Iodine(III) Reagents in Radical Chemistry

Synthesis of NH-Sulfoximines by Using Recyclable Hypervalent Iodine(III) Reagents under Aqueous Micellar Conditions

The synthesis of NH-sulfoximines from sulfides has been first developed under mild conditions in an aqueous solution with surfactant TPGS-750-M as the catalyst at room temperature. In this newly developed process, a simple and convenient recycling strategy to regenerate the indispensable hypervalent iodine(III) is used. The resulting 1,2,3-trifluoro-5-iodobezene can be recovered almost quantitively from the mixture by liquid-liquid extraction and then oxidized to give the corresponding iodine(III) species. This optimized procedure is compatible with a broad range of functional groups and can be easily performed on a gram scale, providing a green protocol for the synthesis of sulfoximines.

Generation of Aryl Radicals from Aryl Hydrazines via Catalytic Iodine in Air: Arylation of Substituted 1,4-Naphthoquinones

Arylated building blocks or heterocycles are key to myriad applications, including pharmaceutical drug discovery, materials sciences, and many more. Herein, we have reported a mild and efficient strategy for generation of aryl radicals by reacting appropriate aryl hydrazines with catalytic iodine in open air. The aryl radicals were quenched by diversely substituted 1,4-napthoquinones present in the reaction mixture to afford diversely substituted 2,3-napthoquinones in moderate to excellent yield. Control experiments provided insights into the putative reaction mechanism.

Photoredox-Mediated Net-Neutral Radical/Polar Crossover Reactions

Radical/Polar Crossover (RPC) chemistry is a rapidly growing subset of photoredox catalysis that is characterized by transformations featuring both radical and ionic modes of reactivity. Net-neutral RPC is particularly interesting in that both the single-electron oxidation and reduction steps occur through interaction with the photocatalyst, thus precluding the need for exogenous oxidants or reductants. As such, these transformations facilitate rapid incorporation of molecular complexity while maintaining mild reaction conditions. This review covers recent advances in photoredox-mediated net-neutral RPC synthetic methods, with a particular emphasis on C-C bond-forming reactions.

Highly Selective Synthesis of 2-tert-Butoxy-1-Arylethanones via Copper(I)-Catalyzed Oxidation/tert-Butoxylation of Aryl Olefins with TBHP

A practical and environmentally friendly protocol for the selective oxidation of aryl olefins to arylethanone derivatives by using a Cu(I) catalyst and tert-butyl hydroperoxide (TBHP) has been developed. A series of 2-tert-butoxy-1-arylethanones were obtained in moderate to good yields under mild conditions with high selectivity. In this method, TBHP acts not only as an oxidant but also as the tert-butoxy and carbonyl oxygen sources. This enables one-step oxidation/tert-butoxylation. Various allyl peroxides were also synthesized from allyl substrates.

Copper-catalyzed enantioselective arylalkynylation of alkenes

Enantioselective aryl and alkynylation of activated/nonactivated alkenes.

A copper-catalyzed enantioselective arylalkynylation of alkenes with diaryliodonium salt and a monosubstituted alkyne is reported. The three-component coupling reactions proceed under mild reaction conditions with a broad substrate scope, leading to synthetically valuable 1,2-diaryl-3-butynes. The key to the success of this chemistry is the employment of the chiral bisoxazoline-phenylaniline (BOPA) ligand. A novel reaction pathway involving the phenyl radical generation under thermal copper catalysis is proposed according to mechanistic studies.

Metal-free late-stage C(sp2)-H functionalization of N-aryl amines with various sodium salts

Metal-free consecutive C(sp2)-X (X = Cl, Br, S, N) bond formations of N-aryl amines (cyclic, fused, carbamate, and aminium radicals) were achieved under mild conditions using [bis(trifluoroacetoxy)iodo]benzene (PIFA) and simple nonharmful sodium salts. This direct and selective C(sp2)-H functionalization showed excellent functional group compatibility, cost effectiveness, and late-stage applicability for the synthesis of biologically active natural products. Two mechanisms were proposed to explain the ortho- or para-preference, as well as the accelerating effect of CH3NO2.

