Rhodium-catalyzed highly enantioselective addition of arylboronic acids to 2-nitrostyrenes by tert-butanesulfinylphosphine ligand

Article abstract of DOI:10.1002/chem.201100135

Particularly picky ligands! An efficient (S/C, substrate/catalyst, up to 500) Rh-catalyzed enantioselective addition of arylboronic acids to 2-nitrostyrenes under mild conditions was developed, which gave excellent yields (up to 99 %) and enantioselectivi

Full text of DOI:10.1002/chem.201100135

DOI: 10.1002/chem.201100135
Rhodium-Catalyzed Highly Enantioselective Addition of Arylboronic Acids
to 2-Nitrostyrenes by tert-Butanesulfinylphosphine Ligand
Feng Lang,[a, b] Guihua Chen,[a, b] Liangchun Li,[a, b] Junwei Xing,[a, b] Fuzhong Han,[a, b]
Linfeng Cun,[a, b] and Jian Liao*[a]
Chiral 2,2-bisarylethylamines constitute a broad class of
framework in biologically important targets.[1] Typical exam-
ples include potential protein kinaseB (PKB) inhibitor 1,[1a]
dihydrexidine 2[1b] (a full dopamine D1 receptor agonist),
and cherylline 3[1d] (an alkaloid isolated from amaryllidaceae
plants, which exhibits central nervous activity). The most
However, the addition of arylboronic reagents to arylnitroal-
kenes is a challenge in the field of asymmetric catalysis and
development of effective catalysts for this transformation is
desirable.[3b,6] Very recently, significant progress was ach-
ieved by Lin and co-workers, who utilized a C2-symmetric
chiral diene as a ligand and potassium trifluoroborates, as
organoboron reagents (generated in situ), to achieve good
to excellent enantioselectivies.[7] In this context, we report a
highly efficient transformation in the Rh/sulfoxide–phos-
phine-catalyzed enantioselective addition of arylboronic
acids to 2-nitrostyrenes under mild conditions with excellent
enantioselectivity. Based on this transformation, the first
asymmetric synthesis of cherylline was reported.[8]
Recent studies in our laboratory have focused on transi-
tion-metal-catalyzed asymmetric reactions by using our con-
cise chiral sulfoxide–phosphine ligands L1–L4[9] and bis-sulf-
oxide ligands L5,[10] which utilize the tert-butylsulfinyl
moiety as a ligating entity and a chiral source.[11] These li-
gands showed excellent results in the Rh-catalyzed 1,4-addi-
tion of arylboronic acids to enones. We expect that these
chiral sulfoxide ligands could also generate exciting results
in the addition of arylboronic acids to 2-nitrostyrenes.
We initially conducted the model reaction of p-CH3O-ni-
trostyrene 4a with PhB(OH)2 5m in the presence of Rh/
sulfoxide–phosphines L1–L4, or Rh/bis-sulfoxide L5
(4 mol%) and aqueous KOH in dichloromethane at 408C.
direct and attractive method to obtain chiral 2,2-bisaryle-
thylamines utilizes the asymmetric 1,4-addition of aryl re-
agents to 2-nitrostyrenes as the key step, followed by a
simple reduction. Although the organocatalytic 1,4-type
Friedel–Crafts reaction demonstrated good enantioselectivi-
ties for this transformation, specific substrates are re-
quired.[2] Among asymmetric transition-metal-catalyzed re-
actions, the Rh-catalyzed 1,4-addition of arylboronic acids
to electron-deficient olefins is one of the most important
[3–4]
À
tools for asymmetric C C bond formation.
