Phenyl iodonium ion

The regiospecific coupling of triphenylphosphine with the alkynyl ligands of alkynyl(phenyl)iodonium ions in such photosubstitution reactions is remarkable. This may be contrasted with the less selective and somewhat unpredictable photolytic decomposition (i.e. high pressure mercury lamp, pyrex) of alkynyl(phenyl)iodonium salts in the absence of nucleophiles (e.g. see equation 70)89. 

The conversion of alkynyl(phenyl)iodonium ions to alkynyl esters with carboxylate ions via the MC mechanism (equation 45) has been proposed81. Evidence for the viability of this process is provided by the generation of 3,3-dimethyl-1-cyclopentenyl benzoate in addition to the expected alkynyl benzoate when the iodonium triflate shown in equation 90 is mixed with sodium benzoate in dichloromethane (yields not reported)3. 

MCI reactions of alkynyliodonium salts with enolates derived from active methylene compounds containing two acidic CH bonds follow a divergent course that leads to furans, presumably via carbenic insertion into enolic OH bonds (equation 122)28. In the reaction of acetylacetonate ion with the l-decynyl(phenyl)iodonium ion, CH insertion is competitive with OH insertion (equation 123)28. 

The a-carbon-hydrogen bonds of vinyl(phenyl)iodonium ions are relatively acidic, two examples of base catalyzed hydrogen-deuterium exchange in -oxavinyliodonium salts having been reported (equations 227 and 228)79,95. Although vinylidene-iodonium ylides... 

Richter, Koser and coworkers investigated the nature of species present in aqueous solutions of phenylio-dine(III) organosulfonates [198]. It was shown by spectroscopic measurements and potentiometric titrations that PhI(OH)OTs and PhI(OH)OMs upon dissolution in water undergo complete ionization to give the hydroxy(phenyl)iodonium ion (Phl+OH in hydrated form) and the corresponding sulfonate ions. The... 

A detailed mechanism of the process shown in Scheme 1.3 is unknown. Two general mechanistic pathways, dissociative and associative, have been proposed for the ligand exchange reactions of X -iodanes (Scheme 1.4) [26, 127]. The dissociative pathway seems to be less likely to occur, because of the low stability of the dico-ordinated iodonium ion [PhIL]+ involved in this mechanism [127]. Such iodonium 8-1-2 species, however, have been frequently observed in the gas phase, for example, in mass spectrometry studies of protonated iodosylbenzene, [PhIOH]+ [101], or in the mass spectra of all known iodonium salts, [ArIR]+. The presence of cationic iodonium species in aqueous solution has been confirmed by spectroscopic measurements and potentiometric titrations of PhI(OH)OTs and PhI(OH)OMs [198] however, all available experimental data show that the iodonium species in solution are coordinated with solvent molecules or with available counteranions. X-Ray diffraction analysis of the protonated iodosylbenzene aqua complexes [PhI(H20)0H]+ isolated from aqueous solutions revealed a T-shaped stmcture, ligated with one water molecule at the apical site of the iodine(III) atom of hydroxy(phenyl)iodonium ion, with a near-linear O-I-O triad (173.96°), which is in agreement with a regular X -iodane structure [178]. 

Oligomeric iodosylbenzenes 26 and 27 have been prepared by ligand exchange in X. -iodanes under moderately acidic conditions. The oligomer 26 was obtained by the treatment of PhI(OAc)2 with aqueous NaHS04 [116,117], while product 27 precipitated from dilute aqueous solutions of PhI(OH)OTs and Mg(C104)2 [118] (Scheme 2.13). The formation of both products can be explained by self-assembly of the hydroxy(phenyl)iodonium ions (Phl+OH in hydrated form) and [oxo(aquo)iodo]benzene PhI+(0H2)0 in aqueous solution under the reaction conditions. 

The proposed mechanism for these catalytic oxidations includes two catalytic redox cycles (i) initial oxidation of iodobenzene with Oxone, producing hydroxy(phenyl)iodonium ion and hydrated iodosylbenzene and (ii) the oxidation of iron(III)-porphyrin to the oxoiron(IV)-porphyrin cation-radical complex by the intermediate iodine(III) species (Scheme 4.58) [93]. The oxoiron(IV)-porphyrin cation-radical complex acts as the actual oxygenating agent toward aromatic hydrocarbons. The presence of the [PhI(OH)]+ and PhI(OH)2 species in solutions containing Phi and Oxone has been confirmed by ESI mass spectrometry [93]. 

