Recyclable Hypervalent Iodine Reagents

The direct chlorination of iodoarenes 9 and 11 has been used for the preparation of 4,4 -bis(dichloroiodo)biphenyl (10) and 3-(dichloroiodo)benzoic acid (12) (Scheme 2.4), which are convenient recyclable hypervalent iodine reagents (Section 5.3) [66]. 

Various [hydroxy(tosyloxy)iodo]arenes 64 can be conveniently prepared by a ligand-exchange reaction of (diacetoxyiodo)arenes with p-toluenesulfonic acid monohydrate in acetonitrile (Scheme 2.26). This method has been apphed to the synthesis of derivatives with various substituted aromatic groups [150,166,186,209, 210], [hydroxy(tosyloxy)iodo]heteroarenes [206] and the recyclable hypervalent iodine reagents 65-68 (also see Chapter 5) [154,155,158]. Similarly, numerous polylluoroalkyl derivatives of the types C F2 +iI(OH)OTs [135,176,211] andC F2 +iCH2l(OH)OTs [176,212] can be prepared from the respective bis(trifluoroacetates) and p-toluenesulfonic acid. 

Iodine in combination with [bis(acyloxy)iodo]arenes is a classical reagent combination for the oxidative iodination of aromatic and heteroaromatic compounds [99], A typical iodination procedure involves the treatment of electron-rich arenes with the PhI(OAc)2-iodine system in a mixture of acetic acid and acetic anhydride in the presence of catalytic amounts of concentrated sulfuric acid at room temperature for 15 min [100,101]. A solvent-free, solid state oxidative halogenation of arenes using PhI(OAc)2 as the oxidant has been reported [102]. Alkanes can be directly iodinated by the reaction with the PhI(OAc)2-iodine system in the presence of f-butanol under photochemical or thermal conditions [103]. Several other iodine(in) oxidants, including recyclable hypervalent iodine reagents (Chapter 5), have been used as reagents for oxidative iodination of arenes [104-107]. For example, a mixture of iodine and [bis(trifluoroacetoxy)iodo]benzene in acetonitrile or methanol iodinates the aromatic ring of methoxy substituted alkyl aryl ketones to afford the products of electrophilic mono-iodination in 68-86% yield [107]. 

Recyclable Hypervalent Iodine Reagents 385 4, AcOEt, 20-60 °C, dark, 4-16 h... 

Various recyclable hypervalent iodine reagents based on the frameworks of 1,3,5,7-tetraphenyladamantane (65-67) [74], tetraphenylmethane (69-71) [75], biphenyl (73 and 74) [76] and terphenyl (76) [76] have been developed (Figure 5.3). Similarly to previously discussed reagent 56 (Scheme 5.23), aryl iodides 64,68,72... 

The reactivity pattern of recyclable hypervalent iodine reagents 65-67,69-71,73,74 and 76 is similar to the common iodobenzene-based reagents [34], For example, the recyclable (diacetoxyiodo)arenes 65,69,73 and 76 can be used instead of (diacetoxyiodo)benzene in the KBr or TEMPO-catalyzed oxidations of alcohols [74-78], while [hydroxy(tosyloxy)iodo]arenes 67, 71 and 74 are excellent reagents for tosyloxylation of ketones (Scheme 5.28) [76]. 

Recyclable Hypervalent Iodine Reagents 409 AcOOH (5 equiv), 147 or 148 (0.2 equiv), (Cp3)2CHOH, rt... 

Yadav JS, Reddy BVS, Basak AK, Narsaiah AV (2004) Recyclable 2nd generation ionic liquids as green solvents for the oxidation of alcohols with hypervalent iodine reagents. Tetrahedron 60 2131-2135... 

A similar oxidative protocol has been used for the oxidation of (fluoroalkyl)alkanols, Rf(CH2) CH20H, to the respective aldehydes [146], in the one-pot selective oxidation/olefination of primary alcohols using the PhI(OAc)2-TEMPO system and stabilized phosphorus ylides [147] and in the chemo-enzymatic oxidation-hydrocyanation of 7,8-unsaturated alcohols [148]. Other [bis(acyloxy)iodo]arenes can be used instead of PhI(OAc)2 in the TEMPO-catalyzed oxidations, in particular the recyclable monomeric and the polymer-supported hypervalent iodine reagents (Chapter 5). Further modifications of this method include the use of polymer-supported TEMPO [151], fluorous-tagged TEMPO [152,153], ion-supported TEMPO [154] and TEMPO immobilized on silica [148],... 

