Highly enantioselective kinetic resolution of axially chiral BINAM derivatives catalyzed by a Bronsted acid

Article abstract of DOI:10.1002/anie.201310562

A highly efficient strategy for the kinetic resolution of axially chiral BINAM derivatives involving a chiral Bronsted acid-catalyzed imine formation and transfer hydrogenation cascade process was developed. The kinetic resolution provides a convenient route to chiral BINAM derivatives in high yields with excellent enantioselectivities. Chiral BINAMs on demand: A highly efficient strategy for the kinetic resolution of axially chiral BINAM derivatives involving a chiral Bronsted acid catalyzed imine formation and transfer hydrogenation cascade process was developed. The kinetic resolution provides a convenient route to chiral BINAM derivatives in high yields with excellent enantioselectivities.

Full text of DOI:10.1002/anie.201310562

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Angewandte
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DOI: 10.1002/anie.201310562
Kinetic Resolution
Highly Enantioselective Kinetic Resolution of Axially Chiral BINAM
Derivatives Catalyzed by a Brønsted Acid**
Dao-Juan Cheng, Liang Yan, Shi-Kai Tian,* Ming-Yue Wu, Lu-Xin Wang, Zi-Li Fan, Sheng-
Cai Zheng, Xin-Yuan Liu,* and Bin Tan*
Abstract: A highly efficient strategy for the kinetic resolution
of axially chiral BINAM derivatives involving a chiral
Brønsted acid-catalyzed imine formation and transfer hydro-
genation cascade process was developed. The kinetic resolution
provides a convenient route to chiral BINAM derivatives in
high yields with excellent enantioselectivities.
of bispirooxindole derivatives with three quaternary chiral
centers with excellent stereocontrol by employing a multi-
functional organocatalyst containing an axially chiral BINAM
skeleton. In this context, a more reliable and straightforward
approach to optically pure BINAM derivatives would greatly
widen the application scope of this particular class of axially
chiral catalyst in asymmetric catalysis.
T
he axially chiral BINAP, BINOL, and their derivatives are
There have only been a few synthetic attempts toward
enantiopure BINAM derivatives, including a direct catalytic
enantioselective oxidative homocoupling of 2-naphthyla-
mines with modest results[6] and [3,3]-sigmatropic rearrange-
ment promoted by a camphor sulfonic acid with poor
enantioselectivity.[7] Quite recently, with Brønsted acids as
efficient organocatalysts, the [3,3]-sigmatropic rearrangement
strategy was further improved by the groups of List[8a] and
Kꢀrti,[8b] respectively (Scheme 1a). Over the past several
decades, the kinetic resolution[9] of racemic starting materials
has been one of the most powerful and reliable strategies for
the synthesis of enantiopure compounds in both academia
and industry, which would serve as a valuable complementary
approach to the traditional synthetic methods, such as
asymmetric synthesis and classic resolution. However, exam-
ples of catalytic kinetic resolution of axially chiral BINAM
derivatives, to the best of our knowledge, have not been
reported. Although during the preparation of this manuscript,
an efficient method for the kinetic resolution of 2-amino-1,1’-
biaryl compounds by phase-transfer-catalyzed N-allylation
was reported by Maruoka and co-workers,[10] only one
BINAM derivative was utilized as the substrate with fairly
good results. Therefore, it still remains a challenging under-
taking for the catalytic kinetic resolution of these axially
chiral compounds.
among the most successful chiral ligands/catalysts in asym-
metric catalysis, with extensive applications in various cata-
lytic enantioselective transformations.[1] As a result, the
construction of these scaffolds has attracted considerable
attention and the relatively practical methods have already
been achieved.[2] In contrast, 1,1’-binaphthyl-2,2’-diamine
(BINAM), with a similar axially chiral skeleton, remains
largely unexplored with respect to the application in asym-
metric synthesis, which is probably due to the rather
expensive production cost resulting from the lack of reliable
synthetic routes.[3] Noteworthy is that several well-established
BINAM-based compounds[4] have recently been developed as
effective chiral organocatalysts (Figure 1).[5] For example, the
Barbas research group[4a] has discovered a highly efficient
organocatalytic domino approach for the direct construction
Figure 1. Examples of BINAM-based organocatalysts.
