cyclization of 2'-alkynylacetophenone oximes, performed in collaboration with the de Lijser group, presented the first reported formation of isoindole N-oxides. That study focused on determining a mechanistic explanation for the unexpected chemistry observed when three 2'-alkynylacetophenone oximes were photo-oxidized with 9,10-dicyanoanthracene (DCA), specifically the derivatives with a phenyl, isopropyl, or n-butyl substituent at the alkynyl group. Here, we use density functional theory to develop a broader understanding of the scope of this chemistry. In particular, we demonstrate that substituents on the alkynyl group and on the central benzene ring can significantly modulate the thermodynamic driving force for oxime radical cation generation when DCA is used as the photosensitizer. In contrast, substituents are shown to have a small impact on the chemical reactivity of the radical cation intermediates. In particular, 5-exo radical cation cyclization, which ultimately results in an isoindole N-oxide product, is always kinetically and sometimes also thermodynamically preferred over 6-endo radical cation cyclization, which would produce an isoquinoline N-oxide product. Overall, this study provides mechanistic insights into the diversity of isoindole N-oxides that can be produced through the photo-oxidative cyclization of 2'-alkynylacetophenone oximes.

Synthesis of Six-Membered N-Heterocycle Frameworks Based on Intramolecular Metal-Free N-Centered Radical Chemistry

The formation of intramolecular C-N bond represents a powerful strategy in organic transformation as the great importance of N-heterocycles in the fields of natural products and bioactive molecules. This personal account describes the synthesis of cyclic phosphinamidation, benzosultam, benzosulfoximine, phenanthridine and their halogenated compounds through transition-metal-free intramolecular oxidative C-N bond formation. Mechanism study reveals that N-X bond is initially formed under the effect of hypervalent halogen, which is very unstable and easily undergoes thermal or light homolytic cleavage to generate nitrogen radical. Then the nitrogen radical is trapped by the arene to give aryl radical. Rearomatization of aryl radical under the oxidant to provide corresponding N-heterocycle. Under suitable conditions, the N-heterocycles can be further transformed to halogenated N-heterocycles.

Photochemical radical cyclization reactions with imines, hydrazones, oximes and related compounds

Photochemical reactions are a key method to generate radical intermediates. Often under these conditions no toxic reagents are necessary. During recent years, photo-redox catalytic reactions considerably push this research domain. These reaction conditions are particularly mild and safe which enables the transformation of poly-functional substrates into complex products. The synthesis of heterocyclic compounds is particularly important since they play an important role in the research of biologically active products. In this review, photochemical radical cyclization reactions of imines and related compounds such as oximes, hydrazones and chloroimines are presented. Reaction mechanisms are discussed and the structural diversity and complexity of the products are presented. Radical intermediates are mainly generated in two ways: (1) electronic excitation is achieved by light absorption of the substrates. (2) The application of photoredox catalysis is now systematically studied for these reactions. Recently, also excitation of charge transfer complexes has been studied in this context from many perspectives.

Mechanistic Investigation of the Formation of Isoindole N-Oxides in the Electron Transfer-Mediated Oxidative Cyclization of 2'-Alkynylacetophenone Oximes

This paper describes a joint experiment-theory investigation of the formation and cyclization of 2'-alkynylacetophenone oxime radical cations using photoinduced electron transfer (PET) with DCA as the photosensitizer. Using a combination of experimental ¹H and ¹³C nuclear magnetic resonance (NMR) spectra, high-resolution mass spectrometry, and calculated NMR chemical shifts, we identified the products to be isoindole N-oxides. The reaction was found to be stereoselective; only one of the two possible stereoisomers is formed under these conditions. A detailed computational investigation of the cyclization reaction mechanism suggests facile C-N bond formation in the radical cation leading to a 5-exo intermediate. Back-electron transfer from the DCA radical anion followed by barrierless intramolecular proton transfer leads to the final product. We argue that the final proton transfer step in the mechanism is responsible for the stereoselectivity observed in experiment. As a whole, this work provides new insights into the formation of complex heterocycles through oxime and oxime ether radical cation intermediates produced via PET. Moreover, it represents the first reported formation of isoindole N-oxides.

