Catalysis of stannane-mediated radical chain reactions by benzeneselenol

Recent Advances in Dearomative Partial Reduction of Benzenoid Arenes

Dearomative partial reduction is an extraordinary approach for transforming benzenoid arenes and has been well-known for many decades, as exemplified by the dehydrogenation of Birch reduction and the hydroarylation of Crich addition. Despite its remarkable importance in synthesis, this field has experienced slow progress over the last half-century. However, a revival has been observed with the recent introduction of electrochemical and photochemical methods. In this Minireview, we summarize the recent advancements in dearomative partial reduction of benzenoid arenes, including dihydrogenation, hydroalkylation, arylation, alkenylation, amination, borylation and others. Further, the intriguing utilization of dearomative partial reduction in the synthesis of natural products is also emphasized. It is anticipated that this Minireview will stimulate further progress in arene dearomative transformations.

Recent advances in cascade radical cyclization of radical acceptors for the synthesis of carbo- and heterocycles

This review is devoted to highlighting main achievements in the development of cascade radical cyclization of radical acceptors for the synthesis of carbo- and heterocycles.

Using the Thiyl Radical for Aliphatic Hydrogen-Atom Transfer: Thiolation of Unactivated C-H Bonds

A metal- and catalyst-free thiyl-radical-mediated activation of alkanes is described. Tetrafluoropyridinyl disulfide is used to perform thiolation of the C-H bonds under irradiation with 400 nm light-emitting diodes. The key C-H activation step is believed to proceed via hydrogen-atom abstraction effected by the fluorinated thiyl radical. Secondary, tertiary, and heteroatom-substituted C-H bonds can be involved in the thiolation reaction. The resulting sulfides have wide potential as photoredox-active radical precursors in reactions with alkenes and heteroarenes.

A theoretical study on the pKa values of selenium compounds in aqueous solution

The pK_a values of different kinds of selenium compounds (R-SeH) were investigated by using the ωB97XD method with a SMD model.

Cascade Radical Cyclization on Alkynyl Vinylogous Carbonates for the Divergent Synthesis of Tetrasubstituted Furans and Dihydrofurans

A single alkynyl vinylogous carbonate was elaborated to tetrasubstituted furan or dihydrofuran via a cascade inter-intramolecular radical reaction by changing the radical being added. The strategy could be used in the synthesis of polycyclic heterocycles as well as bis-furan exhibiting atropisomerism. Installation of a new furan motif on the existing one was feasible by iteration. Stannyl dihydrofuran derivative was used in Stille coupling, whereas intramolecular Friedel-Crafts acylation on the furan gave furanonaphthol.

Radical Arene Addition vs Radical Reduction: Why Organometal Hydride Chain Reactions Stop and How To Make Them Go

Nonideal kinetic chain analysis was used to examine the kinetic limitations of free-radical synthesis. Homolytic aromatic substitution (HAS: ArH + R^• → ArR + H^•) occurs in a chain-terminating side reaction to the tributyltin hydride ( SnH) reduction chain (RX + SnH + ( i^•)_cat. → RH + SnX). Kinetic modeling of premixed and slow reagent addition reactions have clarified the mechanisms of SM HAS, with the azo initiator ( iNN i) acting not only as radical source but also (as an H^• acceptor) as the redox catalyst for aromatization, and/or as a postaddition oxidant. Refractory halides and other hitherto baffling anomalies may arise from the build up of ipso (rather than ortho)-cycloadduct radicals in the steady-state radical population. The implications of these findings for "tin-free" radical chains (and emerging photoredox methods) are considered via historical and recent examples of the effects of chain-degrading radical transfer (to substrate, product, solvent, initiator, and/or reagent ligands) on the reagent's chain.

Organocatalytic reductive coupling of aldehydes with 1,1-diarylethylenes using an in situ generated pyridine-boryl radical

A pyridine-boryl radical promoted reductive coupling reaction of aldehydes with 1,1-diarylethylenes has been established.

