Iodanes

The epoxidation of electron-deficient alkenes, particularly a,P-unsaturated carbonyl compounds, continues to generate much activity in the literature, and this has been the subject of a recent concise review <00CC1215>. Additional current contributions in this area include a novel epoxidation of enones via direct oxygen atom transfer from hypervalent oxido-).3-iodanes (38), a process which proceeds in fair to good yields and with complete retention of... 

With a-A3-iodanyl ketone precursors. Exposure of (2-acetoxyvinyl)phenyl-A3-iodanes 369 to 2-imidazolidinethione and triethylamine in methanol produced the bridgehead heterocycle 370 of type 5,6-dihydro-imidazo[2,l-3]thiazoles (Equation 166) <2003JOC7887>. 

The reactions involved are unimolecular, and the cyclohexenyl derivative 3 undergoes solely the spontaneous heterolysis while both spontaneous heterolysis and ligand coupling occur with the iodane 14. The relative contributions of the two reactions of 14 depend on the solvent polarity. The results summarized in Table I show that the iodonium ion and the counteranion are in equilibrium with the hypervalent adduct, X3-iodane. The equilibrium constants depend on the identity of the anion and the solvent employed, and the iodane is less reactive than the free iodonium ion as the k /k2 raios demonstrate. Spontaneous heterolysis of 3 occurs more than 100 times as fast as th t of the adduct 14 as observed in methanol the leaving ability of the iodonid group is lowered by association by more than 100 times. 

In order to strengthen evidence in favour of the proposition that concerted inplane 5n2 displacement reactions can occur at vinylic carbon the kinetics of reactions of some /3-alkyl-substituted vinyliodonium salts (17) with chloride ion have been studied. Substitution and elimination reactions with formation of (21) and (22), respectively, compete following initial formation of a chloro-A, -iodane reaction intermediate (18). Both (17) and (18) undergo bimolecular substitution by chloride ion while (18) also undergoes a unimolecular (intramolecular) jS-elimination of iodoben-zene and HCl. The [21]/[22] ratios for reactions of (18a-b) increase with halide ion concentration, and there is no evidence for formation of the -isomer of (Z)-alkene (21) iodonium ion (17d) forms only the products of elimination, (22d) and (23). 

SCHEME 100. Oxidation of sulfides with hypervalent (tert-butylperoxy)iodanes 178... 

The electrochemical transformation of 2-methoxy- and 2-methylphenols to give orthoquinoid cyclohexadienone synthons with the aid of iodanes has been reported [173]. Iodanes (or hyperva-lent iodine [174, 175]) are a large family of reagents [176, 177] and often beneficial as clean and recyclable reagents [178]. The two most often utilized reagents are 2-Iodoxybenzoic (IBX) acid and l,l,l-triacetoxy-l,l-dihydro-l,2-benziodoxol-3(lH)-one (Dess-Martin pe-riodinane, DMP) (see Scheme 7). These... 

Further to its ability to perform allylic and benzylic oxidations,149 /-butylpcroxy-iodane (6) effects radical oxidation of 4-alkylphenols to give 2,5-cyclohexadien-l-ones under mild conditions in good yields.150 o,o-Coupling dimers as side products and inhibition of the reaction by added galvinoxyl radical scavenger support a radical oxidation mechanism. 

Negative reaction constants p1 for the oxidation of sulfides by [10-1-3]—(r-butylperoxy)iodanes are consistent with a mechanism involving rate-limiting formation of a sulfonium species by nucleophilic attack of sulfide on the iodine(III) atom followed by attack of water to give sulfoxide.151 However, in dichloromethane, inhibition by galvinoxyl implicates a free radical mechanism perhaps by homolytic cleavage of the weak iodine(III)-peroxy bond. 

Diaryliodonium salts (diaryl-A3-iodanes) are widely used as arylating agents. There are a number of methods available for their synthesis typically involving two or three steps.378,379 A recent one-pot approach, however, offers a simple and high-yielding access to unsymmetrical diaryliodonium triflates using meto-chloroperbenzoic acid (mCPBA) as the oxidant380 [Eq. (4.111)]. Moreover, symmetrical diaryliodonium salts can directly be prepared from iodine and arenes without the use of expensive aryl iodides [Eq. (4.112)]. 

Keywords. Iodane, Hypervalent, Reductive elimination, Ligand coupling, Ligand exchange... 

The term iodane refers to hydrogen iodide (HI), a colorless non-flammable gas. According to IUPAC rules, compounds with nonstandard bonding number are shown by the lambda notation thus, H3I is called A3-iodane and H5I A5-iodane. The most common ArIL2 (L heteroatom ligands) with decet structure is named aryl-A3-iodane and ArIL4 with dodecet structure aryl-A5-iodane. 

