Chiral lanthanum(III)-binaphthyldisulfonate complexes for catalytic enantioselective strecker reaction

Article abstract of DOI:10.1021/ol900680f

A catalytic enantioselective Strecker reaction catalyzed by novel chiral lanthanum(III)-binaphthyl disulfonate complexes was developed. The key to promoting the reactions was a semistoichiometric amount of AcOH or i-PrCO2H, which takes advantage of HCN ge

Full text of DOI:10.1021/ol900680f

ORGANIC
LETTERS
2009
Vol. 11, No. 11
2321-2324
Chiral Lanthanum(III)-Binaphthyldisulfonate
Complexes for Catalytic Enantioselective
Strecker Reaction
Manabu Hatano, Yasushi Hattori, Yoshiro Furuya, and Kazuaki Ishihara*
Graduate School of Engineering, Nagoya UniVersity, Furo-cho,
Chikusa, Nagoya 464-8603, Japan
ishihara@cc.nagoya-u.ac.jp
Received April 2, 2009
ABSTRACT
A catalytic enantioselective Strecker reaction catalyzed by novel chiral lanthanum(III)-binaphthyl disulfonate complexes was developed. The
key to promoting the reactions was a semistoichiometric amount of AcOH or i-PrCO2H, which takes advantage of HCN generation in situ. The
corresponding cyanation products were obtained in high yields and with high enantioselectivities.
The catalytic enantioselective Strecker reaction is one of the
most convenient methods for the synthesis of optically active
natural and unnatural R-amino acids.1 This process usually
involves the addition of hydrogen cyanide (HCN) or tri-
methylsilyl cyanide (TMSCN) to imines, and several chiral
organocatalysts2 and metal catalysts3 have recently been
developed to achieve a high level of enantioselectivity. In
particular, chiral binaphthyl derivatives offer great advantages
in the design of both organocatalysts and metal catalysts for
the Strecker reaction.4,5 In the course of our studies of chiral
binaphthyl chemistry, we recently developed chiral 1,1′-
binaphthyl-2,2′-disulfonic acid (BINSA, 1) as an organo-
catalyst for the enantioselective direct Mannich-type reac-
tion.6 BINSA, which has an extremely simple structure,
should be a highly promising chiral ligand for metal-mediated
(1) For reviews, see: (a) Duthaler, R. O. Tetrahedron 1994, 50, 1539.
(b) Yet, L. Angew. Chem., Int. Ed. 2001, 40, 875. (c) Gro¨ger, H. Chem.
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Chem. Soc. 1996, 118, 4910. (b) Corey, E. J.; Grogan, M. J. Org. Lett.
1999, 1, 157. (c) Sigman, M. S.; Vachal, P.; Jacobsen, E. N. Angew. Chem.,
Int. Ed. 2000, 39, 1279. (d) Vachal, P.; Jacobsen, E. N. Org. Lett. 2000, 2,
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Y.; Gao, B.; Fu, Y.; Dong, S.; Liu, X.; Feng, X. Chem.sEur. J. 2008, 14,
6789.
10.1021/ol900680f CCC: $40.75
Published on Web 04/30/2009
 2009 American Chemical Society

enantioselective catalyses. First, the bidentate electron-
withdrawing sulfonate groups should effectively activate the
metal center after metal complexation (eq 1). Second, high
enantioselectivity may be induced even without any 3,3′-
modification of the 1,1′-binaphthyl skeleton, since the
-SO3H substituents of BINSA are sterically more demanding
than other 2,2′-substituents of 1,1′-binaphthyl such as -OH
and -CO2H. In this paper, we describe the catalytic
enantioselective Strecker reaction of aldimines with the use
of novel chiral lanthanum(III)-1,1′-binaphthyl 2,2′-disul-
fonate complexes, which is the first catalysis with 1-metal
complexes.
