Insertion Reactions of Carbon Monoxide and Isonitriles

Zirconacyclopentadiene shows a different reactivity towards CO as compared with zirco-nacyclopentane and zirconacyclopentene. Zirconacyclopentane and zirconacydopentene readily react with CO at low temperature to give cyclopentanone and cyclopentenone, respectively. The different reactivity of zirconacyclopentadienes can be explained by comparing the reactivity of the Zr—Csp2 bond with that of the Zr—C, bond. Insertion of CO into the Zr—Csp3 bond proceeds readily at low temperature and therefore zirconacyclopentane and zirconacyclopentene, which contain Zr—Csp3 bonds, react directly with CO as shown in Eq. 2.65 [45]. Zirconacyclopentadienes, on the other hand, do not. 

The first successful reaction of zirconacyclopentadienes with CO is shown in Eq. 2.29. However, this reaction gives cyclopentenones rather than cyclopentadienones. In order to form cyclopentadienones from zirconacyclopentadienes by CO insertion, transmetalation to Ni is very effective. Thus, in the presence of NiCl2(PPh3)2, zirconacydopentadienes react with CO to afford cyclopentadienones, as shown in Eq. 2.66 [46], 

Usually, an isonitrile can be inserted without transmetalation. As shown in Eq. 2.67, Majoral et al. have isolated and characterized by X-ray analysis the intermediate involved in isonitrile insertion into zirconaindene complexes [47]. 

In particular cases, such as with sterically hindered substituents in the a-positions of the zirconacyclopentadiene, the addition of CuCl is effective (Eq. 2.68) [46]. 

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Insertion reactions of carbon monoxide and isonitriles. The carbenic character of carbon monoxide and isonitriles, which favors initial donation of an electron pair to the 16-electron Zr atom, is certainly responsible for the success of these processes. 

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