Iodoxybenzene. A remarkably close ozone equivalent

Abstract

Iodoxybenzene is isoelectronic to ozone and several of its reactions proceed via pathways remarkably similar to that of ozone. An attractive feature of iodoxybenzene mediated reactions is the direct formation of products and the transformation of the reagent to iodobenzene. The formation of benzil and iodobenzene in good yields from diphenylacetylene provides support for the anticipated intermediate arising from π⁴s+π²s addition. Iodoxybenzene transforms phenanthrene to phenanthrenequinone as a result of the initially formed 4 + 2 adduct undergoing C-H rupture, in preference to the C-C rupture. This change is parallel to the transformation of phenanthrene 9-carboxylic acid to phenanthrenequinone with ozone. The expected C-C rupture of the initially formed adduct does take place with acenaphthylene leading to naphthalic anhydride. The reaction of pyrene with iodoxybenzene leads to nearly equal amounts of the 4-5, 1-6 and 3-6 quinones involving attack on the bond of the lowest bond localisation energy and the atom of the lowest atom localization energy. This behaviour is similar to the action of ozone on benzopyrene. iodoxybenzene transforms anthracene, just like ozone, to anthraquinone, in good yields. The involvement of transannular addition of iodoxybenzene, as is partly the case with ozone, is proved unlikely by reaction with 9, 10 dimethylanthracene which gave 10-methyl 9-anthraldehyde and 10-carboxy 9-anthraldehyde, involving the insertion of elements of iodoxybenzene to the aryl C-H bond. This tendency of iodoxybenzene has been further demonstrated by the following changes: diphenylmethane→benzophenone; fluorene→fluorenone and tetralin→α-tetralone. Several of the transformations brought about with iodoxybenzene provide attractive synthetic routes, particularly to phenanthrenequinone, pyrenequinones and tetralone. Finally, just like the 2O₃→3O₂ change, iodoxybenzene, and even more so, 4-iodoxybiphenyl, thermally fragment to iodobenzene and 4-iodobiphenyl.