functionality. In the presence of diaryliodonium salts, enolates undergo α-arylation. Bulky diaryliodoniums react more slowly, and enolate homocoupling (see equation () below) begins to compete as the aromatic ring is substituted. α-Oxytosylation facilitates the elaboration of carbonyl compounds into a variety of α-functionalized products. The α-tosyloxycarbonyl compounds that result are more stable than α-halocarbonyl compounds and are not lachrymators. Silyl enol ethers undergo many of the same reactions as carbonyl compounds in the presence of iodine(III) reagents. α-Alkoxylation is possible in the presence of an external alcohol nucleophile, although yields are somewhat variable. When no external or internal nucleophile is present, oxidative homocoupling occurs, yielding 1,4-dicarbonyl compounds. Intramolecularly tethered nucleophiles may displace iodobenzene to afford lactones or other heterocycles. If acidic hydrogens are present in the cyclic product, overoxidation can occur under the reaction conditions. In some cases, rearrangements complicate hypervalent iodine oxidations of carbonyl compounds. Aryl migration may occur under acidic conditions, yielding α-aryl esters from enol ethers. Favorskii rearrangements have also been observed, and these have been particularly useful for steroid synthesis. Synthetic applications Oxidative functionalization of silyl enol ethers in low concentration (to avoid homocoupling) without an external nucleophile leads to dehydrogenation. This can be a useful way to generate α,β-unsaturated carbonyl compounds in the absence of functional handles. For instance, dehydrogenation is employed in steroid synthesis to form unsaturated ketones. Comparison with other methods Few compounds that oxidize carbonyl compounds rival the safety, selectivity, and versatility of hypervalent iodine reagents. Other methods for the α-hydroxylation of carbonyl compounds may employ toxic organometallic compounds (such as lead tetraacetate or osmium tetroxide). One alternative to hypervalent iodine oxidation that does not employ heavy metals is the attack of a metal enolate on dioxygen, followed by reduction of the resulting peroxide (equation ()). The most popular method for the α-hydroxylation of carbonyl compounds is the Rubottom oxidation, which employs silyl enol ethers as substrates and peracids as oxidants. Oxidative rearrangements are generally easier to accomplish using hypervalent iodine reagents than other oxidizing agents. The Willgerodt-Kindler reaction of alkyl aryl ketones, for instance, requires forcing conditions and often gives low yields of amide products. Experimental conditions and procedure Example Procedure Hydroxy(mesyloxy)iodobenzene (3.16 g, 10 mmol) was added to a solution of the methyl trimethylsilyl phenylketene acetal derived from methyl phenylacetate (3.33 g, 15 mmol) in dry dichloromethane (50 mL). The mixture was stirred at room temperature for 2 hours and