Formation of Heterocycles by Substitution Reactions

An important area of progress in zirconacydopentadiene chemistry has been the heteroatom transfer developed by Fagan and Nugent, leading to five-membered heterocydes (Eq. 2.19) [26], 

Heterocycle formation from zirconacyclopentadienes by heteroatom transfer. 

E = PhP, Me2Sn, ClzGe, PhB, PhAs, PhSb, PhBi, Br2Si, S, Se 

Heteroatom transfer in metallacyclopentadienes was first developed in the context of cobalt chemistry in the mid-1970s [27]. Cobaltacyclopentadienes were converted into various five-membered heterocyclic compounds such as pyrrole and thiophene, and into six-mem-bered heterocyclic compounds such as pyridine and pyridone derivatives. In the case of zirconacyclopentadienes, the heteroatom compound must bear at least two halide substituents, since the Cp2Zr moiety is re-converted to the stable Cp2ZrX2. Indeed, this is the driving force behind the heteroatom transfer of zirconacyclopentadienes. 

The reaction of zirconacyclopentadienes with Group 14 compounds such as Me2SiCl2, Me2GeCl2, and Me2SnCl2 should be discussed here. GeCl4 readily reacts with both mono-cyclic and bicyclic zirconacyclopentadienes (Eq. 2.20) [26], 

Heterocycle formation from zircona-cyclopentadienes by heteroatom transfer. 

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