Oxidative alkylation of alkenes with carbonyl compounds through concomitant 1,2-aryl migration by photoredox catalysis

An efficient protocol for the construction of 1,5-diketones was realized in the presence of organic fluorophore 4CzIPN, diaryliodonium salt, and visible light irradiation.

Zinc-catalyzed asymmetric nitrooxylation of β-keto esters/amides with a benziodoxole-derived nitrooxy transfer reagent

Zinc-catalyzed asymmetric nitrooxylation to afford a series of α-nitrooxy β-keto esters/amides in high yields and with low to moderate enantioselectivities has been disclosed.

Gold and hypervalent iodine(iii): liaisons over a decade for electrophilic functional group transfer reactions

Over the last two decades, hypervalent iodine(iii) reagents have evolved from being 'bonding curiosities' to mainstream reagents in organic synthesis, in particular, electrophilic functional group transfer reactions. In this context, gold catalysts have not only emerged as a unique toolbox to facilitate such reactions (especially alkynylations) but also opened new possibilities with their different modes of reactivities for other functional group transfer reactions (acetoxylations and arylations). This feature article critically summarizes hitherto all such Au-catalyzed electrophilic functional group transfer reactions with hypervalent iodine(iii) reagents, emphasizing their mechanistic aspects.

Oxidative ability of organic iodine(iii) reagents: a theoretical assessment

Computational prediction of the oxidative ability of hypervalent iodine reagents was performed based on redox potentials, calculated at the B3LYP/6-311+G(d,p) level of theory with the SDD ECP46MDF pseudopotential for the iodine atom.

Rhenium-catalyzed alkylarylation of alkenes with PhI(O2CR)2via decarboxylation to access indolinones and dihydroquinolinones

Rhenium-catalyzed alkylarylation of alkenes with hypervalent iodine(iii) reagents (HIRs) via decarboxylation to access various 3,3-disubstituted indolinones and trans-3,4-dihydroquinolinones is described.

Transition-metal-free highly regioselective C–H acetoxylation of pyrrolo[2,3-d]pyrimidine derivatives

Transition-metal-free catalyzed direct C(sp²)–H acetoxylation of pyrrolo[2,3-d]pyrimidine derivatives is reported. This protocol provides a variety of acetoxylated pyrrolo[2,3-d]pyrimidines in good to excellent yields.

Fe-Catalyzed decarbonylative cascade reaction of N-aryl cinnamamides with aliphatic aldehydes to construct 3,4-dihydroquinolin-2(1H)-ones

A practical Fe-catalyzed decarbonylative cascade reaction of N-aryl cinnamamides with aliphatic aldehydes to provide C3 alkylated 3,4-dihydroquinolin-2(1H)-ones is developed. Aliphatic aldehydes were oxidatively decarbonylated into 1°, 2° and 3° alkyl radicals conveniently, allowing for the subsequent cascade construction of C(sp3)-C(sp3) and C(sp3)-C(sp2) bonds via radical addition and HAS-type cyclization. The importance of the amide linkage and the selectivity of the 6-endo-trig over 5-exo-trig cyclization pathway were elucidated by experimental results and DFT calculations.

Visible-Light-Enabled Oxidative Coupling of Alkenes with Dialkylformamides To Access Unsaturated Amides

A practical and direct method for oxidative cross-coupling of alkenes with dialkylformamides is established employing visible-light-enabled photoredox catalysis. This strategy allows efficient access to diverse unsaturated amides under mild reaction conditions. The application of an appropriate diaryliodonium salt was demonstrated to be critical to the success of this process. This catalyst system is well tolerant of a variety of useful functional groups.

