Carboxy-directed asymmetric hydrogenation of 1,1-diarylethenes and 1,1-dialkylethenes

Article abstract of DOI:10.1002/anie.201208606

Carboxy marks the spot: A carboxy-directed asymmetric hydrogenation of 1,1-diarylethenes and 1,1-dialkylethenes with chiral iridium/spiro phosphine-oxazoline catalysts has been developed. A wide range of chiral diarylethanes and chiral γ-methyl fatty acids were synthesized with excellent enantioselectivity (see scheme). Copyright

Full text of DOI:10.1002/anie.201208606

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DOI: 10.1002/anie.201208606
Directed Asymmetric Hydrogenation
Carboxy-Directed Asymmetric Hydrogenation of 1,1-Diarylethenes
and 1,1-Dialkylethenes**
Song Song, Shou-Fei Zhu, Yan-Bo Yu, and Qi-Lin Zhou*
Owing to high atom economy, chemoselectivity, and stereo-
selectivity, the transition-metal-catalyzed asymmetric hydro-
=
genation of C C bonds has become one of the most reliable
methods for the preparation of optically active compounds
both in academia and in industry.[1] In the asymmetric
hydrogenation of a double bond, chiral induction arises
from catalyst differentiation of the prochiral faces of the
bond. The Re and Si faces are much easier to differentiate if
the substituents are very different in size, as is the case for
1-aryl-1-alkyl ethenes, and high enantioselectivities can be
achieved.[2] In sharp contrast, differentiation of the Re- and Si-
faces is difficult if the substituents are very similar in size, as is
the case for 1,1-diarylethenes and 1,1-dialkylethenes, and
enantioselectivities with such substrates are low. In the
hydrogenation of 1,1-diarylethenes, high enantioselectivities
have been achieved only when one of the aryl groups has an
ortho-substituent.[2f,g,3] In the asymmetric hydrogenation of
1,1-dialkylethenes, the highest enantioselectivity reported to
date is only 41% ee.[4] Therefore, it is highly desirable to
develop a new strategy to realize the asymmetric hydro-
genation of 1,1-diarylethenes and 1,1-dialkylethenes, because
the products of this reaction, diaryl and dialkylethanes, are
very useful in the synthesis of biologically active compounds.
Recently, we developed highly efficient chiral iridium
catalysts bearing spiro phosphine–oxazoline ligands for the
asymmetric hydrogenation of a,b-unsaturated[5] and b,g-
unsaturated carboxylic acids.[6] The carboxy group of the
unsaturated acids anchors the catalyst and directs the hydro-
genation of the double bond.[5a] This finding encouraged us to
study the carboxy group as a directing group[7] for the
asymmetric hydrogenation of 1,1-diarylethenes and 1,1-dia-
lkylethenes. Herein, we report the highly enantioselective
iridium-catalyzed hydrogenation of 1,1-diarylethenes 1[8] and
1,1-dialkylethenes 4 directed by a carboxy group. When used
in combination with a convenient decarboxylation, the
hydrogenation of 1 provides an effective method for the
construction of chiral diarylethanes 3, which are core
structures for many biologically active compounds, such as
sleep-inducing H1-antihistamines,[9] the antiparasitic agent
demiditraz,[10] and anticancer agents.[11] The hydrogenation of
Scheme 1. Carboxy-directed asymmetric hydrogenation of 1,1-diaryl-
ethenes and 1,1-dialkylethenes catalyzed by chiral iridium/spiro phos-
phine–oxazoline complexes. BArFÀ =tetrakis[3,5-bis(trifluoromethyl)-
phenyl]borate.
