Highly effective asymmetric hydrogenation of cyclic N -alkyl imines with chiral cationic Ru-MsDPEN catalysts

Article abstract of DOI:10.1021/ol201679f

A range of cyclic N-alkyl imines were efficiently hydrogenated by using a chiral cationic Ru(η⁶-cymene)(MsDPEN)(BArF) complex (MsDPEN = N-(methanesulfonyl)-1,2-diphenylethylenediamine) in high yields and up to 98% ee. A one-pot synthesis of chiral 2-phenylpyrrolidine via reductive amination was also developed.

Full text of DOI:10.1021/ol201679f

ORGANIC
LETTERS
2011
Vol. 13, No. 16
4348–4351
Highly Effective Asymmetric
Hydrogenation of Cyclic N-Alkyl Imines
with Chiral Cationic Ru-MsDPEN Catalysts‡
Fei Chen, Ziyuan Ding, Jie Qin, Tianli Wang, Yanmei He, and Qing-Hua Fan*
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Molecular
Recognition and Function, Institute of Chemistry and Graduate School, Chinese
Academy of Sciences (CAS), Beijing 100190, P. R. China
fanqh@iccas.ac.cn
Received June 22, 2011
ABSTRACT
A range of cyclic N-alkyl imines were efficiently hydrogenated by using a chiral cationic Ru(η6-cymene)(MsDPEN)(BArF) complex (MsDPEN =
N-(methanesulfonyl)-1,2-diphenylethylenediamine) in high yields and up to 98% ee. A one-pot synthesis of chiral 2-phenylpyrrolidine via reductive
amination was also developed.
Chiral amines are ubiquitous in natural products and
serve as an important type of building block for the syn-
thesis of many pharmaceutical and agrochemical substances.1
Asymmetric hydrogenation of the corresponding imines is an
elegant and efficient method for obtaining optically active
amines.2 In contrast to the great progress recently achieved in
the asymmetric hydrogenation of activated and N-aryl
imines,3,4 there have been few reports on the highly enantio-
selective hydrogenation of the often-problematic N-alkyl
imines, particularly the cyclic N-alkyl imines.5,6 Buchwald
and co-workers first reported the Titanocene-catalyzed asym-
metric hydrogenation of cyclic N-alkyl imines with good
reactivity and high enantioselectivity.6a Most recently, Xiao
et al. found that the ionic Rh-TsDPEN complex was an
efficient catalyst for the asymmetric hydrogenation of cyclic
N-alkyl imines to afford tetrahydroisoquinolines and
(4) For selected examples on asymmetric hydrogenation of N-aryl
imines, see: (a) Chan, Y. N. C.; Osborn, J. A. J. Am. Chem. Soc. 1990,
112, 9400. (b) Spindler, F.; Pugin, B.; Blaser, H.-U. Angew. Chem., Int.
Ed. Engl. 1990, 29, 558. (c) Xiao, D.; Zhang, X. Angew. Chem., Int. Ed.
2001, 40, 3425. (d) Pugin, B.; Landert, H.; Spindler, F.; Blaser, H.-U.
Adv. Synth. Catal. 2002, 344, 974. (e) Trifonova, A.; Diesen, J. S.;
Chapman, C. J.; Andersson, P. G. Org. Lett. 2004, 6, 3825. (f) Moessner,
C.; Bolm, C. Angew. Chem., Int. Ed. 2005, 44, 7564. (g) Imamoto, T.;
Iwadate, N.; Yoshida, K. Org. Lett. 2006, 8, 2289. (h) Shang, G.; Yang,
Q.; Zhang, X. Angew. Chem., Int. Ed. 2006, 45, 6360. (i) Cheemala,
M. N.; Knochel, P. Org. Lett. 2007, 9, 3089. (j) Li, C.; Wang, C.; Villa-
ꢀ ꢁ
Marcos, B.; Xiao, J. J. Am. Chem. Soc. 2008, 130, 14450. (k) Mrsic, N.;
‡ Dedicated to Professor Christian Bruneau on the occasion of his 60th
birthday.
(1) For recent review, see: Chiral Amine Synthesis: Methods, Devel-
opments and Applications; Nugent, T. C., Ed.; Wiley-VCH: Weinheim,
2010.