Copper-Catalyzed Amino-oxymethylation of Ynamides with N,O-Acetals

A copper-catalyzed 1,2-difunctionalization of ynamides has been developed by using readily available N,O-acetals as the bifunctional reagents. Importantly, contrary to the initially expected 1,2-oxy-aminomethylation reaction, an unprecedented 1,2-amino-oxymethylation has been observed. This reaction is atom-economical and tolerates a broad substrate scope, affording the difunctionalization products in good yield with unique Z configuration.

Unified and practical access to ɤ-alkynylated carbonyl derivatives via streamlined assembly at room temperature

The development of a unified and straightforward method for the synthesis of ɤ-alkynylated ketones, esters, and amides is an unmet challenge. Here we report a general and practical protocol to access ɤ-alkynylated esters, ketones, and amides with diverse substitution patterns enabled by dual-catalyzed spontaneous formation of Csp3–sp3 and Csp3–sp bond from alkenes at room temperature. This directing-group-free strategy is operationally simple, and allows for the straightforward introduction of an alkynyl group onto ɤ-position of carbonyl group along with the streamlined skeleton assembly, providing a unified protocol to synthesize various ɤ-alkynylated esters, acids, amides, ketones, and aldehydes, from readily available starting materials with excellent functional group compatibility.

Alkene Synthesis by Photocatalytic Chemoenzymatically Compatible Dehydrodecarboxylation of Carboxylic Acids and Biomass

Direct conversion of renewable biomass and bioderived chemicals to valuable synthetic intermediates for organic synthesis and materials science applications by means of mild and chemoselective catalytic methods has largely remained elusive. Development of artificial catalytic systems that are compatible with enzymatic reactions provides a synergistic solution to this enduring challenge by leveraging previously unachievable reactivity and selectivity modes. We report herein a dual catalytic dehydrodecarboxylation reaction that is enabled by a crossover of the photoinduced acridine-catalyzed O-H hydrogen atom transfer (HAT) and cobaloxime-catalyzed C-H-HAT processes. The reaction produces a variety of alkenes from readily available carboxylic acids. The reaction can be embedded in a scalable triple-catalytic cooperative chemoenzymatic lipase-acridine-cobaloxime process that allows for direct conversion of plant oils and biomass to long-chain terminal alkenes, precursors to bioderived polymers.

Cu-Mediated arylselenylation of aryl halides with trifluoromethyl aryl selenonium ylides

An unprecedented arylselenylation of aryl halides with trifluoromethyl aryl selenonium ylides in the presence of copper is described. The reaction proceeded at 100-140 °C under ligand- and additive-free conditions for 3-20 h to form a variety of unsymmetrical diaryl selenides in good to high yields. Arylselenylation is easy to operate, has good functional group tolerance, and demonstrates the different reaction profiles of trifluoromethyl aryl selenonium ylides from the homologous trifluoromethyl aryl sulfonium ylides.

Practical, metal-free remote heteroarylation of amides via unactivated C(sp3)-H bond functionalization

Development of practical methods for the production of multi-functionalized amides is one of the most important topics in both synthetic chemistry and drug discovery. Disclosed herein is a new, efficient, site-selective heteroarylation of amides via C(sp3)-H bond functionalization. Amidyl radicals are directly generated from the amide N-H bonds under mild conditions, which trigger the subsequent 1,5-HAT process. A wide scope of aliphatic amides including carboxamides, sulfonamides, and phosphoramides are readily modified at remote C(sp3)-H bonds by installing diverse heteroaryl groups. Borne out of pragmatic consideration, this protocol can be used for the late-stage functionalization of amides.