lanthanide complexes showed moderate enantioselectivities
(entries 3-5), while Sc(O-i-Pr)3, Y(O-i-Pr)3, and late lan-
thanide complexes showed lower enantioselectivities (entries
1, 2, 7-9).8,9 When more polar EtCN was used in place of
toluene, the solubility of the heterogeneous catalysts was
slightly improved to give the same enantioselectivity (entry
10). La(OPh)3 was found to be most effective among the
La(III) precursors (LaX) examined (entry 11).10 Moreover,
the enantioselectivity was improved to 65% ee when the ratio
of 1 to La(OPh)3 was optimized as 1:1. As expected, the
activities of 1 without La(OPh)3 (entry 13) and of La(OPh)3
without 1 (entry 14) were low. A poor result was also
observed when La(OPh)3 and (R)-BINOL were used (entry
15).
Further optimization was examined by adding protic
compounds, since the actual cyanide source has been
previously shown to be HCN rather than TMSCN.3c,d,h,4c,e,i
The addition of protic compounds such as H2O and PhOH
improved the yields of 3a (Table 2, entries 2 and 3).
First, we examined the metal tuning for 1 (10-15 mol
%) in the reaction of aldimine 2a with TMSCN (1.5 equiv)
at -20 °C for 20 h (Table 1).7 Monovalent, divalent, and
Table 2. Effect of Protic Additivesa
Table 1. 1-M(III)X3-Catalyzed Strecker Reaction of 2a with
TMSCNa
additive
(mol %)
yield ee
(%) (%) entry
additive
(mol %)
yield ee
(%) (%)
entry
1
2
3
4
5
6
22
46
42
98
65
65
53
84
64
84
7
8
9
i-PrCO2H (70)
i-PrCO2H (100) 89
i-PrCO2H (150) 69
88
74
73
23
49
38
73
H2O (50)
PhOH (50)
AcOH (50)
10 HCO2H (50)
11 CF3CO2H (50)
12 t-BuCO2H (50)
46
34
62
i-PrCO2H (30) 58
i-PrCO2H (50) 86
entry
M(III)X3
1 (mol %) solvent yield (%) ee (%)
a Prior to the reaction, each catalyst was prepared in situ from 1 and
La(OPh)3 in EtCN at 60 °C for 1 h. Then, 2a, the additive, and TMSCN
were poured into the heterogeneous suspension at -20 °C.
1
2
3
4
5
6
7
8
Sc(Oi-Pr)3
Y(Oi-Pr)3
La(Oi-Pr)3
Pr(Oi-Pr)3
Nd(Oi-Pr)3
Sm(Oi-Pr)3
Dy(Oi-Pr)3
Er(Oi-Pr)3
Yb(Oi-Pr)3
La(Oi-Pr)3
La(OPh)3
La(OPh)3
15
15
15
15
15
15
15
15
15
15
15
10
10
0
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
EtCN
56
28
34
24
29
35
6
12
29
27
38
22
26
16
9
18
0
54
46
49
34
20
10
10
55
57
65
13
Fortunately, we found that 50 mol % of AcOH or i-PrCO2H
significantly improved the catalytic activity, and 3a was
obtained with 84% ee in good yields (entries 4 and 6). The
amount of i-PrCO2H was also important, and less or more
than 50 mol % of i-PrCO2H decreased the catalytic activity
(entries 5-9). Other carboxylic acids such as HCO2H,
CF3CO2H, and t-BuCO2H gave less improvement than AcOH
or i-PrCO2H (entries 10-12).
9
10
11
12
13b
14c
15d
EtCN
EtCN
EtCN
EtCN
La(OPh)3
La(OPh)3
0
EtCN
0
(4) (a) Ishitani, H.; Komiyama, S.; Kobayashi, S. Angew. Chem., Int.
Ed. 1998, 37, 3186. (b) Byrne, J. J.; Chavarot, M.; Chavant, P.-Y.; Valle´e,
Y. Tetrahedron Lett. 2000, 41, 873. (c) Takamura, M.; Hamashima, Y.;
Usuda, H.; Kanai, M.; Shibasaki, M. Angew. Chem., Int. Ed. 2000, 39, 1650.
