Iodine(III) Reagents in Radical Chemistry

Metal-Free Direct C-H Cyanation of Alkenes

A metal‐free and direct alkene C−H cyanation is described. Directing groups are not required and the mechanism involves electrophilic activation of the alkene by a cyano iodine(III) species generated in situ from a [bis(trifluoroacetoxy)iodo]arene and trimethylsilyl cyanide as the cyanide source. This C−H functionalization can be conducted on gram scale, and for noncyclic 1,1‐ and 1,2‐disubstuted alkenes high stereoselectivity is achieved, thus rendering the method highly valuable.

Metal-free alkene oxy- and amino-perfluoroalkylations via carbocation formation by using perfluoro acid anhydrides: unique reactivity between styrenes and perfluoro diacyl peroxides

A practical metal-free perfluoroalkylation using acid anhydrides with unique reaction mode via carbocation has been developed.

We present a strategy for metal-free, alkene difunctionalization-type, oxy- and amino-perfluoroalkylations, using perfluoro acid anhydrides as practical and user-friendly perfluoroalkyl sources. This method provides efficient access to oxy-perfluoroalkylation products via carbocation formation due to the unique reactivity between styrenes and bis(perfluoroacyl) peroxides generated in situ from perfluoro acid anhydrides. This reaction is also applicable to metal-free intramolecular amino-perfluoroalkylation of styrenes bearing a pendant amino group. Synthetic utility of the oxy-trifluoromethylation products was confirmed by demonstrating derivatization via hydrolysis, elimination, and acid-catalyzed substitution with carbon nucleophiles. The mechanism of the carbocation formation was investigated experimentally and theoretically.

A Nitrido-bridged Heterometallic Ruthenium(IV)/Iron(IV) Phthalocyanine Complex Supported by A Tripodal Oxygen Ligand, [Co(η5-C5H5){P(O)(OEt)2}3]-: Synthesis, Structure, and Its Oxidation to Give Phthalocyanine Cation Radical and Hydroxyphthalocyanine Complexes

Dinuclear iron nitrido phthalocyanine complexes are of interest owing to their applications in catalytic oxidation of hydrocarbons. While nitrido-bridged diiron phthalocyanine complexes are well documented, the oxidation chemistry of heterodinuclear iron(IV) phthalocyanine nitrides has not been well explored. In this paper we report on the synthesis of a heterometallic Fe^IV/Ru^IV phthalocyanine nitride and its oxidation to yield phthalocyanine cation radical and hydroxyphthalocyanine complexes. Treatment of [Fe^II(Pc)] (Pc^2- = phthalocyanine dianion) with [Ru^VI(L_OEt)(N)Cl₂] (L_OEt^- = [Co(η⁵-C₅H₅){P(O)(OEt)₂}₃]^-) (1) afforded the heterometallic μ-nitrido complex [Cl₂(L_OEt)Ru^IV(μ-N)Fe^IV(Pc)(H₂O)] (2) that contains an Ru^IV=N = Fe^IV linkage with the Ru-N and Fe-N distances of 1.689(6) and 1.677(6) Å, respectively, and Ru-N-Fe angle of 176.0(4)°. Substitution of 2 with 4- tert-butylpyridine (Bupy) gave [Cl₂(L_OEt)Ru^IV(μ-N)Fe^IV(Pc)(Bupy)]. The cyclic voltammogram of 2 displayed a reversible Pc-centered oxidation couple at +0.18 V versus Fc^+/0 (Fc = ferrocene). The oxidation of 2 with [N(4-BrC₆H₄)₃]SbCl₆ led to isolation of the cationic complex [Cl₂(L_OEt)Ru^IV(μ-N)Fe^IV(Pc^·+)(H₂O)][SbCl₆]_0.85[SbCl₅(OH)]_0.15 (2[SbCl₆]_0.85[SbCl₅(OH)]_0.15), whereas that with PhICl₂ yielded the chloride complex [Cl₂(L_OEt)Ru^IV(μ-N)Fe^IV(Pc^·+)Cl] (3). Complexes 2[SbCl₆]_0.85[SbCl₅(OH)]_0.15 and 3 have been characterized by X-ray crystallography. The UV/visible spectra of 2⁺ (λ_max = 515 and 747 nm) and 3 (λ_max = 506 and 748 nm) displayed absorption bands that are characteristic of Pc cation radical. The EPR spectrum of 3 showed a signal with the g value of 2.0012 (width = 5 G) that is consistent with an organic radical. The spectroscopic data support the formulation of 2⁺ and 3 as Ru^IV-Fe^IV Pc cation radical complexes. The reaction of 2 with PhI(CF₃CO₂)₂ in dried CH₂Cl₂ afforded a mixture of [Cl₂(L_OEt)Ru^IV(μ-N)Fe^IV(Pc^·+)(CF₃CO₂)] (4) and a hydroxyphthalocyanine complex, [Cl₂(L_OEt)Ru^IV(μ-N)Fe^IV(Pc-OH)(H₂O)](CF₃CO₂) (5), whereas that in wet CH₂Cl₂ (containing ca. 0.5% water) led to isolation of 5 as the sole product. Complex 4 was independently prepared by salt metathesis of 3 with AgCF₃CO₂.