A pyridine-boryl radical promoted reductive coupling reaction of aldehydes with 1,1-diarylethylenes has been established via a combination of computational and experimental studies. Density functional theory calculations and control experiments suggest that the ketyl radical from the addition of the pyridine-boryl radical to aldehydes is the key intermediate for this C–C bond formation reaction. This metal-free reductive coupling reaction features a broad substrate scope and good functional compatibility.

Fragment Coupling and the Construction of Quaternary Carbons Using Tertiary Radicals Generated From tert-Alkyl N-Phthalimidoyl Oxalates By Visible-Light Photocatalysis

The coupling of tertiary carbon radicals with alkene acceptors is an underdeveloped strategy for uniting complex carbon fragments and forming new quaternary carbons. The scope and limitations of a new approach for generating nucleophilic tertiary radicals from tertiary alcohols and utilizing these intermediates in fragment coupling reactions is described. In this method, the tertiary alcohol is first acylated to give the tert-alkyl N-phthalimidoyl oxalate, which in the presence of visible-light, catalytic Ru(bpy)3(PF6)2, and a reductant fragments to form the corresponding tertiary carbon radical. In addition to reductive coupling with alkenes, substitution reactions of tertiary radicals with allylic and vinylic halides is described. A mechanism for the generation of tertiary carbon radicals from tert-alkyl N-phthalimidoyl oxalates is proposed that is based on earlier pioneering investigations of Okada and Barton. Deuterium labeling and competition experiments reveal that the reductive radical coupling of tert-alkyl N-phthalimidoyl oxalates with electron-deficient alkenes is terminated by hydrogen-atom transfer.

Toward the ABCD Core of the Calyciphylline A-Type Daphniphyllum Alkaloids: Solvent non-Innocence in Neutral Aminyl Radical Cyclizations

The Daphniphyllum alkaloids remain an attractive target in the synthetic community because of their unique framework and promising biological activities. We have shown that the ABC core of the calyciphylline A-type alkaloids can be rapidly accessed via the tandem cyclization of a neutral aminyl radical with a polarized cyclic olefin. Deuterium labeling experiments and reactions omitting a tin hydride reagent suggest that the solvent is the major source of the terminating hydrogen atom in the cyclization cascade. Incorporation of an internal alkyne in the radical pathway was tolerated in the reaction, and it provided the necessary atoms to enable completion of the D ring of the calyciphylline A-type alkaloids.

Constructing Quaternary Carbons from N-(Acyloxy)phthalimide Precursors of Tertiary Radicals Using Visible-Light Photocatalysis

Tertiary carbon radicals have notable utility for uniting complex carbon fragments with concomitant formation of new quaternary carbons. This article explores the scope, limitations, and certain mechanistic aspects of Okada's method for forming tertiary carbon radicals from N-(acyloxy)phthalimides by visible-light photocatalysis. Optimized conditions for generating tertiary radicals from N-(acyloxy)phthalimide derivatives of tertiary carboxylic acids by visible-light irradiation in the presence of 1 mol % of commercially available Ru(bpy)3(PF6)2, diethyl 1,4-dihydro-2,6-dimethylpyridine-3,5-dicarboxylate (8), and i-Pr2NEt and their coupling in dichloromethane at room temperature with alkene acceptors were developed. Four representative tertiary N-(acyloxy)phthalimides and 15 alkene radical acceptors were examined. Both reductive couplings with electron-deficient alkenes and radical substitution reactions with allylic and vinylic bromides and chlorides were examined with many such reactions occurring in good yield using only a slight excess (typically 1.5 equiv) of the alkene. In general, the yields of these photocatalytic reactions were higher than the analogous transformations of the corresponding N-phthalimidoyl oxalates. Deuterium labeling and competition experiments reveal that the reductive radical coupling of tertiary N-(acyloxy)phthalimides with electron-deficient alkenes can be terminated by both hydrogen-atom transfer and single-electron reduction followed by protonation, and that this mechanistic duality is controlled by the presence or absence of i-Pr2NEt.

