X’-iodane

The reactions involved are unimolecular, and the cyclohexenyl derivative 3 undergoes solely the spontaneous heterolysis while both spontaneous heterolysis and ligand coupling occur with the iodane 14. The relative contributions of the two reactions of 14 depend on the solvent polarity. The results summarized in Table I show that the iodonium ion and the counteranion are in equilibrium with the hypervalent adduct, X -iodane. The equilibrium constants depend on the identity of the anion and the solvent employed, and the iodane is less reactive than the free iodonium ion as the k /k2 raios demonstrate. Spontaneous heterolysis of 3 occurs more than 100 times as fast as thnt of the adduct 14 as observed in methanol the leaving ability of the iodonio group is lowered by association by more than 100 times. 

A series of mixed phosphonium-iodonium ylides (26) and (27), compounds that contain both phosphoranyl and hypervalent iodine (X, -iodanes), have been prepared and characterised (Scheme 5). These compounds might prove to be synthetically valuable reagents. ... 

The overwhelming majoritiy of currently known alkynyliodonium species are prepared by interaction of a terminal, sila- or tin-acetylene with an electrophilic X -iodane, also referred to as a 10-1-3 hypervalent species [14]. Key reagents are iodosobenzene (4), [hydroxyftosyl-oxy)iodo]benzene (5, HTIB) [15] the -oxo-X -iodane 6 [16], and cyano(phenyl)iodonium triflate 7 [17]. 

Few, if any, direct mechanistic investigations have been reported in the literature on the formation of alkynyliodonium salts. However, all available evidence suggests that the reaction involves initial electrophilic addition of a highly polar or ionic X -iodane to the triple bond and formation of a vinyl cation 24 (or vinyl cation-like intermediate). The reaction of the cyano species, 7, is illustrative, as summarized in Scheme 3-2. 

Iodine trichloride or trichloro-X -iodane (Dichloroiodo)benzene... 

Diphenyliodonium chloride or chloro(diphenyl)->. -iodane [/V-(4-Methylphenylsulfonyl) i m i no] pheny I- iodane Iodine pentafluoride or pentafluoro-X -iodane Iodic acid lodylbenzene 1 -Hydroxy-1 -oxo-1 H-IX -benzo [t/j [ 1,2 ] iodoxo I -3 -one... 

Iodine heptafluoride or heptafluoro-X -iodane Periodic acid... 

The I-C bond lengths in iodonium salts Ral X and X -iodanes RIX2 are approximately equal to the sum of the covalent radii of iodine and carbon, ranging generally from 2.00 to 2.10 A. 

Several important spectroscopic structural studies of polyvalent iodine compounds in solution have been published [108-112, 189]. Reich and Cooperman reported low-temperature NMR study of triaryl-X -iodanes 27 (Scheme 1.1), which demonstrated that these compounds have a nonsymmetrical planar orientation of iodine-carbon bonds and that the barrier to unimolecular degenerate isomerization between 27 and 27 is greater than 15 kcal mol The exact mechanism of this degenerate isomerization is unknown both pseudorotation on iodine(III) and intermolecular ligand exchange may account for the isomerization of these compounds [189]. 

Scheme 1.1 Degenerate isomerization of triaryl-X -iodanes 27 in solution.

Most reactions of X -iodanes PhIL2 involve the initial exchange of ligands on the iodine atom with external nucleophiles (Nu ) followed by reductive elimination of iodobenzene (Scheme 1.3). The second step in this simplified scheme can also proceed as ligand coupling [1], if it occurs as a concerted process. A similar general mechanistic description can also be applied to the reactions of X -iodanes. 

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]. 

Scheme 1.3 Simplified description of the reactions of X -iodanes with nucleophiles Nu.

The associative pathway of ligand exchange starts from the addition of a nucleophile to the positively charged iodine atom of a X -iodane with the initial formation of a trans hypervalent 12-1-4 square-planar species. This intermediate species isomerizes to the cis 12-1-4 square-planar intermediate and eliminates the ligand L to afford the final product (Scheme 1.4). Such a mechanism has been validated by the isolation and X-ray structural identification of several stable 12-1-4 species. For example, the interaction of ICI3 with chloride anion affords tetrachloroiodate anion, ICl4, which has a distorted square-planar structure as established by X-ray analysis of the trichlorosulfonium salt, CI3S+ ICU [209]. 

