MECHANISTIC STUDIES OF THE ZIRCONIUM-TRIISOPROPANOLAMINE-CATALYZED ENANTIOSELECTIVE ADDITION OF AZIDE TO CYCLOHEXENE OXIDE

The mechanism of the enantioselective ring-opening of cyclohexene oxid e by Me3SiN3, catalyzed by zirconium complexes of the C-3-symmetric li gand (+)-(S,S,S)-triisopropanolamine, has been investigated. Measureme nts of molecular weights of precatalyst species show that complexes ar e formed with average trimeric aggregation. Kinetics measurements reve al the overall process to be approximately half order in total zirconi um, epoxide, and Me3SiN3 components. The reaction also shows a strong nonlinear relationship between enantiomeric excess of product azido et her vs the incorporation of (R,S,S)-triisopropanolamine ligand in the catalyst mixture. On the basis of these and other results, a preequili brium interconversion of dimeric and tetrameric zirconium-triisopropan olamine species is proposed to occur rapidly with respect to the rate of epoxide ring-opening, with the dimeric form being the active cataly st. The reaction is accelerated by silyl ethers or by small amounts of water or alcohol, whereas larger amounts of protic additives inhibit the reaction. Enantioselectivity is eroded at catalyst concentrations less than 1 mole-percent and at high concentrations of cyclohexene oxi de. Both enantioselectivity and rate are influenced to a small extent by the nature of the silyl azide employed for the first catalytic turn over, suggesting that a silyl fragment becomes irreversibly incorporat ed in the catalyst structure. It is proposed that catalytic activity r equires the cooperative action of two zirconium centers for the bindin g and delivery of azide to epoxide.