Hypervalent iodine, reaction with

The formation of carbon-heteroatom bonds can be effected by reactions of hypervalent iodine reagents with a wide range of organic substrates and inorganic nucleophiles, and represents one of the most popular applications of organoiodine(III) compounds [1-10]. Except for C-I(III) bond forming reactions used for the synthesis of iodanes and iodonium salts, C-heteroatom bond formation is almost always accompanied by reduction of the hypervalent iodine reagents to iodine(I) compounds. 

Deprez NR, Sanford MS (2007) Reactions of hypervalent iodine reagents with palladium mechanisms and applications in organic synthesis. Inorg Chem 46 1924—1935... 

Rearrangement reactions of three- and four-membered cyclo-alkanols have been demonstrated in a number of notable contexts. In the former case, fragmentation occurs with PhI(OAc)2 in MeOH to yield the unsaturated ester product (eq 50). Other combinations of hypervalent iodine reagents with Br0nsted acids have also proven effective for this transformation. 

Oxidative iodination of aromatic compounds by the combination of a hypervalent iodine reagent with iodine is a synthetically important reaction (Section 3.1.4) [34]. Polymer-supported diacetate 4 is a particularly convenient reagent for oxidative iodination since it can be regenerated and reused many times. Reagent 4 gives the best results for the iodination of electron-rich arenes 13, with predominant formation of the para-substituted products 14 (Scheme 5.8) [12,21]. 

In the area of oxidation catalysis, several interesting reactions have been developed by Waser using a Koser-type hypervalent iodine 71 with an incorporated acetylene ligand (Scheme 16.17). Under catalysis with palladium hexafluoroacetylacetonate, nucleopalladation with both a phenol (e.g., 70) and an acid (e.g., 73) followed by oxidative C(sp )-C(sp) coupling led to new products 72 and 74. Initially, this reaction was developed for cyclization of alcohols or acids, respectively. Both five-and six-membered ring cyclization were successfully employed, and a total of 20 examples with 34-82% yield demonstrate the broad scope of this approach [58]. 

Scheme Ic). This type of mechanism is common in the synthesis of biaryls by the coupling of two electron-rich aryl moieties (see Sect. 4.2.1) [28]. Kita and coworkers discovered that the fluorinated alcohols 2,2,2-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) are excellent solvents in many reactions with At2IX and other hypervalent iodine reactions because of their ability to stabilize carbocation radicals in SET reactiOTis [29, 30]. 

In 2008 Wirth developed the first example of the a-oxysulfonylation of ketones using hypervalent iodine catalysts with mCPBA and TsOH (Scheme 19.17) [117]. The best results were achieved with precatalysts containing esters with two chiral centers however, long reaction times of up to 2-4 days were needed. The reaction is proposed to proceed through a chiral Koser-type chiral hypervalent iodine active catalyst (ArlOTs) formed from the reaction of the precatalyst 39 with mCPBA and TsOH. 

CF,C02)2lPh, H2O, CH3CN, 85-99% yield. In the presence of ethylene glycol the dithiane can be converted to a dioxolane (91% yield) or in the presence of methanol to the dimethyl acetal. The reaction conditions are not compatible with primary amides. Thioesters are not affected. A phenylthio ester is stable to these conditions, but amides are not. The hypervalent iodine derivative l-(t-butylperoxy)-l,2-benziodoxol-3(l/f)-one similarly cleaves thioketals."... 

A similar approach was carried out reacting an enohzable ketone with amides in the presence of the hypervalent iodine(III) reagent (hydroxy(tosyl-oxy)iodobenzene, HDNIB) [48]. The reaction was carried out imder solvent-... 

Following a similar strategy, an ingenious mixed resin bed quench and purification strategy was devised for the Dess-Martin periodinane mediated conversion of alcohols to carbonyls. This hypervalent iodine oxidant was viewed as containing an inherent masked carboxylic acid functionality that was revealed at the end of the reaction (Species (11) Scheme 2.30). Therefore purification was easily achieved by treatment of the reaction mixture with a mixed-resin bed containing both a thiosulfate resin and a polymeric base. The thiosulfate polymer was used to reduce excess hypervalent iodine lodine(V) and (III) oxidation states species to 2-iodoben-zoic acid (11), which was in turn scavenged by the polymeric base [51]. 

A hypervalent iodine(III) reagent, Ph—1=0, together with TMS-azide, promotes direct a-azidation of cyclic sulfides the reaction opens up a route to unstable N,S-acetals. ... 

The step common to both of these reactions is electrophilic attack of a hypervalent iodine species at the a-carbon of the carbonyl compounds to yield an intermediate 3. Nucleophilic attack of methoxide ion or tosy-loxy ion with the concomitant loss of iodobenzene results in a-functionalized carbonyl compounds (Scheme 2). 

While the silver and zinc salts were effective Lewis acids for these cyclizations, Kikugawa and coworkers reported that the alkoxynitrenium ions could be generated directly from hydroxamic esters (4) using hypervalent iodine oxidants such as hydroxy(tosyloxy) iodobenzene (HUB) and phenyliodine(lll)bis(trifluoroacetate) (PIFA) . Presumably, with such reagents the reactions proceed through A-(oxoiodobenzene) intermediates (54), which can themselves be regarded as anomeric hydroxamic esters and sources of alkoxynitrenium ions (55) (Scheme 11). 

