A two carbon homologation of Friedel-Crafts alkylation enabled by photochemical alkene stitching: modular assembly of cyclolignans

Herein we report an efficient two-carbon homologated variant of Friedel-Crafts alkylation via photochemical radical alkene stitching. Readily available feedstock alkenes are used as bridges between photogenerated alkyl radicals and arenes, opening a route to γ-aryl-carbonyls for chemo-divergent access to aryltetralone and γ-lactones, a gateway to 2,7'-cyclolignans.

Direct Synthesis of Esters from Alkylarenes and Carboxylic Acids: The C-H Bond Dehydroesterification

Herein, a reaction in which the benzyl C-H bonds of alkylarenes are directly esterified by carboxylic acids to produce benzyl esters in high yields is reported. This reaction is catalyzed by Pd nanoparticles (NPs) on N-doped carbon (CN) composites based on a carbonizing Al-MIL-101(NH₂) material, and no oxidants or hydrogen acceptors are required. Use of o-alkylbenzoic acids as substrates leads to phthalides, whereas with carboxylic acids and alkylarenes as the feedstock, the reaction produces the benzyl esters. These reactions that use readily available alkylarenes instead of benzyl halides or benzyl alcohols as raw materials for one-step synthesis of benzyl esters without oxidants are inherently atom- and step-efficient. The CN composites and the CN-supported Pd NP catalysts were prepared and are well characterized. The proposed mechanism involves dehydrogenation of both the carboxylic groups and the benzylic groups and the transformation of benzylic C-H bond into the C-O bond via hydrogen abstraction from the benzylic group through an organopalladium intermediate. The kinetic isotope effect (k_H/k_D = 2.77) indicated that C(sp³)-H bond cleavage of the alkane aromatics is the rate-determining step.

Electrochemical C(sp3)-H Lactonization of 2-Alkylbenzoic Acids toward Phthalides

Herein, we report direct electrochemical C(sp³)-H lactonization of 2-alkylbenzoic acids toward phthalides. The reaction provides a wide substrate scope of 2-alkylbenzoic acids bearing primary to tertiary C(sp³)-H bonds by utilizing a graphite anode, dichloromethane (DCM) solvent, hexafluoroisopropanol (HFIP) cosolvent, and n-Bu₄NClO₄ electrolyte. Our synthetic approach offers a simple, intuitive, and atom-economical protocol to synthesize various phthalides (25 examples, up to 92% yield) and obtain other 5- and 6-membered lactones (10 examples, up to 83% yield).

HFIP in Organic Synthesis

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) is a polar, strongly hydrogen bond-donating solvent that has found numerous uses in organic synthesis due to its ability to stabilize ionic species, transfer protons, and engage in a range of other intermolecular interactions. The use of this solvent has exponentially increased in the past decade and has become a solvent of choice in some areas, such as C-H functionalization chemistry. In this review, following a brief history of HFIP in organic synthesis and an overview of its physical properties, literature examples of organic reactions using HFIP as a solvent or an additive are presented, emphasizing the effect of solvent of each reaction.

Diradicals Photogeneration from Chloroaryl‐Substituted Carboxylic Acids

With the aim of generating new, thermally inaccessible diradicals, potentially able to induce a double‐strand DNA cleavage, the photochemistry of a set of chloroaryl‐substituted carboxylic acids in polar media was investigated. The photoheterolytic cleavage of the Ar−Cl bond occurred in each case to form the corresponding triplet phenyl cations. Under basic conditions, the photorelease of the chloride anion was accompanied by an intramolecular electron‐transfer from the carboxylate group to the aromatic radical cationic site to give a diradical species. This latter intermediate could then undergo CO₂ loss in a structure‐dependent fashion, according to the stability of the resulting diradical, or abstract a hydrogen atom from the medium. In aqueous environment at physiological pH (pH=7.3), both a phenyl cation and a diradical chemistry was observed. The mechanistic scenario and the role of the various intermediates (aryl cations and diradicals) involved in the process was supported by computational analysis.

Aroyl Fluorides as Bifunctional Reagents for Dearomatizing Fluoroaroylation of Benzofurans

The 2,3-dihydrobenzofuran scaffold is widely found in natural products and biologically active compounds. Herein, dearomatizing 2,3-fluoroaroylation of benzofurans with aroyl fluorides as bifunctional reagents to access 2,3-difunctionalized dihydrobenzofurans is reported. The reaction that occurs by cooperative NHC/photoredox catalysis provides 3-aroyl-2-fluoro-2,3-dihydrobenzofurans with moderate to good yield and high diastereoselectivity. Cascades proceed via radical/radical cross-coupling of a benzofuran radical cation generated in the photoredox catalysis cycle with a neutral ketyl radical formed through the NHC catalysis cycle. The redox-neutral transformation exhibits broad substrate scope and high functional group compatibility. With anhydrides as bifunctional reagents, dearomatizing aroyloxyacylation of benzofurans is achieved and the strategy can also be applied to N-acylated indoles to afford 3-aroyl-2-fluoro-dihydroindoles.

Copper-catalyzed aerobic benzylic C(sp3)-H lactonization of 2-alkylbenzamides via N-centered radicals

Herein, we describe the copper-catalyzed aerobic C(sp³)-H functionalization of 2-alkylbenzamides for the synthesis of benzolactones. This reaction proceeds via 1,5-hydrogen atom transfer of N-centered radicals directly generated by N-H bond cleavage and does not require the synthesis of pre-functionalized N-centered radical precursors or the use of strong stoichiometric oxidants.