(2) For recent reviews, see: (a) Tang, W.; Zhang, X. Chem. Rev. 2003,
103, 3029. (b) Nugent, T. C.; EI-Shazly, M. Adv. Synth. Catal. 2010, 352,
753. (c) Xie, J.-H.; Zhu, S.-F.; Zhou, Q.-L. Chem. Rev. 2011, 111, 1713.
(3) For selected examples on asymmetric hydrogenation of activated
imines, see: (a) Charette, A.; Giroux, A. Tetrahedron Lett. 1996, 37,
6669. (b) Spindler, F.; Blaser, H.-U. Adv. Synth. Catal. 2001, 343, 68.
(c) Yang, Q.; Shang, G.; Gao, W.; Deng, J.; Zhang, X. Angew. Chem., Int.
Ed. 2006, 45, 3832. (d) Wang, Y.-Q.; Lu, S.-M.; Zhou, Y.-G. J. Org. Chem.
2007, 72, 3729. (e) Wang, Y.-Q.; Yu, C.-B.; Wang, D.-W.; Wang, X.-B.;
Zhou, Y.-G. Org. Lett. 2008, 10, 2071. (f) Yu, C.-B.; Wang, D.-W.; Zhou,
Y.-G. J. Org. Chem. 2009, 74, 5633.
Minnaard, A. J.; Feringa, B. L.; de Vries, J. G. J. Am. Chem. Soc. 2009,
131, 8358. (l) Han, Z.; Wang, Z.; Zhang, X.; Ding, K. Angew. Chem., Int.
Ed. 2009, 48, 5345. (m) Kutlescha, K.; Irrgang, T.; Kempe, R. Adv.
Synth. Catal. 2010, 352, 3126. (n) Baeza, A.; Pfaltz, A. Chem.;Eur. J.
2010, 16, 4003. (o) Chen, M.-W.; Duan, Y.; Chen, Q.-A.; Wang, D.-S.;
Yu, C.-B.; Zhou, Y.-G. Org. Lett. 2010, 12, 5075.
(5) For selected examples on asymmetric hydrogenation of acyclic N-
alkyl imines, see refs 4l and 10: (a) Willoughby, C. A.; Buchwald, S. L.
J. Am. Chem. Soc. 1992, 114, 7562. (b) Lensink, C.; Rijnberg, E.;
de Vries, J. G. J. Mol. Catal. A: Chem. 1997, 116, 199. (c) Schnider,
ꢁ ^
€
P.; Koch, G.; Pretot, R.; Wang, G.; Bohnen, F. M.; Kruger, C.; Pfaltz,
A. Chem.;Eur. J. 1997, 3, 887. (d) Jiang, X.-B.; Minnaard, A. J.;
Hessen, B.; Feringa, B. L.; Duchateau, A. L. L.; Andrien, J. G. O.;
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r
10.1021/ol201679f
Published on Web 07/18/2011
2011 American Chemical Society

tetrahydro-β-carbolines with up to 99% ee.6b Zhang and
co-workers used an electron-donating bisphosphine-con-
taining Ir(I) catalyst for the hydrogenation of cyclic imines
to afford chiral 2-aryl pyrrolidine and 2-aryl piperidine
with up to 89% ee.6c
results, we hope to expand the substrate scope from acyclic
to cyclic N-alkyl imines, which will realize the asymmetric
synthesis of chiral 2-arylpyrrolidine derivatives,11,12 ubi-
quitousstructuralmoietiesinbiologicallyactivemolecules,
and natural products. Recently, Wills et al. developed a
one-pot synthesis of cyclic amines via direct asymmetric
reductive amination (DARA) with similar chiral Ru catalyst
3 under transfer hydrogenation conditions (Scheme 1).13
However, only racemic products were observed. Herein,
we disclose the details of the asymmetric hydrogena-
tion of cyclic N-alkyl imines with Ru-MsDPEN com-
plexes, including a scaled-up one-pot synthesis of chiral
N-Boc-2-phenylpyrrolidine via DARA.