In 2000, Hay-
ashi reported the first example of a Rh–BINAP-complex-
catalyzed asymmetric addition of arylboronic acids to ali-
phatic nitroalkenes and impressive results were achieved.[5]
[a] F. Lang,+ G. Chen,+ Dr. L. Li, J. Xing, F. Han, L. Cun,
Prof. Dr. J. Liao
Natural Products Research Centre
Chengdu Institute of Biology
Chinese Academy of Sciences
Chengdu, 610041 (P.R. China)
Fax : (+86)28-8522-9250
To our delight, with the exception of L1, ligands L2–L4 can
promote the reaction to generate the desired product 6am
with moderate yields (75–78%) and good enantioselectivi-
ties within 5 h (79–84% enantiomeric excess (ee)), Table 1,
entries 1–4). Unfortunately, the C2-symmetric chiral bis-sulf-
oxide L5 did not work well in this reaction, although it is an
excellent ligand in the addition of arylboronic reagents to
enones.[10] Similarly, the classical Rh–BINAP system also
gave a poor result (Table 1, entries 5 and 6). Next, the reac-
E-mail: jliao@cib.ac.cn
[b] F. Lang,+ G. Chen,+ Dr. L. Li, J. Xing, F. Han, L. Cun
Chengdu Institute of Organic Chemistry
Chinese Academy of Sciences, Chengdu 610041
China; Graduate School of Chinese Academy of Sciences
Beijing, 100049 (P.R. China)
[+] These two authors contributed equally to this work.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201100135.
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Chem. Eur. J. 2011, 17, 5242 – 5245

COMMUNICATION
Table 1. Rh-catalyzed asymmetric 1,4-addition of phenylboronic acid to
4-MeO-2-nitrostyrene.[a]
corresponding products with excellent yields and ee values
(Table 2, entries 17 and 18 and 21–24).
DFT calculations[13] using the BH and H/6-31G(d)
method[14] have been applied to understand why this catalyst
system can induce high enantioselectivity. The geometries
and the relative energies of initial coordination of the olefin
onto the L2–Rh–phenyl species are shown in Figure 1, and
Entry
Ligand
Solvent
Base
t
Yield[b]
[%]
ee[c]
[%]
[h]
1
2
3
4
L1
L2
L3
L4
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
dioxane
THF
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KF
5
5
5
5
5
8
5
5
5
n.r.[d]
78
75
78
14
40
trace
17
75
82
70
50
75
62
56
99
n.d.[e]
84
79
82
25
5
L5
6[f]
7
(R)-BINAP
10
L2
L2
L2
L2
L2
L2
L2
L2
L2
L2
n.d.[e]
91
8
9
dioxane
Et2O
82
90
91
92
95
95
94
95(R)
10
11
12
13
14
15
16
toluene
EtOAc
MeOH
EtOH
EtOH
EtOH
EtOH
5
5
10
10
10
10
10
K2CO3
Et3N
[a] Reaction conditions: 4a (0.25 mmol), 5m (0.5 mmol), [RhACTHNUGTRNE(UNG C2H4)2Cl]2
(2.0 mg, 0.005 mmol), ligand (0.012 mmol), base (inorganic base
(50 mol%) in 0.1 mL water), solvent (1.0 mL), and 408C. [b] Yield of iso-
lated product. [c] Determined by chiral HPLC analysis. [d] No reaction.
[e] Not determined. [f] [RhACHTNUTRGEN(UNG C2H4)2Cl]2 (1.5 mol%), (R)-BINAP
(3.0 mol%), KOH (30 mol%), 1008C.
tion conditions were investigated intensely. The solvent
showed some effect on yield and ee value; careful screening
indicated that ethanol was the best solvent for enantioselec-
tivity, however a prolonged reaction time was needed to fa-
cilitate the conversion (95% ee, 10 h, Table 1, entry 13). No-
tably, replacing the inorganic base with an organic base
(triethylamine) dramatically promoted the conversion to
obtain a quantitative yield (Table 1, entry 16).
Figure 1. The geometries and the relative energies of initial coordination
of the olefin onto the L2–Rh-phenyl species optimized with BH and H/6-
31G(d) (all hydrogen atoms were merged for clarity): a) and c) the struc-
ture leading to the R product; b) and d) the structure leading to the S
product.
