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N-tert-Butylimidozirconocene (Cp2Zr=Nt-Bu) and its analogue (ebthi)Zr=NAr (ebthi = ethylenebis(tetrahydroindenyl), Ar = 2,6-dimethylphenyl) undergo overall cycloaddition reactions at ambient temperature with epoxides that lack accessible beta-hydrogens. The process results in addition of the Zr=N bond to an epoxide C-O bond, leading to azazirconacyclopentanes. The regio- and stereochemistry of the products implicate a stepwise mechanism, with the intermediacy of zwitterions having substantial carbocation character at the substituted carbon of the ring-opened epoxide-derived fragment. The azametallacycles undergo facile cleavage to beta-amino alcohols upon addition of mild acid.
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Blum et al. (2003) studied this question.
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