Processes involving a single-electron transfer (SET) step and cation-radical intermediates can occur in the reactions of X - or X -iodanes with electron-rich organic substrates in polar, non-nucleophilic solvents. Kita and coworkers first found that the reactions of p-substituted phenol ethers 29 with [bis(trifluoroacetoxy)iodo]benzene in the presence of some nucleophiles in fluoroalcohol solvents afford products of nucleophilic aromatic substitution 31 via a SET mechanism (Scheme 1.5) [212,213]. On the basis of detailed UV and ESR spectroscopic measurements, it was confirmed that this process involves the generation of cation-radicals 30 produced by SET oxidation through the charge-transfer complex of phenyl ethers with the hypervalent iodine reagent [213,214],... 

The SET mechanism was also proposed for some oxidations involving X -iodanes. In particular, mechanistic studies involving isotope labeling, kinetic studies, cyclic voltammetry measurements and NMR spectroscopic analysis confirm that SET is a rate-determining step in the IBX-promoted oxidative cyclization of unsaturated anilides in THE-DMSO solutions [216], The analogous mechanism was proposed for the oxidation of alkylbenzenes at the benzylic position under similar conditions [217]. 

Organic iodosyl compounds usually have a polymeric structure, (RIO) , with a typical, for X -iodanes, T-shaped geometry at the iodine atom no structural evidence supporting the existence of an 1=0 double bond has been reported. Most known iodosyl compounds have low thermal stability and some are explosive upon heating. Iodosyl compounds can be prepared by direct oxidation of organic iodides, or, more commonly, by basic hydrolysis of other iodine(III) compounds. Table 2.4 summarizes the preparation methods for organic iodosyl compounds. 

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. 

Halobenziodoxoles l-Chloro-l,2-benziodoxol-3-(l//)-one (88, 2X = O, Y = Cl) can be easily prepared by direct chlorination of 2-iodobenzoic acid [233], or by the oxidation of 2-iodobenzoic acid with sodium chlorite (NaC102) in aqueous hydrochloric acid media [267]. The original X-ray single-crystal analysis of l-chloro-l,2-benziodoxol-3-(l//)-one reported in 1976 was relatively imprecise [268]. More recently, Koser and coworkers reported the single-crystal X-ray structure of a 1 1 complex of l-chloro-1,2-benziodoxol-3-(l/7)-one and tetra-n-butylammonium chloride [262], The primary bond distances at iodine in this compound are consistent with expectations for a X -iodane. In particular, the I—Cl and I—O bond distances of 2.454 and 2.145 A, respectively, are greater than the sums of the appropriate covalent radii and reflect the... 

Azidobenziodoxoles The noncyclic azido X -iodanes, for example, PhI(N3)OAc or PhI(N3)2, in general lack stability and rapidly decompose at -25 to 0 °C with the formation of iodobenzene and dinitrogen (Section 2.1.12.1). The incorporation of hypervalent iodine atom into a five-membered heterocycle leads to a significant stabilization of the azidoiodane. Stable azidobenziodoxoles 122-124 can be prepared by the reaction of hydroxybenziodoxoles 121 with trimethylsilyl azide in acetonitrile [251, 292], or by treatment of acetoxybenziodoxoles 125 with trimethylsilyl azide in dichloromethane in the presence of catalytic trimethylsilyl triflate (Scheme 2.43) [259]. All three azides 122-124 were isolated as thermally stable, non-explosive, microcrystalline solids that can be stored indefinitely in a refrigerator. 

Trifluoromethylbenziodoxoles The noncyclic CF3-substituted X -iodanes in general lack stability and cannot be isolated at room temperature however, the incorporation of a hypervalent iodine atom into a five-membered heterocycle has a significant stabilization effect. The first synthesis of stable trifluoromethylbenziodoxoles 157 and 159-161 by treatment of corresponding methoxybenziodoxole 156 or acetoxybenziodoxole 158 with trimethyl(trifluoromethyl)silane was reported by Togni and coworkers in 2006 (Scheme 2.52) [248]. 