Under the optimal reaction conditions, the scope and limi-
tation of the Rh–L2 complex in the addition of arylboronic
acids to 2-nitrostyrenes was explored and the results are
summarized in Table 2. Most of the 2-nitrostyrenes and aryl-
boronic acids reacted smoothly under mild conditions to
give the corresponding products in high yields (up to 99%)
and excellent enantioselectivities (up to 98%). The steric
hindrance of the substituent on the aromatic ring (for exam-
ple, in 2-nitrostyrenes and arylboronic acids) had a signifi-
cant effect on the enantioselectivity, but little effect on the
activity (Table 2, entries 2–6). Furthermore, electron-with-
drawing substituents had a slightly negative impact on the
enantioselectivity (86–90% ee, Table 2, entries 7 and 8). As
expected, electron-donating substituents are good for this
transformation. Furthermore, for electron-poor nitrostyr-
enes, the Rh–L2 system can also generate good results, for
example, p-F-2-nitrostyrene (Table 2, entry 14, 88% yield,
86% ee) and p-Cl-2-nitrostyrene (Table 2, entry 15, 81%
yield, 82% ee).[12] In particular, substrates with a hydroxyl or
a dimethylamino group on the aromatic ring afforded the
the initial coordination of the nitroalkene sitting trans to the
phosphine group (Figure 1a and b) are favored by at least
6.0 kcalmolÀ1 compared with the structures of the nitroal-
kene sitting cis to the phosphine group (Figure 1c and d).
Further calculations showed that the insertion of the nitroal-
À
kene into the Rh Ph bond determined the enantioselectivi-
ty. The computed geometries and the energy barriers of
transition states (TS) for the model reaction are given in
Figure 2. Calculations showed that the TS leading to the S
product is about 5.0 kcalmolÀ1 higher in energy than that
leading to the R product, which is in agreement with the ex-
perimentally observed preferential formation of the R prod-
uct with high enantioselectivity, in which complex Rh-L2
was used as catalyst.
The synthetic usage of our methodology is illustrated by
the synthesis of cherylline, as shown in Scheme 1. The addi-
tion was carried out with 2-nitrostyrene 4j (1 mmol scale) in
the presence of the Rh–L2 catalyst to produce adduct 6jz in
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5243

J. Liao et al.
Table 2. Scope of the Rh-catalyzed asymmetric addition of arylboronic acids to 2-nitrostyrenes.[a]
Significantly,
our
Rh–L2
system demonstrated remark-
able catalytic activity.[17] The
model reaction can be scaled
up to tenfold with only
Entry
4
Ar1
5
Ar2
Yield[b]
[%]
ee[c]
[%]
0.2 mol%
catalyst
loading,
without affecting the yield or ee
value (Scheme 2).
1
2
3
4
5
6
7
8
4a
4b
4c
4a
4a
4a
4a
4a
4a
4a
4a
4a
4a
4d
4e
4 f
4g
4h
4i
4-MeOC6H4
3-MeOC6H4
2-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-FC6H4
5m
5m
5m
5n
5o
5p
5q
5r
5s
5t
5u
5v
5w
5m
5m
5m
5m
5m
5m
5x
5x
5x
5y
5y
C6H5
C6H5
C6H5
2-MeC6H4
1-naph
99
90
87
81
96
99
99
99
99
99
99
99
99
88
81
99
93
99
99
99
92
95
99
99
99
95(R)
87
81
58
72
95
86
90
97
96
95
97
96
86
82
94
97
97
91
96(S)
98
98
98
97
98
In summary, we have devel-
oped an efficient (S/C, sub-
strate/catalyst, up to 500) Rh-
catalyzed addition of arylboron-
ic acids to 2-nitrostyrenes under
mild conditions, which gives ex-
cellent yields (up to 99%) and
2-naph
4-ClC6H4
4-FC6H4
4-MeC6H4
3-MeC6H4
3-MeOC6H4
4-tBuC6H4
3,5-diMeC6H3
C6H5
C6H5
C6H5
C6H5
C6H5
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
enantioselectivities
(up
to
98%) by using tert-butanesulfi-
nylphosphine as a ligand. This
process provides a promising
approach for the synthesis of
4-ClC6H4
3,4-diMeOC6H3
4-Me2NC6H4
4-HOC6H4
4-MeC6H4
C6H5
4-HOC6H4
4-Me2NC6H4
4-HOC6H4
4-HO-3-MeOC6H3
4-BnO-3-MeOC6H3
chiral
2,2-bisarylethylamines
with excellent yields and ee
values. In addition, DFT calcu-
lations were used to rationalize
the high enantioselectivity. This
method has been successfully
applied to the synthesis of (R)-
cherylline in excellent overall
yield (70% for four steps).