4 provides an efficient method for preparing chiral g-methyl
fatty acids 5, which are important intermediates for natural
products such as the pheromone of rhinoceros beetles
(Scheme 1).[12]
Substrates 1 and 4 were easily prepared by Wittig
olefination of the corresponding ketones (for details, see the
Supporting Information). First, we chose 2-(1-phenylvinyl)-
benzoic acid (1a) as a model substrate to evaluate various
chiral iridium/spiro phosphine–oxazoline catalysts 6[13] for the
carboxy-directed asymmetric hydrogenation (Scheme 1). Cat-
alyst (Sa)-6a, which has phenyl groups on the phosphorous
atom and no substituent on the oxazoline ring, showed 100%
conversion and afforded 2a with 99% ee within 5 h (Table 1,
entry 1). The substituents on the aryl groups of the phospho-
rous atom affected the reaction rate and enantioselectivity
only slightly (entries 2 and 3 vs. entry 1). The use of catalyst
(Sa)-6b, which has 3,5-dimethylphenyl groups on the phos-
phorous atom, decreased the reaction time from 5 to 4 h, but
the enantioselectivity remained high (99% ee; entry 2).
However, catalyst (Sa)-6c, which has bulkier 3,5-di-tert-
butylphenyl groups on the phosphorous atom, required
[*] Dr. S. Song, Prof. S.-F. Zhu, Y.-B. Yu, Prof. Q.-L. Zhou
State Key Laboratory and Institute of Elemento-organic Chemistry
Nankai University, Tianjin 300071 (China)
E-mail: qlzhou@nankai.edu.cn
[**] We thank the National Natural Science Foundation of China, the
National Basic Research Program of China (2011CB808600), and
the Ministry of Education of China (B06005) for financial support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201208606.
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Table 1: Comparison of catalysts for the carboxy-directed asymmetric
Table 2: Substrate scope of the carboxy-directed asymmetric hydro-
genation of 1,1-diarylethenes.[a]
hydrogenation of 1,1-diarylethenes.[a]
Ar1; R (1)
Product (2)
Yield [%]
ee [%]
Entry
Catalyst
t [h]
Conv. [%][b]
ee [%][c]
Entry
1
2
3
4
5
6
(Sa)-6a
(Sa)-6b
(Sa)-6c
(Sa,S)-6d
(Sa,R)-6d
(Sa,S)-6e
(Sa)-6b
(Sa)-6b
(Sa)-6b
(Sa)-6b
(Sa)-6b
(Sa)-6b
5
4
10
4
20
20
20
4
100
100
100
100
71
99
99
97
95
91
51
–
98
96
–
1
2
3
4
5
6
7
C6H5; H (1a)
2a
2b
2c
2d
2e
2 f
2g
2h
2i
2j
2k
2l
2m
2n
2o
2p
98
97
96
98
97
97
97
99
99
97
98
96
99
97
97
97
98 (R)
98 (R)
98
98 (R)
97
4-MeC6H4; H (1b)
4-MeOC6H4; H (1c)
4-ClC6H4; H (1d)
3,4-Me2C6H3; H (1e)
3-BrC6H4; H (1 f)
2,5-Me2C6H3; H (1g)
2-naphthyl; H (1h)
2-thiophenyl; H (1i)
2-furanyl; H (1j)
Ph; 4-Br (1k)
Ph; 4-Me (1l)
Ph; 5-Br (1m)
Ph; 4,5-Cl2 (1n)
Ph; 6-Me (1o)
Ph; 4,5-(CH)4 (1p)
55
28
97
7[d]
8[e]
9[f]
10[g]
11[h]
12[i]
99.6
98 (R)
97
99
96
98
96
97
99.3
98 (R)
8[b]
9[c]
10[c]
11
12
13
14
15[d]
16
100
100
<5
100
100
6
20
6
98
98
20
[a] Reaction conditions: 0.5 mmol scale, [substrate]=0.25 molLÀ1, sub-
strate/catalyst=200:1, H2 (6 atm), Et3N (1.0 equiv) as an additive.
[b] Determined by 1H NMR spectroscopy. [c] Determined by HPLC
analysis on a chiral stationary phase. [d] At ambient hydrogen pressure.
[e] H2 (50 atm). [f] Na2CO3 (1.0 equiv) was used as base. [g] Without
base. [h] substrate/catalyst=400:1, 608C. [i] Substrate/cata-
lyst=1000:1, 708C.