Scheme 1. Enantioselective Synthesis of Cyclic Amines via Ru-
Catalyzed Asymmetric Reduction
We started our study with 2-phenyl-1-pyrroline (1a) as a
standard substrate. The initial hydrogenation experiment
was carried out under 50 atm of H2 at 40 °C in DCM
(CH2Cl2) with (R,R)-4a as catalyst (Table 1, entry 1). Un-
fortunately, it was found that the catalytic activity is very low.
Considering the possible catalyst deactivation caused by a
pyrrolidine product,14 (Boc)2O ((Boc)2O = di-tert-butyl
dicarbonate) was added to eliminate the inhibition via in situ
protection of the resulting pyrrolidine.10 Expectedly, full
conversion and excellent enantioselectivity (92% ee) were
observed in the presence of 1.1 equiv of (Boc)2O under
otherwise identical reaction conditions (Table 1, entry 2).
Encouraged by this exciting result, we subsequently
investigated the effect of different catalysts and other
reaction conditions on this reaction (Table 1 and Table
S1 in Supporting Information). After a survey of a variety
of catalysts in the hydrogenation of 1a, it was found that
the weakly coordinating counterions influenced the enan-
tioselectivity, and the highest ee was obtained with BArFÀ
(tetrakis(3,5-bis-trifluoromethylphenyl)borate) as the coun-
terion (Table 1, entries 2À7).10,15 So catalyst 4f turned out
to be optimal in terms of both reactivity and enantioselec-
tivity. In addition, the solvent effect was studied. Notably,
aprotic solvents, such as DCM, DCE (ClCH2CH2Cl), and
toluene, gave higher enantioselectivities (Table 1, entries
7À10). It was observed that the enantioselectivity is in-
sensitive to hydrogen pressure and temperature (Table 1,
entries 7 and 11À14). Furthermore, the reaction proceeded
smoothly at a low catalyst loading of 0.2 mol % in full
conversion with only slightly low enantioselectivity (93%
Recently, we have found that the cationic ruthenium
complexes of chiral monotosylated diamines7 were very
efficient catalysts for the asymmetric hydrogenation8 of
quinoline derivatives, providing chiral 1,2,3,4-tetrahydro-
quinolines with up to 99% ee.9 This catalytic system was
also demonstrated to be highly enantioselective for the
asymmetric hydrogenation of a broad range of acyclic N-
alkyl ketimines, evenunder solvent-freeconditions, afford-
ingchiralamineswith upto99% ee.10 Encouragedby these
(6) For selected examples on asymmetric hydrogenation of cyclic N-
alkyl imines, see: (a) Willoughby, C. A.; Buchwald, S. L. J. Am. Chem.
Soc. 1994, 116, 8952. (b) Li, C.; Xiao, J. J. Am. Chem. Soc. 2008, 130,
13208. (c) Chang, M.; Li, W.; Zhang, X. Adv. Synth. Catal. 2010, 352,
3121. (d) Zhu, G.; Zhang, X. Tetrahedron: Asymmetry 1998, 9, 2415.
(e) Morimoto, T.; Nakajima, N.; Achiwa, K. Synlett 1995, 748.
(f) Cobley, C. J.; Henschke, J. P. Adv. Synth. Catal. 2003, 345, 195.
(g) Jackson, M.; Lennon, I. C. Tetrahedron Lett. 2007, 48, 1831.
(7) For asymmetric transfer hydrogenation catalyzed by metal com-
plexes of chiral monotosylated diamines, see: (a) Noyori, R.; Hashiguchi,
S. Acc. Chem. Res. 1997, 30, 97. (b) Gladiali, S.; Alberico, E. Chem. Soc.
Rev. 2006, 35, 226. (c) Hashiguchi, S.; Fujii, A.; Takehara, J.; Ikariya, T.;
Noyori, R. J. Am. Chem. Soc. 1995, 117, 7562. (d) Fujii, A.; Hashiguchi, S.;
Uematsu, N.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1996, 118, 2521.
(e) Uematsu, N.; Fujii, A.; Hashiguchi, S.; Ikariya, T.; Noyori, R. J. Am.
Chem. Soc. 1996, 118, 4916.
(8) For pioneering work on use of transition metal complexes of
diamine for asymmetric hydrogenation, see: (a) Ito, M.; Hirakawa, M.;
Murata, K.; Ikariya, T. Organometallics 2001, 20, 379. (b) Ohkuma, T.;
Utsumi, N.; Tsutsumi, K.; Murata, K.; Sandoval, C.; Noyori, R. J. Am.