(d) Chavarot, M.; Byrne, J. J.; Chavant, P. Y.; Valle´e, Y. Tetrahedron:
Asymmetry 2001, 12, 1147. (e) Nogami, H.; Matsunaga, S.; Kanai, M.;
Shibasaki, M. Tetrahedron Lett. 2001, 42, 279. (f) Rueping, M.; Sugiono,
E.; Azap, C. Angew. Chem., Int. Ed. 2006, 45, 2617. (g) Ooi, T.; Uematsu,
Y.; Maruoka, K. J. Am. Chem. Soc. 2006, 128, 2548. (h) Ooi, T.; Uematsu,
Y.; Fujimoto, J.; Fukumoto, K.; Maruoka, K. Tetrahedron Lett. 2007, 48,
1337. (i) Hou, Z.; Wang, J.; Liu, X.; Feng, X. Chem.sEur. J. 2008, 14,
4484.
a Prior to the reaction, each catalyst was prepared in situ from 1 and
MX3 in the solvent at 60 °C for 1 h. b In the absence of MX3. c Reaction
time was 20 h. d 10 mol % of (R)-BINOL was used in place of 1.
tetravalent MXn precursors, such as AgOAc, CuOAc,
Cu(OAc)2, Cu(OMe)2, Pd(OAc)2, Ti(O-i-Pr)4, Zr(O-t-Bu)4,
Hf(O-t-Bu)4, etc., were not effective, and the corresponding
product (3a) was obtained in low yields with low enanti-
oselectivities (<20% ee). Interestingly, however, we found
that trivalent precursors improved both the yields and the
enantioselectivities. In particular, La(O-i-Pr)3 and early
(5) Our preliminary reports of catalytic enantioselective trimethylsilyl-
cyanation of aldehydes and ketones with BINOL derivatives: (a) Hatano,
M.; Ikeno, T.; Miyamoto, T.; Ishihara, K. J. Am. Chem. Soc. 2005, 127,
10776. (b) Hatano, M.; Ikeno, T.; Matsumura, T.; Torii, S.; Ishihara, K.
AdV. Synth. Catal. 2008, 350, 1776.
2322
Org. Lett., Vol. 11, No. 11, 2009

A direct comparison with HCN was also examined (eq
2). The reaction proceeded smoothly when 3 equiv of HCN
was used, and 3a was obtained in 86% yield with 56% ee.
Although the enantioselectivity was not as high as under the
conditions using TMSCN (Table 2, entry 1), these results
strongly suggest that HCN is a key reagent in this reaction.
Next, we examined the scope of the aldimine substrates
in the presence of 10 mol % each of 1 and La(OPh)3,
i-PrCO2H or AcOH (50 mol %), and TMSCN (1.5 equiv) in
EtCN at -20 °C (Table 3). For N-protection, a CHPh2 group
Figure 1. Nonlinear effect between 1 and 3a.