C6H5
4j
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
4-MOMOC6H4
4-MOMOC6H4
4-BnOC6H4
4h
4g
4h
4k
4l
5z
[a] Reaction conditions: 4 (0.25 mmol), 5 (0.5 mmol), [RhACHTNUGTRNE(UNG C2H4)2Cl]2 (2.0 mg, 0.005 mmol), L2 (0.012 mmol),
Et3N (50 mol%), EtOH (1.0 mL), 408C, and 10 h. [b] Yield of isolated product. [c] Determined by chiral
HPLC analysis. The absolute configuration was determined by comparison with literature data, see ref. [7].
Scheme 1. Synthesis of (R)-cherylline.
Figure 2. The geometries and energy barriers (DE) of the transition states
(TS) optimized with BH and H/6-31G(d) (all hydrogen atoms were
merged for clarity): a) the TS leading to the R product; b) the TS leading
to the S product.
quantitative yield and with 98% ee. After reduction of the
nitro moiety (with NaBH4/NiCl2·6H2O[15]) and cyclization,[16]
the key intermediate 7 was obtained in 73% yield and
>99% ee. After treating 7 with hydrochloride in ethanol,
optically pure (R)-cherylline was obtained in excellent yield
(70% overall yield). This is the first asymmetric synthesis of
cherylline based on asymmetric catalysis.
Scheme 2. Catalyst-loading test.
Experimental Section
[RhACTHNUTRGNEUGN(C2H4)2Cl]2 (2.0 mg, 0.005 mmol) and tert-butanesulfinylphosphine L2
(5.2 mg, 0.012 mmol) were added to a 10 mL Schlenk tube under an
argon atmosphere at room temperature, followed by addition of dichloro-
methane (1.0 mL) and the mixture was stirred at room temperature for
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Enantioselective Addition of Arylboronic Acids to 2-Nitrostyrenes
COMMUNICATION
30 min. Then, the dichloromethane was removed under reduced pressure
and 2-nitrostyrene 4 (0.25 mmol) and arylboronic acid 5 (0.5 mmol) were
added. After purging with argon, ethanol (1.0 mL) and TEA (18 mL,
0.125 mmol) were added successively. The mixture was stirred at 408C
for 10 h, then the solvents were removed in vacuo and the residue was
purified by flash chromatography on silica gel (with petroleum ether/
ethyl acetate, 20:1 as eluent) to afford the desired product 6; the enantio-
meric excess was determined by chiral HPLC analysis.
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Acknowledgements
We thank the National Natural Science foundation of China (No.
20872139 and 21072186), the West-Light foundation of Chinese Academy
of Sciences, Chengdu Institute of Biology of Chinese Academy of Scien-
ces (Y0B1051100) and the Major State Basic Research Development
Program (973 program, 2010CB833300) for their generous financial sup-
port. The support of the Supercomputing Center, CNIC, CAS, for com-
puter time is also acknowledged.
Keywords: asymmetric
synthesis
·
cherylline
·
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Received: January 14, 2011
Published online: April 6, 2011
Chem. Eur. J. 2011, 17, 5242 – 5245
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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