Page 3 - Chiral N,N-Dioxides: New Ligands and Organocatalysts for Catalytic Asymmetric Reactions

Chiral N,N -Dioxides Liu et al.
0

FIGURE 2. Representative chiral N,N -dioxides.
0

FIGURE 3. Crystal structures and stereoviews of N,N -dioxide and the metal complexes.
0
1 0
H NMR and X-ray analysis of N,N -dioxides. However, free (Figure 3bd) reveals that the spatial arrangement of
0
L2f exhibits a conformation in which the two N-oxide amide N,N -dioxide varies, especially that of the amide moieties.
0
moieties are located away from each other (Figure 3a), 10 The N,N -dioxide behaves as a neutral tetradentate ligand
casting doubt as to whether this soft backbone would that both oxygens of N-oxide as well as both carbonyl
promote good stereoinduction for asymmetric reactions. 1 oxygens coordinate to the metal center, and the two amide
As a result of the chelate effect, the structure of the groups on the opposite sides are disposed in a bibrachial
N,N -dioxides becomes compacted in the presence of a manner. The structure also features a metal-centered spiro-
0
Lewis acid, which serves to attract the four oxygen atoms cycle that the two (CdO)metal(ON) six-membered rings
together. The chirality-at-nitrogen is maintained in the lie in perpendicular planes. A cycle generated from the alkyl
formed complex, even though the original intramolecular linkage breaks the C 2 -symmetry of the complex and ef-
hydrogen bonds are disrupted. X-ray analysis of the L1fSc- fectively shields one quadrant. Two unoccupied positions
(OTf) 3 , 10,11 L2fNi(BF 4 ) 2 , 10,12 andL3fMg(OTf) 2 10 complexes in the quadrilateral bipyramid offer opportunities for the

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