Page 11 - Chiral N,N-Dioxides: New Ligands and Organocatalysts for Catalytic Asymmetric Reactions

Chiral N,N -Dioxides Liu et al.
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was achieved with chiral N,N -dioxideSc(OTf) 3 through a
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bromonium-based mechanism (Figure 9). 39 Chiral R-bro-
mo-β-amino ketones with up to 99% ee and 99:1 diaster-
eomeric ratio (dr) were obtained using 0.05 mol %
L2dSc(OTf) 3 in the presence of 4 Å molecular sieves. Most
of the chalcone derivatives were tolerant, but chalcones
with 2-methoxy and 4-methoxy substituents at the β-phen-
yl group were exceptions with poor enantioselectivity,
FIGURE 10. Miscellaneous nucleophilic addition reactions.
probably due to the facile racemization of the electron-rich
bromonium intermediate through the bimolecular olefin-to- Controlled reversal of enantioselectivity using Sm(OTf) 3 /
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olefin transfer path. Later, we harnessed TsNCl 2 /TsNH 2 as a AgAsF 6 and N,N -dioxides from the same chiral source were
new effective system of reagents to perform the catalytic observed in the reaction of indole to β,γ-unsaturated
asymmetric chloroamination reaction of R,β-unsaturated γ- R-ketoesters (Scheme 19). 42 This phenomenon was ex-
ketoesters and chalcones. Excellent results were achieved with plained by the different coordination capability between
0.050.5 mol % of the L2bSc(OTf) 3 complex (Figure 9). 40 Ag(I) and Sm(III).
FriedelCrafts Alkylation Reaction. Asymmetric nucleo- Miscellaneous Reactions. Systematic studies were also
philic addition of indoles to prochiral electrophiles provides carried out with the complexes of Cu(I), Cu(II), and In(III) as
a useful strategy for accessing chiral indole frameworks that shown in Figure 10. The addition of nitromethane to alde-
represent a privileged class of biologically interesting com- hydes, R-ketoesters, aldimines, and ketimines (for the first
pounds (Scheme 18). R-Indoly(hydroxyl)acetates were ob- time) was achieved using N,N -dioxideCuOTf catalysts. 43
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tained using the L3fSc(OTf) 3 catalyst with good outcomes The Mannich-type reaction of a glycine Schiff base with
and to the exclusion of the bisindole byproduct. The reactions aldimines was catalyzed by L1dCu(OAc) 2 to afford a wide
between indoles and alkylidene malonates 11 or chalcones range of anti-R,β-diamino acid esters. 44 The chiral indium
were performed smoothly to yield the indole derivatives in complex was useful for asymmetric allylation of ketone

moderate to good results. Pyrrole was also tolerable in L2d derivatives. 20 Catalyst L3f-Sc(OTf) 3 was found to be effective
Sc(OTf) 3 catalyzed asymmetric transformation of chalcones. 41 for use with 2-aminophenol-derived aldimines in the addi-

SCHEME 19. Reversal of the Enantioselectivity in FriedelCrafts Reaction

SCHEME 20. Nucleophilic Additions of N-Arylimine

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