Hypervalent iodine ligand exchange

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 associative pathway of ligand exchange starts from the addition of a nucleophile to the positively charged iodine atom of a X -iodane with the initial formation of a trans hypervalent 12-1-4 square-planar species. This intermediate species isomerizes to the cis 12-1-4 square-planar intermediate and eliminates the ligand L to afford the final product (Scheme 1.4). Such a mechanism has been validated by the isolation and X-ray structural identification of several stable 12-1-4 species. For example, the interaction of ICI3 with chloride anion affords tetrachloroiodate anion, ICl4, which has a distorted square-planar structure as established by X-ray analysis of the trichlorosulfonium salt, CI3S+ ICU [209]. 

Various [hydroxy(tosyloxy)iodo]arenes 64 can be conveniently prepared by a ligand-exchange reaction of (diacetoxyiodo)arenes with p-toluenesulfonic acid monohydrate in acetonitrile (Scheme 2.26). This method has been apphed to the synthesis of derivatives with various substituted aromatic groups [150,166,186,209, 210], [hydroxy(tosyloxy)iodo]heteroarenes [206] and the recyclable hypervalent iodine reagents 65-68 (also see Chapter 5) [154,155,158]. Similarly, numerous polylluoroalkyl derivatives of the types C F2 +iI(OH)OTs [135,176,211] andC F2 +iCH2l(OH)OTs [176,212] can be prepared from the respective bis(trifluoroacetates) and p-toluenesulfonic acid. 

Hydroxybenziodoxole 104 can be readily converted into its acetoxy derivative, l-acetoxy-1,2-benziodoxole-3(l//)-one (88, 2X = O, Y = OAc), by heating 104 with acetic anhydride and the acetoxy derivative can be further converted into the alkoxy derivatives by treatment with an appropriate alcohol [280], The tetrabutylanmionium salt of hydroxybenziodoxole 104 has been prepared by the reaction of hydroxybenziodoxole with tetrabutylammonium fluoride in THF it is a mild oxidant that is useful for the preparation of epoxides from a,p-unsaturated carbonyl compounds [281], l-Hydroxy-l,2-benziodoxole-3(l//)-one and l-acetoxy-l,2-benziodoxole-3(l//)-one have found wide application as starting compounds for the synthesis of various benziodoxole-based hypervalent iodine reagents by ligand exchange on iodine [239]. 

The proposed catalytic cycle for this reaction includes initial formation of [hydroxyl(tosyloxy)iodo]benzene 37 by oxidation of iodobenzene in the presence of toluenesulfonic acid followed by its conversion into the bromoiodane 38 via ligand exchange and then the bromination of arene to form the aryl bromide (Scheme 4.19). The reduced by-product, iodobenzene, is again transformed into the hypervalent iodine reagent by oxidation with mCPBA [46]. 

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