REACTIVITY OF ZIRCONOCENE AZAMETALLACYCLOBUTENES - INSERTION OF ALDEHYDES, CARBON-MONOXIDE, AND FORMATION OF ALPHA,BETA-UNSATURATED IMINES - FORMATION AND TRAPPING OF [CP(2)ZR=O] IN A [4-RETROCYCLOADDITION(2])

Azametallacyclobutene Cp(2)ZrN-t-BuCEt=CEt (1) underwent an insertion reaction with CO to form the acyl complex 2 (Cp(2)Zr(N-t-BuCEtCEtCO), 67% yield). The addition of acetone to azametallacyclobutene 3 (Cp(2)Z r(NArCMeCPh), Ar=2,6-dimethylphenyl) yielded the N-bonded enamine and O-bonded enolate complex of zirconocene 4 (Cp(2)Zr(NArCMeCPhH)(OCMeCH( 2)), 76% yield). The addition of aldehydes RCOH to metallacycle 3 resu lted in the insertion of the aldehyde into the Zr-C bond to form compl exes Cp(2)Zr(NArCMeCPhCRHO) (8a) and Cp(2)Zr(NArCMeCPhC(i-Pr)HO (9) in 85% (R=Ph) and 73% yields, respectively. Similarly, treatment of meta llacycle 10 (Cp(2)Zr(NArCEtCEt)) with benzaldehyde yielded the inserti on product 11 (Cp(2)Zr(NArCEtCEtCPhHO)) in 56% isolated yield. The str ucture of complex 11 was confirmed by an X-ray crystallographic study. Heating the insertion products 8a and 9 led to elimination of the alp ha,beta-unsaturated imines 13 and 14a (ArN=CMeCPh=CRH) in 53% and 72% yields, respectively, and the formation of oxozirconocene oligomer (Cp (2)ZrO)(n). The oxozirconocene monomer was trapped by dimethylzirconoc ene, preventing the formation of oligomer and resulting in the isolati on of product 15. A kinetic study of this retrocycloaddition produced the following activation parameters: Delta H double dagger=26.5 kcal/m ol, Delta S double dagger=3.48 eu. A Hammett sigma/rho study showed th at electron-donating groups alpha to the metallacycle oxygen accelerat e the retrocycloaddition (rho=-0.8).