Aryl-A5-iodanes ArIL4 have a square pyramid structure with an aryl group in an apical position and four heteroatom ligands in basal positions. Two orthogonal hypervalent 3c -4e bonds accommodate all of the heteroatom ligands and the apical aryl group has a character of a normal covalent bond using hybridized 5sp orbital [3]. 

Structure 1 1 10-1-3 chloro(diphenyl)-X3-iodane 2 8-I-2 diphenyliodonium chloride... 

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. 

Heteroatom ligands of A3-iodanes are readily displaced with external nucleophiles. Detailed mechanism for ligand exchange on iodine(III) is not known, but two mechanistic pathways, associative and dissociative, are considered for the process [9]. There are many evidences supporting the associative mecha-... 

Bis(trifluoromethyl) A3-iodane 6a undergoes degenerate ligand exchange with added alkoxide PhC(CF3)2OK more rapidly (second-order rate constant = 49 M 1s 1 at 56 °C) than that of dimethyl A3-iodane 6b (second-order rate constant =61 M 1s 1 at 93 °C), in which an associative mechanism involving the formation of [12-1-4] species was proposed [16]. The CF3 substituents, which lower the electron density on iodine(III) relative to the CH3 substituents, make the iodine of 6a more susceptible to attack by alkoxide ion. Dynamic 19F NMR of A3-iodane 7 showed an intramolecular ligand exchange via intermediacy of bicyclic tetracoordinated iodate with a AG of ca. 12 kcal/mol at - 80 °C [17]. 

Ligand exchange provides a route for the synthesis of chiral A3-iodanes. [(+)-10-Camphorsulfonyl]oxy-A3-iodane 8 was prepared from the reaction of 4 with (+)-10-camphorsulfonic acid in aqueous acetonitrile [22]. Concentration of a solution of [methoxy(tosyloxy)iodo]benzene and (+)-menthol in dichloro-methane under vacuum results in facile ligand exchange on iodine to give the chiral A3-iodane 9 [23]. 

Oxidation of (R)-(+)-2-iodo-a-methylbenzhydrol with f-BuOCl gives chiral chloro-A3-iodane, which on ligand exchange with NaN3 affords (+)-azido-A3-iodane [Eq. (6)] [26]. 

Formation of the A3-iodane 13 with two I-S bonds was proposed when (diace-toxyiodo)benzene 4 was treated with electron deficient 2,3,5,6-tetrafluorothio-phenol in pyridine [27]. The A3-iodane 13 reacts with terminal alkynes to give l,2-bis(arylthio)alkenes. 

Exposure of alkenes to a combination of 4 and trimethylsilyl isothiocyanate leads to the formation of 1,2-dithiocyanates [Eq. (8)] [28]. The reaction involves the formation of bis(thiocyanato)-A3-iodane by a ligand exchange. The decomposition of this iodane leads to the formation of thiocyanogen, which in turn undergoes the anti electrophilic addition to olefins. 

Reaction of diarylhalo-A3-iodanes with sodium AT,AT-dialkyldithiocarbamates results in the formation of yellow or orange dialkylcarbamoyl(diaryl)-A3-iodanes, which are stable in the dark but decompose to aryl iodides and aryl dialkyldithiocarbamates in daylight via light-promoted homolytic pathway [29]. For ligand exchange of A3-iodanes with sulfides, see Section 3.2.5.4. 

Exchange reaction between heteroatom ligands of A3-iodanes probably occurs [Eq. (9)] 13C NMR spectra of a mixture of 4 and (dichloroiodo)benzene 15 in CDC13 showed a rapid ligand exchange and formation of a new A3-iodane, presumably 16, was detected as a major component in a nearly statistical ratio 1 1 2 of 4 15 16 [30]. 

Ligand exchange on iodine(III) with carbon nucleophiles provides a useful method for synthesis of A3-iodanes with two carbon ligands. Koser and coworkers found that exposure of aryltrimethylsilanes to [hydroxyltosyloxy)-iodo]benzene 17 in refluxing acetonitrile allows the directed synthesis of diaryl-A3-iodanes [31]. The reaction involves silicon-directed ipso carbon attack on the positively charged iodine and, therefore, is regiospecific. 

Alkynyl(trimethyl)silanes, germanes, and stannanes produce alkynyl-(phenyl)-A3-iodanes via ligand exchange on iodine under similar conditions [36]. Stang and co-workers developed a useful procedure for the preparation of diverse /3-functionalized alkynyl-A3-iodanes, which involves a ligand exchange of cyano-A3-iodane 19 with alkynylstannanes [37]. 

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