Since its discovery by Christoph Hartmann and Victor Meyer in 1893, 2-iodoxybenzoic acid (IBX) has emerged as a rather ubiquitous oxidant for organic synthesis. The past decade has seen the development of a large variety of applications that go far beyond the simple oxidation of alcohols. This Review is concerned with the synthetic potential of IBX, with particular emphasis on uncommon reactivity patterns and novel fields of application.

Au‐spicious! Under silver‐free conditions, simple 3‐silyloxy‐1,5‐enynes were converted into complex cyclopentenes by a gold(I)‐catalyzed sequence that likely proceeds through a carbocyclization followed by a pinacol rearrangement. For the final demetalation step, isopropyl alcohol and N‐iodosuccinimide are effectively utilized, and the resulting products are set up for a wealth of further reactions.

On activation with catalytic amounts of gold(I) complexes, 3-silyloxy 1,6-enynes can react through two alternative pathways. In one, a cascade reaction consisting of carbocyclization and subsequent pinacol rearrangement takes place. In the second pathway, a heterocyclization is followed by a Claisen rearrangement. The reaction outcome differs depending on the substitution pattern of the 3-silyloxy 1,6-enynes and, more importantly, the electronic properties of the gold-bound phosphane ligand.

Over the past years, the value of 2-iodoxybenzoic acid [1] (IBX, 1) as a mild, practical, and synthetically useful oxidizing agent has been continually increased.[2] Even though the simple oxidation of alcohols to the corresponding carbonyl compounds still remains the primary application of IBX, [3,4] a huge variety of unique oxidative transformations has been developed.[5] Thus, IBX has emerged as an oxidant being of rather ubiquitous use in organic synthesis. Of particular significance, IBX induces the dehydrogenation of simple carbonyl compounds as described by Nicolaou and co-workers.[6] Typically, such reactions of ketones and aldehydes with IBX require elevated temperatures (65-85 8C) to produce the corresponding a,b-unsaturated products.[6b] On the other hand, the direct dehydrogenation of 2-oxonitriles proceeds smoothly at ambient temperature in a highly selective manner and in excellent yields, as we found recently in the case of nitrile 2 [Eq. (1)]. [7] We also found in the course of our previous investigations in the field that the behavior of enolizable carbonyl compounds, when subjected to IBX, strongly depends on the type of additional functional groups in the a-position. This becomes apparent in the reactivity of a-alkynyl carbonyl compounds, which in the presence of IBX predominantly yield tertiary alcohols resulting from aoxygenation rather than undergoing dehydrogenation [Eq.(2)]. [8] In fact, oxidative processes involving the transfer of an oxygen atom from IBX are not uncommon.[9] Hence, it seems all the more remarkable that only the selective oxidation of benzylic positions [10] and most notably the hydroxylative dearomatization of phenols [11] have attained major synthetic importance. The related direct a-hydroxylation of carbonyl compounds upon exposure to IBX has received much less attention [e.g., Eq. (2)]. [4b,c, 8] In part, this neglect is due to the fact that there are rigorous structural constraints on those carbonyl compounds that are currently known to react this way. The true value of IBX-mediated a-hydroxylation of carbonyls can only be garnered when the basic factors are determined that favor the oxygenation pathway rather than other oxidative processes such as dehydrogenation. In this context, we expected that ketones A lacking an a-alkynyl substituent might still be able to participate in oxygen-transfer reactions with IBX provided that they are sufficiently activated by a suitable functional group Z (Scheme 1).[12]Herein, we present novel IBX-induced oxygenations of various easily accessible substrate classes by use of an operationally simple and broadly applicable protocol.To probe the feasibility of the projected transformation for Z = CO 2 R, we first examined the conversion of keto ester 6 a into tertiary alcohol 7 a in the presence of IBX (Table 1). Due to the low solubility of IBX in most common solvents, conversion of 6 a was only detectable when using the polar solvent DMSO.[13] The reaction in pure DMSO was invariably accompanied by the formation of significant amoun...