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c
c
c
cycle
1
2
3
4
5
a
yield (%)
85(92)
99
77(80)
99
94(97)
99
88(92)
99
85(89)
99
b
ee (%)
a
The yield was determined by isolation from a small fraction of a
crude mixture. The yield determined by 1H NMR analysis was shown
b
c
in parentheses. Determined by HPLC analysis. The recovered
catalyst was heated at 150 °C for 6 h before use.
in every cycle. The yield decreased slightly in the second cycle
after washing the recovered catalyst with water and an organic
solvent followed by drying. The recovered catalyst was then
reactivated by heating at 150 °C for 6 h, which resulted in the
high yield and outstanding enantioselectivity being maintained
for the following three cycles (cycles 3−5).
In summary, heterogeneous chiral Rh NP catalysts for
asymmetric addition of arylboronic acids to aldimines have
been developed. The catalyst worked efficiently in aqueous
media, and the highest TON (700) in heterogeneous catalysts
yet described for this reaction was achieved. A wide variety of
substrates could be utilized to afford the desired products in
high yields and with outstanding enantioselectivities. Remark-
ably, in spite of the aqueous conditions, even water-unstable
aliphatic imines could be converted into the desired products
with excellent enantioselectivities. Moreover, the reusability of
the catalyst was confirmed and a simple heating treatment
could be used to maintain the catalytic activity for five cycles.
We believe that heterogeneous chiral NPs are active and
practical catalyst systems for the synthesis of important chiral
amine compounds.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
■
S
Experimental procedures, characterization data, and
copies of NMR and HPLC charts (PDF)
(9) (a) Yasukawa, T.; Miyamura, H.; Kobayashi, S. Chem. Soc. Rev.
2014, 43, 1450. (b) Ranganath, K. V. S.; Kloesges, J.; Schafer, A. H.;
̈
Glorius, F. Angew. Chem., Int. Ed. 2010, 49, 7786. (c) Heitbaum, M.;
Glorius, F.; Escher, I. Angew. Chem., Int. Ed. 2006, 45, 4732. (d) Jansat,
AUTHOR INFORMATION
Corresponding Author
■
́
S.; Gomez, M.; Philippot, K.; Muller, G.; Guiu, E.; Claver, C.;
Castillon, S.; Chaudret, B. J. Am. Chem. Soc. 2004, 126, 1592.
(e) Orito, Y.; Imai, S.; Niwa, S.; Nguyen, G. H. Yuki Gosei Kagaku
Kyokaishi 1979, 37, 173.
Notes
(10) Kobayashi, S.; Miyamura, H. Aldrichimica Acta 2013, 46, 3.
(11) (a) Yasukawa, T.; Suzuki, A.; Miyamura, H.; Nishino, K.;
Kobayashi, S. J. Am. Chem. Soc. 2015, 137, 6616. (b) Yasukawa, T.;
Miyamura, H.; Kobayashi, S. J. Am. Chem. Soc. 2012, 134, 16963.
(12) The structure of the chiral diene was previously optimized in the
asymmetric 1,4-addition to α,β-unsaturated esters. See ref 11a.
(13) We assumed that a redox process between Rh/Ag NP and
arylboronic acids might occur during the “preheating” stage to
generate active species. A similar shorter induction period was
observed when preheating was carried out in the presence of the
catalyst and a catalytic amount of sodium citrate (5 mol%) as a
reductant. See ref 11a for more details.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partially supported by a Grant-in-Aid for Science
Research from the Japan Society for the Promotion of Science
(JSPS), Global COE Program, The University of Tokyo,
MEXT, Japan, and the Japan Science and Technology Agency
(JST).
REFERENCES
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(1) (a) Ameen, D.; Snape, T. J. MedChemComm 2013, 4, 893.
(b) Marques, C. S.; Burke, A. J. ChemCatChem 2011, 3, 635.
C
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