Iodinating reagent, cationic

An insoluble cationic iodinating reagent, combined with a chiral binol-derived lipophilic phosphoric acid catalyst, has been found to act as an efficient source of chiral iodine that performs the semipinacol transposition of strained allylic alcohols to -iodo spiroketones B (Scheme 85). (Se)... 

Collections of fundamental and thermodynamic data can be found in an earlier review [158] and in standard resources [13, 14]. However, due to the reactivity of iodine there are many less common or more reactive forms of iodine that have been less well characterized. For example, a blue 12 cation, a brown I3+, or a green I5+ cation are formed in concentrated sulfuric acid and 1+ is stabilized in donor environments such as pyridine [159]. So-called hypervalent iodine reagents have been developed as a versatile oxidation tool in organic synthesis and often iodine derivatives are employed as electron transfer catalysts. Some fundamental thermodynamic data and typical applications of iodine are summarized in Scheme 5. 

The purpose of present review is to summarize the application of different classes of iodine(III) compounds in carbon-carbon bond forming reactions. The first two sections of the review (Sects. 2 and 3) discuss the oxidative transformations induced by [bis(acyloxy)iodo] arenes, while Sects. 4 through 9 summarize the reactions of iodonium salts and ylides. A number of previous reviews and books on the chemistry of polyvalent iodine discuss the C-C bond forming reactions [1 -10]. Most notable is the 1990 review by Moriarty and Vaid devoted to carbon-carbon bond formation via hypervalent iodine oxidation [1]. In particular, this review covers earlier literature on cationic carbocyclizations, allyla-tion of aromatic compounds, coupling of /1-dicarbonyl compounds, and some other reactions of hypervalent iodine reagents. In the present review the emphasis is placed on the post 1990s literature. 

The same reaction takes place with other iodinating reagents such as Me3SiI1099 or Nal combined with a cationic exchange resin1100. 

Pirkuliev et al. examined the reaction of cyclobutene oxide 23 with a hypervalent iodine reagent and isolated a mixture of products consistent with the initial formation of cation 24 which can rearrange to give precursors to the other products (Scheme 12) <2002RJ01066>. 

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],... 

Activation of phenol derivatives 263 with a hypervalent iodine reagent promotes the formation of bicyclic and tricyclic (264) products via a cationic cyclization process (Scheme 3.109). The method allows efficient one-step syntheses of scaffolds present in several natural products and occurs with total stereocontrol [329],... 

Cationic cyclizations, induced by hypervalent iodine reagents, are particularly useful in the synthesis of het-erocycles. Tellitu and Dominguez have developed a series of [bis(trifluoroacetoxy)iodo]benzene-promoted intramolecular amidation reactions, generalized in Scheme 3.134, leading to various five, six and seven-membered heterocycles 335 [388,389]. Experimental evidence supports the ionic mechanism of these reactions, involving A -acylnitrenium intermediates 334 generated in the initial reaction of the amide 333 with the hypervalent iodine reagent [390]. 

On the basis of control experiments a mechanism for the a-amination of aldehydes catalyzed by in situ generated hypoiodite has been proposed (Scheme 4.79) [123]. In the first step, the active cationic iodine species, hypoiodite acid, which is thought to function as a one-electron oxidizing reagent or electrophilic... 

Oxidative isomerization of vinylidenecyclopropanes gives dimethylenecyclopropane aldehydes using tetrapropylammonium perruthenate (TPAP)/4-methylmorpholine A-oxide (NMO) as a catalytic system. A Witkop-Winterfeldt oxidation with ozone has been reported to convert tetrahydropyridoindoles into pyrroloquinolones and cinnolines. Activation of phenol derivatives with a hypervalent iodine reagent has been reported to promote the formation of bicycUc and tricyclic products via a cationic cyclization process (Scheme 64). ... 

Allyl silanes are a species of carbon nucleophile that has performed well in combination with oxidative dearomatization induced by hypervalent iodine reagents [65]. Inter- and intramolecular additions of allyl silanes to putative phenoxonium cations have been reported, and the latter reaction type provided a key transformation in the synthesis of the potent antibiotic (-)-platensimycin (Scheme 15.23) [66]. 

In addition, Trauner and co-workers recently presented the short total synthesis of racemic merochlorin B by means of this biomimetic [3+2] cationic condensation between phenol ring carbons and the internal nucleophilic alkene moiety mediated by hypervalent iodine reagents (Scheme 20) [107]. 

The initial realization involved a catalytic system of 4-tolyl iodide and mCPBA as terminal oxidant, which generates a hypervalent iodine reagent that promotes the formation of a cationic nitrogen. Subsequent nucleophilic attack of the anisole ring engages in C-N bond formation and ultimately in dearomatization. 

Not only do chiral hypervalent iodine reagents have the potential for such conversions, achiral iodanes in combination with chiral auxiliaries can also be used in asymmetric oxidative protocols. The Kita group performed the controlled oxidation of sulfides to sulfoxides using iodoxybenzene (PWO2) in a cationic reversed micellar system. High chemical yields and good stereoselectivities... 

Kita anployed a hypervalent iodine species for the oxidative cyclization of phenol 310 as shown in Equation 12.54-1, Scheme 12.54 [190]. When allowed to react with phenyliodinedll) bisftrifluoroacetate) (FIFA), 310 delivered the desired galantamine skeleton in 40% yield. The cyclization most likely proceeds via the initial reaction of the phenolic oxygen with the hypervalent iodine reagent and subsequent nucleophilic attack of the second electron-rich aromatic ring onto the intermediary formed cation radical intermediate. Kita and coworkers outlined the importance of the fluorinated solvent. When carried out in benzene or dichloromethane, the desired product was not formed. A similar oxidative coupling protocol has been successfully employed by the authors for other Amaryllidaceae alkaloids [191]. 

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