Oxidative desulfurization of acylthiosemicarbazides 97 and bisdiarylthioxneas 99 involving hydroxy(phenyl) iodonium ion was effectively applied by Telvekar et al. to deliver the corresponding 2-arylaminooxadiazoles 98 and 2-arylaminothiadiazoles 100 in good-to-excellent yields. This active iodine species, hydroxy(phenyl)iodonium ion was generated by the oxidation of iodobenzene using inexpensive and readily available Oxone as a co-oxidant at room temperature (Scheme 20) [36]. 

Most N-phenyl quaternary salts are not prepared by direct quater-nization but rather by introducing the nitrogen substituent before ring closure. It has recently been found that diphenyl iodonium boro-fluoride reacts smoothly with pyridine the phenyl carbonium ions formed give the 1-phenylpyridinium ion good yield. ... 

When 4-/-butylcyclohex-1 -enyl(phenyl)iodonium tetrafluoroborate (3) is heated at 60 °C in chloroform, 1-fluorocyclohexene 4, 1-chlorocyclohexene 5 and l-(o-iodophenyl)cyclohexene 6 are formed with accompanying iodobenzene leaving group (eq 2).3 These three substitution products are best accounted for by formation of an ion pair involving cyclohexenyl cation 7. The cyclohexenyl cation 7 formed picks up fluoride from tetrafluoroborate and chloride from chloroform solvent, and recombines with the iodobenzene generated (eq 3). This kind of reactions with a counteranion and solvent are characteristic of unstable carbocations and are known in the case of phenyl cation generated from the diazonium salt in the Schiemann-type reaction.4... 

Reactions of 3 in alcoholic and aqueous solvents result in a normal solvolysis product 8 (and cyclohexanone 9) as well as the recombination product 6 (eq 4).5 This is again rationalized by an ion-pair mechanism. 2-Methylcyclohex-l-enyl(phenyl)iodonium tetrafluoroborate (10) undergoes solvolysis about 250 times as fast as 3, and gives some rearranged product 12 in accord with the SnI solvolysis mechanism (Scheme 1). 

Interesting results concerning phenyl group participation were observed with ( )-styryl(phenyl)iodonium tetrafluoroborate (26) using a deuterated substrate (eq 12)16 When 26-ad was heated in trifluoroethanol (TFE) at 60 °C, slow reaction gave die E isomer of substitution product 28 quantitatively, but the deuterium was completely scrambled between the a and p positions. This strongly indicates that a symmetrical intermediate is involved during the reaction and the most reasonable one is vinylenebenzenium ion (27) formed by phenyl participation. This intermediate also explain the exclusive formation of the retained ( )-28. 

Reactions of (ii)-l-decenyl(phenyl)iodonium salt (6a) with halide ions have been examined under various conditions. The products are those of substitution and elimination, usually (Z)-l-halodec-l-ene (6b) and dec-l-yne (6c), as well as iodobenzene (6d), but F gives exclusively elimination. In kinetic studies of secondary kinetic isotope effects, leaving-group substituent effects, and pressure effects on the rate, the results are compatible with the in-plane vinylic mechanism for substitution with inversion. The reactions of four ( )-jS-alkylvinyl(phenyl)iodonium salts with CP in MeCN and other solvents at 25 °C have been examined. Substitution with inversion is usually in competition with elimination to form the alk-l-yne. 

Stang and co-workers361-363 have reported the synthesis of the triflate salts of dialkynyliodonium ions (142), the phenyl(cyano)iodonium ion 143 and the dicya-noiodonium ion 144. 

Kinetic and spectroscopic studies of reactions of several ( )-/J-alkyl-vinyl(phenyl)iodonium tetrafluorob orates with tetrabutylammonium chloride indicate that the vinyliodonium chlorides, generated by anion exchange, are present in equilibrium with the corresponding vinyl(chloro)iodanes (Scheme 48) [140]. Both species undergo vinylic Sn2 reactions with chloride ion, and although the chloroiodanes are less reactive, they are by far the dominant species at equilibrium and account for most of the (Z)-l-chloroalkene production. 