A Tunable Trifluoromethyliodonium Reagent

Four complexes of MCl₄ (M=Ti, Zr, Hf) with the hypervalent trifluoromethyl iodine reagent trifluoromethyl-1,3-dihydro-3,3-dimethyl-1,2-benziodoxole (1,=L) are described. With TiCl₄ , an I-O bond cleavage occurs, leading to the formation of the trifluoromethyliodonium alcoholate complexes [Ti₂ Cl₆ (L)₄ ]Cl₂ (2 a) and Ti₂ Cl₈ (L) (2 b). Reactions with ZrCl₄ and HfCl₄ form the complexes ZrCl₄ (L)₂ (3) and HfCl₄ (L)₂ (4), respectively, wherein the original I-O bond is retained and elongated compared to that in free 1. Therefore, the reactivity of 1 can be easily and practically fine-tuned by addition of different metal chlorides, following the order ZrCl₄ /HfCl₄ <TiCl₄ <2 TiCl₄ . Complexes 2 a, 3, and 4 are remarkably bench-stable forms of activated reagent 1, while 2 b is readily accessible in situ. 2 a and 2 b represent the first "real" trifluoromethyliodonium reagents derived from iodanes, that is, with the I-O bond being completely cleaved. The new complexes were shown to be useful for the trifluoromethylation of para-toluenesulfonate under aprotic conditions.

Direct Intermolecular Anti-Markovnikov Hydroazidation of Unactivated Olefins

We herein report a direct intermolecular anti-Markovnikov hydroazidation method for unactivated olefins, which is promoted by a catalytic amount of bench-stable benziodoxole at ambient temperature. This method facilitates previously difficult, direct addition of hydrazoic acid across a wide variety of unactivated olefins in both complex molecules and unfunctionalized commodity chemicals. It conveniently fills a synthetic chemistry gap of existing olefin hydroazidation procedures, and thereby provides a valuable tool for azido-group labeling in organic synthesis and chemical biology studies.

Hypercoordinate iodine for catalytic asymmetric diamination of styrene: insights into the mechanism, role of solvent, and stereoinduction

Stereoselectivity in the asymmetric diamination of styrene catalyzed by chiral hypercoordinate iodine originates from the prochiral face recognition when the substrate binds to the catalyst.

Hypercoordinate iodine has evolved as an impressive class of catalysts for various organic transformations. Extension of this idea to asymmetric applications, such as in the asymmetric difunctionalization of styrene or its derivatives, constitutes an important reaction. In this study, the mechanism and origin of stereoinduction in styrene diamination, with a sulfonimide (HNMs₂) as the diaminating agent and iodoresorcinol (((iPr)₂N(CO)-CH(Me)-O)₂Ar–I) based chiral hypercoordinate iodine as the catalyst, are investigated using density functional theory calculations. The energetically preferred catalytic pathway has been found to involve, among other steps, two very important mechanistic events: (a) the formation of a catalyst–substrate complex by the action of styrene on the catalyst ArI(NMs₂)₂, resulting in the displacement of one of the imidates (NMs₂^–); and (b) a rebound of the departed imidate on the iodine-bound styrene to form an iodonium ion intermediate with a N–C bond. Explicit interaction of the imidate ion with hexafluoroisopropanol (HFIP), used as a solvent additive, lowers the barrier for the formation of the iodonium ion. The P helical fold of the chiral arms of the iodoresorcinol catalyst is found to offer a chiral environment for the reactants. Coordination of the iodine catalyst to the styrene double bond is found to make the benzylic carbon more electrophilic and hence makes it the preferred site for the nucleophilic addition. In the chiral environment of the catalyst, an enhanced polarization of the styrene double bond is noticed when the double bond coordinates through the si prochiral face than the re face. Nucleophilic addition on the re face of the catalyst–substrate complex is associated with a lower activation barrier leading to the experimentally observed S enantiomeric product. The stereoselective model developed in this study can be employed to related asymmetric styrene difunctionalizations using similar hypercoordinate iodine catalysts.

Copper-catalyzed three-component reaction of N-heteroaryl aldehydes, nitriles, and water

An efficient and straightforward method for the synthesis of N-heteroaroyl imides has been successfully developed involving a copper-catalyzed radical-triggered three-component reaction of N-heteroaryl aldehydes, nitriles, and water. Mechanistic studies indicate that the reaction may undergo a radical-triggered Ritter-type reaction in which water serves as the oxygen source for the formation of the C-O bond. The reaction has advantages such as a broad substrate scope for the N-heteroaryl aldehydes, atom economy, and simple operation.