Selective carboxylation of reactive benzylic C-H bonds by a hypervalent iodine(III)/inorganic bromide oxidation system

An oxidation system comprising phenyliodine(III) diacetate (PIDA) and iodosobenzene with inorganic bromide, i.e., sodium bromide, in an organic solvent led to the direct introduction of carboxylic acids into benzylic C–H bonds under mild conditions. The unique radical species, generated by the homolytic cleavage of the labile I(III)–Br bond of the in situ-formed bromo-λ³-iodane, initiated benzylic carboxylation with a high degree of selectivity for the secondary benzylic position.

Preparation, structure, and reactivity of bicyclic benziodazole: a new hypervalent iodine heterocycle

A new bicyclic organohypervalent iodine heterocycle derivative of benziodazole was prepared by oxidation of 2-iodo-N,N’-diisopropylisophthalamide with m-chloroperoxybenzoic acid under mild conditions. Single crystal X-ray crystallography of this compound revealed a five-membered bis-heterocyclic structure with two covalent bonds between the iodine atom and the nitrogen atoms. This novel benziodazole is a very stable compound with good solubility in common organic solvents. This compound can be used as an efficient reagent for oxidatively assisted coupling of carboxylic acids with alcohols or amines to afford the corresponding esters or amides in moderate yields.

Promoting Intermolecular C-N Bond Formation under the Auspices of Iodine(III)

The quest for the development of new protocols that provide general conditions for oxidative carbon-nitrogen bond formation has grown over recent years. Within this context, due to feasibility and benignity considerations in biochemical sciences, reactions that rely on main group oxidants as the only promoters have received particular interest. We have recently found that simple protonolysis events enable the incorporation of nitrogenated groups of the bissulfonimide family into the coordination sphere of common iodine(III) complexes such as diacetoxy iodobenzene. The products of the type ArI(OAc)(NTs₂) represent rare examples of iodine(III) compounds displaying reactive iodine-nitrogen single bonds. Further protonolysis furnishes the corresponding iodine(III) compounds ArI(NTs₂)₂ containing two defined iodine-nitrogen single bonds for unprecedented dual transfer of both nitrogenated groups. It is of great synthetic importance that these new compounds contain iodine-nitrogen entities, which upon dissociation in solution lead to electrophilic iodine centers and nucleophilic nitrogen groups. This has enabled the development of a body of conceptually new amination reactions, which do not rely on conventional electrophilic nitrogen reagents but rather employ iodine(III) as an electrophilic activator and bissulfonimides as the source of subsequent nucleophilic amination. Additional diversification arises from the ambident nature of bissulfonimines enabling oxygenation pathways. The exciting chemistry covered in this Account comprises structural features of the reagents (including X-ray analysis), scope and limitation in synthetic amination of different hydrocarbons (including sp-, sp²-, and sp³-hybridized centers as in acetylenes, alkenes, enols, butadienes, allenes, arenes, and alkylketones), and physical-organic and theoretical analysis of the underlying reaction mechanisms. The oxidative transformations with all their rich diversifications originate from the versatile redox chemistry of the iodine(III) and iodine(I) pair, which shares several aspects of transition metal high oxidation state chemistry. For the present aryliodine(III) reagents, steric and electronic fine-tuning is possible through accurate engineering of the arene substituent. In addition to the general reactivity of the I-N bond, chiral aryliodine(III) reagents with defined stereochemical information in the aryl backbone are conceptually compatible with this approach. Thus, the development of enantioselective amination reactions with up to 99% ee was also successful. Several of the active enantioselective reagents have been isolated and structurally characterized. Following this approach for the important class of chiral vicinal diamines, an unprecedented direct diamination of alkenes could be conducted in an enantioselective catalytic manner under full intermolecular reaction control. This latter reaction is based on the precise engineering of a chiral aryliodine(III) catalyst in combination with bismesylimide as nitrogen source. It is the consequence of the precise understanding of the reaction behavior of structurally defined bisimidoiodine(III) reagents.

Palladium-Catalyzed Ring-Forming Alkene Aminoaroylation of Unsaturated Hydrazones and Sulfonamides

The first example of a Pd(OAc)₂-catalyzed ring-forming alkene aminoaroylation of unsaturated hydrazones and sulfonamides is described. This protocol features the use of diaryliodonium salts as both oxidants and aryl sources, thus enabling mild reaction conditions, good chemoselectivity, a broad substrate scope, and high functional group tolerance. A wide range of synthetically and biologically important functionalized dihydropyrazoles and isoxazolidines have been obtained in good yields.

