Photocatalyzed Site-Selective C(sp3)-H Functionalization of Alkylpyridines at Non-Benzylic Positions

Radical reactions enabled by polyfluoroaryl fragments: photocatalysis and beyond

Polyfluoroarenes have been known for a long time, but they are most often used as fluorinated building blocks for the synthesis of aromatic compounds. At the same time, due to peculiar fluorine effect, they have unique properties that provide applications in various fields ranging from synthesis to materials science. This review summarizes advances in the radical chemistry of polyfluoroarenes, which have become possible mainly with the advent of photocatalysis. Transformations of the fluorinated ring via the C-F bond activation, as well as use of fluoroaryl fragments as activating groups and hydrogen atom transfer agents are discussed. The ability of fluoroarenes to serve as catalysts is also considred.

Membrane-based TBADT recovery as a strategy to increase the sustainability of continuous-flow photocatalytic HAT transformations

Photocatalytic hydrogen atom transfer (HAT) processes have been the object of numerous studies showcasing the potential of the homogeneous photocatalyst tetrabutylammonium decatungstate (TBADT) for the functionalization of C(sp³)-H bonds. However, to translate these studies into large-scale industrial processes, careful considerations of catalyst loading, cost, and removal are required. This work presents organic solvent nanofiltration (OSN) as an answer to reduce TBADT consumption, increase its turnover number and lower its concentration in the product solution, thus enabling large-scale photocatalytic HAT-based transformations. The operating parameters for a suitable membrane for TBADT recovery in acetonitrile were optimized. Continuous photocatalytic C(sp³)-H alkylation and amination reactions were carried out with in-line TBADT recovery via two OSN steps. Promisingly, the observed product yields for the reactions with in-line catalyst recycling are comparable to those of reactions performed with pristine TBADT, therefore highlighting that not only catalyst recovery (>99%, TON > 8400) is a possibility, but also that it does not happen at the expense of reaction performance.

Visible-light-mediated one-pot efficient synthesis of 1-aryl-1H,3H-thiazolo[3,4-a]benzimidazoles: a metal-free photochemical approach in aqueous ethanol

A new metal-free approach to construct medicinally valuable 1-aryl-1H,3H-thiazolo[3,4-a]benzimidazoles under visible light irradiation in aqueous ethanol medium at room temperature has been developed. The present process was performed with 1,2-phenylenediamines, aromatic aldehydes and 2-mercaptoacetic acid utilizing a simple household 22 W compact fluorescent lamp to generate C-S, C-N bonds through radical intermediates. This visible-light-promoted synthesis provides lower cost, operation simplicity and high functional groups tolerating ability with short reaction time and high yield under mild reaction conditions.

Direct Photocatalyzed Hydrogen Atom Transfer (HAT) for Aliphatic C-H Bonds Elaboration

Direct photocatalyzed hydrogen atom transfer (d-HAT) can be considered a method of choice for the elaboration of aliphatic C–H bonds. In this manifold, a photocatalyst (PC_HAT) exploits the energy of a photon to trigger the homolytic cleavage of such bonds in organic compounds. Selective C–H bond elaboration may be achieved by a judicious choice of the hydrogen abstractor (key parameters are the electronic character and the molecular structure), as well as reaction additives. Different are the classes of PCs_HAT available, including aromatic ketones, xanthene dyes (Eosin Y), polyoxometalates, uranyl salts, a metal-oxo porphyrin and a tris(amino)cyclopropenium radical dication. The processes (mainly C–C bond formation) are in most cases carried out under mild conditions with the help of visible light. The aim of this review is to offer a comprehensive survey of the synthetic applications of photocatalyzed d-HAT.

A visible-light mediated ring opening reaction of alkylidenecyclopropanes for the generation of homopropargyl radicals

Classical cyclopropylcarbinyl radical clock reactions have been widely applied to conduct mechanistic studies for probing radical processes for a long time; however, alkylidenecyclopropanes, which have a similar molecular structure to methylcyclopropanes, surprisingly have not yet attracted researcher's attention for similar ring opening radical clock processes. In recent years, photocatalytic NHPI ester activation chemistry has witnessed significant blooming developments and provided new synthetic routes for cross-coupling reactions. Herein, we wish to report a non-classical ring opening radical clock reaction using innovative NHPI esters bearing alkylidenecyclopropanes upon photoredox catalysis, providing a brand-new synthetic approach for the direct preparation of a variety of alkynyl derivatives. The potential synthetic utility of this protocol is demonstrated in the diverse transformations and facile synthesis of bioactive molecules or their derivatives and medicinal substances.

Decatungstate-Catalyzed C(sp3)-H Sulfinylation: Rapid Access to Diverse Organosulfur Functionality

Here we report the direct conversion of strong, aliphatic C(sp3)-H bonds into the corresponding alkyl sulfinic acids via decatungstate photocatalysis. This transformation has been applied to a diverse range of C(sp3)-rich scaffolds, including natural products and approved pharmaceuticals, providing efficient access to complex sulfur-containing products. To demonstrate the broad potential of this methodology for the divergent synthesis of pharmaceutically relevant molecules, procedures for the diversification of the sulfinic acid products into a range of medicinally relevant functional groups have been developed.

