Hypervalent iodine species

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). 

The use of TMOF as a solvent provides strong acetalizing conditions (323 330). This allows the generation of enol ether 331, which on electrophilic attack of hypervalent iodine species [PhI(OMe)2] (83IC1563) gives intermediate 332. Nucleophilic attack of the solvent at the C(4)-position of 332, followed by migration of ring A, results in the formation of 326. The minor product 327 is resulted by a Sn2 attack of methanol at the C(3)-position of 333 (Scheme 85). 

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... 

A formal equivalent of Woodward reaction has been developed, which is based on the addition of hypervalent iodine species (Scheme 3).28... 

The Dess-Martin periodinane 30 (1,1,1 -triacetoxy-1,1 -dihydro-1,2-benziodoxol-3(7H)-one) was originally described in 1983 and has become a widespread reagent for the oxidation of complex, sensitive and multifunctional alcohols.15 The periodinane is a hypervalent iodine species and a number of related compounds also serve as oxidizing agents. 

Mixtures of 4-(difluoroiodo)toluene and anhydrous phosphoric acid form a nonisolated hypervalent iodine species, probably PhI(0H)(0P03H2), which reacts with silyl enol ethers producing tris-ketol phosphates. Interestingly, mono- or bis-ketol phosphates could not be obtained using the appropriate stoichiometry. 

Alkynyliodonium ions, 1 and 2, are hypervalent iodine species in which one or two alkynyl ligands are bound to a positively charged iodine(III) atom. They are sensitive to nucleophiles, especially at the /1-carbon atom(s) of the alkynyl ligand(s), and for that reason, the isolation of stable alkynyliodonium salts generally requires the incorporation of nucleofugic anions. A list of known alkynyliodonium compounds (i.e. as of 4/1/94), containing 134 iodonium salts derived from 103 iodonium ions, and references (5-45) to their preparation and characterization are presented in Table 1. Among these compounds, alkynyl(phenyl)iodonium sulfonates and tetrafluoroborates are the most common, while alkynyl(alkyl)iodonium salts of any kind are unknown. 

Vinyliodonium ions, 35 and 36, are hypervalent iodine species in which one or two alkenyl ligands are bound to a positively charged iodine(III) atom. Although they are reactive with nucleophilic reagents, they are less labile than alkynyliodonium ions, and stable halide salts of vinyliodonium ions can be prepared. The first vinyliodonium compounds [i.e. (a, / -dichlorovinyl)iodonium salts] were synthesized by the treatment of silver acetylide-silver chloride complexes with (dichloroiodo)arenes or l-(dichloroiodo)-2-chloroethene in the presence of water (equation 152). The early work was summarized by Willgerodt in 1914115. This is, of course, a limited and rather impractical synthetic method, and some time elapsed before the chemistry of vinyliodonium salts was developed. Contemporary synthetic approaches to vinyliodonium compounds include the treatment of (1) vinylsilanes and vinylstannanes with 23-iodanes, (2) terminal alkynes with x3-iodanes, (3) alkynyliodonium salts with nucleophilic reagents and (4) alkynyliodonium salts with dienes. 

THF, PhI(OAc)2, 10-68% yield. These results show that hypervalent iodine species should probably not be used in THF as a solvent. 

Reactions of alkenes with hypervalent iodine compounds lead mostly to vicinally functionalised alkanes. This is the case with PhI(OAc)2, PhIO, PhI(OH)OTs, PhI(OTf)0(TfO)IPh and other related reagents.230,231,233,239-247 poj. example, treatment of alkenes with PhI(OH)OTs, (HTIB), affords vie bis(tosyloxy)alkanes with a syn stereospecificity.239,241 n generally admitted that this reaction proceeds by the electrophilic attack of the hypervalent iodine species on the ethylenic double bond to afford a carbonium ion intermediate (140). This intermediate undergoes two consecutive Sn2 substitution reactions to eventually give the final products. (Scheme 5.17)... 

Amano Y, Nishiyama S (2006) Oxidative synthesis of azacyclic derivatives through the nitrenium ion application of a hypervalent iodine species electrochemically generated from iodobenzene. Tetrahedron Lett 47 6505-6507... 

Alkylcarboxamides can be converted into the respective amines by Hofinann rearrangement using hypervalent iodine species generated in situ from iodobenzene and a terminal oxidant, such as Oxone (2KHSO5 KHSO4 K2SO4) or m-chloroperoxybenzoic acid (mCPBA). In particular, a convenient experimental procedure for the preparation of alkylcarbamates using Oxone as the oxidant in the presence of iodobenzene in methanol (Scheme 3.166) has been developed [359]. 