Some comments should be added for a few cases. The reaction of diphenyliodo-nium tetrafluoroborate with sodium thiocyanate, and sodium phenylsulphinate, was also carried out in chloroform-water, at reflux, with very high yields similar efficient phenylation occurred with other anions [84]. Although these reactions were not performed on a preparative scale, it is likely that similar conditions may be applicable to other iodonium salts as well, for example in the thiocyanation of di-(3-thienyl)iodonium ion which gave 3-thiocyanatothiophene (43%), the best yield compared with other methods [18]. Concerning the preparation of triarylsulphonium... 

Alkynyliodonium ions, 1 and 2, are hypervalent iodine species in which one or two alkynyl ligands are bound to a positively charged iodine(III) atom. They are sensitive to nucleophiles, especially at the /1-carbon atom(s) of the alkynyl ligand(s), and for that reason, the isolation of stable alkynyliodonium salts generally requires the incorporation of nucleofugic anions. A list of known alkynyliodonium compounds (i.e. as of 4/1/94), containing 134 iodonium salts derived from 103 iodonium ions, and references (5-45) to their preparation and characterization are presented in Table 1. Among these compounds, alkynyl(phenyl)iodonium sulfonates and tetrafluoroborates are the most common, while alkynyl(alkyl)iodonium salts of any kind are unknown. 

When y-CH bonds are present in the R group of the alkynyliodonium ion, cyclopentenyl sulfones predominate. For example, the treatment of 5-phenyl-1-pentynyl(phenyl)-iodonium tetrafluoroborate with te/ra- -butylammonium benzenesulfinate in THF (i.e. homogeneous conditions) affords a moderate yield of l-phenylsulfonyl-3-phenylcy-clopentene and a low yield of the corresponding alkynyl sulfone (equation 51)32. With appropriately constructed alkynyliodonium ions, annulated cyclopentenyl sulfones are obtained (equations 52 and 53)32. 

The reactions of / -ketoethynyl- and ) -amidoethynyl(phenyl)iodonium triflates, 17 and 18, with sodium / -toluenesulfinate illustrate the synthetic potential of alkynyliodonium salts33. Although the direct attachment of a carbonyl group to the / -carbon atom of the triple bond in alkynyliodonium ions might be expected to facilitate alkynyl sulfone formation via the Ad-E mechanism, this mode of reactivity has not been observed. Instead, the MC pathway with carbenic insertion dominates and affords sulfones containing the... 

Dimethyl-l-butynyl(phenyl)iodonium tosylate has been employed with bis-(diphenylphosphino)methane to give the (terr-butyl)phospholium tosylate shown in equation 7892. The initial formation of an alkynylphosphonium ion with a free phosphino group has been proposed. Intramolecular cyclization of this intermediate followed by a 1,3-prototropic shift would lead to the observed product. Evidence for the probability of such an intermediate is provided by the fact that the alkynyliodonium tosylate, like the... 

When / -dicarbonyl enolates are allowed to react with alkynyliodonium salts, typically in ter/-butyl alcohol or THF, alkynyl- and/or cyclopentenyl- -dicarbonyl compounds are obtained. The product compositions are largely regulated by the migratory aptitude of R in the alkynyl moiety and the availability of alkyl side chains for the MC-insertion (MCI) pathway (equation 45). These divergent modes of reactivity are nicely illustrated by the reactions of the 2-phenyl-1,3-indandionate ion with ethynylfphenyl)- and 4-methyl-1-hexynyl(phenyl)iodonium tetrafluoroborates (equation 1 15)27 2. 

Because the hydrogen atom and phenyl group migrate so readily, the reactions of / -dicarbonyl enolates with ethynyl- and (phenylethynyl)iodonium salts can be expected to result in alkynylation. It has already been noted that the 2- -hexyl-l,3-indandionate ion undergoes alkynylation with (phenylethynyl)phenyliodonium tetrafluoroborate (equation 43), despite the availability of the -hexyl group for [2 + 3] annulation. Ethynylations of six / -dicarbonyl enolates and the anion of 2-nitrocyclohexane with ethynyl(phenyl)-iodonium tetrafluoroborate in THF have also been reported27. For example, admixture of the ethynyliodonium salt and the anion of ethyl 2-cyclopentanone-l-carboxylate in THF affords the 1-ethynyl derivative in 71% isolated yield (equation 124)27. 

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