A simple method for the preparation of a-azido ketones and esters 500 in good yields by direct azidation of carbonyl derivatives 499 at the a-carbon using 4,4 -bis-(dichloroiodo)biphenyl and sodium azide has been reported (Scheme 3.198) [575]. The hypervalent iodine reagent, 4,4 -bis-(dichloroiodo)biphenyl, can be easily recycled from the reaction mixture. 

Iodosylbenzoic acid (85) is also a convenient recyclable hypervalent iodine oxidant for the synthesis of a-iodo ketones by oxidative iodination of ketones [88], Various ketones and p-dicarbonyl compounds can be iodinated by this reagent system under mild conditions to afford the respective a-iodo substituted carbonyl compounds in excellent yields. The final products of iodination are conveniently separated from by-products by simple treatment with anionic exchange resin Amberlite IRA 900 HCOs" and are isolated with good purity after evaporation of the solvent. The reduced form of the hypervalent iodine oxidant, 3-iodobenzoic acid (59), can be recovered in 91-95% yield from the Amberlite resin by treatment with aqueous hydrochloric acid followed by extraction with ethyl acetate [88]. 

Two recyclable nonpolymeric hypervalent iodine reagents, 3-[bis(trifluoroacetoxy)iodo]benzoic acid (90) and 3-[hydroxy(tosyloxy)iodo]benzoic acid (91) can be conveniently prepared in high yield by treatment of 3-iodosylbenzoic acid (85) with trifluoroacetic acid or p-toluenesulfonic acid, respectively (Scheme 5.31) [89,90]. 

Figure 5.4 shows several examples of ionic-liquid-supported (also called ion-supported) hypervalent iodlne(III) reagents [93], Ion-supported (diacetoxyiodo)arenes 98-100 are prepared by the peracetic oxidation of appropriate ion-supported aryl iodides [93-95], The ion-supported tosylates 101 and 102 are made by treatment of the appropriate acetates with toluenesulfonic acid in acetonitrile [95,96] and the ion-supported iodosylbenzenes 103 and 104 are prepared by treatment of appropriate diacetates with sodium hydroxide in water [93], Ion-supported derivatives 98-104 are thermally stable solids or viscous liquids that can be used as efficient recyclable hypervalent iodine(III) reagents. Reactions of these reagents with organic substrates produce the corresponding ion-supported aryl iodides as by-products, which can be easily recovered from the reaction mixture either by extraction into an ionic liquid phase or by simple filtration. 

The first nonpolymeric recyclable hypervalent iodine(V) reagents based on the readily available derivatives of 2-iodopyridine were reported in 2011 [109], 2-Iodylpyridines 134 were prepared by oxidation of the respective 2-iodopyridines 133 with 3,3-dimethyldioxirane (Scheme 5.40). Structures of 2-iodylpyridines 134 (R = H, OPr and OBu) were established by single crystal X-ray crystallography [109]. 

Another recyclable hypervalent iodine(V) reagent, potassium 4-iodylbenzenesulfonate (138), was prepared by the oxidation of 4-iodobenzensulfonic acid (137) with Oxone (2BCHS0s KHS04 K2S04) in water (Scheme 5.42) [110]. A single-crystal X-ray structure analysis of product 138 revealed the presence of polymeric chains in the solid state due to a combination of numerous intra- and intermolecular interactions. 

Reactions of Hypervalent Iodine Reagents in Recyclable Organic Solvents... 

Handy and Okello prepared ionic liquid-supported stable and recyclable hypervalent iodine(III) reagents 19 and 20 (Figure 6) and used in the preparation of a-fxmctionalized ketones [8]. The by-product supported monovalent iodine could be easily separated from the product and recycled without compromising the product yield. In 2006, Zhang et al. reported an environmentally benign ionic liquid-supported iodine(III) reagent 21 (Figure 6) which was foxmd to be quite effective in the selective oxidation of primary alcohols [9]. 

C. Zhu, A. Yoshimura, Y. Wei, V.N. Nemykin, V.V. Zhdankin, Facile preparation and reactivity of bifunctional ionic liquid-supported hypervalent iodine reagent a convenient recyclable reagent for catalytic oxidation. Tetrahedron Lett. 53 (2012) 1438-1444. 

Oxidation by the Dess-Martin Reagent. Another reagent that has become important for laboratory synthesis is known as the Dess-Martin reagent,28 which is a hypervalent iodine(V) compound.29 The reagent is used in inert solvents such as chloroform or acetonitrile and gives rapid oxidation of primary and secondary alcohols. The by-product, o-iodosobenzoic acid, can be extracted with base and recycled. 

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