Vinyl carbocations: solution studies of alkenyl(aryl)iodonium triflate fragmentations

Generation of vinyl cations is facile by fragmentation of alkenyl(aryl)iodonium trifluoromethanesulfonates. Kinetics and electronic effects were probed by (1)H NMR spectroscopy in CDCl(3). Products of fragmentation include six enol triflate isomers in addition to iodoarenes. The enol triflates arise from direct reaction of a triflate anion with the starting iodonium salts as well as triflate reaction with rearranged secondary cations derived from those salts. G2 calculations of the theoretical isodesmic hydride-transfer reaction between secondary vinyl cation 7 and primary vinyl cation 6 reveal that cation 6 is 17.8 kcal/mol higher in energy. Activation parameters for fragmentation of (Z)-2-ethyl-1-hexenyl(3,5-bis-trifluoromethylphenyl)iodonium triflate, 17e, were calculated using the Arrhenius equation: E(a) = 26.8 kcal/mol, Delta H(++) = 26.2 kcal/mol, and Delta S(++) = 11.9 cal/mol x K. Added triflate increases the rate of fragmentation slightly, and it is likely that for most beta,beta-dialkyl- substituted vinylic iodonium triflates enol triflate fragmentation products are derived from three competing mechanisms: (a) vinylic S(N)()2 substitution; (b) ligand coupling (LC); and (c) concerted aryliodonio departure and 1,2-alkyl shift leading to secondary rather than primary vinyl cations.     

Copper-Catalyzed One-Pot Arylation and Cyclization of Diaryliodonium Salts Derived from o-Iodoanilines for Indolo[2,3-b]indoles Syntheses

One-pot synthesis of various indolo[2,3-b]indoles has been accomplished from a copper-catalyzed transformation of easily accessible 2-(substituted-amino)aryl)(mesityl)iodonium salts and indole derivatives. Reaction offers great functional group toleration, providing a series of indolo[2,3-b]indole derivatives in good yields. Additionally, intramolecular cyclization of 2-(substituted-amino)aryl)(mesityl)iodonium salts in the presence of copper-catalyst provided direct access to benzoxazole derivatives.     

Preparation of bis(heteroaryl)iodonium salts via an iodonium transfer reaction between di(cyano)iodonium triflate and organostannes

Di(cyano)iodonium triflate 5 generated in situ from iodosyl triflate (0 = I-OTf) and cyanotrimethylsilane is a preferred reagent for the preparation of various bis(heteroaryl)iodonium triflate salts 11–15 via an iodonium transfer reaction with the corresponding tributyltin substituted heterocycles 6–10 . Novel heteroaromatic iodonium salts 3, 11–15 were isolated in good yields as relatively stable microcrystalline solids and characterized by multinuclear nmr, ir and hrms data.     

香豆素酮类化合物的瞬态研究——Ⅲ.香豆素酮和碘鎓盐间的电子转移

本工作采用激光闪光光解法对香豆素酮类化合物敏化碘鎓盐化合物问题进行了研究。结果表明:碘鎓盐化合物能通过电子转移机理猝灭香豆素酮的激发三重态。工作中还用甲基紫精(PQ²⁺)为模型化物,观察到它也能使香豆素酮的三重态猝灭,同时可看到在位于610nm处的PQ⁺生成。这些结果说明,在发生电子转移的反应中香豆素酮是电子给体,按Weller公式的计算结果也表明它们之间可发生电子转移反应。     

Single‐Electron‐Transfer (SET)‐Induced Oxidative Biaryl Coupling by Polyalkoxybenzene‐Derived Diaryliodonium(III) Salts

Metal‐free oxidative C? C coupling by using polyalkoxybenzene‐derived diaryliodonium(III) salts as both the oxidant and aryl source has been developed. These salts can induce single‐electron‐transfer (SET) oxidation to yield electron‐rich arenes and subsequently transfer the polyalkoxyphenyl group into in situ generated aromatic radical cations to produce biaryl products. The reaction is promoted by a Lewis acid that activates the iodonium salts. It has been revealed that the reactivity of the salts under acidic conditions is quite different to their known behavior under basic conditions. The reactivity preference of a series of iodonium salts in the SET oxidation and their ligand transfer abilities have been systematically investigated and the results are summarized in this report.     

