3160
K. Kurihara et al. / Tetrahedron Letters 50 (2009) 3158–3160
methyl or ethyl 2Àacetamidocinnamates possessing a substituent
at the para, meta, or ortho carbon, hydrogenation of these sub-
strates was carried out in a mixed solvent of toluene and CH2Cl2
or in CH2Cl2 (runs 12À28). The conversions and selectivities were
almost perfect for most substrates, but 4-methoxy, 4-fluoro,
4-acetyl, and 3,4-methylenedioxy cinnamate resulted in selectivi-
ties less than 95% ee (runs 15, 21, 24, 27, and 28) and ethyl 2-ace-
tamidocinnamates showed higher selectivities than those of
methyl analogs (runs 15/16 and 17/18).
the Global COE Program (No. B01, Catalysis as the Basis for Innova-
tion in Materials Science) from the Ministry of Education, Culture,
Sports, Science, and Technology, Japan.
References and notes
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J.; Matt, D. Chem. Soc. Rev. 2008, 839–864.
The results of hydrogenation of
a-aryl enamides are shown in
Table 2. The effects of solvents on enantioselectivities and reaction
rates were analogous to those shown in Table 1, but the reaction
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was significantly slower than that of
a-dehydroamino esters. The
reaction was completed in CH2Cl2 with 1 mol % catalyst loading
(run 2), but slow reaction in toluene showed the best enantioselec-
tivity (95% ee, run 1), and the use of such a catalyst prepared in situ
resulted in yields and selectivities comparable to those of a rho-
dium/Me-BIPAM complex (2b) (run 6). The conversion in toluene
was quantitative when 2 mol % of catalyst was used (run 5), but
increase of hydrogen pressure to 0.6 MPa in the presence of a rho-
dium(I)/Me-BIPAM complex (2b) finally achieved practical conver-
sion and selectivity (run 7) without increasing the catalyst loading
(run 5). Thus, hydrogenation of other derivatives of a-aryl enamide
was conducted under 0.6 MPa hydrogen in the presence of 2b
(1 mol %) (runs 8–16). Although the reaction was highly sensitive
to steric hindrance of ortho-substituents on the aromatic ring (runs
10 and 15), analogs possessing a para and a meta-substituent were
hydrogenated under these conditions in high yields. Aliphatic ena-
mide, such as 2-acetamide-3,3-dimethyl-1-butene, similarly gave
low conversion (15%).
Hydrogenation of (E)-b-acylamino methyl crotonate (7) and
that of dimethyl itaconate (9) are shown in Eqs. 2 and 3. Hydroge-
nation of 7 under the optimal conditions shown in Tables 1 and 2
suffered from low conversions and low enantioselectivities (8À57%
yields and 35À87% ee). The reaction finally achieved 90% ee with
94% conversion when [Rh(cod)2]SbF6 (1 mol %) and Me-BIPAM in
acetone were used at 50 °C under 0.8 MPa of hydrogen (Eq. 2).
On the other hand, hydrogenation of dimethyl itaconate (9) took
place smoothly at room temperature to yield the corresponding
chiral diester with 97% ee (Eq. 3). Under 0.3 MPa hydrogen, the
reaction was also completed quantitatively at 25 °C with
0.2 mol % loading of 2a.
9. (a) Iguchi, Y.; Itooka, R.; Miyaura, N. Synlett 2003, 1040–1042; (b) Boiteau, J.-G.;
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3113; (d) Boiteau, J.-G.; Minnerd, A. J.; Feringa, B. L. J. Org. Chem. 2003, 68,
9481–9484; (e) Duursma, A.; Boiteau, J.-G.; Lefort, L.; Boorgers, J. A. F.; de Vries,
A. H. M.; de Vries, J. G.; Minnerd, A. J.; Feringa, B. L. J. Org. Chem. 2004, 69, 8045–
8052; (f) Martina, S. L. X.; Minnerd, A. J.; Hesssen, B.; Feringa, B. L. Tetrahedron
Lett. 2005, 46, 7159–7163.
