Selective and Nonselective Aza-Michael Additions Catalyzed by a Chiral Zirconium Bis-Diketiminate Complex

Reaction of the chiral bis-diketiminate
complex rac- or (R,R)-C6H10(nacnacXyl)2ZrCl2 with AgOTf yielded the corresponding
bis-triflate complex. The complex
geometry changes from distorted octahedral in the dichloride complex
to a pseudotetrahedral coordination involving π coordination
of the diketiminate ligands. The bis-triflate complex is highly active
for aza-Michael additions with turnover frequencies of 20000/h for
the addition of morpholine to acrylonitrile and 1000/h for the addition
of morpholine to methacrylonitrile. The enantioselectivities of the
latter reaction in various solvents were low, never surpassing 19%
ee. The reaction is first-order in olefin concentration and second
order in amine concentration, which is explained by its participation
as a base in the reaction mechanism. The presence of catalytic amounts
of triethylamine slightly increases the observed rate constants and
reduces the reaction order in amine to first order. Other activated
alkenes such as methacrylonitrile, crotonitrile, methyl acrylate,
and cyclohexenone can be employed, but no reactivity is observed toward
styrene or vinyl ethers. Primary amines, secondary amines, and anilines
can be employed as nucleophiles with activities correlating with their
nucleophilicity, but the catalyst is unstable in the presence of alcohols.