[a] Reaction conditions: 0.5 mmol scale, [substrate]=0.25 molLÀ1, sub-
strate/catalyst=400:1, 608C, H2 (6 atm), Et3N (1.0 equiv) as an additive.
Full conversion was obtained within 12 h. [b] (Sa)-6b (0.5 mol%) was
used. [c] (Sa)-6b (1 mol%) was used. [d] (Sa)-6c (1 mol%) was used.
a longer reaction time (10 h) and gave slightly lower
enantioselectivity (97% ee; entry 3). We also studied the
effect of the substituent on the oxazoline ring of the catalyst
on the enantioselectivity of the reaction. Introduction of
a benzyl group at the 4-position, (Sa,S)-6d, resulted in slightly
lower enantioselectivity (95% ee; entry 4). When catalyst
(Sa,R)-6d, a diastereoisomer of (Sa,S)-6d, was used, both the
conversion and the enantioselectivity were reduced (to 71%
and 91% ee, respectively; entry 5). This result demonstrates
that the chirality in catalyst (Sa,S)-6d was matched for
achieving high enantioselectivity in the hydrogenation of
1,1-diarylethenes. However, replacement of the benzyl group
on the oxazoline ring with a phenyl group, (Sa,S)-6e,
dramatically diminished the rate, conversion, and enantiose-
lectivity of the reaction (entry 6). Experiments on hydrogen
pressure showed that the reaction gave significantly lower
conversion under ambient hydrogen pressure (entry 7). The
catalyst has the same activity and slightly lower enantiose-
lectivity under 50 atm of hydrogen (entry 8). The choice of the
base also played an important role in the reaction. Et3N
provided the best results. An inorganic base, Na2CO3, gave
1,1-diarylethenes 1 had a negligible influence on the yield and
enantioselectivity of the reaction: all of the tested substrates
afforded essentially the same yields (96–99%) and enantio-
selectivities (96–99.6% ee; entries 1–16). The highest enan-
tioselectivity was obtained in the hydrogenation of 1g (99.6%
ee; entry 7).
Because the carboxy group could be easily removed by
decarboxylation, it could serve as a traceless directing group.
The combination of copper-promoted decarboxylation and
carboxy-directed asymmetric hydrogenation of 1,1-diaryl-
ethenes 1 allowed us to synthesize chiral diarylethanes 3 in
one pot (Table 3). The methanol solvent was evaporated after
the hydrogenation, and the residue was heated with copper
Table 3: One-pot preparation of chiral diarylethanes 3.[a]
a
slightly reduced reaction rate and enantioselectivity
(entry 9), whereas the conversion was less than 5% in the
absence of base (entry 10). Catalyst (Sa)-6b was active enough
that the reaction could be performed at a catalyst loading of
0.25 mol% at 608C (entry 11) or 0.1 mol% at 708C without
any reduction in the enantioselectivity (entry 12).
Under the optimized reaction conditions, various 1,1-
diarylethenes 1 bearing an ortho-carboxy group were hydro-
genated (Table 2). The carboxy-directed hydrogenation
showed a wide substrate scope and a strong tolerance for
various functional groups, including heterocycles such as
furans and thiophenes. The substituents on the aryl rings of
Entry
Ar1; R (1)
Product (3)
Yield [%]
ee [%][b]
1
2
3
4
5
4-MeC6H4; H (1b)
4-ClC6H4; H (1d)
2-naphthyl; H (1h)
Ph; 4,5-(CH)4 (1p)
2-thiophenyl; H (1i)
3b
3d
3h
3h
3i
80
81
83
76
77
98 (S)
98 (S)
98 (S)
98 (R)
97[c]
[a] The hydrogenation reactions were followed by copper-powder-pro-
moted decarboxylation in a one-pot procedure (see the Supporting
Information for details). [b] Determined by HPLC analysis on a chiral
stationary phase, or supercritical fluid chromatography. [c] The ee shown
is that of 2i.