Catalytic reactions of this type usually involve the reoxidation of iodoarene to aryliodine(III) species in situ using oxidants such as peroxycarboxylic acids, hydrogen peroxide, sodium perborate, or Oxone at room temperature. The choice of oxidant is critically important the oxidant must not react with the substrate, as the substrate should only be oxidized by the hypervalent iodine species. The stoichiometric oxidant has to be carefully selected to achieve the re-oxidation of the iodine compound under homogeneous reaction... 

The mechanism of this reaction involves initial oxidation of aryl iodide 42 by peracetic acid to the active hypervalent iodine species 45, followed by ligand substitution at iodine(III) by 40 or 43 to generate species 46, which undergo oxidative fragmentation to form nitrenium ions 47. Reaction of an arene with the electron-deficient nitrenium ion 47 affords the final products of amination and the reduced intermediate 48, which is reoxidized to the species 45 (Scheme 4.22) [47]. 

Several examples of cyclizations through intramolecular C-N bond formation catalyzed by hypervalent iodine species have been reported. Antonchick and coworkers developed an efficient organocatalytic method for the preparation of carbazoles through catalytic oxidative C-N bond formation [48]. The best yields of products were obtained in hexafluoro-2-propanol using 2,2 -diiodo-4,4, 6,6 -tetramethylbiphenyl (42) as the catalyst and peracetic acid as the oxidant, as illustrated by a representative example shown in Scheme 4.23. 

Several catalytic systems involving two or more sequential catalytic cycles and utilizing hypervalent iodine species as catalysts have been developed. An efficient, catalytic aerobic oxidation of primary and secondary alcohols to the corresponding aldehydes and ketones by using catalytic amounts of iodylbenzene, bromine and sodium nitrite in water (Scheme 4.55) has been reported by Liu and coworkers [86]. 

Hypervalent iodine species were demonstrated to have a pronounced catalytic effect on the metalloporphyrin-mediated oxygenations of aromatic hydrocarbons [93]. In particular, the oxidation of anthracene (114) to anthraquinone (115) with Oxone readily occurs at room temperature in aqueous acetonitrile in the presence of 5-20 mol% of iodobenzene and 5 mol% of a water-soluble iron(llI)-porphyrin complex (116) (Scheme 4.57) [93]. 2-ferf-Butylanthracene and phenanthrene also can be oxygenated under similar conditions in the presence of 50 mol% of iodobenzene. The oxidation of styrene in the presence of 20 mol% of iodobenzene leads to a mixture of products of epoxidation and cleavage of the double bond. Partially hydrogenated aromatic hydrocarbons (e.g., 9,10-dihydroanthracene, 1,2,3,4-tetrahydronaphthalene... 

Isoxazoles display a range of biological activities, such as anti-inflammatory, antimicrobial, anticancer, and antinociceptive, that justify a constant effort in the development of new synthetic strategies. New syntheses of isoxazoles 1 and isQxazolines 2 via 1,3-dipolar cycloaddition (1,3-DC) of alkynes and alkenes with nitrile oxides were described (130L4010). The 1,3-dipoles were generated by oxidation of aldoximes catalyzed with hypervalent iodine species formed in situ from catalytic iodoarene and oxone as a terminal oxidant, in the presence of hexafluoroisopropanol (HFIP) in aqueous methanol solution. 

A closely related chiral template is L-P-malamidic acid 5.182), which was converted to 5.183 using a hypervalent iodine species. Acid hydrolysis gave S-isoserine (3-amino-2-hydroxypropanoic acid, 5.35). 

SCHEME 12.7 Alternative hypervalent iodine species 7 as benzyne precursor. 

Oxidation of 1,4-Dihydropyridines to Pyridines with Stoichiometric Oxidants Many of the known stoichiometric oxidants were examined in the oxidation of 1,4-dihydropyridines to the corresponding pyridines. Thus, halogen-based oxidants, such as potassium bromate [251], sodium chlorite [252], calcium hypochlorite [253], iodic acid [254], its anhydride [255], iodine chloride [256], or hypervalent iodine species [257], give good results (Scheme 13.144). 

This reaction cycle is characterized by the use of hypervalent iodine species XII as the stoichiometric oxidant and a catalytic quantity, around 0.2 mol, of the oxidizing azo reagent XIII. The benefit of using XII is that the by-products, iodobenzene and acetic acid, are relatively simple to remove, while at the same time, the amount of hydrazine by-product XI formed is dramatically reduced. Under optimized conditions, the authors obtained aryl-carboxylic acid esters with chiral alcohols in 65% yield and with 100% e.e. 

In the presence of hypervalent iodine species such as iodosoben-zene, TMS-Br can be used to introduce bromine atoms onto electron-rich ring systems. As shown in eqs40 and41,bromina-tion may be quickly and efficiently affected in combination with iodobenzene diacetate. 

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