General Papers ARKIVOC 2006 (ix) 26-58
transformations effected by hypervalent iodine reagents have recently been developed by many synthetic chemists. These protocols include catalytic imidations with iodonium imides, hypervalent iodine mediated oxidative coupling of phenols and related compounds, applications of iodine(III) compounds as useful carbene and nitrene precursors and the broad synthetic applications of hypervalent iodine heterocycles derived from benziodoxoles and benziodazoles. Many reviews, some comprehensive, but most dealing with specific aspects of hypervalent organoiodine chemistry, have been published just in the last 5-6 years.1-34 Most notable are the monograph by Varvoglis on the application of hypervalent iodine compounds in organic synthesis1 and the volume of Topics in Current Chemistry on hypervalent iodine chemistry.2
Despite being less developed in comparison with trivalent iodine reagents, the chemistry of iodine(V) compounds (.5-iodanes) has also attracted substantial attention in recent years. This widespread practical interest to .5-iodanes is mainly due to Dess-Martin periodinane (DMP) and, especially, to 2-iodoxybenzoic acid (IBX), both of which are mild and useful oxidizers for alcohols and amines, for conversions of carbonyl compounds to the respective a,ß-unsaturated derivatives and for effecting a number of other unique and useful synthetic transformations. Various IBX analogs, having better solubility profile and/or being recyclable, have emerged recently. Several aspects of .5-iodanes have been highlighted in chemical literature.25-26, 32-34 However, the chemistry of iodine(V) reagents have never been systematically reviewed. The purpose of the present review is to summarize the recent literature data on synthetically useful hypervalent iodine(V) reagents; literature coverage is through the first half of 2005.
2. Iodylbenzene and other noncyclic reagents The noncyclic iodyl (also known as iodoxy) compounds, RIO2, in general have found only very limited practical application due to their low stability. While the aryl derivatives, ArIO2, can form relatively stable compounds, iodylalkanes are extremely unstable and can exist only at very low temperatures. Thus, Clark and coworkers reported the matrix isolation and FTIR spectra of the unstable iodyl derivatives, RIO2, generated by the co-deposition and photolysis of ozone with iodoethane, 2-iodopropane, pentafluoroiodoethane, 1,1,1-trifluoroiodoethane, 1,1,2,2tetrafluoroiodoethane, 1,1,1,2-tetrafluoroiodoethane, or iodine cyanide in an argon matrix at 14
16 K.35-37
Several noncyclic ArIO2 have been reported in the literature. These compounds possess a polymeric structure, which makes them insoluble in the majority of organic solvents, with the exception of DMSO. Structural investigations revealed infinite polymeric chains with strong I···O secondary intermolecular interactions.38-39 Also noncyclic iodylarenes are explosive under excessive heating (> 200 oC) or mechanical impact. Despite their low solubility and explosive character, iodylarenes have found some practical application as oxidizing reagents. Among various ArIO2, iodylbenzene PhIO2 is the most popular reagent.40
ISSN 1424-6376 Page 27 ©ARKAT
