Alcohol Etherification via Alkoxy Radicals Generated by Visible-Light Photoredox Catalysis

Electrochemical Deconstructive and Ring-Expansion Functionalization of Unstrained Cycloalkanols

An efficient and sustainable electrochemical method for the synthesis of cyclic ethers and acyclic aldehydes from alkanols has been reported. This strategy has been successfully applied to cycloalkanols bearing different ring sizes and different types of nucleophiles. In addition, mechanistic investigations show that the reactions undergo sequential processes, including anodic oxidation, β-scission, and nucleophilic addition. This method provides a new synthetic approach to constructing cyclic ethers and terminal aldehydes from cycloalkanols and nucleophiles.

Synthesis of Sterically Hindered Dialkyl Ethers via Palladium-Catalyzed Fluoro-alkoxylation of gem-Difluoroalkenes

Organofluorine compounds are of high value in medicinal and agricultural chemistry. Herein, we report a palladium-catalyzed fluoro-alkoxylation of gem-difluoroalkenes for the synthesis of much more challenging sterically hindered ethers. This reaction represents a direct synthesis method for α-trifluoromethyl ethers with a broad functional group tolerance and excellent regioselectivity. This system employs N-fluorobenzenesulfonimide (NFSI) as an electrophilic fluorine source and alcohols as nucleophilic donors, including but not limited to sterically hindered tert-substituted alcohols.

Photo-Induced Generation of Oxygenated Quaternary Centers via EnT Enabled Singlet O2 Addition to C3-Maleimidated Quinoxaline: A Reagent-Less Approach

Demonstrated here is an external photo-sensitizer-free (auto-sensitized) singlet oxygen-enabled solvent-dependent tertiary hydroxylation and aryl-alkyl spiro-etherification of C3-maleimidated quinoxalines. Such "reagent-less" photo-oxygenation at Csp3-H and etherification involving Csp3-H/Csp2-H are unparalleled. Possibly, the highly π-conjugated N-H tautomer allows the substrate to get excited by irradiation, and subsequently, it attains the triplet state via ISC. This excited triplet-state sensitized molecule then transfers its energy to a triplet-state oxygen (3O2) generating reactive singlet oxygen (1O2) for hydroxylation and spirocyclization depending on the solvent used. In HFIP, the generated alkoxy radical accepts a proton via HAT giving hydroxylated product. In contrast, in an aprotic PhCl it underwent a radical addition at the ortho-position of the C2 aryl to provide spiro-ether. An unprecedented orthogonal spiro-etherification was observed via the displacement of o-substitutents for ortho (-OEt, -OMe, -F, -Cl, -Br) substituted substrates. The order of ipso substitution follows the trend -OMe>-OEt>-F>-H>-Cl>-Br. Both these oxygenation reactions can be carried out with nearly equal ease using direct sunlight without the requirement of any elaborate reaction setup. Demonstration of large-scale synthesis and a few interesting transformations have also been realized. Furthermore, several insightful control experiments and quantum chemical computations were performed to unravel the mechanism.

Organohypervalent heterocycles

This review summarizes the structural and synthetic aspects of heterocyclic molecules incorporating an atom of a hypervalent main-group element. The term "hypervalent" has been suggested for derivatives of main-group elements with more than eight valence electrons, and the concept of hypervalency is commonly used despite some criticism from theoretical chemists. The significantly higher thermal stability of hypervalent heterocycles compared to their acyclic analogs adds special features to their chemistry, particularly for bromine and iodine. Heterocyclic compounds of elements with double bonds are not categorized as hypervalent molecules owing to the zwitterionic nature of these bonds, resulting in the conventional 8-electron species. This review is focused on hypervalent heterocyclic derivatives of nonmetal main-group elements, such as boron, silicon, nitrogen, carbon, phosphorus, sulfur, selenium, bromine, chlorine, iodine(III) and iodine(V).

Access to Chromenopyrrolidines Enabled by Organophotocatalyzed [2 + 2 + 1] Annulation of Chromones with N-Arylglycines

A facile approach toward chromenopyrrolidines was achieved under mild conditions via organophotocatalyzed aerobic decarboxylative [2 + 2 + 1] annulation of chromones with N-arylglycines, in which N-arylglycines perform dual roles (i.e., radical precursor and methylene provider). Mechanistic studies suggested that a Giese-type radical addition and consequent Mannich pathway were likely responsible for the annulation reaction.

