Chemistry of Polyvalent Iodine
Introduction Starting from the early 1990's, the chemistry of polyvalent iodine organic compounds has experienced an explosive development. This surging interest in iodine compounds is mainly due to the very useful oxidizing properties of polyvalent organic iodine reagents, combined with their benign environmental character and commercial availability. Iodine(III) and iodine (V) derivatives are now routinely used in organic synthesis as reagents for various selective oxidative transformations
... complex organic molecules. Several areas of hypervalent organoiodine chemistry have recently attracted especially active interest and research activity. These areas, in particular, include the synthetic applications of 2-iodoxybenzoic acid (IBX) and similar oxidizing reagents based on the iodine(V) derivatives, the development and synthetic use of polymer-supported and recyclable polyvalent iodine reagents, the catalytic applications of organoiodine compounds, and structural studies of complexes and supramolecular assemblies of polyvalent iodine compounds. The chemistry of polyvalent iodine has previously been covered in four books 1-4 and several comprehensive review papers. 5-17 Numerous reviews on specific classes of polyvalent iodine compounds and their synthetic applications have recently been published. Most notable are the specialized reviews on [hydroxy(tosyloxy)iodo]benzene, 41 the chemistry and synthetic applications of iodonium salts, 29, 36, 38, 42, 43, 46, 47, 54, 55 the chemistry of iodonium ylides, 56-58 the chemistry of iminoiodanes, 28 hypervalent iodine fluorides, 27 electrophilic perfluoroalkylations, 44 perfluoroorgano hypervalent iodine compounds, 61 the chemistry of benziodoxoles, 24,45 polymer-supported hypervalent iodine reagents, 30 hypervalent iodinemediated ring contraction reactions, 21 application of hypervalent iodine in the synthesis of heterocycles, 25,40 application of hypervalent iodine in the oxidation of phenolic compounds, 32,34,50-53,60 oxidation of carbonyl compounds with organohypervalent iodine reagents, 37 application of hypervalent iodine in (hetero)biaryl coupling reactions, 31 phosphorolytic reactivity of o-iodosylcarboxylates, 33 coordination of hypervalent iodine, 19 transition metal catalyzed reactions of hypervalent iodine compounds, 18 radical reactions of hypervalent iodine, 35,39 stereoselective reactions of hypervalent iodine electrophiles, 48 catalytic applications of organoiodine compounds, 20,49 and synthetic applications of pentavalent iodine reagents. 22, 23, 26, 59 Iodine(III) compounds (structures 1 and 2), or λ 3 -iodanes according to the IUPAC nomenclature, are commonly classified by the type of ligands attached to the iodine atom. 2,3, 5,6 This section of the review is organized according to the traditional classification and will cover the preparation, structure, and reactivity of iodosylarenes, aryliodine(III) halides, carboxylates, sulfonates, cyclic λ 3 -iodanes, iodonium salts, ylides, and imides with emphasis on their synthetic application. Iodosylarenes Preparation- The most important representative of iodosylarenes, iodosylbenzene, is best prepared by alkaline hydrolysis of (diacetoxy)iodobenzene. 113 The same procedure can be used for the preparation of a variety of ortho-, meta-, and para-substituted iodosylbenzenes from the respective (diacetoxy)iodoarenes (Scheme 1).    108, 114 This procedure, for example, was recently used for the preparation of 4-methoxyiodosylbenzene, 108 4nitroiodosylbenzene 108 and pseudocyclic iodosylarenes bearing tert-butylsulfonyl 91 or diphenylphosphoryl 92 groups in the ortho-position. An alternative general procedure for the preparation of iodosylarenes 7 employs the alkaline hydrolysis of (dichloroiodo)arenes under conditions similar to the hydrolysis of (diacetoxyiodo)arenes. 115 A modified procedure employs aqueous tetrahydrofuran as the solvent for the hydrolysis of (dichloroiodo)arenes 6 (Scheme 2). 116 Iodosylbenzene is a yellowish amorphous powder, which cannot be recrystallized due to its polymeric nature; it dissolves in methanol with depolymerization affording PhI(OMe) 2 . 117 Heating or extended storage at room temperature results in disproportionation of iodosylbenzene to PhI and a colorless, explosive iodylbenzene, PhIO 2 . Drying iodosylbenzene at elevated temperatures should be avoided; a violent explosion of 3.0 g PhIO upon drying at 110 °C in vacuum has recently been reported. 118 Zhdankin and Stang 3.1.2. Structural Studies-Based on spectroscopic studies, it was suggested that in the solid state iodosylbenzene exists as a zigzag polymeric, asymmetrically bridged structure, in which monomeric units of PhIO are linked by intermolecular I•••O secondary bonds. 6 The I-O bond distances of 2.04 and 2.37 Å and the C-I-O bond angle near 90° have been deduced from EXAFS analysis of polymeric iodosylbenzene. 119 The polymeric structure of iodosylbenzene was also theoretically analyzed by density functional theory computations at the B3LYP level and, in particular, the importance of the presence of a terminal hydration water in its zigzag polymeric structure HO-(PhIO) n -H was established. 120 The zigzag asymmetrically bridged structure of (PhIO) n has recently been confirmed by single crystal Xray diffraction studies of the oligomeric sulfate 8 and perchlorate 9 derivatives. 87,121 In particular, iodine atoms in the (PhIO) 3 fragment of the oligomeric sulfate 8 exhibit a typical of trivalent iodine T-shaped intramolecular geometry with O-I-O and O-I-C bond angles close to 180° (166.54-177.99) and 90° (79.18-92.43), respectively. The I-O bond distances in the (PhIO) 3 fragment of sulfate 8 vary in a broad range of 1.95 to 2.42 Å. 121 The single crystal Xray crystal study of the oligomeric perchlorate 9 revealed a complex structure consisting of pentaiodanyl dicationic units joined by secondary I•••O bonds into an infinite linear structure of 12-atom hexagonal rings. 87 The oligomer 8 was prepared by the treatment of PhI(OAc) 2 with aqueous NaHSO 4 , while product 9 precipitated from dilute aqueous solutions of PhI(OH) OTs and Mg(ClO 4 ) 2 . The formation of both products can be explained by self-assembly of the hydroxy(phenyl)iodonium ions (PhI + OH in hydrated form) and [oxo(aquo)iodo]benzene PhI + (OH 2 )O − in aqueous solution under reaction conditions.