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Y.; Schudde, E. P.; de Vries, J. G.; Feringa, B. L.; Minnaard, A. J. J. Comb. Chem.
2007, 9, 407–414.
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Tetrahedron: Asymmetry 2001, 12, 1929–1937; (b) Jagt, R. B. C.; Toullec, P. Y.; de
Vries, J. G.; Feringa, B. L.; Minnaard, A. J. Org. Biomol. Chem. 2006, 4, 773–775;
(c) Eberhardt, L.; Armspach, D.; Matt, D.; Oswald, B.; Toupet, L. Org. Biomol.
Chem. 2007, 5, 3340–3346.
H2 (0.8 MPa)
MeOOC
MeOOC
(2)
[Rh(cod)2]SbF6 (1 mol%)
NHAc
NHAc
(R)-Me-BIPAM (1.1 mol%)
7
8 (94%, 90%ee (S))
acetone, 50 °C, 20 h
12. (a) Matsunaga, S.; Das, J.; Roles, J.; Vogl, E. M.; Yamamoto, N.; Iida, T.;
Yamaguchi, K.; Shibasaki, M. J. Am. Chem. Soc. 2000, 122, 2252–2260; (b)
Kumagai, N.; Matsunaga, S.; Kinoshita, T.; Harada, S.; Okada, S.; Sakamoto, S.;
Yamaguchi, K.; Shibasaki, M. J. Am. Chem. Soc. 2003, 125, 2169–2178.
13. Majima, K.; Takita, R.; Okada, A.; Ohshima, T.; Shibasaki, M. J. Am. Chem. Soc.
2003, 125, 15837–15845.
COOMe
COOMe
H2 (0.3 MPa)
COOMe
COOMe
(3)
[Rh((R)-Me-bipam)(cod)]PF6
(0.2 mol%)
9
10 (>99%, 97%ee (S))
CH2Cl2, 25 °C, 20 h
14. (a) Yamamoto, Y.; Kurihara, K.; Sugishita, N. N.; Oshita, K.; Piao, D.; Miyaura, N.
Chem. Lett. 2005, 34, 1224–1225; (b) Kurihara, K.; Yamamoto, Y.; Sugishita, N.;
Oshita, K.; Piao, D.; Miyaura, N. J. Organomet. Chem. 2007, 692, 428–435.
15. Kurihara, K.; Yamamoto, Y.; Miyaura, N. Adv. Synth. Catal., in press.
16. To a solution of Me-BIPAM (0.05 mmol) in CD2Cl2 was added [Rh(nbd)2]BF4
(0.05 mmol) under argon atomosphere. The solvent was evaporated in vacuo to
give solids of [Rh(nbd)(Me-BIPAM)]BF4. 31P NMR (CD2Cl2) 142.4 (d,
JRhÀP = 248.9 Hz); HRMS (FAB) calcd for C53H46O5P2Rh+ (M+ÀBF4) 955.1932,
found 955.1938.
17. General procedure for hydrogenation: A stainless-steel autoclave charged with
rhodium catalyst and Me-BIPAM was flushed with nitrogen. Solvent was then
added. After being stirred for 1 h at room temperature, alkene was added. The
hydrogenation was performed under hydrogen for 20 h. The hydrogen pressure
was passed carefully in a hood, and the reaction mixture was evaporated. The
conversion and enantiomeric excesses were measured by 1H NMR and HPLC
without purification.
In conclusion, we demonstrated high performance of Me-BIPAM
for enantioselective hydrogenation of -dehydroamino ester,
enamides, and itaconate, the results of which are comparable to
representative C2 symmetric chiral auxiliaries reported for hydro-
genation of alkenes.1 Further application of Me-BIPAM to other
enantioselective transformations is in progress in our group.
a
Acknowledgments
This work was supported by a Grant-in-Aid for Scientific Re-
search in Priority Areas (No. 18064001, Synergy of Elements) and