higher-generation catalyst 5 led to product formation with
slightly reduced enantioselectivity.
4 For a review on chirality in dendritic architectures, see: H. W. I.
Peerlings and E. W. Meijer, Chem. Eur. J., 1997, 3, 1563.
5
H. Brunner, J. Organomet. Chem., 1995, 500, 39; H. Brunner and S.
Altmann, Chem. Ber., 1994, 127, 2285; H. Brunner and J. Fürst,
Tetrahedron, 1994, 50, 4303; H. Brunner and G. Net, Synthesis, 1995,
The highest ee values were found in reductions of a-
chloroacetophenone giving the corresponding alcohol with up
to 96% ee (Table 1, entries 11 and 12). These results are
particularly important because a-(chloromethyl)benzyl alcohol
is a valuable precursor for the synthesis of phenyloxirane.18
The potential of these new dendritic catalysts which operate
homogeneously is also highlighted by the fact that under
heterogeneous conditions with polymeric 1c significantly lower
enantioselectivities were achieved (cf. acetophenone: 76% ee;
4
23.
6
M. S. T. H. Sanders-Hovens, J. F. G. A. Jansen, J. A. J. M. Vekemans
and E. W. Meijer, Polym. Mater. Sci. Eng., 1995, 73, 338.
7 D. Seebach, R. E. Marti and T. Hintermann, Helv. Chim. Acta, 1996, 79,
1710; P. B. Rheiner, H. Sellner and D. Seebach, Helv. Chim Acta, 1997,
80, 2027; P. B. Rheiner and D. Seebach, Polym. Mater. Sci. Eng., 1997,
7
7, 130.
14,19
8 T. Suzuki, Y. Hirokawa, K. Ohtake, T. Shibata and K. Soai,
Tetrahedron: Asymmetry, 1997, 8, 4033.
propiophenone: 79% ee; a-chloroacetophenone: 84% ee).
In summary, we have demonstrated the feasibility of
performing highly enantioselective borane reductions of pro-
chiral ketones using dendritic catalysts (hccs), and we are
currently investigating their possible applications in con-
tinuously operating reactor systems.2
9
S. Yamago, M. Furukawa, A. Azuma and J.-I. Yoshida, Tetrahedron
Lett., 1998, 39, 3783.
1
0 C. Köllner, B. Pugin and A. Togni, J. Am. Chem. Soc., 1998, 120,
0274.
1
0,21
11 C. Bolm, N. Derrien and A. Seger, Synlett, 1996, 387.
12 (a) G. Giffels, J. Beliczey, M. Felder and U. Kragl, Tetrahedron:
Asymmetry, 1998, 9, 691; (b) J. Beliczey, G. Giffels, U. Kragl and C.
Wandrey, Tetrahedron: Asymmetry, 1997, 8, 1529; (c) M. Felder, G.
Giffels and C. Wandrey, Tetrahedron: Asymmetry 1997, 8, 1975; (d) U.
Kragl and C. Dreisbach, Angew. Chem., 1996, 108, 684; Angew. Chem.,
Int. Ed. Engl., 1996, 35, 642; (e) U. Kragl, C. Dreisbach and C.
Wandrey, Applied Homogeneous Catalysis with Organometallic Com-
pounds, ed. B. Cornils and W. A. Herrmann, VCH, Weinheim, 1996,
p. 832.
We are grateful to the Bundesministerium für Forschung und
Technologie (BMFT) and the Fonds der Chemischen Industrie
for financial support of this work. We thank the Degussa-Hüls
for the generous donation of chemicals.
Notes and references
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For a selection of general reviews, see: H.-F. Chow, T. K.-K. Mong,
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4 S. Itsuno, M. Nakano, K. Ito, A. Hirao, M. Owa, N. Kanda and S.
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6 Details of the syntheses will be published elsewhere. See also: N.
Derrien, dissertation, RWTH Aachen 1998.
7 Reviews: E. J. Corey and C. J. Helal, Angew. Chem., 1998, 110, 2092;
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2
3
J. W. J. Knapen, A. W. van der Made, J. C. de Wilde, P. W. N. M. van
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8 E. J. Corey, S. Shibata and R. K. Bakshi, J. Org. Chem., 1988, 53,
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For later examples of achiral dendritic catalysts, see: (a) C. C. Mak and
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9 Soluble polymers containing 1 give comparable results; see ref. 12(a).
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1997, 38, 6461; (e) N. J. Hovestad, E. B. Eggeling, H. J. Heidbüschel,
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Chem. Commun., 1999, 2087–2088