Transition metal-free visible light photoredox-catalyzed remote C(sp3)-H borylation enabled by 1,5-hydrogen atom transfer

The borylation of unreactive carbon-hydrogen bonds is a valuable method for transforming feedstock chemicals into versatile building blocks. Here, we describe a transition metal-free method for the photoredox-catalyzed borylation of unactivated C(sp³)-H bond, initiated by 1,5-hydrogen atom transfer (HAT). The remote borylation was directed by 1,5-HAT of the amidyl radical, which was generated by photocatalytic reduction of hydroxamic acid derivatives. The method accommodates substrates with primary, secondary and tertiary C(sp³)-H bonds, yielding moderate to good product yields (up to 92%) with tolerance for various functional groups. Mechanistic studies, including radical clock experiments and DFT calculations, provided detailed insight into the 1,5-HAT borylation process.

Utilizing a needle as a source of iron in synergistic dual photoredox catalytic generation of alkoxy radicals

A visible-light mediated alkoxy radical generation is described, which allows for a structurally divergent oxidative C(sp³)-H bond functionalization. This protocol employs a photoredox/iron dual catalysis allowing for an unprecedented chemoselective single-step transformation of alcohol derivatives giving access to two valuable sets of products, tetrahydrofurans and γ-bromoketones, under one set of conditions. Addition of iron, by slow corrosion of a needle, provides superior reaction efficiency as compared to various iron(III) complexes.

Hypervalent Iodine Reagents Enable C-H Alkynylation with Iminophenylacetic Acids via Alkoxyl Radicals

Here we report δ-C-H alkynylation to synthesize various δ-alkynols from iminophenylacetic acids. The hypervalent iodine-coordinated benziodoxole-alkoxyl-iminophenylacetic acid complex was the key intermediate and was characterized by X-ray crystallography for the first time. δ-C-H alkynylation is compatible with sensitive functional groups, including azides, aldehydes, and free alcohols, for the synthesis of δ-alkynols with diversified substituents in excellent regioselectivity. This reaction extends to δ-hydroxylalkene and δ-hydroxylnitrile synthesis, and the δ-alkynol products are easily derivatized to other valuable bifunctional building blocks.

Alkoxy Radicals See the Light: New Paradigms of Photochemical Synthesis

Alkoxy radicals are highly reactive species that have long been recognized as versatile intermediates in organic synthesis. However, their development has long been impeded due to a lack of convenient methods for their generation. Thanks to advances in photoredox catalysis, enabling facile access to alkoxy radicals from bench-stable precursors and free alcohols under mild conditions, research interest in this field has been renewed. This review comprehensively summarizes the recent progress in alkoxy radical-mediated transformations under visible light irradiation. Elementary steps for alkoxy radical generation from either radical precursors or free alcohols are central to reaction development; thus, each section is categorized and discussed accordingly. Throughout this review, we have focused on the different mechanisms of alkoxy radical generation as well as their impact on synthetic utilizations. Notably, the catalytic generation of alkoxy radicals from abundant alcohols is still in the early stage, providing intriguing opportunities to exploit alkoxy radicals for diverse synthetic paradigms.

Hypervalent Iodine(III)-Promoted Radical Oxidative C-H Annulation of Arylamines with α-Keto Acids

A novel catalyst-free radical oxidative C-H annulation reaction of arylamines with α-keto acids toward benzoxazin-2-ones synthesis under mild conditions was developed. This hypervalent iodine(III)-promoted process eliminated the use of a metal catalyst or additive with high levels of functional group tolerance. Hypervalent iodine(III) was both an oxidant and a radical initiator for this reaction. The synthetic utility of this method was confirmed by the synthesis of the natural product cephalandole A.

Visible light photoredox-catalysed remote C–H functionalisation enabled by 1,5-hydrogen atom transfer (1,5-HAT)

The generation of heteroatom-centred radicals followed by intramolecular 1,5-HAT and functionalisation of the translocated carbon-centred radical is an efficient way to functionalize chemo- and regio-selectively the remote unactivated C(sp³)–H bond.