Preparation A common synthetic approach to alkynyliodonium salts involves the reaction of an electrophilic X -iodane with a terminal alkyne or its silylated, stannylated, or lithiated derivative. In the early 1980s, Koser and coworkers found that [hydroxy(tosyloxy)iodo]benzene 75 reacts with terminal alkynes 344 upon gentle heating in chloroform or dichloromethane to form alkynyliodonium tosylates 345 in moderate to low yield (Scheme 2.98) [199,471,476]. 

Within the broad field of hypervalent iodine chemistry, five-membered iodine(V) heterocycles occupy a special place. There has been significant interest in the cyclic X -iodanes, mainly 2-iodoxybenzoic acid (IBX)... 

The reaction of difluoroiodotoluene with a four-, flve-, or six-membered carbocycles 47 affords the ring-expanded ( )-S-fluoro-3-halovinyliodonium tetrafluoroborates 48 stereoselectively in high yields (Scheme 3.17) [42], This reaction proceeds via a sequence of X -iodanation-1,4-halogen shift-ring enlargement-fluorination steps. 

In contrast with X, -iodanes (Section 3.2), hypervalent iodine(III) reagents are not effective oxidants of alcohols in the absence of catalysts. Kita and coworkers were the first to find that, in the presence of bromide salts, iodosylbenzene or (diacetoxyiodo)benzene can be used as an efficient reagent for selective oxidation of alcohols [136,137]. The iodosylbenzene-KBr system is applicable to the oxidation of various primary and secondary alcohols, even in the presence of sensitive functional groups such as ether, ester, sulfonamide and azido groups. Primary alcohols under these conditions afford carboxylic acids (Scheme 3.50), while the oxidation of various secondary alcohols under similar conditions affords the appropriate ketones in almost quantitative yield [136]. 

Alkenes and alkynes can be oxidatively functionalized by electrophilic X -iodanes, such as iodosylbenzene, [bis(acyloxy)iodo]arenes and organoiodine(lll) derivatives of strong acids. Iodosylbenzene itself has a low reactivity to alkenes due to the polymeric structure. However, the relatively weak electrophilic reactivity of (PhlO) can be increased considerably in the presence of BF3-Et20 or other Lewis acids. This activation... 

A very mild procedure for the Hofmann rearrangement of aromatic and aliphatic carboxamides 409 is based on the use of (tosylimino)phenyl-X. -iodane, PhINTs, as the oxidant (Scheme 3.165) [506]. Owing to the mild reaction conditions, this method is particularly useful for the Hofmann rearrangement of substituted benzamides 409 (R = aryl), which usually afford complex reaction mixtures with other hypervalent iodine oxidants. The mild reaction conditions and high selectivity in the reaction of carboxamides with PhINTs allow the isolation of the initially formed labile isocyanates 410, or their subsequent conversion into stable carbamates 411 by treatment with alcohols. Based on the previously reported mechanistic studies of the Hofmann rearrangement using other hypervalent iodine reagents [489,490,496], it is assumed that the reaction... 

Hypervalent iodine(III) compounds have found wide application for the oxidation of organic derivatives of nitrogen, sulfur, selenium, tellurium and other elements. Reactions of X -iodanes with organonitrogen compounds leading to the electron-deficient nitrenium intermediates and followed by cyclizations and rearrangements (e.g., Hofmann rearrangement) are discussed in Section 3.1.13. Several other examples of oxidations at a nitrogen center are shown below in Schemes 3.168-3.170. 

The chemistry of X -iodanes in general has been less developed than with the X -iodanes [1102]. Significant interest in these compounds originated in 1983, when Dess and Martin reported a simple two-step preparation of the triacetate 800 via the bromate oxidation of 2-iodobenzoic acid to 2-iodoxybenzoic acid (IBX, 799) followed by heating with acetic anhydride (Scheme 3.316) [1103]. The authors have also found that the... 

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