Ring-enlargement and ring-opening reactions of benzoxazole, thiazoles, and benzisoxazole by zirconocene-alkyne complexes

Zirconocene alkyne complexes Cp(2)Zr(L)(Me(3)SiC(2)R) (L = Py, THF; R = SiMe(3), tBu) react with heterocyclic compounds like benzoxazole and related thiazoles to yield ring-expanded adducts Cp(2)Zr-C(SiMe(3))=C(R)-CH=N-o-C6H4-X (1-3) and Cp(2)Zr-C(SiMe(3))=C(SiMe(3))-CH=N-C(R')=C(R')-X (R' = Me, H) (4, 5) by formal C-X (X = O, S) bond cleavage and coupling with the coordinated alkyne. In the case of benzisoxazole, the alkyne is not coupled but eliminated, and with ring-enlargement of the benzisoxazole a N-bridged dimer [Cp(2)Zr-N=CH-o-C6H4-O](2) (6) is formed. The obtained complexes 1, 3, and 6 were characterized by NMR spectra and crystal structure analysis.