Page 10 - Chiral N,N-Dioxides: New Ligands and Organocatalysts for Catalytic Asymmetric Reactions

Chiral N,N -Dioxides Liu et al.
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1 mol % catalyst loading (Figure 7). The autocatalytic process
SCHEME 17. Asymmetric Intramolecular Oxa-Michael Reaction
was excluded, and a strong positive nonlinear effect implied
the presence of polymeric lanthanum species. Further
studies on the active intermediates are still ongoing. Michael
addition of nitroalkanes to nitroolefins to form the optically

active1,3-dinitroalkaneswaseffectivelymediatedbyLa(OTf) 3
L1c with high outcomes using mild procedure (Figure 8). 37
The bidentate chelating nature of the dicarbonyl com-
pounds with nickel complex was utilized for the intramole- SCHEME 18. Asymmetric FriedelCrafts Reaction
cular oxa-Michael addition of tert-butylester activated
R,β-unsaturated ketones to synthesize pharmaceutically
active chromanone derivatives (Scheme 17). 38 It was found
that the counterion of the Ni(II) complex greatly affected the
activity, and the substituent on the amide subunits as well as
the amino acid backbone of the ligand had a notable impact
on the enantioselectivity of the reaction. The association
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of N,N -dioxide L3dNi(Tfacac) 2 yielded chromanones in
8499% ee.

Haloamination Reaction. The asymmetric electrophilic
haloamination reaction has the potential to generate key
synthetic intermediates, vicinal halo-amine compounds.
However, the asymmetric methods face the difficulties of
control over the regioselectivity and enantioselectivity. The
FIGURE 8. Asymmetric synthesis of 1,3-dinitroalkanes. first asymmetric bromoamination reaction of chalcones

FIGURE 9. Asymmetric haloamination reactions.

Vol. 44, No. 8 ’ 2011 ’ 574–587 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 583