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powder in quinoline at 2208C for 3 h to produce 3 in good
yields (76–83%) with excellent enantioselectivities (97–98%
ee). It is remarkable that chiral diarylethanes with opposite
configurations, for example, (S)-3h and (R)-3h, could be
obtained from different substrates by means of this one-pot
procedure (entries 3 and 4). Because chiral diarylethanes 3
are generally prepared by means of multistep transforma-
tions,[14] the present one-pot procedure provides an efficient
method for the synthesis of these bioactive compounds.
The carboxy-directing strategy can also be utilized in the
asymmetric hydrogenation of 1,1-dialkylethenes 4, which are
even more difficult substrates and for which there are no
highly enantioselective hydrogenation methods reported. In
the hydrogenation of 1,1’-dialkylethenes, the iridium complex
(Sa,S)-6 f, which bears an iso-butyl group on the oxazoline ring
and 3,5-di-tert-butylphenyl groups on the phosphorous atom,
was the most effective catalyst (for a comparison of catalysts,
see the Supporting Information). By using catalyst (Sa,S)-6 f
(1.5 mol% in MeOH under 6 atm of H2 at 458C), various 1,1-
dialkylethenes 4 were hydrogenated to the corresponding
chiral g-methyl fatty acids 5 in high yields (91–97%) and high
enantioselectivities (89–99% ee; Table 4). This asymmetric
Scheme 2. Transformations of hydrogenation products. a) Trifluoroace-
tic anhydride, CH2Cl2, 08C. b) N,O-dimethylhydroxylamine hydrochlo-
ride, (3-dimethylaminopropyl)ethyl-carbodiimide hydrochloride,
1-hydroxybenzotriazole, iPr2NEt, CH2Cl2, 08C. c) MeMgBr, THF, 08C.
d) LiAlH4, THF, 08C. e) Dess–Martin periodinane, CH2Cl2, 08C;
f) EtMgBr, THF, 08C.
We performed additional experiments to investigate the
role of the carboxy group in the reactions. When 1,1-
diphenylethene 12 and esters 13 and 14 were subjected to
the standard hydrogenation conditions, no reaction was
observed (Scheme 3), thus showing that the carboxy group
Table 4: Asymmetric hydrogenation of 1,1-dialkylethenes 4.[a]
Entry
R (4)
Product
Yield [%]
ee [%][b]
1
2
3
4
Et (4a)
nBu (4b)
nC6H13 (4c)
nC7H15 (4d)
iPr (4e)
iBu (4 f)
PhCH2CH2 (4g)
MeO2CCH2CH2 (4h)
5a
5b
5c
5d
5e
5 f
97
96
95
96
95
94
97
91
97 (R)
96 (R)
95 (R)
96 (R)
99
89
94
97
5
6[c]
7
Scheme 3. Substrate studies.
5g
5h
8
[a] Reaction conditions: 0.5 mmol scale, [substrate]=0.25 molLÀ1
,
(Sa,S)-6 f (1.5 mol%) as catalyst, H2 (6 atm), NEt3 (1.0 equiv) as an
additive. Full conversions were obtained in all cases. [b] Determined by
HPLC analysis of the corresponding anilide on a chiral stationary phase.
[c] Using (Sa,S)-6 f (2 mol%) as catalyst.
is indispensable. Moreover, the presence of a base was
necessary for the hydrogenation of 1a (Table 1, entry 10). The
substrates meta-carboxy diphenylethene and para-carboxy
diphenylethene could not be hydrogenated under the stan-
dard hydrogenation conditions. These experiments indicate
that, upon interaction with a base, the carboxy group of the
substrates acts as an anchor, coordinating with the iridium of
the catalyst and endowing the catalyst with the ability to
discriminate between the prochiral faces of the substrates and
to catalyze their hydrogenation with high enantioselectivity.