Chem. Soc. 2006, 128, 8724.
(9) (a) Zhou, H.; Li, Z.; Wang, Z.; Wang, T.;Xu, L.; He, Y.; Fan, Q.-H.;
Pan, J.; Gu, L.; Chan, A. S. C. Angew. Chem., Int. Ed. 2008, 47, 8464. (b) Li,
Z.-W.; Wang, T.-L.; He, Y.-M.; Wang, Z.-J.; Fan, Q.-H.; Pan, J.; Xu, L.-J.
Org. Lett. 2008, 10, 5265. (c) Wang, Z.-J.; Zhou, H.-F.; Wang, T.-L.; He,
Y.-M.; Fan, Q.-H. Green Chem. 2009, 11, 767. (d) Wang, T.; Zhuo, L.-G.;
Li, Z.; Chen, F.; Ding, Z.; He, Y.; Fan, Q.-H.; Xiang, J.; Yu, Z.-X.; Chan,
A. S. C. J. Am. Chem. Soc. 2011, 133, 9878.
(11) To the best of our knowledge, there are only three papers
reporting the enantioselective synthesis of chiral pyrrolidine derivatives
via asymmetric hydrogenation of the corresponding cyclic imines. For
details, see refs 6a, 6c, and 6d.
(12) For examples of enantioselective synthesis of chiral pyrrolidine
derivatives with other methods, see: (a) Hou, G.-H.; Xie, J.-H.; Yan,
P.-C.; Zhou, Q.-L. J. Am. Chem. Soc. 2009, 131, 1366. (b) Kuwano, R.;
Kashiwabara, M.; Ohsumi, M.; Kusano, H. J. Am. Chem. Soc. 2008,
130, 808. (c) Becker, R.; Brunner, H.; Mahboobi, S.; Wiegrebe, W.
Angew. Chem., Int. Ed. Engl. 1985, 24, 995. (d) Verdaguer, X.; Lange,
U. E. W.; Reding, M. T.; Buchwald, S. L. J. Am. Chem. Soc. 1996, 118,
6784. (e) Nishibayashi, Y.; Takei, I.; Uemura, S.; Hidai, M. Organome-
tallics 1998, 17, 3420.
(13) Williams, G. D.; Pike, R. A.; Wade, C. E.; Wills, M. Org. Lett.
2003, 5, 4227.
(14) (a) Heiden, Z. M.; Gorecki, B. J.; Rauchfuss, T. B. Organome-
tallics 2008, 27, 1542. (b) Hansen, K. B.; Rosner, T.; Kukryk, M.;
Dormer, R. G.; Armstrong, J. D. Org. Lett. 2005, 7, 4935.
(15) Smidt, S. P.; Zimmermann, N.; Studer, M.; Pfaltz, A. Chem.;
Eur. J. 2004, 10, 4685.
(10) Chen, F.; Wang., T.; He, Y.; Ding, Z.; Li, Z.; Xu, L.; Fan, Q.-H.
Chem.;Eur. J. 2011, 17, 1109.
Org. Lett., Vol. 13, No. 16, 2011
4349

ee) upon prolonged reaction time (Table 1, entry 15).
After a single recrystallization from petroleum ether, 2a
with >99% ee could be obtained. Very interestingly, full
conversion was also observed when the hydrogenation was
carried out at a substrate/catalyst ratio of 1000 under
solvent-free conditions, but obviously low enantioselectiv-
ity (78% ee) was obtained (Table 1, entry 16).
with a diphosphine-containing Ir-catalyst (Table 2, entry 15).6c
Notably, on increasing the ring size from 5-membered to 6-
and 7-membered cyclic imines, the enantiomeric excess
slightly dropped (Table 2, entries 16À17).
