Communication
ChemComm
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pyrazolone enol and pyrenyl were found to stabilize Ts-major. In
Ts-major-a, weaker aromatic C–HÁ Á ÁO hydrogen bonding inter-
action may account for higher free energies than Ts-major-b.
For Ts-minor, the aromatic C–HÁ Á ÁO hydrogen bonding interac-
tions and p–p stacking interactions between pyrazolone enol
and pyrenyl are relatively weaker than in Ts-major. But the p–p-
interaction between the two substrates (Fig. S4, ESI†) and weak
pyrazolone enol a-C–HÁÁÁO interaction (Scheme 3) counterbalanced
part of the energy differences. For Ts-minor-b, weaker aromatic
C–HÁ Á ÁO hydrogen bonding interaction and weaker dispersion
interactions between 1-pyrenyl and azonaphthalenes (Fig. S4,
ESI†) may account for higher free energies than Ts-minor-a.
In conclusion, we have established the first catalytic asym-
metric reactions of azonaphthalenes with pyrazolones by using
chrial phosphoric acid catalysts. This protocol allowed the
synthesis of a variety of axially chiral pyrazole derivatives in
good yields and excellent enantioselectivities. This reaction
provides a useful method for constructing axially chiral pyrazole
scaffolds. Theoretical calculations were also carried out to
elucidate the origins of enantioselectivity.
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We are grateful for financial support from the National
Natural Science Foundation of China (21502043), the Special
Funds of the Taishan Scholar Program of Shandong Province
(tsqn201812047), and the Natural Science Foundation of Shandong
Province (ZR2017JL011). We also thank the State Key Laboratory on
Elemento-organic Chemistry.
Conflicts of interest
There are no conflicts to declare.
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16 For experimental details see ESI†.
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See ESI† for full citation and calculation details.
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0.3 kcal molÀ1 higher than the energy of pyrazolone 2a keto form
and the calculated activation energy for tautomerization catalyzed
by chiral phosphoric acid 4f is 9.0 kcal molÀ1 (Fig. S3, ESI,† relative
to pyrazolone), which indicated that the tautomerization process
under experimental conditions is quick and reversible.
12718 | Chem. Commun., 2019, 55, 12715--12718
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