Synthesis of Chiral, Enantiopure Zirconocene Imido Complexes: Highly Selective Kinetic Resolution and Stereoinversion of Allenes, and Evidence for a Stepwise Cycloaddition/Retrocycloaddition Reaction Mechanism

Conversion of a racemic mixture of a chiral allene into a single enantiomer requires selective inversion of the absolute configuration of one species. This can be accomplished as shown in Equation (1): The enantiopure imido complex 1 reacts more rapidly with one enantiomer of chiral 1,3-disubstituted allenes, and more slowly with the other, so a deficiency of 1 gives an efficient kinetic resolution. For equimolar amounts of 1 and a 1,3-disubstituted allene, both allene enantiomers are transformed into the same metallacycle. The enantioenriched allene, which now has the same configuration as the fast reacting enantiomer, is released in high yield and high ee recovery upon treatment of the metallacycle with 1,2-propadiene.