C-H Alkylation of Aldehydes by Merging TBADT Hydrogen Atom Transfer with Nickel Catalysis

Catalyst controlled site-selective C-H functionalization is a challenging but powerful tool in organic synthesis. Polarity-matched and sterically controlled hydrogen atom transfer (HAT) provides an excellent opportunity for site-selective functionalization. As such, the dual Ni/photoredox system was successfully employed to generate acyl radicals from aldehydes via selective formyl C-H activation and subsequently cross-coupled to generate ketones, a ubiquitous structural motif present in the vast majority of natural and bioactive molecules. However, only a handful of examples that are constrained to the use of aryl halides are developed. Given the wide availability of amines, we developed a cross-coupling reaction via C-N bond cleavage using the economic nickel and TBADT catalyst for the first time. A range of alkyl and aryl aldehydes were cross-coupled with benzylic and allylic pyridinium salts to afford ketones with a broad spectrum of functional group tolerance. High regioselectivity toward formyl C-H bonds even in the presence of α-methylene carbonyl or α-amino/oxy methylene was obtained.

A computational study of site-selective hydrogen abstraction by sulfate radical anion

Many hydrogen abstraction reactions on sp3 carbons with oxyradicals take place site-selectively (regioselectively). To investigate this selectivity, ab initio and density functional theory (DFT) calculations were carried out using cyclopentanone and SO4-˙ as the substrate and oxyradical, respectively. At the ωB97XD/6-311+G(d,p) level, the energy barriers for the forward process (ΔE1‡) of both α- and β-hydrogen abstraction were predicted to be 54.6 and 50.9 kJ mol-1, respectively. Consideration of solvent effects (acetonitrile) decreased these energy barriers to 33.2 and 26.1 kJ mol-1, respectively. These calculation outcomes suggested that β-hydrogen abstraction would be favourable, which supports experimental findings (i.e. β-selective abstraction). At the ωB97XD level, investigations into hydrogen abstraction from cyclohexanone with SO4-˙ confirmed the regioselectivity observed experimentally. Hydrogen abstractions from 2-propylpyridine and 3-methyl-1-butanol using SO4-˙, which are unknown reactions, were also calculated using the DFT method, and the predicted regioselectivity was consistent with that in the known reactions using tetrabutylammonium decatungstate (TBADT). In addition, regioselectivities in unexplored hydrogen abstractions of cyclopentanone by several oxyradicals were predicted. Natural bond orbital (NBO) analysis carried out at the ωB97XD level indicated that the transferred hydrogen atom is partially positively charged when abstracted by an oxyradical. Interestingly, hydrogens bonded to the most positively charged carbon in the substrate were predominantly abstracted by oxyradicals in practice, which should be a simple compass for predicting regioselectivity in the functionalisation of C(sp3)-H bonds with oxyradicals.

Site-Selective Alkenylation of Unactivated C(sp3 )-H Bonds Mediated by Compact Sulfate Radical

A broad variety of unactivated acyclic and alicyclic substrates cleanly undergo site-selective alkenylation of unactivated C(sp³ )-H bonds with 1,2-bis(phenylsulfonyl)ethene in the presence of persulfate. This simple transformation furnishes (E)-2-alkylvinylphenylsulfones in up to 88 % yield. In contrast with the previously reported decatungstate protocol, the current method is applicable to alkenylation of sterically hindered C-H bonds. This important advantage significantly broadens the substrate scope, and is attributed to the compact size of the sulfate radical employed in the C-H activation and cleavage.

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.

Optimization of a Decatungstate-Catalyzed C(sp3)-H Alkylation Using a Continuous Oscillatory Millistructured Photoreactor

Tetrabutylammonium decatungstate (TBADT) has emerged as an efficient and versatile photocatalyst for hydrogen atom transfer (HAT) processes that enables the cleavage of both activated and unactivated aliphatic C-H bonds. Using a recently developed oscillatory millistructured continuous-flow photoreactor, investigations of a decatungstate-catalyzed C(sp³)-H alkylation protocol were carried out, and the results are presented here. The performance of the reactor was evaluated in correlation to several chemical and process parameters, including residence time, light intensity, catalyst loading, and substrate/reagent concentration. In comparison with previously reported batch and flow protocols, conditions were found that led to considerably higher productivity, achieving a throughput up to 36.7 mmol/h with a residence time of only 7.5 min.

One-Pot Formal Dehydrogenative Ketone Synthesis from Aldehydes and Non-activated Hydrocarbons

Ketones are a fundamental functionality found throughout a range of natural and synthetic compounds, making their synthesis essential throughout the chemical disciplines. Herein, we describe a one-pot synthesis of ketones via decatungstate-mediated formal dehydrogenative coupling between aldehydes and non-activated hydrocarbons. A variety of substituted benzaldehydes and cycloalkanes could be used in the optimized reaction to produce the desired ketones in moderate yields. The decatungstate photocatalyst functions in two reactions in this synthesis, catalyzing both the coupling and oxidation steps of the process. Notably, the reaction displays both high atom economy and sustainability, as it uses light and oxygen as key energy sources.

Selective C(sp3 )-H Aerobic Oxidation Enabled by Decatungstate Photocatalysis in Flow

A mild and selective C(sp3 )-H aerobic oxidation enabled by decatungstate photocatalysis has been developed. The reaction can be significantly improved in a microflow reactor enabling the safe use of oxygen and enhanced irradiation of the reaction mixture. Our method allows for the oxidation of both activated and unactivated C-H bonds (30 examples). The ability to selectively oxidize natural scaffolds, such as (-)-ambroxide, pregnenolone acetate, (+)-sclareolide, and artemisinin, exemplifies the utility of this new method.