Synthesis and study of iodonium borate salts as photoinitiators

Iodonium butyltriphenylborate salts (A I⁺ Ar′Ph₃B⁻ R), (Bu) were found to be more efficient than iodonium tetraphenylborate salts (RPh) when used as photoinitiators for the polymerization of acrylates. Relative photodecomposition rates were also different. It was found from a study of the photoreaction of iodonium borate salts with a model monomer, methyl methacrylate, that iodonium butyltriphenylborate salts simultaneously produce a butyl radical from the borate anion and an aryl radical from the iodonium cation upon irradiation. Both radicals initiate polymerization. Iodonium tetraphenylborate salts were found to release an aryl radical, but only from the iodonium cation. Iodonium borate salts exhibit strong absorption below 300 nm with a tail absorption above 400 nm. Thus, iodonium butyltriphenyl borate salts are efficient photoinitiators even when used with visible light. When a photosensitizer such as 5,7-diiodo-3-butoxy-6-fluorone is employed, iodonium butyltriphenylborate salts are rendered much more efficient for visible light photopolymerization. © 1998 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 36: 1667–1677, 1998     

A Selective C−H Deprotonation Strategy to Access Functionalized Arynes by Using Hypervalent Iodine

Described here is an efficient method to access highly functionalized arynes from unsymmetrical aryl(mesityl)iodonium tosylate salts. The iodonium salts are prepared in a single pot from either commercially available aryl iodides or arylboronic acids. The aryne intermediates are generated by ortho‐C?H deprotonation of aryl(mesityl)iodonium salt with a commercially available amide base and trapped in a cycloaddition reaction with furan in moderate to good yields. Coupling partners for the aryne intermediates beyond furan are also described, including benzyl azide and alicyclic amine nucleophiles. The regio‐ and chemoselectivity of this reaction is discussed and evidence for the spectator aryl ligand of the iodonium salt as a critical control element in selectivity is presented.     

Cyclotetraphosphinophosphonium ions: synthesis, structures, and pseudorotation

The first derivatives of catenated cyclotetraphosphinophosphonium cations, [(PhP)4PPhMe]+ (8a), [(MeP)4PMe2]+ (8b), [(CyP)4PPh2]+ (8d), [(CyP)4PMe2]+ (8e), [(PhP)4PPh2]+ (8f), [(PhP)4PMe2]+ (8g), are synthesized as trifluoromethanesulfonate (triflate, OSO2CF3-) salts through the reaction of cyclopentaphosphines (PhP)5 (4a) or (MeP)5 (4b) with methyl triflate (MeOTf) or by a net phosphenium ion [PR2+, R = Ph, Me; from R2PCl and trimethylsilyltriflate (Me3SiOTf)] insertion into the P-P bond of either cyclotetraphosphine (CyP)4 (3c) or cyclopentaphosphines (PhP)5 (4a) or (MeP)5 (4b). Although more conveniently prepared from 4a, compound 8a[OTf] can also be formed from (PhP)4 (3a) and MeOTf, and derivatives 8f[OTf] and 8g[OTf] are also accessible through reactions of 3a and R2PCl/Me3SiOTf with R = Ph or Me, respectively. A tetrachlorogallate salt of [(PhP)4PPhtBu]+ (8c) has been synthesized by alkylation of 4a with tBuCl/GaCl3. 31P[1H] NMR parameters for all derivatives of 8 have been determined by iterative simulation of experimental data. Derivatives 8a[OTf], 8b[OTf], 8c[GaCl4], 8e[OTf], 8f[OTf], and 8g[OTf] and have been characterized by X-ray crystallography, showing the most favorable all-trans configuration of substituents for the phosphine centers, thus minimizing steric interactions. Each derivative adopts a unique envelope or twist conformation of C1 symmetry. The effective C2 symmetry observed for 8b, d, e, f, and g in solution, signified by their 31P[1H] NMR AA'BB'X spin systems, implies a rapid conformational exchange for derivatives of 8. The core frameworks of the cations in the solid state are viewed as snapshots of different conformational isomers within the solution-phase pseudorotation process.     

Regioselective Synthesis of 1,2,3,4-Tetrasubstituted Arenes by Vicinal Functionalization of Arynes Derived from Aryl(Mes)iodonium Salts**

Herein, the synthesis of 1,2,3,4-tetrasubstituted benzenoid rings, motifs found in pharmaceutical, agrochemical, and natural products, is described.^[1] In the past, the regioselective syntheses of such compounds have been a significant challenge. This work reports a method using substituted arynes derived from aryl(Mes)iodonium salts to access a range of densely functionalized 1,2,3,4-tetrasubstituted benzenoid rings. Significantly, it was found that halide substituents are compatible under these conditions, enabling post-synthetic elaboration via palladium-catalyzed coupling. This concise strategy is predicated on two regioselective events: 1) ortho- deprotonation of aryl(Mes)iodonium salts to generate a substituted aryne intermediate, and 2) regioselective trapping of said arynes, thereby improving previously reported reaction conditions to generate arynes at room temperature and in shorter reaction times. Density functional theory (DFT) computations and linear free energy relationship (LFER) analysis suggest the regioselectivity of deprotonation is influenced by both proximal and distal ring substituents on the aryne precursor. A competition experiment further reveals the role of arene substituents on relative reactivity of aryl(Mes)iodoniums as aryne precursors.     