Table 2. Asymmetric Hydrogenation of 2-Substituted Cyclic
Imines Catalyzed by (R,R)-4fa
Table 1. Optimization of Reaction Conditionsa
yield
(%)b
ee
(%)c
entry
R
n
H2 (atm)/
conv
(%)b
ee
(%)c
entry
catalyst
solvent
DCM
temp (°C)
1
C6H5 (1a)
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
3
92
93
95
92
96
95
92
90
93
95
93
90
96
92
92
93
95
97
97
98
97
98
97
96
96
98
97
96
67
96
97
91e
95
92
2
4-Me-C6H4 (1b)
4-MeO-C6H4 (1c)
4-F-C6H4 (1d)
4-Cl-C6H4 (1e)
4-Br-C6H4 (1f)
3-Me-C6H4 (1g)
3-MeO-C6H4 (1h)
3-F-C6H4 (1i)
3-Cl-C6H4 (1j)
3,4-(MeO)2-C6H3 (1k)
2-MeO-C6H4 (1l)
2-naphthyl (1m)
2-thienyl (1n)
n-Bu (1o)
1
(R,R)-4a
(R,R)-4a
(R,R)-4b
(R,R)-4c
(R,R)-4d
(R,R)-4e
(R,R)-4f
(R,R)-4f
(R,R)-4f
(R,R)-4f
(R,R)-4f
(R,R)-4f
(R,R)-4f
(R,R)-4f
(R,R)-4f
(R,R)-4f
50; 40
50; 40
50; 40
50; 40
50; 40
50; 40
50; 40
50; 40
50; 40
50; 40
80; 40
10; 40
50; 60
50; 20
50; 40
50; 40
<5
Nd
3
2
DCM
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
>95
92
4
3
DCM
93
5
4
DCM
94
6
5
DCM
94
7
6
DCM
95
8
7
DCM
toluene
DCE
97
9
8
96
10
11
12d
13
14
15
16
17
9
95
10
11
12
13
14
15d
16f
MeOH
DCM
41
97
DCM
97
DCM
97
DCM
97
DCM
93 (>99e)
78
C6H5 (1p)
C6H5 (1q)
solvent-free
a Reaction conditions: substrate (1aÀo; 0.2 mmol) in dichloro-
methane (1 mL), (R,R)-4f (1.0 mol %), H2 (50 atm), (Boc)2O (1.1 equiv),
stirred at 40 °C for 10 h. b Isolated yield. c The enantiomeric excesses were
determined by HPLC with chiral AD-H or OJ-H column. d (R,R)-4f
(2.0 mol %), stirred at 60 °C for 16 h. e Determined by GC with a chiral
CP7502 column.
a Reaction conditions: 1a (0.2 mmol) in solvent (1 mL), Ru-catalyst
(1.0 mol %), 10 h. (Boc)2O (1.1 equiv) was added as additive except for
entry 1. b The conversions were determined by 1H NMR spectroscopy of
the crude reaction mixture. c The enantiomeric excesses were determined
by HPLC with a chiral AD-H column. d With 0.2 mol % catalyst (145 mg
1a in 1 mL DCM), 20 h. e Recrystallized from petroleum ether. f With
0.1 mol % catalyst (286 mg 1a), 20 h.
To further evaluate the synthetic potential of the cata-
lytic system, a scaled-up one-pot synthesis of chiral 2-phe-
nylpyrrolidine 2a via direct asymmetric reductive amina-
tion (DARA) was performed. DARA is believed to be the
most convenient, economic, and eco-benign method for
the synthesis of chiral amines. However, few labrotary
methods are thus far known for enatioselectivity reductive
amination catalyzed by homogeneous metal complexes.16
Although racemic products were obtained with Wills’
catalytic system under transfer hydrogenation conditions,
Under the optimized reaction conditions (Table 1, entry
7), a variety of cyclic N-alkyl imines were efficiently
hydrogenated in the presence of 1.0 mol % (R,R)-4f to
afford the corresponding chiral N-Boc-protected amines
with extraordinarily high enantioselectivities (92À98% ee,
Table 2). It was evident that the electronic properties of the
substituents at the para or meta position of the phenyl ring
had no apparent effect on activity and enantioselectivity
(Table 2, entries 1À11). But when the substituent is located
at the ortho position, both reactivity and enantioselectivity
dropped significantly (Table 2, entry 12). Excellent results
were also achieved with 2-naphthyl pyrroline 1m (Table 2,
entry 13). 2-Thienyl pyrroline 1n was converted to the
chiral product 2n with excellent enantioselectivity, leaving
the heteroaromatic ring intact (Table 2, entry 14). In addi-
tion, hydrogenation of n-Bu-substituted pyrroline 1o pro-
ceeded smoothly under identical conditions, providing the
chiral amine with unprecedentedly high enantioselectivity
(91% ee), which is obviously higher than that obtained
(16) For selected examples of DARA: (a) Blaser, H.-U.; Buser, H.-P.;
Jalett, H.-P.; Pugin, B.; Spindler, F. Synlett 1999, 867. (b) Kadyrov, R.;
Riermeier, T. H. Angew. Chem., Int. Ed. 2003, 42, 5472. (c) Chi, Y.;
Zhou, Y.-G.; Zhang, X. J. Org. Chem. 2003, 68, 4120. (d) Kadyrov, R.;
€
Riermeier, T. H.; Dingerdissen, U.; Tararov, V.; Borner, A. J. Org.
