Reaction of (η⁴-butadiene)-zirconocene and -hafnocene with 2-methyl-2-nitrosopropane

(η⁴-Butadiene)-zirconocene and -hafnocene each react with two molar equivalents of 2-methyl-2-nitrosopropane to give nine-membered metallacycles Cp₂MON(R)CH₂CH=CHCH₂N(R)O (8a: R = t-Bu; M = Zr; 8b: R = t-Bu, M = Hf), which have a trans-carbon-carbon double bond in the ring. In solution two different dynamic processes can be observed by variable temperature NMR spectroscopy. Enantiomerization of the chiral ring system (Δ_ent^‡ (390 K) ≈ 19 kcal/mol) is observed at high temperature, whereas at low temperature an equilibration process involving hindered N-CMe₃ rotation becomes slow on the NMR time scale (ΔG_rot^‡ (190 K) ≈ 8 kcal/mol).