Tandem Reaction Approaches to Isoquinolones from 2-Vinylbenzaldehydes and Anilines via Imine Formation-6π-Electrocyclization-Aerobic Oxidation Sequence

Two distinctive transition-metal-promoted aerobic oxidation protocols have been developed for the synthesis of isoquinolones from 2-vinylbenzaldehydes and aniline derivatives. Thus, the one-pot tandem reaction sequence of imine formation, thermal 6π-electrocyclization, followed by either Cu(OAc)₂-mediated or Pd(OAc)₂-catalyzed aerobic oxidation protocol allowed the ready access to isoquinolone derivatives. The control experiments revealed that the 1,2-dihydroisoquinoline intermediates from the 6π-electrocyclization of 1-azatrienes were aerobically oxidized to isoquinolones in the presence of either Cu(OAc)₂ or Pd(OAc)₂ catalyst.

Energy transfer-driven regioselective synthesis of functionalized phenanthridines by visible-light Ir photocatalysis

A photocatalytic strategy for selective synthesis of 2-substituted phenanthridines from N-iminylpyridinium salts has been developed.

Ruthenium(ii)-catalyzed [5 + 1] annulation reaction: a facile and efficient approach to construct 6-ethenyl phenanthridines utilizing a primary amine as a directing group

A ruthenium(ii)-catalyzed [5 + 1] annulation reaction between 2-arylanilines and cyclopropenones employing a free amine as a directing group has been developed.

Computational Investigation of Substituent Effects on the Formation and Intramolecular Cyclization of 2'-Arylbenzaldehyde and 2'-Arylacetophenone Oxime Ether Radical Cations

This work describes our computational study of substituent effects on the formation and cyclization of 2'-arylbenzaldehyde and 2'-arylacetophenone oxime ether radical cations. Recent experimental work by de Lijser and co-workers has demonstrated that these reactive intermediates, which are generated through photoinduced electron transfer (PET) with a photosensitizer, undergo intramolecular cyclization to yield substituted phenanthridines. The experimental study further showed correlations between the yield of cyclized products and the Hammett σ_Para⁺ parameter of the substituent on the aryl group, with both strongly electron-withdrawing and electron-donating substituents shown to significantly reduce the product yield. By analyzing the ΔG_PET associated with radical cation formation as well as the thermodynamics and kinetics of radical cation cyclization, we provide an explanation for these observations. We then computationally extend this mechanistic analysis to 2'-arylbenzaldehyde oxime ethers with substituents also present on the central benzene ring and show that such substituents generally have a larger impact on the PET-induced cyclization than those on the aryl group. Overall, this work extends our understanding of the overall scope of this photooxidative route toward substituted phenanthridines as well as makes clear predictions as to how the formation of oxime ether radical cations can be tuned by substituents.

Probing the hydrolytic reactivity of 2-difluoromethyl pyrroles

α-Difluoromethyl pyrroles were found to be stable while N-protected with an electron-withdrawing group. Due to the propensity of pyrroles to access azafulvenium-like intermediates, the C-F bonds of an α-difluoromethyl substituent are labile under hydrolytic conditions. The presence of certain electron-withdrawing substituents about the pyrrolic ring can accelerate this process, as determined through a kinetic comparison of the deprotection and subsequent hydrolysis reactions of N-protected β-aryl α-difluoromethyl pyrroles.