Common alcohol oxidation methods employ stoichiometric amounts of toxic and reactive oxidants like Cr03, hypervalent iodine reagents (Dess-Martin) and peracids that pose severe safety and environmental hazards in large-scale industrial reactions. Therefore, a variety of catalytic methods for the oxidation of alcohols to aldehydes, ketones or carboxylic acids have been developed employing hydrogen peroxide or alkyl hydroperoxides as stoichiometric oxygen sources in the presence of catalytic amounts of a metal catalyst. The commonly used catalysts for alcohol oxidation are different MoAV(VI), Mn(II), Cr(VI), Re(Vn), Fe(II) and Ru complexes . A selection of published known alcohol oxidations with different catalysts will be presented here. 

Oxidative C-H amination has been an area of intensive research since the publication of CHEC-II(1996). This methodology has been applied to the synthesis of a variety of 1,2-thiazine 1,1-dioxides. In the simple cases, substrates containing an aromatic C-H can be cyclized in the presence of hypervalent iodine. For instance, the reaction of A-methoxy(2-arylethane)sulfonamide 202 with [hydroxyl(tosyloxy)iodo]benzene rapidly affords benzenesulfon-amide 203 in excellent yield (Equation 30) <20030BC1342> see also <2000JOC926> and <2000JOC8391>. 

Oxathiane 2-oxides aie fonned by the oxidative ring expansion of 2-alkylthio-2-benzylthiolane 1-oxides brought about by [bis(trifluoroacetoxy)iodo]benzene. That the reaction is only successful with the (lR )-diastereoisomeis is attributed to chelation between the nucleophilic S and O atoms and the hypervalent iodine <99EJ0943>. A diazo-mediated thiolane ring expansion is the key step in a synthesis of the acenaphtho-[U-b][l,4]oxathiine system <99JCS(P2)755>. 

The first reaction step involves a method developed by Stork use of the hypervalent-iodine species bis(tnfluoroacetoxy)iodobenzene (26), which effects oxidative removal of the dithiane.11 Methylace-lal 25 a is formed in methanol solution in the presence of traces of acid. Subsequent silylalion of the secondary alcohol is accomplished using TBS-lrifiate with lutidine as base. The third reaction... 

Similar hypervalent iodine radicals (9-1-2) are formed in the reaction of alkyl radicals with alkyliodides (R + RI — R2I ), and as an intramolecular complex they are stable enough that a reaction with 02 is only low (Miranda et al. 2000). Such 9-X-2 radicals have also been postulated as intermediates in the reduction of alkylhalides by a-hydroxyalkyl radicals (Lemmes and von Sonntag 1982). 

As described above, two fundamental modes of the reaction of organo-A3-iodanes involve ligand exchange, occurring at iodine(III) with no change in the oxidation state, and reduction of hypervalent A3-iodane to iodide, called reductive elimination. These processes are discussed in detail. 

Once formed, hypervalent iodine compounds, i.e. A3- and A5-iodanes, can exchange readily their ligands with nucleophiles, sometimes with assistance from electrophiles. When only nucleophiles are involved, reactions follow an associative pathway, in which an iodate(III) or (V) species is formed. The mixed iodane initially formed is sometimes isolable but usually this procedure takes place with both ligands so that eventually the new species has two... 

The use of hypervalent iodine reagents in carbon-carbon bond forming reactions is summarized with particular emphasis on applications in organic synthesis. The most important recent methods involve the radical decarboxylative alkylation of organic substrates with [bis(acyloxy)iodo]arenes, spirocyclization of para- and ortho-substituted phenols, the intramolecular oxidative coupling of phenol ethers, and the reactions of iodonium salts and ylides. A significant recent research activity is centered in the area of the transition metal-mediated coupling reactions of the alkenyl-, aryl-, and alkynyliodonium salts. 

The five-membered hypervalent iodine heterocycles, benziodoxoles, are commonly used as convenient radical precursors [3,33]. The main advantage of benziodoxoles over the non-cyclic hypervalent iodine reagents is the higher thermal stability allowing the preparation of otherwise unstable derivatives with I-Br, I-OOR, I-N3, and I-CN bonds. The stable cyanobenziodoxoles 36-38 are prepared in one step by the reaction of cyanotrimethylsilane with the respective hydroxybenziodoxoles 35 (Scheme 16) [34, 35], or from acetoxybenziodoxole... 

Recent progress on the use of hypervalent iodine reagents for the construction of carbon-het-eroatom (N, O, P, S, Se, Te, X) bonds is reviewed. Reactions of aryl-A3-iodanes with organic substrates are considered first and are loosely organized by functional group, separate sections being devoted to carbon-azide and carbon-fluorine bond formation. Arylations and alkenyla-tions of nucleophilic species with diaryliodonium and alkenyl(aryl)iodonium salts, and a variety of transformations of alkynyl(aryl)iodonium salts with heteroatom nucleophiles are then detailed. Finally, the use of sulfonyliminoiodanes as aziridination and amidation reagents, and reactions of iodonium enolates formally derived from monoketones are summarized. 

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