Iodine readily forms compounds with most other elements in the periodic table. It occurs most commonly in monovalent form with an oxidation state of 1. It forms relatively weak bonds with first-row elements, including carbon, the typical CI bond dissociation energy being only about 55 kcal per mole. Organoiodine compounds have been used since the mid-1800s, notably in Wurtz coupling reactions, the Williamson ether synthesis, and Hofmann's alkylation of amines. Currently, the most important and common use of organoiodine compounds involves various metal-mediated cross-coupling reactions where they serve as premier electrophilic partners in Heck, Negishi, Suzuki, Sonogashira, Stille, and similar cross-coupling protocols. These metal-catalyzed cross-coupling reactions are extensively employed in preparative organic chemistry, the synthesis of complex natural products, and the manufacture of drugs, as well as in supramolecular and materials chemistry. Because iodine is the largest, least electronegative, and most polarizable of the common halogens, it is also capable of forming stable polycoordinate high-valent (with a value of up to 7, IF7) compounds. The most common polyvalent organic iodine compounds are I(III) and I(V) species. The first stable polyvalent organic iodine compound, the trivalent PhICl2, was prepared by the German chemist C. H. C. Willgerodt in 1886. In the 1980s, we and others developed alkynyliodonium salts, RCCI+PhX (X=OTs, OTf, BF4, etcetera), the newest member of the family of polyvalent organoiodine compounds, which may serve as electrophilic acetylene equivalents. This has engendered a renaissance in polyvalent organoiodine chemistry. Arguably, the most useful and widely employed contemporary polyvalent organoiodine compound is the I(V) Dess-Martin periodinane that has emerged as the reagent of choice for the oxidation of primary and secondary alcohols to aldehydes and ketones, respectively. Because of its ready availability; its convenience of use; its unique, selective oxidizing property; and, most importantly, its functional group tolerance, the Dess-Martin periodinane is widely employed in the synthesis of complex natural products of biological and medicinal interest. Among the more common, everyday uses of iodine are the following: in halogen lamps, as a salt additive (to prevent goiter), and in ink pigments. Tincture of iodine is used as a topical antiseptic to kill bacteria. Silver iodide is used in the preparation of some photographic films.
Peter J. Stang is distinguished professor of chemistry and dean of the College of Science at the University of Utah. He is a member of the National Academy of Sciences and a fellow of the American Academy of Arts & Sciences. Since 2002, he has been the editor of the Journal of the American Chemical Society.
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