[*] Dr. D.-J. Cheng, M.-Y. Wu, L.-X. Wang, Z.-L. Fan, Dr. S.-C. Zheng,
Prof. Dr. X.-Y. Liu, Prof. Dr. B. Tan
Department of Chemistry, South University of Science and
Technology of China, Shenzhen, 518055 (China)
E-mail: liuxy3@sustc.edu.cn
tanb@sustc.edu.cn
L. Yan, Prof. Dr. S.-K. Tian
Department of Chemistry, University of Science and Technology of
China, Hefei, Anhui, 230026 (China)
E-mail: tiansk@ustc.edu.cn
[**] Financial support from the National Natural Science Foundation of
China (Nos. 21232007, 21302088, 21302087), the National Basic
Research Program of China (973 Program 2010CB833300), Shenz-
hen special funds for the development of biomedicine, Internet, new
energy, and new material industries (JCYJ20130401144532131,
JCYJ20130401144532137), and South University of Science and
Technology of China is greatly appreciated.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201310562.
Scheme 1. General strategies for creating axially chiral BINAM deriva-
tives.
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Table 1: Screening results of reaction conditions.[a]
Recently, organocatalytic asymmetric transfer hydroge-
nation with organic hydride donors[11] has emerged as
a powerful method for the construction of diverse cyclic and
acyclic amines by reduction of imines in the presence of
Brønsted acids. Based on the “three-point contact model” for
asymmetric transfer hydrogenation reported recently by
Goodman and co-workers,[12] we envisioned that an organo-
catalytic kinetic resolution through a phosphoric acid cata-
lyzed[13] imine formation and transfer hydrogenation cascade
process could potentially be a powerful and general approach
toward chiral BINAM derivatives. This strategy consists of
consecutive asymmetric kinetic resolution process by means
of chiral phosphoric acid catalysis, in which installing a bulky
group in one of amines of racemic BINAM was presumed to
increase the rotation energy barrier and effective steric
interaction for improved enantioselectivity (Scheme 1b).
Herein, we present the details of our studies with this strategy.
To validate the feasibility of the proposed process, we
selected N-[2’-amino(1,1’-binaphthalen)-2-yl]naphthalene-2-
sulfonamide rac-1a with a bulky naphthalenesulfonamide
group as the model substrate for the optimization of reaction
conditions. We initiated our studies by evaluating the reaction
between rac-1a and 0.6 equiv of 2-naphthaldehyde 2a using
10 mol% of chiral phosphoric acid 4a as the Brønsted acid
catalyst and diethyl 2,6-dimethyl-1,4-dihydropyridine-3,5-
dicarboxylate (HEH1) as the hydride source in ethyl acetate
(EA) at room temperature for 48 h. To our delight, the
reaction proceeded smoothly and afforded the desired
product 3a in 43% yield with 93% ee, albeit with only
moderate ee (53%) of the recovered starting material (R)-
1a[14] (Table 1, entry 1). After calculation, a selectivity fac-
tor[9a] (S = kfast/kslow) of 47 was obtained (entry 1). Encouraged
by this, a detailed optimization study was first done with
different phosphoric acid catalysts. Several BINOL, H8-
BINOL, SPINOL, and VAPOL-derived catalysts were inves-
tigated, which displayed remarkable effects on the outcome of
the reaction. It should be noted that the yield and ee of
product 3a was always excellent with different BINOL-
derived catalysts (Table 1, entries 2–6), even in the presence
of the catalyst with phenyl group on the 3,3’-positions
(Table 1, entry 3), while ee of the recovered 1a varied. It is
interesting to find that the kinetic resolution reaction did not
proceed with VAPOL 4i as catalyst, which is most likely due
to the steric hindrance with the bulky starting material
(Table 1, entry 9). Considering catalyst 4e with the best
results (S = 354) for both the transfer hydrogenation product
3a (50% yield, 98% ee) and the recovered starting material
(R)-1a (41% yield, 94% ee) (Table 1, entry 5), this catalyst
was selected for further optimization. Predictably, the solvent