In conclusion, we have developed a new strategy to realize
the asymmetric hydrogenation of 1,1-diarylethenes and 1,1-
dialkylethenes by using carboxy as a directing group. Under
mild reaction conditions, a wide range of 1,1-diarylethenes
and 1,1-dialkylethenes were hydrogenated to chiral 1,1-
diarylethanes and chiral g-methyl fatty acids with high
enantioselectivities by chiral iridium/spiro phosphine–oxazo-
line catalysts. This carboxy-directing strategy should be
applicable for the asymmetric hydrogenation of other sub-
strate types.
hydrogenation of 1,1-dialkylethenes 4 provides a new and
short route to optically active g-methyl chiral fatty acids.[12]
Product 5b, (R)-4-methyloctanoic acid, is a pheromone for
rhinoceros beetles.[12a]
The carboxy group could also be conveniently converted
to other functional groups (Scheme 2). For example, hydro-
genation product 2b was transformed into chiral anthrone 7 in
81% yield with 96% ee by means of a Friedel–Crafts
acylation. Furthermore, chiral diarylethanes bearing an
ortho-acetyl group (8) or an ortho-formyl group (9) were
obtained from 2a by means of simple transformations, with
the complete preservation of enantiomeric purity. (R)-6-
Methyl-3-octanone (11), a component of the alarm phero-
mone of Grematogaster ants,[15] was synthesized in 59%
overall yield starting from acid 5a.
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General hydrogenation procedure:
A hydrogenation tube was
charged with a stir bar, 1,1-diaryethene 1 (0.5 mmol), and catalyst
(Sa)-6b (2.2 mg, 0.00125 mmol) in an argon-filled glove box. MeOH
(2 mL) and Et3N (51 mg, 0.5 mmol) were injected into the hydro-
genation tube with a syringe while stirring. The hydrogenation tube
was placed in an autoclave, which was purged five times with
hydrogen gas. The autoclave was then charged with hydrogen gas to
6 atm, and the reaction mixture was stirred at 608C for 12 h before
releasing the hydrogen. The reaction mixture was treated with 5% aq.
NaOH (10 mL). The aqueous layer was separated and washed with
Et2O (10 mL), acidified with HCl (3m, pH 1), and extracted with
Et2O (3 ꢀ 10 mL). The combined extracts were washed with a satu-
rated solution of NaCl, dried over MgSO4, and evaporated in vacuo to
give the hydrogenation product. The ee values of the products were
determined by means of supercritical fluid chromatography or HPLC
analysis on a chiral stationary phase.
[7] Recently, a carboxy group was successfully used as a directing
À
group in transition-metal-catalyzed C H functionalization reac-
tions. For examples, see: a) K. M. Engle, T.-S. Mei, M. Wasa, J.-
Q. Yu, Acc. Chem. Res. 2012, 45, 788 – 802, and references
therein; b) J. Cornella, M. Righi, I. Larrosa, Angew. Chem. 2011,
123, 9601 – 9604; Angew. Chem. Int. Ed. 2011, 50, 9429 – 9432;
c) M. A. Bigi, S. A. Reed, M. C. White, Nat. Chem. 2011, 3, 216 –
222.
Received: October 26, 2012
[8] For highly enantioselective rhodium-catalyzed hydrogenation of
1,1’-diarylethenes directed by a hydroxy group, see Ref. [3].
[9] a) W. J. Moree, B.-F. Li, F. Jovic, T. Coon, J. Yu, R. S. Gross, F.
Tucci, D. Marinkovic, S. Zamani-Kord, S. Malany, M. J. Brad-
bury, L. M. Hernandez, Z. OꢅBrien, J. Wen, H. Wang, S. R. J.
Hoare, R. E. Petroski, A. Sacaan, A. Madan, P. D. Crowe, G.
Beaton, J. Med. Chem. 2009, 52, 5307 – 5310; b) G. Beaton, W. J.
Moree, F. Jovic, T. Coon, J. Yu, US Patent 20060014797, 2006.
[10] N. A. L. Chubb, M. R. Cox, J. S. Dauvergne, R. A. Ewin, C.
Lauret, US Patent 20070167506, 2007.
Published online: December 17, 2012
Keywords: asymmetric hydrogenation · chiral spiro ligands ·
.
diarylethanes · directing groups · iridium
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