Radical cyclizations of cyclic ene sulfonamides occur with β-elimination of sulfonyl radicals to form polycyclic imines

Radical cyclizations of cyclic ene sulfonamides provide stable bicyclic and tricyclic aldimines and ketimines in good yields. Depending on the structure of the precursor, the cyclizations occur to provide fused and spirocyclic imines with five-, six-, and seven-membered rings. The initial radical cyclization produces an α-sulfonamidoyl radical that undergoes elimination to form the imine and a phenylsulfonyl radical. In a related method, 3,4-dihydroquinolines can also be produced by radical translocation reactions of N-(2-iodophenylsulfonyl)tetrahydroiso-quinolines. In either case, very stable sulfonamides are cleaved to form imines (rather than amines) under mild reductive conditions.

Barton esters for initiator-free radical cyclisation with heteroaromatic substitution

S-(1-Oxido-2-pyridinyl)-1,1,3,3-tetramethylthiouronium hexafluorophosphate (HOTT) facilitates the first examples of efficient radical cyclisation with (hetero)aromatic substitution via Barton ester intermediates. Cyclopropyl and alkyl radicals allow access to five, six and seven-membered alicyclic-ring fused heterocycles with and without an additional fused cyclopropane, including the skeleton of the anti-cancer agent, cyclopropamitosene, expanded, and diazole analogues. Radical initiators are not required for cyclisation from carboxylic acid precursors.

2-(Selenocyanatomethyl)-2-propenol — A convenient synthon for ligation via the deselenative allylic rearrangement of allyl selenosulfides: preparation, functional group compatibility, and application

The preparation and reactions of 2-(selenocyanatomethyl)-2-propenol are described and reveal the compatibility of the allylic selenocyanate group with a range of mild oxidizing and Lewis acidic conditions. 2-(Selenocyanatomethyl)-2-propenol and its derivatives are employed in the functionalization of simple and amino acid derived thiols in methanolic solution at room temperature to give 2-(hydroxymethyl)allyl sulfides in good to excellent yield.

Polarity reversal catalysis in radical reductions of halides by N-heterocyclic carbene boranes

Otherwise sluggish or completely ineffective radical reductions of alkyl and aryl halides by N-heterocyclic carbene boranes (NHC-boranes) are catalyzed by thiols. Reductions and reductive cyclizations with readily available 1,3-dimethylimidazol-2-ylidene borane and a water-soluble triazole relative are catalyzed by thiophenol and tert-dodecanethiol [C(9)H(19)C(CH(3))(2)SH]. Rate constants for reaction of the phenylthiyl (PhS•) radical with two NHC-boranes have been measured to be ~10(8) M(-1) s(-1) by laser flash photolysis experiments. An analysis of the available evidence suggests the operation of polarity reversal catalysis.

A mild deuterium exchange reaction of free carboxylic acids by photochemical decarboxylation

Deuterium exchange of a carboxy group was achieved by photochemical decarboxylation of free carboxylic acids in the presence of thiol and a small amount of D(2)O, and a deuterated product with excellent deuterium content was obtained; this reaction is a practical means of synthesizing regioselective deuterium-labelled compounds under mild reaction conditions.

Radical migration of substituents of aryl groups on quinazolinones derived from N-acyl cyanamides

A newly designed radical cascade involving N-acyl cyanamides is reported. It builds on aromatic homolytic substitutions as intermediate events and leads to complex heteroaromatic structures via an unprecedented radical migration of a substituent on aryl groups of quinazolinones (hydrogen or alkyl). Mechanistic considerations are detailed, which allowed us to devise fine control over the domino processes. The latter could be predictably stopped at several stages, depending on the reaction conditions. Finally, a surgical introduction of a trifluoromethyl substituent on a quinazolinone was achieved via the reported migration.