played an important role in this transformation, especially as
to the enantioselectivity of the recovered starting material,
and EA was proved to be the best choice (Table 1, entries 10–
14). Inspired by a recent review on Hantzsch esters as the
hydride sources for transfer hydrogenations,[16] we screened
different Hantzsch esters (Table 1, entries 15–17) and found
that HEH3 gave rise to a better result (S = 303) with 98% ee
of the recovered 1a and 97% ee of the product (Table 1,
entry 16). Finally, we investigated the effects of the reaction
Entry cat. HEH Solvent
3a
1a
S[d]
yield
[%][b]
ee
yield
ee
[%][c] [%][b]
[%][c]
1
2
3
4
5
6
7
8
4a
4b
4c
4d
4e
4 f
4g
4h
4i
4e
4e
4e
4e
4e
4e
4e
4e
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
3
4
3
3
3
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc NR
DCE 27
toluene 40
MTBE 36
43
48
42
52
50
44
42
46
93
94
98
95
98
50
46
49
37
41
46
50
45
53
73
47
71
87 283
48 63
94 354
83 259
98
À97
À74 146
98
88 290
9
ND NR
ND
24
73
75
38
–
8
42
48
28
10
11
12
13
14
15
16
17
73
90
91
90
98
98
97
99
98
98
97
69
39
44
50
49
47
44
43
46
44
46
CH3CN 25
iPrOAc 41
86 276
80 244
98 303
97 844
92 328
64 192
99 327
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
EtOAc
48
47
45
50
46
47
18[e] 4e
19[f] 4e
20[g] 4e
[a] Reaction conditions: rac-1a (0.050 mmol), Hantzsch ester
(0.70 equiv), catalyst 4 (10 mol%), EtOAc (0.6 mL). [b] Determined by
chiral stationary phase HPLC analysis. [c] Yield of isolated product.
[d] The selectivity factor was calculated as S=ln[(1ÀC)(1Àee(1a))]/
ln[(1ÀC)(1+ee(1a))], C=ee(1a)/(ee(3a)+ee(1a)). [e] 10 mg of 3 ꢀ MS
was added. [f] The reaction was run at 508C. [g] The reaction was
performed at 108C for 72 h.
temperature and 3 ꢁ molecular sieves as the additive without
significant improvement (Table 1, entries 18–20).
With the optimal reaction conditions in hand, we set out to
explore the substrate generality of this procedure. As shown
in Table 2, the functional groups on aldehydes, such as 1-
naphthyl, 2- naphthyl, 6-OMe-2-naphthyl, and 4-chlorobenzyl
groups, were well-tolerated, giving the corresponding prod-
ucts 3a-3d in good to excellent enantioselectivities with good
yields (S = 15–303, Table 2, entries 1–4). Notably, the 2-
naphthaldehyde exhibited the best performance in terms of
stereoselectivity and yield for both of the product 3a and
recovered starting material 1a, indicating an aromatic stack-
ing interaction during the transfer hydrogenation step.
Based on this observation, we next examined other bulky
protecting groups for this process. This reaction shows
excellent compatibility with different types of protecting
groups, such as sulfonyl, benzoyl, 2-naphthylmethyl, Fmoc,
amido and thioamido groups (S = 7-340, Table 2, entries 5–
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Table 2: Reaction scope for the kinetic resolution of BINAM deriva-
(Scheme 2), which further demonstrated that aromatic stack-
ing interaction may play an important role in achieving good
stereocontrol.
tives.[a]
Scheme 2. Kinetic resolution of H8-BINAM derivative.
To demonstrate the utility of the current procedure, the
bulky protecting groups of the resulting optically pure
transfer hydrogenation product (S)-3a and recovered starting
material (R)-1a[17] could be readily removed to produce (S)-
BINAM and (R)-BINAM with moderate to high yields, and
the enantiopurity can be completely retained (Scheme 3),
which has been suggested to have wide applications as the
effective asymmetric organocatalysis.[4,5] The optically pure
product (up to 100% ee) can be obtained after a single
recrystallization process of the enantiomerically enriched (S)-
BINAM from toluene.[8b]
Entry rac-1
2
3
1
S[d]
Yield
[%][b]
ee
Yield
ee
[%][c] [%][b]
[%][c]
1
2
3
rac-1a
rac-1a
rac-1a
rac-1a
rac-1b 2a 52 (3e)
rac-1c 2a 47 (3 f)
2a 47 (3a)
97
44 (1a)
98
80
90
94
303
246
21
15
31
2b 43 (3b) 98 47 (1a)
2c 51 (3c) 75
2d 58 (3d) 64
43 (1a)
40 (1a)
41 (1b) 94
46 (1c) 93
43 (1d) 96
4[e]
5
80
98
6
7
340
94
Scheme 3. Access to enantiopure BINAM.