ꢁ
ꢁ
Chem. 2003, 68, 4067. (e) Rubio-Perez, L.; Perez-Flores, F. J.; Sharma,
P.; Velasco, L.; Cabrera, A. Org. Lett. 2009, 11, 265. (f) Li, C.; Villa-
Marcos, B.; Xiao, J. J. Am. Chem. Soc. 2009, 131, 6967. (g) Villa-
Marcos, B.; Li, C.; Mulholland, K. R.; Hogan, P. J.; Xiao, J. Molecules
2010, 15, 2453. (h) Bondarev, O.; Bruneau, C. Tetrahedron: Asymmetry
2010, 21, 1350.
4350
Org. Lett., Vol. 13, No. 16, 2011

we reasoned that effective asymmetric reductive amination
could be achieved under hydrogenation conditions due to
the excellent enantioselectivity obtained in the reduction of
isolated cyclic N-alkyl imines.17
neutralization with anhydrous NaHCO3. To this resulting
imine mixture, (R,R)-4f and (Boc)2O in DCM was added
under a nitrogen atmosphere in a glovebox (for details, see
Supporting Information). To our delight, hydrogenation
was performed smoothly under otherwise identical condi-
tions, giving 2-phenylpyrrolidine 2a in 94% isolated yield
and 96% ee, very similar to those obtained from the
hydrogenation of isolated imine 1a (Table 2, entry 1).
In summary, we have shown that the chiral cationic Ru-
MsDPEN complexes were highly efficient catalysts for the
asymmetric hydrogenation of a range of cyclic N-alkyl
imines. Excellent isolated yields and enantioselectivities
(up to 98% ee) were achieved. Moreover, a one-pot
synthesis of chiral 2-phenylpyrrolidine via reductive ami-
nation was developed. Investigation of the underlying
mechanistic aspects that account for the highly enantiose-
lective control is in progress.
Scheme 2. One-Pot Direct Asymmetric Reductive Amination of
5 Catalyzed by (R,R)-4f
We thus chose N-Boc-protected amino ketone 5 as the
model substrate for our study (Scheme 2). Following
Wills’s procedure,13 the formation of cyclic imine 1a via
a deprotection/cyclization sequence with formic acid was
Acknowledgment. Financial support from the National
Natural Science Foundation of China (Grant No. 20973178),
the National Basic Research Program of China (973 Pro-
gram, No. 2010CB833300), and the Chinese Academy of
Sciences is greatly acknowledged.
1
highly efficient (>95% yield, monitored by H NMR).
However, our initial attempt of further asymmetric hydro-
genation of the resulting crude imine 1a failed. In view of
the negative effect of a large excess of acid on the
catalysis,18 we then removed the excess formic acid by
evaporation under reduced pressure and further complete
Supporting Information Available. Experimental pro-
cedures, characterization data for all compounds, de-
scriptions of stereochemical assignments, and copies of
1H and 13C NMR spectra for all compounds reported in
the text. This material is available free of charge via the
Internet at http://pubs.acs.org.
(17) Racemic product was also obtained with Wills’ catalytic system
under transfer hydrogenation conditions when the isolated imine was
used; see ref 13.
(18) Sandoval, C. A.; Ohkuma, T.; Utsumi, N.; Tsutsumi, K.;
Murata, K.; Noyori, R. Chem.;Asian J. 2006, 1, 102.
Org. Lett., Vol. 13, No. 16, 2011
4351