Diaryl-A3-iodanes

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)]. 

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. 

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. 

Studies on the solution structure of A3-iodanes are relatively limited. In polar solvents, cryoscopic and conductance measurements have shown extensive dissociation of diaryl-A3-iodanes (Ar2IL L = BF4, Cl, Br, OAc) into the solvated iodo-nium ions (Ar2I+S S = polar solvents such as H20, MeOH, and DMSO) [3,210]. Even in dichloromethane, bis(4-methylphenyl)-A3-iodane (Ar2IBF4 Ar=p-MeC6H4) dissociates into the solvated iodonium ions with dissociation constant Kdissoc=4.7xlO-6M[211]. 

We propose a mechanism of carbon-halogen bond-forming ligand coupling of diaryl(halo)-A3-iodanes, which involves a highly polarized apical-equatorial transition state. Ligand coupling mechanism of p-chlorophenyl(p-methyl-phenyl)bromo-A3-iodane 77, which exists as an equilibrium mixture of 77a and... 

Recently, Widdowson and coworkers reported the ab initio MO calculations for the ligand coupling of diaryl(fluoro)-A3-iodanes [125]. 

---