rac-1d 2a 49 (3g) 92
8
9
rac-1e
rac-1 f
rac-1 f
rac-1g 2d 59 (3k)
rac-1h 2d 58 (3l)
rac-1i
rac-1j
rac-1k
rac-1l
2a 44 (3h) 88
45 (1e)
41 (1 f)
38 (1 f)
35 (1g) 92
40 (1h) 93
42 (1i)
43 (1j)
45 (1k)
47 (1l)
43 (1m) 96
40 (1n) 98
99
82
97
82
23
17
7
10
8
38
307
194
108
11
Furthermore, the unsatisfactory results obtained in the
kinetic resolution of simple non-protected BINAM under the
standard conditions demonstrated that the bulky protecting
group is necessary for good interaction between the catalyst
and the substrate. In this case, the isolation of product 3d with
93% ee further indicated the importance of the presence of
a bulky group (Scheme 4). To better understand this cascade
kinetic resolution process, we also tested the reductive step
with the use of sodium borohydride (NaBH4) as hydrogen
source. The poor enantioselectivity of the product might not
only suggest that the first imine formation step was not the
determining process of the kinetic resolution, but also
indicate that the cooperation of Hantzsch ester and phos-
phoric acid for the transfer hydrogenation should be crucial
for the kinetic resolution process, which is in accordance with
previous reports.[15]
2a 55 (3i)
2d 59 (3j)
80
62
40
52
10[e]
11[e]
12[e]
13
14
15
16
17
18
2a 55 (3m) 44
2a 50 (3n) 85
2a 46 (3o) 98
2a 45 (3p) 96
92
90
90
96
rac-1m 2a 47 (3q) 93
rac-1n 2a 55 (3r) 47
[a] Reaction conditions: rac-1 (0.050 mmol), 2 (0.030 mmol), Hantzsch
ester (0.035 mmol), catalyst 4e (10 mol%), EtOAc (0.6 mL), RT. [b] Yield
of isolated product. [c] Determined by chiral stationary phase HPLC
analysis. [d] The selectivity factor was calculated as S=ln[(1ÀC)-
(1Àee(1))]/ln[(1ÀC)(1+ee(1))], C=ee(1)/(ee(3)+ee(1)). [e] HEH1 was
used instead of HEH3.
13). Furthermore, investigations into the effect of substituents
on the BINAM skeleton of the substrates revealed that the
reaction was also efficient for substrates with functional
groups (R3 = Br, Cl, TMS) at the 6-position or 6,6’-positions
(R2 = R3 = Br or TMS) with good yields and good to excellent
enantioselectivity (S = 11–307, Table 2, entries 14–18). The
resultant halogen-containing (Br, Cl) products can be easily
transformed into other substituted compounds by cross-
coupling reactions,[16] providing opportunities for the efficient
construction of more complicated BINAM derivatives. To
further probe the substrate scope, under the similar reaction
conditions described above, the current reaction system was
extended to rac-H8-BINAM 1o with moderate results
In conclusion, we have developed a highly efficient and
practical strategy for the kinetic resolution of axially chiral
Scheme 4. Control experiments by using simple reagents.
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BINAM derivatives in high yields with excellent enantiose-
lectivities, involving a chiral Brønsted acid-catalyzed imine
formation and transfer hydrogenation cascade process. This
method features a general strategy by employing the well-
known reactions for the kinetic resolution of structurally
useful compounds with a high level of enantioselectivity.
Further applications of this strategy toward the kinetic
resolution of similar axially chiral compounds are currently
under active investigation.
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Keywords: axial chirality · BINAM · Brønsted acids ·
.
enantioselectivity · kinetic resolution
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