Sigmatropic Rearrangements of Hypervalent-Iodine-Tethered Intermediates for the Synthesis of Biaryls

Metal-free dehydrogenative couplings of aryliodanes with phenols to afford 2-hydroxy-2'-iodobiaryls have been developed. This reaction proceeds through ligand exchange on the hypervalent iodine atom followed by a [3,3] sigmatropic rearrangement and with complete regioselectivity. This coupling, in combination with in situ oxidation by mCPBA, enables the convenient conversion of iodoarenes into desirable biaryls. The obtained biaryls have convertible iodo and hydroxy groups in close proximity, and are thus synthetically useful, as exemplified by the controlled syntheses of π-extended furans and a substituted [5]helicene. DFT calculations clearly revealed that the rearrangement is sigmatropic, with C-C bond formation and I-O bond cleavage proceeding in a concerted manner. Acetic acid, which was found to be the best solvent for this protocol, renders the iodine atom more cationic and thus accelerates the sigmatropic rearrangement.

Palladium/Light Induced Radical Alkenylation and Allylation of Alkyl Iodides Using Alkenyl and Allylic Sulfones

Alkenylation and allylation of alkyl iodides with alkenyl and allyl sulfones, respectively, took place under Pd/photoirradiation system. The initial alkyl radical, derived from a single electron transfer between Pd(0) and RI, underwent the title transformations. Pd(0) was regenerated through a reductive elimination of PhSO₂PdI, which is formed by the combination of the sulfonyl radical and the palladium radical. The addition of water was effective, presumably by pushing the equilibrium through hydrolysis of PhSO₂I.

Selective P-C(sp3 ) Bond Cleavage and Radical Alkynylation of α-Phosphorus Alcohols by Photoredox Catalysis

Herein the first P-C(sp³ ) bond cleavage and radical alkynylation of α-phosphorus alcohols to construct phosphonoalkynes is reported. The phosphorus radical is generated upon P-C bond cleavage reaction via the alkoxyl radical through photoredox catalysis with cyclic iodine(III) reagents. Various arylphosphinoyl-, alkylphosphinoyl-, phosphonate-, and phosphonic amide alcohols serve as radical phosphorus precursors to construct phosphonoalkynes for the first time.

Site-selective synthesis of functionalized dibenzo[f,h]quinolines and their derivatives involving cyclic diaryliodonium salts via a decarboxylative annulation strategy

An unprecedented site-selective synthesis of dibenzo[f,h]quinolines and their derivatives was reported via an acid directed decarboxylative annulation cascade.

A metal-free direct C (sp3)–H cyanation reaction with cyanobenziodoxolones

Metal-free direct C(sp³)–H cyanation reaction: cyanobenziodoxolones as cyanating reagents and oxidants.

Nickel(ii)-catalyzed enantioselective α-alkylation of β-ketoamides with phenyliodonium ylideviaa radical process

Enantioselective α-alkylation of β-ketoamides with iodonium ylide was realizedviaa radical process. In the presence of a chiralN,N′-dioxide/Ni(OTf)₂complex, the corresponding products were obtained with good results. Control experiments and EPR studies supported a radical process.

A strategy for generating aryl radicals from arylborates through organic photoredox catalysis: photo-Meerwein type arylation of electron-deficient alkenes

Generation of a variety of aryl radicals from arylboronic acids through metal-free photoredox catalysis under mild conditions.

PhI(OAc)2-mediated dearomative C–N coupling: facile construction of the spiro[indoline-3,2′-pyrrolidine] skeleton

A facile construction of the spiro[indole-3,2′-pyrrolidine] skeleton, through diacetoxyiodobenzene (PIDA) mediated C–N bond-forming dearomatization of C3 sulfonamide linked indole derivatives, has been developed.

Catalytic, metal-free sulfonylcyanation of alkenes via visible light organophotoredox catalysis

A catalytic, redox-neutral group transfer radical addition of olefins with tosyl cyanide via visible light-induced organophotoredox catalysis has been described.

Bromide-catalyzed electrochemical trifluoromethylation/cyclization of N-arylacrylamides with low catalyst loading

An electro-generated trifluoromethyl radical mediated by a bromide ion is efficiently applied to the trifluoromethylarylation/cyclization of N-arylacrylamides.

Regio- and Stereoselective Iron-Catalyzed Oxyazidation of Enamides Using a Hypervalent Iodine Reagent

A novel regio- and diastereoselective iron-catalyzed intermolecular oxyazidation of enamides using various azidobenziodoxolone (ABX) derivatives is presented. A variety of α-N₃ amino derivatives and of α-N₃ piperidines were synthesized in good yields and under mild reaction conditions. The reaction involves a radical process using cheap FeCl₂ as the initiator.