Synthesis of Visible-Light-Activated Hypervalent Iodine and Photo-oxidation under Visible Light Irradiation via a Direct S0→Tn Transition

Heavy atom-containing molecules cause a photoreaction by a direct S₀ → T_n transition. Therefore, even in a hypervalent iodine compound with a benzene ring as the main skeleton, the photoreaction proceeds under 365-400 nm wavelength light, where UV-visible spectra are not observed by usual measurement method. Some studies, however, report hypervalent iodine compounds that strongly absorb visible light. Herein, we report the synthesis of two visible light-absorbing hypervalent iodines and their photooxidation properties under visible light irradiation. We also demonstrated that the S₀ → T_n transition causes the photoreaction to proceed under wavelengths in the blue and green light region.

Visible Light-Promoted Magnesium, Iron, and Nickel Catalysis Enabling C(sp3)-H Lactonization of 2-Alkylbenzoic Acids

A mild and practical C(sp³)-H lactonization protocol has been achieved by merging photocatalysis and magnesium (iron, nickel) catalysis. A diverse range of 2-alkylbenzoic acids with a variety of substitution patterns could be transformed into the corresponding phthalide products. Based on the mechanistic experimentation and reported prior studies, a possible mechanism for the benzylic oxidative lactonization reaction was proposed with the hypothetic photoactive ternary complex formed between the 2-alkylbenzoic acid substrate, magnesium ion, and bromate anion.

Benzylic C(sp3)-C(sp2) cross-coupling of indoles enabled by oxidative radical generation and nickel catalysis

A mechanistically unique functionalization strategy for a benzylic C(sp3)-H bond has been developed based on the facile oxidation event of indole substrates. This novel pathway was initiated by efficient radical generation at the benzylic position of the substrate, with subsequent transition metal catalysis to complete the overall transformation. Ultimately, an aryl or an acyl group could be effectively delivered from an aryl (pseudo)halide or an acid anhydride coupling partner, respectively. The developed method utilizes mild conditions and exhibits a wide substrate scope for both substituted indoles and C(sp2)-based reaction counterparts. Mechanistic studies have shown that competitive hydrogen atom transfer (HAT) processes, which are frequently encountered in conventional methods, are not involved in the product formation process of the developed strategy.

A Direct S0 →Tn Transition in the Photoreaction of Heavy-Atom-Containing Molecules

According to the Grotthuss-Draper law, light must be absorbed by a substrate to initiate a photoreaction. There have been several reports, however, on the promotion of photoreactions using hypervalent iodine during irradiation with light from a non-absorbing region. This contradiction gave rise to a mystery regarding photoreactions involving hypervalent iodine. We demonstrated that the photoactivation of hypervalent iodine with light from the apparently non-absorbing region proceeds via a direct S₀ →T_n transition, which has been considered a forbidden process. Spectroscopic, computational, and synthetic experimental results support this conclusion. Moreover, the photoactivation mode could be extended to monovalent iodine and bromine, as well as bismuth(III)-containing molecules, providing new possibilities for studying photoreactions that involve heavy-atom-containing molecules.

Iodine-catalyzed efficient synthesis of xanthene/thioxanthene-indole derivatives under mild conditions

An iodine-catalyzed nucleophilic substitution reaction of xanthen-9-ol and thioxanthen-9-ol with indoles has been developed, providing an efficient procedure for the synthesis of xanthene/thioxanthene-indole derivatives with good to excellent yields. This protocol offers several advantages, such as short reaction times, green solvent, operational simplicity, easily available catalyst and mild reaction conditions. Moreover, this method showed good tolerance of functional groups and a wide range of substrates.

An efficient synthesis of xanthene/thioxanthene-indole derivatives has been developed through iodine-catalyzed nucleophilic substitution reaction of xanthen-9-ols and thioxanthen-9-ol with indoles.

Design and application of diimine-based copper(i) complexes in photoredox catalysis

Structurally different bis(imino)copper(i) complexes were prepared in a highly modular manner and utilized as copper-based photocatalysts in the ATRA reactions of styrenes and alkyl halides. The new photocatalysts showed good catalytic activity and ensured efficient chemical transformations.

trans-Diastereoselective Syntheses of γ-Lactones by Visible Light-Iodine-Mediated Carboesterification of Alkenes

This study aims to develop an intermolecular lactonization reaction of alkenes with carbonyls mediated by visible light and molecular iodine. The one-step reaction involved the carboesterification of alkenes to produce the corresponding lactones in moderate to good yield. It was also revealed that it is possible to control the diastereoselectivity of the reaction by altering the base used and the reaction conditions. When water was added as a solvent, the reaction resulted in the formation of lactones with trans-selectivity. A mechanistic investigation was undertaken and it was found that the reaction requires the generation of an iodine radical from molecular iodine, driven by visible light irradiation, and proceeds via the formation of an iodine radical alkene adduct. The proposed reaction is an example of a rare-metal free intermolecular addition cyclization reaction, which is an environment-friendly chemical process that only uses molecular iodine. In addition, since diastereoselectivity was observed without the use of any specific reagents, the developed methodology is an example of a novel stereoselective transformation using only cost-effective reagents.