Copper‐Catalyzed Oxy‐Alkenylation of Homoallylic Alcohols to Generate Functional syn‐1,3‐Diol Derivatives

A novel method for the synthesis of a wide range of functionalized 1,3‐diol derivatives is reported. Employing a copper‐catalyzed oxy‐alkenylation strategy, a range of readily available, substituted homoallylic alcohol derivatives and alkenyl(aryl) iodonium salts combine to form syn‐1,3‐carbonates in excellent yield and with high selectivity. Furthermore, the products formed are amenable to an iterative reaction sequence, thus affording highly complex polyketide‐like fragments.     

Arylation with Unsymmetrical Diaryliodonium Salts: A Chemoselectivity Study

Phenols, anilines, and malonates have been arylated under metal‐free conditions with twelve aryl(phenyl)iodonium salts in a systematic chemoselectivity study. A new “anti‐ortho effect” has been identified in the arylation of malonates. Several “dummy groups” have been found that give complete chemoselectivity in the transfer of the phenyl moiety, irrespective of the nucleophile. An aryl exchange in the diaryliodonium salts has been observed under certain arylation conditions. DFT calculations have been performed to investigate the reaction mechanism and to elucidate the origins of the observed selectivities. These results are expected to facilitate the design of chiral diaryliodonium salts and the development of catalytic arylation reactions that are based on these sustainable and metal‐free reagents.     

Fluoroalkylations and Fluoroalkenylations with Iodonium Salts

Synthesis and applications of fluoroalkyl and fluoroalkenyliodonium salts are summarized in this account article, focusing preferably to the reagents designed in our laboratory in the last decade. Among these reagents trifluoroethyl(aryl)iodonium salts have been used most frequently to build carbon-carbon and carbon-heteroatom bonds in simple nucleophilic substitutions and through transition metal catalyzed coupling reactions. Iodonium salts equipped with unsaturated fluorinated function showed diverse reactivity due to their electron deficient character, and these molecular motifs enable cycloadditions and nucleophilic additions to prepare fluorinated carbo- and heterocyclic molecules. Beyond the overview of existing transformations, with the presented collection, we aim to inspire future developments of iodonium reagents and their application in organic synthesis.     

Covalent grafting of glassy carbon electrodes with diaryliodonium salts: new aspects

The applicability and versatility of the recently communicated procedure for the grafting of conducting carbon substrates by diaryliodonium salts is expanded. We have found that several types of organic arylic layers can be formed on the carbon surface and that the chemical functionalities of the thus formed layers can be varied extensively over electron withdrawing (for example, -NO2) to electron donating (for example, -OMe) groups. A comparative study involving the grafting of aryldiazonium salts reveals that, despite the two approaches being similar, iodonium salts exhibit spontaneous grafting to a significantly lower extent. Nevertheless, the grafted layer becomes less accessible to proton transport as visualized from a greater reluctance toward the reduction of surface-confined nitro groups to amino groups in acidic medium. Employment of unsymmetrical iodonium salts opens up the interesting possibility of forming organic films consisting of a mixture of two different aryl groups. Alternatively, such composite layers may be prepared by selecting iodonium and diazonium salts with comparable reduction properties. Analysis of the surfaces is carried out by means of cyclic voltammetry, X-ray photoelectron spectroscopy, and ToF-SIMS (time-of-flight secondary-ion mass spectrometry). The ToF-SIMS analysis primarily serves to provide unambiguous evidence for the covalent attachment of the organic layers to the surface.     

New synthesis of 3,5-disubstituted-5H-thiazolo[3,2-a]pyrimidine via ring annulation of 3,4-dihydropyrimidin-2(1H)-thione using alkynyl(aryl)iodonium salts

A transition metal-free protocol for the synthesis of biologically active thiazolo[3,2-a]pyrimidine derivatives has been achieved by the cyclocondensation of 3,4-dihydropyrimidin-2(1H)-thiones with alkynyl(aryl)iodonium tosylates. This reaction demonstrates another useful application of alkynyl(aryl)iodonium tosylates as synthon of alkynyl cation.     