Table 3. 1-La(OPh)3-Catalyzed Strecker Reactiona
oligomeric complexes. However, ESI-MS analysis of the
catalyst, which was prepared in situ from La(OPh)3 (1 equiv)
and 1 (1 equiv) in AcOH (5 equiv) and MeCN, suggested
the dominant generation of monomeric La(III) complexes,
[Ar*(SO3)2La(MeCN)n]+ (Ar*(SO3H)2 ) 1, n ) 1-4). The
observed species may be derived from a parent complex
[Ar*(SO3)2La(OAc)(MeCN)n]+,11 since the pKa value of
HCN is smaller than that of AcOH.12 Although further
investigation is necessary to obtain a clear understanding,
postulated catalytic cycles are shown in Figure 2 as a working
model. Acidic R′CO2H (R′ ) Me or i-Pr) provides a
counteranion for La(III) and generates HCN via proton-
reaction
timeb (h)
yieldb
(%)
eeb c (%)
,
entry
R
1
Ph
20 [20]
20 [24]
20 [16]
20 [20]
20 [18]
86 [96]
24 [20]
96 [20]
86 [72]
92 [96]
20 [48]
86 [98]
92 [78]
97 [97]
95 [96]
99 [96]
97 [83]
97 [96]
64 [28]
97 [99]
68 [79]
99 [98]
84 (S) [84 (S)]
88 (S) [88 (S)]
90 (S) [85 (S)]
86 [74]
86 [80]
92 (R) [76 (R)]
83 [84]
2
4-ClC6H4
4-MeOC6H4
3,4-OCH2OC6H3
3-furyl
2-thienyl
3-thienyl
2-Naph
3
4
5
6
7
8
92 (S) [85 (S)]
80 (S) [64 (S)]
52 [66]
9
PhCHdCH
PhCtC
10
11
(8) Shibasaki’s pioneering work and a review with La(III) catalysts: (a)
Sasai, H.; Suzuki, T.; Arai, S.; Arai, T.; Shibasaki, M J. Am. Chem. Soc.
1992, 114, 4418. (b) Shibasaki, M.; Yoshikawa, N. Chem. ReV. 2002, 102,
2187. Recent work: (c) Kato, N.; Mita, T.; Kanai, M.; Therrien, B.; Kawano,
M.; Yamaguchi, K.; Danjo, H.; Sei, Y.; Sato, A.; Furusho, S.; Shibasaki,
M. J. Am. Chem. Soc. 2006, 128, 6768. (d) Tosaki, S.; Hara, K.;
Gnanadesikan, V.; Morimoto, H.; Harada, S.; Sugita, M.; Yamagiwa, N.;
Matsunaga, S.; Shibasaki, M. J. Am. Chem. Soc. 2006, 128, 11776. (e)
Morimoto, H.; Lu, G.; Aoyama, N.; Matsunaga, S.; Shibasaki, M. J. Am.
Chem. Soc. 2007, 129, 9588. (f) Handa, S.; Nagawa, K.; Sohtome, Y.;
Matsunaga, S.; Shibasaki, M. Angew. Chem., Int. Ed. 2008, 47, 3230. (g)
Lu, G.; Morimoto, H.; Matsunaga, S.; Shibasaki, M. Angew. Chem., Int.
Ed. 2008, 47, 6847. (h) Sone, T.; Yamaguchi, A.; Matsunaga, S.; Shibasaki,
M. J. Am. Chem. Soc. 2008, 130, 10078.
t-Bu
41 (S) [33 (S)]
a Prior to the reaction, the catalyst was prepared in situ from 1 and
La(OPh)3 in EtCN at 60 °C for 1 h. Then, 2, i-PrCO2H, and TMSCN were
poured into the heterogeneous suspension at -20 °C. b Data in brackets
are the result when AcOH was used in place of i-PrCO2H. c Absolute
configurations are shown in parentheses.
gave the best result (Supporting Information).2a,b,j,3c,d,g The
reactions of aromatic and heteroaromatic aldimines bearing
electron-withdrawing and electron-donating groups pro-
ceeded in high yields with moderate to high enantioselec-
tivities (entries 1-8). In particular, 4-MeOC6H4-
CHdNCHPh2 and 2-thienylCHdNCHPh2 gave each of the
products with over 90% ee in high yields (entries 3 and 6).
R,ꢀ-Unsaturated aldimines gave the corresponding products
with moderate to good enantioselectivities (entries 9 and 10),
although aliphatic aldimine gave poor enantioselectivity
(entry 11).
