Halogenolysis

Halogenolysis of zirconacyclopentadienes affords 1,4-dihalodienes (Eq. 2.14) [20]. Solvent effects on halogenolysis are remarkable. Indeed, whereas the iodination of zirconacyclopentadienes in THF with 2 equivalents of I2 affords mainly the monoiodinated diene, the diiododiene can be the major product in CH2C12. For example, 2,3,4,5-tetraethylzirco-nacyclopentadiene reacts with 2 equivalents of I2 in THF at room temperature to give the monoiodinated 3-iodo-4,5-diethylocta-3,5-diene 23 in 70% yield along with only an 18% yield of 3,6-diiodo-4,5-diethylocta-3,5-diene 24 (Eq. 2.14). 

When the same iodination reaction is carried out in CH2C12 at room temperature, the diiodination product 24 is obtained in 73 % yield as the major product, accompanied by an 18% yield of the monoiodination product 23 (Eq. 2.15) [20]. 

Transmetalation of zirconacyclopentadienes to copper will be discussed later. However, it is noteworthy here that when the iodination is carried out in THF in the presence of 2 equivalents of CuCl, the product of diiodination (24) is obtained in 95 % yield without any trace of the monoiodinated adduct 23 (Eq. 2.16) [24], 

Selective formation of the diiodination product from a zircona-cyclopentadiene using CuCl. 

This transmetalation of zirconacyclopentadienes to copper is very effective. Table 2.2 shows several examples of diiodination reactions of zirconacyclopentadienes performed in the presence of CuCl. 

The addition of bromine or iodine to ( )-alkenylboronic acids or esters afforded intermediates, which were converted by base into (Z)-l-halo-l-alkenes.543 On the other hand, ( )-iodo-l-alkenes of high stereochemical purity were prepared by treatment of ( )-l-alkenylboronic acids or esters with sodium hydroxide followed by 1 molar equiv. of iodine.544 Iodine monochloride was used in place of iodine for such a base-induced iodination with retention or inversion of the stereochemistry (Equation (110)).545 Chloramine-T/Nal (Equation (111)),546,547 CuX2 (X = Cl, Br) (Equation (112)),548 and iV-halosuccinimides (Equation (113))549 halogenated the C-B bond with retention of stereochemistry. 

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