Table 3 Substrate scope
(b) H. Ishibashi, K. Ishihara and H. Yamamoto, Chem. Rec.,
2002, 2, 177.
7 For asymmetric protonation reactions with LBA, see:
(a) K. Ishihara, M. Kaneeda and H. Yamamoto, J. Am. Chem.
Soc., 1994, 116, 11179; (b) K. Ishihara, S. Nakamura, M. Kaneeda
and H. Yamamoto, J. Am. Chem. Soc., 1996, 118, 12874;
(c) K. Ishihara, Y. Ishida, S. Nakamura and H. Yamamoto,
Synlett, 1997, 758; (d) K. Ishihara, S. Nakamura and
H. Yamamoto, Croat. Chem. Acta, 1998, 69, 513;
(e) S. Nakamura, M. Kaneeda, K. Ishihara and H. Yamamoto,
J. Am. Chem. Soc., 2000, 122, 8120; (f) K. Ishihara, D. Nakashima,
Y. Hiraiwa and H. Yamamoto, J. Am. Chem. Soc., 2003, 125, 24;
(g) D. Nakashima and H. Yamamato, Synlett, 2006, 150.
8 For asymmetric polyene cyclization reactions with LBA, see:
(a) K. Ishihara, S. Nakamura and H. Yamamoto, J. Am. Chem.
Soc., 1999, 121, 4906; (b) S. Nakamura, K. Ishihara and
H. Yamamoto, J. Am. Chem. Soc., 2000, 122, 8131;
(c) K. Ishihara, H. Ishibashi and H. Yamamoto, J. Am. Chem.
Soc., 2001, 123, 1505; (d) K. Ishihara, H. Ishibashi and
H. Yamamoto, J. Am. Chem. Soc., 2002, 124, 3647;
(e) K. Kumazawa, H. Ishibashi, K. Ishihara and H. Yamamoto,
Org. Lett., 2004, 6, 2551; (f) H. Ishibashi, K. Ishihara and
H. Yamamoto, J. Am. Chem. Soc., 2004, 126, 11122;
(g) M. Uyanik, H. Ishibashi, K. Ishihara and H. Yamamoto,
Org. Lett., 2005, 7, 1601.
9 Other examples of LBAs derived from different metals, see:
(a) with Pd, see: M. Sugiura and T. Nakai, Angew. Chem., Int.
Ed. Engl., 1997, 36, 2366; (b) with Ag, see: A. Yanagisawa,
T. Touge and T. Arai, Angew. Chem., Int. Ed., 2005, 44, 1546.
10 Catalytic usage of LBA for protonation reaction in the presence of
stoichiometric amount of bulky phenol derivatives was reported,
see: ref. 7g.
11 Other chiral ligands were also investigated, see ESIw.
12 For reviews of asymmetric protonation reaction, see: (a) C. Fehr,
Angew. Chem., Int. Ed. Engl., 1996, 35, 2566; (b) J. T. Mohr,
A. Y. Hong and B. M. Stoltz, Nat. Chem., 2009, 1, 359.
13 For metal-free Brønsted acid catalyzed asymmetric protonation
reaction of silyl enol ethers, see: ref. 5b.
Entry
n
R
Time/h
% Yielda
ee (%)b
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
2
2
Ph (1a)
18
18
18
18
12
18
12
16
18
97
96
94
96
95
92
95
93
96
75
72
70
52
42
54
32
54
34
4-MeC6H4 (1b)
4-MeOC6H4 (1c)
4-ClC6H4 (1d)
2-MeOC6H4 (1e)
2-naphthyl (1f)
1-naphthyl (1g)
Ph (1h)
2-naphthyl (1i)
a
b
Isolated yield after column chromatography separation. ee was
determined by HPLC analysis using chiral column.
products in much lower enantioselectivity than those of
six-membered rings counterparts (entries 1, 6, 8, and 9).
In conclusion, we have developed a LBA derived from
(S)-HOP with La(OTf)3 as a Lewis acid activator. The LBA
is Lewis-base tolerant and was successfully applied to the
catalytic enantioselective protonation reaction of silyl enol
ether of 2-aryl cyclic ketones in the presence of a super-
stoichiometric amount of methanol. The enantioselectivity of
the protonation product was increased by decreasing the
amount of MeOH activated by La(OTf)3 and by preventing
oxidation of chiral Brønsted acid to the corresponding phos-
phine oxide ligand. Further application of this Lewis base
tolerant LBA is currently underway in our laboratory.
This research is supported by NIH (grant number:
GM086145-02S1) and Uehara Memorial Foundation for a
postdoctoral fellowship to T.I.
14 Various other Lewis acids were investigated in the same condition.
Yb(OTf)3 and Eu(OTf)3 provided the protonation product in 23,
20% ee, respectively, whereas Sc(OTf)3, In(OTf)3, Zn(OTf)2, and
Cu(OTf)2 provided only the racemic product. For more detailed
information, see ESIw.
15 For other applications of La(OTf)3 in organic reactions, see:
S. Kobayashi and I. Hachiya, J. Org. Chem., 1994, 59, 3590.
16 LBA derived from methylated (S)-HOP, (S)-Me-HOP, and
La(OTf)3 provided only racemic product in the protonation
reaction.
Notes and references
1 For a review of Brønsted acid catalysis, see: P. M. Pihko, Angew.
Chem., Int. Ed., 2004, 43, 2062.
2 For a review of hydrogen bond organic catalysis, see: A. G. Doyle
and E. N. Jacobsen, Chem. Rev., 2007, 107, 5713.
3 Y. Huang, A. K. Unni, A. N. Thadani and V. H. Rawal, Nature,
2003, 424, 146.
17 The Yanagisawa group reported an excellent example of
asymmetric protonation reaction of silyl enol ethers through the
protonation with activated MeOH by chiral LBA, see: ref. 9b.
4 For reviews of chiral phosphoric acid catalysis, see:
(a) T. Akiyama, Chem. Rev., 2007, 107, 5744; (b) M. Tereda, Chem.
Commun., 2008, 4097.
5 Recent examples of reactivity enhancement in Brønsted acid
catalysis, see: (a) D. Nakashima and H. Yamamoto, J. Am. Chem.
Soc., 2006, 128, 9626; (b) C. H. Cheon and H. Yamamoto, J. Am.
Chem. Soc., 2008, 130, 9246.
6 For reviews of combined acids, see: (a) H. Yamamoto and
K. Futatsugi, Angew. Chem., Int. Ed., 2005, 44, 1924;
18 For a chemical shift in 31P NMR of (S)-HOP, see: W. Zhang and
M. Shi, Tetrahedron: Asymmetry, 2004, 15, 3467.
19 For more detailed information, see ESIw.
c
6982 Chem. Commun., 2010, 46, 6980–6982
This journal is The Royal Society of Chemistry 2010