Ligand exchanges, benzene

Quinquecovalent phosphoranes readily undergo ligand exchange on treatment with organolithium compounds. With alkyl-lithiums the resulting alkylphosphoranes give alkylidenephosphoranes, e.g. (2), by loss of benzene. 

The yellow solid may be stored at — 20°C it slowly darkens over a period of weeks but is still useful in syntheses. The complex is soluble in a range of organic solvents and pyridazine/ligand exchange reactions have been satisfactorily accomplished in acetone, benzene, dichloromethane, and chloroform, but solutions of the complex are unstable in the absence of added ligand. 

Attempt to prepare Jt-complexes of triafulvenes and related methylene cyclopro-parenes285,427 428 directly by ligand exchange reaction with transition metal complexes resulted in metal insertion into the sigma bond, forming metallacyclic complexes. Thus reaction of the electron-poor triafulvene l,2-diphenyl-3-dicyanomethylenecyclopropene with (ethylene)bis(triphenylphosphine)platinum in refluxing benzene gave two crystalline products whose platinacyclobutene structure was confirmed by X-ray structure analysis (equation 364)429. 

Rapid ligand exchange of [methoxy(tosyloxy)iodo]benzene with methanol-d4 on the NMR (360 MHz) time scale was observed in CD2C12 at room temperature [15]. The formation of tetracoordinated species was assumed to be involved in this reaction. 

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]. 

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. 

These important compounds are prepared either directly from iodoarenes and peroxyacids or through ligand exchange reactions. The first method is suitable mainly for diacetoxy-derivatives which are readily formed upon oxidation of iodoarenes with H202/Ac0H/Ac20. A detailed procedure for the parent (diace-toxyiodo)benzene can be found in Organic Syntheses (Scheme 4) [18]. 

The second method involves ligand exchange, usually between (diacetoxyio-do)benzene and a carboxylic acid. The reaction is best performed in mildly heated chlorobenzene under reduced pressure. This simple approach is suitable for the preparation of several [bis(acyloxy)iodo]arenes including dibenzoyloxy derivatives and also 1-adamantylcarboxy derivatives which are formed in high yields (Scheme 5) [24,25]. 

---