Regiochemical Control and Suppression of Double Bond Isomerization in the Heck Arylation of 1-(Methoxycarbonyl)-2,5-dihydropyrrole

Arylation of 1-(methoxycarbonyl)-2,5-dihydropyrrole under standard Heck reaction conditions produces a mixture of compounds. The olefin undergoes two types of palladium-catalyzed reactions: (a) arylation to provide C-3 arylated derivatives and (b) competing double bond isomerization. Addition of silver carbonate and thallium acetate fully suppressed the isomerization, and good yields of C-3 substituted compounds were achieved after arylation with aryl halides. With regard to aryl triflates as arylating agents, addition of lithium chloride was necessary to promote the Heck reaction. This additive excluded the use of silver and thallium salts, but high regioselectivity and good yields could be obtained by employing tri-2-furylphosphine as ligand. Arylation was rendered both regioselective and enantioselective (58% ee) with 1-naphthyl triflate as substrate utilizing a (R)-BINAP/thallium acetate combination. The C-3 arylated enamides were converted further into the corresponding 3-arylpyrrolidines.     

One-Pot Synthesis and Applications of N-Heteroaryl Iodonium Salts

An efficient one-pot synthesis of N-heteroaryl iodonium triflates from the corresponding N-heteroaryl iodide and arene has been developed. The reaction conditions resemble our previous one-pot syntheses, with suitable modifications to allow N-heteroaryl groups. The reaction time is only 30 min, and no anion exchange is required. The obtained iodonium salts were isolated in a protonated form, these salts can either be employed directly in applications or be deprotonated prior to use. The aryl groups were chosen to induce chemoselective transfer of the heteroaryl moiety to various nucleophiles. The reactivity and chemoselectivity of these iodonium salts were demonstrated by selectively introducing a pyridyl moiety onto both oxygen and carbon nucleophiles in good yields.     

Modern synthetic methods for copper-mediated C(aryl)[bond]O, C(aryl)[bond]N, and C(aryl)[bond]S bond formation

The copper-mediated C(aryl)[bond]N, C(aryl)[bond]O, and C(aryl)[bond]S bond formation is an important transformation and has been developed to include a wide range of substrates. This Review highlights the recent developments in the copper-mediated (both stoichiometric and catalytic) reactions of aryl boronic acids, aryl halides, iodonium salts, siloxanes, stannanes, plumbanes, bismuthates, and trifluoroborate salts as aryl donors. In particular, the recent introduction of boronic acids as reaction partners in both O- and N-arylation has been a significant discovery and will occupy centre-stage in this review. Clear improvements can be obtained by the correct choice of copper source, base, ligands, and other additives. Mechanistic investigations should provide insight into the catalytically active species, which would aid in the development of milder, more-efficient methods.     

New synthesis of spirocycles by utilizing in situ forming hypervalent iodine species

A very effective spirocyclization procedure for installing nucleophiles (Nu = N(3), NO(2), SCN, SO(2)Tol, and halogens) via iodonium(III) salts has been developed using the combination of iodoarene and mCPBA. The high-yielding syntheses of the cyclohexadienone-type spirocyclic compounds 2 having varied functionalities in the skeletons have been achieved from the aryl alkynes 1 with the optimized bis(iodoarene) 3h.     

Synthesis,Characterization and Unusual Reactivity of Vinylbenziodoxolones—Novel Hypervalent Iodine Reagents

A novel type of hypervalent iodine(III) reagents, vinylbenziodoxolones (VBX), has been synthesized in a one-pot reaction from 2-iodobenzoic acid. VBX is bench stable, has been thoroughly characterized and the cyclic structure is supported by X-ray analysis. The reactivity of VBX was investigated in vinylation of nitrocyclohexane, and delivered vinylated products with opposite regioselectivity compared to acyclic vinyl(aryl)iodonium salts. The reagents could become a powerful tool in vinylation reactions under both metal-free and metal-catalyzed conditions.     

O‐Arylation of Carboxylic Acids using (Phenyl)[2‐(trimethylsilyl)phenyl]iodonium Triflate as a Precursor of Arynes

Using (phenyl)[2‐(trimethylsilyl)phenyl]iodonium triflate as a precursor of arynes, Larock's method for O‐arylation of carboxylic acids and arynes was developed. A variety of acids including simple aliphatic carboxylic acids, aromatic carboxylic acids, allenoic acids, and p‐toluenesulfonic acid under mild reaction conditions could generate the aryl esters.