(9) Reviews of asymmetric catalyses with chiral lanthanide complexes:
(a) Kobayashi, S. Synlett 1994, 689. (b) Aspinall, H. C. Chem. ReV. 2002,
102, 1807. (c) Molander, G. A.; Romero, A. C. Chem. ReV. 2002, 102,
2161. (d) Shibasaki, M.; Yokikawa, N. Chem. ReV. 2002, 102, 2187. (e)
Inanaga, J.; Furuno, H.; Hayano, T. Chem. ReV. 2002, 102, 2211. (f)
Kobayashi, S.; Sugiura, M.; Kitagawa, H.; Lam, H. W.-L. Chem. ReV. 2002,
102, 2227. (g) Mikami, K.; Terada, M.; Matsuzawa, H. Angew. Chem., Int.
Ed. 2002, 41, 3554.
(10) La2(CO3)3, La(OTs)3, and La(NO3)3·6H2O showed low catalytic
activity with poor enantioselectivity. Moreover, other La(OAr)3 complexes
(Ar ) Ph, 3,5-xylyl, mesityl, 2,6-Ph2C6H3) did not affect the enantioselec-
tivity of 3.
(11) The possibility of the complexes, [Ar*(SO3)2La(CN)(MeCN)n]+,
cannot be denied.
Finally, we examined the mechanistic aspects. Based on
the observation of a positive nonlinear effect between 1 and
3a (Figure 1), the structure of enantiomerically pure La(III)
catalysts in situ might involve a mixture of monomeric and
(12) pKa of HCN is 12.9 in DMSO and 9.1 in H2O. The pKa of AcOH
is 12.3 in DMSO and 4.75 in H2O: Taft, R. W.; Bordwell, F. G. Acc. Chem.
Res. 1988, 21, 456.
(13) For the full generation of HCN (1.5 equiv) in situ, a ratio of R′CO2H
(0.5 equiv) and TMSCN (1.5 equiv) is disputable. PhOH (0.3 equiv) released
via ligand exchange and adventitious water also may be involved as protone
sources. Moreover, it is notable that the equiliblium among a product,
TMSCN, PhOTMS, HCN, and PhOH has been proposed by Shibasaki et
al. in ref 4c.
(6) Hatano, M.; Maki, T.; Moriyama, K.; Arinobe, M.; Ishihara, K. J. Am.
Chem. Soc. 2008, 130, 16858.
(7) Solubility of the corresponding complexes was low, and the reaction
conditions were heterogeneous.
Org. Lett., Vol. 11, No. 11, 2009
2323

Figure 3. Possible transition states.
binaphthyldisulfonate complexes. The key to promoting the
reactions was a semistoichiometric amount of AcOH or
i-PrCO2H, which takes advantage of HCN generation in situ.
Further applications of 1-metal salts to other catalytic
enantioselective reactions are now underway.
Figure 2. Postulated catalytic cycles.
exchange reactions.13 First, a possible active EtCN-solvate
catalyst [Ar*(SO3)2La(OCOR′)(EtCN)n] (4) would be gener-
ated in situ from the mixture of 1, La(OPh)3, R′CO2H, and
TMSCN, and then aldimine 2 would be continuously
activated at the La(III) center. HCN is then added to 2,14
and the catalyst would be regenerated and the product (3) is
released. In the transition states, the re-face attack, which
leads to (S)-products, should be favored without conspicuous
steric repulsion between the substrate and the ligand (Figure
3).
Acknowledgment. Financial support for this project was
provided by JSPS, KAKENHI (20245022), Grant-in-Aid for
Young Scientists B of MEXT (19750072 and 21750094),
the G-COE Program of MEXT, Toray Science Foundation,
and Shorai Foundation for Science and Technology.
Supporting Information Available: Experimental pro-
cedures and spectral data, as well as copies of NMR spectra.
This material is available free of charge via the Internet at
http://pubs.acs.org.
In summary, we have developed a catalytic enantioselec-
tive Strecker reaction catalyzed by chiral lanthanum(III)-
(14) Simo´n, L.; Goodman, J. M. J. Am. Chem. Soc. 2009, 131, 4070.
OL900680F
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