Page 12 - Chiral N,N-Dioxides: New Ligands and Organocatalysts for Catalytic Asymmetric Reactions

Chiral N,N -Dioxides Liu et al.
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parameters appear to be important. First, the basicity of the N-
oxide contributes greatly to its acting as a neutral ligand
forming a wide range of metal complexes. Its good stability
and solubility in both aprotic and protic solvents ensure
resistance under various reaction conditions. Importantly,
the use of a C 2 -symmetric scaffold favors the geometrical
factor which reduces the conformational obscurity of the
catalytic intermediate. A comparison of the complexes of
privileged ligands such as pybox and salen with that of N,N -
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dioxide indicates that a metal-centered spirocycle could cre-
ate a chiral environment in which the specific coordination of
the substrate occurs to favor the attack of the reagent on a
preponderant face. Additionally, the effects of variation of the
subunits of N,N -dioxide confirm the importance of flexiblility
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of an ideal ligand. Further studies on the structureactivity
relationship, kinetics experiments, and semiempirical calcula-
FIGURE 11. Possible stereorecognition model.
tions should prove valuable for understanding the features
tions of allyltributyltin, 45 diphenylphosphite, silyl dienoles- that account for the broad applicability of this class of cata-
ter, and ketene silylacetal (Scheme 20). Asymmetric re- lysts and for developing new catalysts.
actions arising from other intriguing metal complexes of
N,N -dioxide are still under way. Summmary
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Mechanism Consideration. The examples of cycloaddi- It has been demonstrated that C 2 -symmetric N,N -dioxide
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tions, nucleophilic additions, conjugate additions, and other amide compounds could participate in a wide range of chiral
reactions illustrated above 46 are important processes that ligandmetal-catalyzed or organocatalyzed asymmetric re-
respond strongly to Lewis acid activation. Although the de- actions under mild reaction conditions. The extensive ver-
mands of each reaction vary in terms of reactivity and stereo- satility of this catalyst library in promoting both traditional
selectivity employing different central metal ion, counterion, reactions and new asymmetric reactions is rather gratifying
chiral backbone, or amide substituent of the ligand, and the after our long-standing endeavors. The practical benefits of
ratio of metal to ligand, the sense of asymmetric induction in these reactions include excellent enantioselectivity and
some respects follows a similar trend. The metal complex of activity, cheap and available materials, mild reaction tem-
the precursor bisamide proved comparatively inefficient. perature, relative insensitivity to moisture, operational sim-
X-ray crystallography of the chiral complexes 10 revealed a plicity, and facile preparative-scale applications. The future
propensity for the coordination of the incoming substrate of the N-oxide family of catalysts in asymmetric transforma-
(Figure 3). For example, in the conjugate additions catalyzed tion and discrimination is promising.
by chiral scandium complexes, the nucleophilic attack of nitro-
methane or malonate was initiated from the β-Si face of the
BIOGRAPHICAL INFORMATION
enones because the amide moiety in the rear tightly shielded
Xiaohua Liu received her B.S. degree from Hubei Normal Uni-
the Re face (Figure 11a). 33,34 In the carbonyl-ene and aza-ene
versity in 2000, and M.S. degree and Ph.D. from Sichuan University
reactions using the chiral nickel complex, the glyoxal deriva- in 2003 and 2006 sequentially. She was appointed as an associate
tives tended to coordinate to the central metal ion in a bidentate professor and joined Prof. Feng's group. Her current research
fashion in which the Re face was blocked by the neighboring interests cover the asymmetric catalysis and organic synthesis.
amide group (Figure 11b). 12,31 The nucleophile preferred to Lili Lin received her B.S. degree and Ph.D. from Sichuan Uni-
versity in 2003 and 2008 sequentially. She currently works at
approach from the Si face to give the target products.
Sichuan University. Her interests include the development of new
The results of these studies highlight the attractive attri-
methodologies in asymmetric catalysis.
butes of a family of chiral Lewis acids in bonding and activat-
Xiaoming Feng was born in 1964. He received his B.S. degree in
ing substrates for asymmetric reactions. The question of how
1985 and M.S. degree in 1988 from Lanzhou University. Then he
the chirality transfer proceeds with N,N -dioxidemetal cata- worked at Southwest Normal University (19881993) and be-
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lysts is at the moment hard to rationalize; however, some came an associate professor in 1991. In 1996, he received his
Vol. 44, No. 8 ’ 2011 ’ 574–587 ’ ACCOUNTS OF CHEMICAL RESEARCH ’ 585