Atom-Transfer Radical Addition to Unactivated Alkenes by using Heterogeneous Visible-Light Photocatalysis

Heterogeneous visible-light photocatalysis represents an important route toward the development of sustainable organic synthesis. In this study visible light-induced, heavy metal-free atom-transfer radical addition to unactivated terminal olefins is carried out by using the combination of heterogeneous titanium dioxide as photocatalyst and a hypervalent iodine(III) reagent as co-initiator. The reaction can be applied to a range of substrates with good functional-group tolerance under very mild conditions. In addition to a number of commonly used atom-transfer reagents, the relatively challenging chloroform is also suitable.

Benziodoxole Triflate as a Versatile Reagent for Iodo(III)cyclization of Alkynes

The utility of benziodoxole triflate, derived from α,α-bis(trifluoromethyl)-2-iodobenzyl alcohol, as a versatile reagent for iodo(III)cyclization via electrophilic activation of alkyne, is reported herein. The reagent promotes cyclization of alkynes tethered to a variety of nucleophilic moieties, affording benziodoxole-appended (hetero)arenes such as benzofurans, benzothiophenes, isocoumarins, indoles, and polyaromatics under mild conditions. This unprecedented class of (hetero)aryl-I^III compounds proved easy to purify, stable, and amenable to various synthetic transformations.

Iodosylbenzene-Pseudohalide-Based Initiators for Radical Polymerization

Iodosylbenzene reacts with various (pseudo)halides (trimethylsilyl azide or isocyanate or potassium azide, cyanate, and bromide) to yield unstable hypervalent iodine(III) compounds, PhIX₂ (X = (pseudo)halide), that undergo rapid homolysis of the hypervalent I-X bonds and generate (pseudo)halide radicals, which can initiate the polymerization of styrene, (meth)acrylates, and vinyl esters. Polymers are formed containing (pseudo)halide functionalities at the α-chain end but, depending on the termination mechanism and the occurrence of transfer of (pseudo)halide groups from the initiator to the propagating radicals, also at the ω-chain end. With slowly polymerizing monomers (styrene and methyl methacrylate) and initiators that were generated rapidly at high concentrations and were especially unstable, the reactions proceeded via a "dead-end" polymerization mechanism, and only low to moderate monomer conversions were attained. When the initiator was generated more slowly and continuously throughout the polymerization (using the combination of iodosylbenzene with the poorly soluble potassium (pseudo)halide salts), typically higher conversions and higher molecular weights were reached. The presence of (pseudo)halide functionalities in the polymers was proved by elemental analysis, IR, and NMR spectroscopy. The azide-containing polymers underwent click-type coupling reactions with dialkynes, while the (iso)cyanate-containing polymers reacted with diamines to afford high-molecular-weight polymers with triazole- and urea-type interchain links, respectively.

Co(OAc)2-Catalyzed Trifluoromethylation and C(3)-Selective Arylation of 2-(Propargylamino)pyridines via a 6-Endo-Dig Cyclization

A Co(OAc)₂-catalyzed trifluoromethylation and subsequent C(3)-selective arylation of 2-(propargylamino)pyridines has been developed. A new 6-endo-dig cyclization involving an unprecedented C(3) selective arylation of the pyridines instead of a commonly observed 5-exo-dig cyclization with "N" is realized. Moreover, the study presents the first case of the installation of a trifluoromethyl group into electron-deficient azaarenes. The process delivers an efficient cascade approach to new trifluoromethylated 1,8-naphthyridine structures with a broad substrate scope.

Hypervalent Iodine Mediated Chemoselective Iodination of Alkynes

Reported herein are practical approaches for the chemoselective mono-, di-, and tri-iodination of alkynes based on efficient oxidative iodinations catalyzed by hypervalent iodine reagents. The reaction conditions were systematically optimized by altering the iodine source and/or the hypervalent iodine reagent system. The tetrabutylammonium iodide (TBAI)/(diacetoxyiodo)benzene (PIDA) system is specific for monoiodination, while the KI/PIDA system results in di-iodination. Combining the TBAI/PIDA and KI/PIDA systems in one pot provided the corresponding tri-iodination products efficiently. These reaction conditions can be applied to the synthetically important iodination of aromatic and aliphatic alkynes, which was accomplished in good yield (up to 99%) and excellent chemoselectivity. These synthetic methods can also be applied to the efficient chemoselective synthesis of iodoalkyne derivatives, intermediates, and related biologically active compounds.

Metal-free intermolecular cyclopropanation between alkenes and iodonium ylides mediated by PhI(OAc)2·Bu4NI

A simple and highly effective synthesis of poly substituted cyclopropanes was developed. This metal-free intermolecular reaction between iodonium ylides and alkenes is mediated by PhI(OAc)₂ and Bu₄NI.