A-iodanes

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). 

Primary vinyl cations are highly labile species. Detailed study on solvolysis of chiral A -iodane (R)-69 in methanol was carried out by Okuyama and Fujita [108]. They concluded that the solvolysis does not involve the generation of the primary vinyl cation 71, but instead the chiral secondary vinyl cation 70, produced via the reductive elimination of iodobenzene with concomitant trans fiC-C migration. Lodder and coworkers, however, found that photochemical solvolysis of 2-phenyl-l-propenyl(phenyl)-A3-iodane produces a primary vinyl cation via an SN1 pathway [109]. 

Both the lack of regioselectivity observed for the intramolecular insertion of alkylidene carbenes derived from (E)- and (Z)-A -iodanes 108 and the high degree of stereo convergence of the olefin geometry of vinylsulfonium salts indicate the intermediacy of the free alkylidene carbene [Eq. (99)] [53,172]. 

The reactions of the vinyl iodonium ions 15a-e with chloride ion (Scheme 12) have been theoretically examined at the MP2/DZ + d levelThe transition structures calculated for the reaction of divinyliodonium ion 15e with chloride are shown in Fig. 3. The CP first coordinates to the positive iodine to form a A -iodane intermediate next, the TSs for the in-plane and out-of-plane attack (SnVtt)... 

An efficient, mild oxidation of alcohols to the corresponding aldehydes or ketones with potassium per-oxodisulfate and TEMPO in the presence of catalytic amounts of iodobenzene has been reported [89], The oxidation proceeds in aqueous acetonitrile at 40 °C to afford carbonyl compounds in high yields. Likewise, the oxidation of alcohols with wt-chloroperoxybenzoic acid and Al-hydroxyphthalimide (NHPI) in the presence of catalytic iodobenzene proceeds in aqueous acetonitrile at room temperature to afford the respective carbonyl compounds in excellent yields [90], The mechanism of these oxidations is similar to the TEMPO-promoted reaction shown in Scheme 4.56 and involves the oxidation of iodobenzene by mCPBA or K2S2O8 in situ to form a -iodane species, which act as reoxidant of NHPI or TEMPOH. 

Given the variety of iodine valence states in organoiodine compounds, the A nomenclature (see Section 1.26) will prove useful for our discussion. Dichloroiodobenzene is thus a A -iodane, the superscript being essentially the valence of the atom in question. As shown below, a number of other A -iodanes such as iodobenzene diacetate are also readily accessible from iodobenzene the pentavalent iodine compound iodoxybenzene is shown below in a different color to distinguish it from the other trivalent compounds. 

Aryl-A -iodanes have gained popularity as oxidants on account of their stability, ease of handling, and selectivity. One of their chief applications is functionalization of enolizable carbonyl compounds ... 

Ligand exchange of monoaryl-A -iodanes with an arylsilane or arylstannane leads to analogous diaryl reagents, which have unique uses. For example, diaryl-A -iodanes bring about a-arylation of enolates, as shown below ... 

Under the same conditions, arylsulfonamides are converted to phenyliodonium imides or iminoaryl-A -iodanes. 

Two of the most important A -iodane reagents are 2-iodoxybenzoic acid (IBX) and Dess-Martin periodinane (DMP). The best current syntheses of the two reagents are as follows, where KHSO5 is potassium hydrogen peroxysulfate ... 

Abstraction of an a-proton from the alkoxide then results in a ketone and an aryl-A -iodane, as shown below. 

As expected from the higher electronegativity of bromine relative to iodine, aryl-A -bromanes are both more electrophilic and more susceptible to reductive elimination than analogous aryl-A -iodanes. Thus, the aryl-A -bromanes have allowed both faster and more convenient versions of known reactions, as well as a number of new reactions. 

The analogous reaction with tosylimido-A -iodanes, by contrast, typically requires a transition-metal catalyst, as well as an excess of the olefin. 

Phthalimides 23 are transformed into a mixture of carbamates 25a and arylamines 25b by reaction with iodobenzene in the presence of /mCPBA and TsOH H20 (Scheme 9) [9]. The suggested mechanism implies the formation of imide-A, -iodane intermediates 24 which undergo alcoholysis followed by Hofmann-type rearrangement to yield 25. 

Reactions of chiral A, -iodanes, amino acid-derived benziodazole oxides 58 [76], (5)-proline based reagents 59 [77], and iodylarenes 60 bearing ester motives [78] with non-symmetric sulfides to give asymmetric sulfoxide formation further recognized the importance of such transformations. 

The enantioselective lactonization of 5-oxo-5-phenylpentanoic acids 64 was studied using chiral A, -iodanes 66 to yield 5-benzoyldihydrofuran-2(3/7)-ones 65, albeit with low ee values (Scheme 26) [83]. Scope of this asymmetric lactmiizatirHi process was further studied by employing other chiral aryl iodides such as 67 [84]. 

Diaryl-A, -iodanes are well-known iodine(lll) compounds and can work as excellent electrophilic arylating agents because of the remarkable electron deficiency at iodine. Because of their relatively low cost and toxicity, they are promising substitutes for transition metal-based reagents [61]. 

A T-shaped pattern of covalent bonds aroxmd the iodine similar to that in A -iodane derivatives described above is also observed when the nucleophilic... 

Some A -iodane derivatives are very useful oxidizing reagents in organic synthesis, e.g., 2-iodoxy-benzoic acid (IBX) and the Dess-Martin reagent (DMP) effectively convert primary and secondary alcohols to aldehydes and ketones, respectively [104]. Also directional preferences of short contacts observed in the sohd between electron rich sites and iodine of some A -iodane derivatives are nicely consistent with an asymmetric electron density distribution on iodine resulting in the presence of most positive regions on the covalent bonds extensions, and the preferential entrance of nucleophiles in these regions [105]. For instance, the o-hole model predicts that in the spirocyclic compound shown in Fig. 17, the electrostatic... 

Diaryl-A, -iodanes (often referred to as diaryliodonium salts) such as [PhjIJOTf or derivatives can couple the phenyl group with another aromatic ring to form a... 

There is a lack of clarity regarding the nomenclature associated with these species. Conventionally, they are known as iodonium salts due to their dissociation in polar solvents, although their solid-state structures resemble typical A -iodanes. Depending on the nature of the diorganoiodine(III) species, the third ligand and the solvent, aggregation phenomena and/or dissociation of the covalent A -dior-ganoiodane structure into iodonium ions in polar solvents (Scheme 2) have been observed. ... 

The Hofmann-type rearrangement of aromatic and aliphatic imides using KBr as the catalyst gives aromatic and aliphatic amino acid derivatives. Using a hypervalent iodine(III) reagent generated in situ, amino acid derivatives are formed via alcoholysis followed by a Hofmann rearrangement promoted by the formation of an imide-A -iodane intermediate (Scheme 170). ... 

A catalytic version of hypervalent aryl-A -iodane-induced Hofmann rearrangement of primary carboxamides has been reported, which is thought to involve in situ generation of a tetracoordinated bis(aqua)(hydroxy)phenyl-A -iodane complex. ... 

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