The search for novel oxidizing agents, especially those that not only transform a broad range of diverse functional groups, but also do so with a high degree of selectivity, remains the focus of intense exploration. In recent years, the use of hypervalent iodine compounds as oxidizing agents has gained attention in synthetic organic chemistry. The α-oxidation of carbonyl compounds by hypervalent iodine oxidants has been most widely investigated and has broad synthetic utility. Although this area has recently been reviewed in various forms and scope, a comprehensive coverage that focuses on synthetic applications is absent and the present chapter addresses this need.
In this chapter the oxidation of carbonyl compounds with hypervalent iodine reagents is discussed. These reagents effect the electrophilic α-oxidation of large numbers of ketones, β-diketones, α,β-unsaturated ketones, and their derivatives such as silyl enol ethers. The discussion is restricted mainly to the reactions of organoiodine(III) reagents; inclusion of other hypervalent iodine reagents such as organoiodine(V) reagents, periodic acid, or purely inorganic iodate reagents is beyond the scope of this chapter. Furthermore, reactions brought about with other oxidizing agents in combination with a catalytic amount of a hypervalent iodine reagent are not considered here. The chapter also excludes discussion of a considerable amount of work based upon the reaction of iodonium ylides, a distinct feature of organoiodine(III) reagents. The common abbreviations and names of the reagents used in this chapter are listed.
A general feature of the reactions of carbonyl compounds described in this chapter is electrophilic attack of the organoiodine(III) reagent at the α-carbon atom of a carbonyl group to yield a presumed tricoordinate iodine(III) intermediate.
The Intermediate reacts further to yield products via various pathways depending upon the reaction conditions. The major processes that occur are summarized. These include oxidation of ketones under basic conditions leading to α-hydroxydimethylacetals, oxidation of α,β-unsaturated ketones leading to α-hydroxy-β-methoxydimethylacetals, oxidation of ketones under neutral conditions, and related oxidation of silyl enol ethers under various conditions leading to α-functionalization, intramolecular participation reactions leading to oxygen-containing heterocyclic compounds including cyclic α-keto ethers and lactones, and rearrangement processes such as 1,2-aryl shifts in alkyl aryl ketones leading to 2-arylalkanoates. All of these reactions are driven by the reduction of iodine(III) to iodine(I) (iodobenzene).