Correlating the para-Substituent Effects on Ru(II)-Polypyridine Photophysical Properties and on the Corresponding Hybrid P450 BM3 Enzymes Photocatalytic Activity

Ru(II)-diimine complexes covalently attached near the heme active site of P450 BM3 enzymes have been used to rapidly inject electrons and drive selective C-H functionalization upon visible light irradiation. Herein, we have generated a series of hybrid P450 BM3 enzymes containing a photosensitizer of general formula [Ru(4,4'-X₂bpy)₂(PhenA)]²⁺ where X = Cl, H, tBu, Me OPhe, OMe, or NMe₂, bpy = 2,2'-bipyridine, and PhenA = 5-acetamido-1,10-phenanthroline. We then probed the effect of electron-withdrawing and -donating groups at the para position of the 4,4'-X₂bpy ligands on the corresponding hybrid enzymes photocatalytic activity. A 3-fold improvement in initial reaction rate was noted when varying the substituent from Cl to tBu, however, the reaction rates decrease thereafter with the more electron donating groups. In order to rationalize those effects, we investigated the variation of the substituent on the photophysical properties of the corresponding [Ru(4,4'-X₂bpy)₂(bpy)]²⁺ model complexes. Several linear correlations were established between the E(III/II) potential, the MLCT emission, and absorption energies as well as the logarithm of the luminescence quenching rate vs the summative Brown-Okamoto parameter (Σσ_p+). Moreover, a downward curved Hammett plot is observed with the hybrid enzyme initial reaction rate revealing mechanistic details about the overall light-driven enzymatic process.

9,10-Dicyanoanthracene Catalyzed Decarboxylative Alkynylation of Carboxylic Acids under Visible-Light Irradiation

A metal-free, visible-light-induced photocatalytic procedure for decarboxylative alkynylation of carboxylic acids was reported. With 9,10-dicyanoanthracene as the photoredox catalyst, the reaction covered a broad scope of α-amino acids, α-oxo acids, and α-keto acids with blue LED irradiation at room temperature under an atmosphere of argon, delivering alkynyl products in moderate to excellent yields. Natural sun light also promoted this alkynylation strategy. This work represents the first example of an organophotocatalytic method for decarboxylative alkynylation of carboxylic acids.

Synthetic Strategies for 5- and 6-Membered Ring Azaheterocycles Facilitated by Iminyl Radicals

The totality of chemical space is so immense that only a small fraction can ever be explored. Computational searching has indicated that bioactivity is associated with a comparatively small number of ring-containing structures. Pyrrole, indole, pyridine, quinoline, quinazoline and related 6-membered ring-containing aza-arenes figure prominently. This review focuses on the search for fast, efficient and environmentally friendly preparative methods for these rings with specific emphasis on iminyl radical-mediated procedures. Oxime derivatives, particularly oxime esters and oxime ethers, are attractive precursors for these radicals. Their use is described in conventional thermolytic, microwave-assisted and UV-vis based preparative procedures. Photoredox-catalyzed protocols involving designer oxime ethers are also covered. Choice can be made amongst these synthetic strategies for a wide variety of 5- and 6-membered ring heterocycles including phenanthridine and related aza-arenes. Applications to selected natural products and bioactive molecules, including trispheridine, vasconine, luotonin A and rutaecarpine, are included.

Functionalised Oximes: Emergent Precursors for Carbon-, Nitrogen- and Oxygen-Centred Radicals

Oxime derivatives are easily made, are non-hazardous and have long shelf lives. They contain weak N-O bonds that undergo homolytic scission, on appropriate thermal or photochemical stimulus, to initially release a pair of N- and O-centred radicals. This article reviews the use of these precursors for studying the structures, reactions and kinetics of the released radicals. Two classes have been exploited for radical generation; one comprises carbonyl oximes, principally oxime esters and amides, and the second comprises oxime ethers. Both classes release an iminyl radical together with an equal amount of a second oxygen-centred radical. The O-centred radicals derived from carbonyl oximes decarboxylate giving access to a variety of carbon-centred and nitrogen-centred species. Methods developed for homolytically dissociating the oxime derivatives include UV irradiation, conventional thermal and microwave heating. Photoredox catalytic methods succeed well with specially functionalised oximes and this aspect is also reviewed. Attention is also drawn to the key contributions made by EPR spectroscopy, aided by DFT computations, in elucidating the structures and dynamics of the transient intermediates.