Enantioselective synthesis of macrocyclic heterobiaryl derivatives of molecular asymmetry by molybdenum-catalyzed asymmetric ring-closing metathesis

Article abstract of DOI:10.1002/anie.201500459

Winding vine-shaped molecular asymmetry is induced by enantioselective ring-closing metathesis with a chiral molybdenum catalyst. The reaction proceeds under mild conditions through an E-selective ring-closing metathesis leading to macrocyclic bisazoles w

Full text of DOI:10.1002/anie.201500459

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201500459
German Edition: DOI: 10.1002/ange.201500459
Asymmetric Catalysis
Enantioselective Synthesis of Macrocyclic Heterobiaryl Derivatives of
Molecular Asymmetry by Molybdenum-Catalyzed Asymmetric Ring-
Closing Metathesis**
Yoichi Okayama, Satoru Tsuji, Yuka Toyomori, Atsunori Mori,* Sachie Arae, Wei-Yi Wu,
Tamotsu Takahashi, and Masamichi Ogasawara*
Abstract: Winding vine-shaped molecular asymmetry is
induced by enantioselective ring-closing metathesis with
a chiral molybdenum catalyst. The reaction proceeds under
mild conditions through an E-selective ring-closing metathesis
leading to macrocyclic bisazoles with enantioselectivities of up
to 96% ee.
C
hiral molecules devoid of stereogenic centers have
attracted considerable attention in organic chemistry. These
stereochemical issues, often referred to as molecular asym-
metry or non-centrochirality, include axial-, planar-, or helical
[1]
chirality, etc. Among them, biarylic axial chirality has been
widely utilized in various asymmetric reactions as chiral
ligands in metal catalysts or as key substructures in organo-
Scheme 1. Synthesis of macrocyclic bisbenzimidazole 1a by ring-clos-
ing metathesis reaction of acyclic 2a.
[
2]
[3]
catalysts showing excellent enantioselection abilities. In
general, axial chirality in biaryls is induced by the introduc-
tion of sterically demanding substituents at the positions
proximal to the carbon–carbon bond between the two aryl
moieties, which inhibit free rotation about the single bond. If
one could freeze the free rotation of the carbon–carbon single
bond by macrocyclic ring formation, analogous axial chirality
is induced in a biaryl molecule. We recently reported the
synthesis and characterization of bisbenzimidazole derivative
be chiral even without a stereogenic carbon atom as shown in
[5]
Scheme 1. The E-alkenylene moiety bridging the two
benzimidazoles inhibits the free rotation about the carbon–
carbon bond in 1a, and, indeed, two enantiomers of the
molecule can be separated by chiral HPLC. Compound 1a,
whose skewed shape resembles a vine winding around
a branch, shows unique molecular asymmetry, and this
unprecedented chirality can be explained in several other
ways in addition to the biarylic axial chirality: 1) helical
chirality of the winding alkenylene chain along the rigid axis
1
a. Compound 1a was prepared by the ruthenium-catalyzed
[
4]
ring-closing metathesis (RCM) of acyclic precursor 2a,
which possesses two 3-butenyl substituents at both sp -
3
[
6]
nitrogen atoms. The RCM reaction took place predominantly
in the E-selective fashion, and RCM product 1a was found to
of the bisimidazole, 2) planar chirality of the trans-cyclo-
[7]
alkene (trans-5,8-diazacyclodecene) whose conformational
freedom is constrained by the fused imidazoles, or 3) central
[8]
chirality based on the distorted stereogenic nitrogen atoms
[
*] Y. Okayama, S. Tsuji, Y. Toyomori, Prof. Dr. A. Mori
Department of Chemical Science and Engineering, Kobe University
whose inversion is retarded by placement at the bridgeheads.
In this report, we focus our attention on the catalytic
enantioselective synthesis of this peculiar chiral molecule,
which would be plausible if an appropriate chiral metal–
1
-1 Rokkodai, Nada, Kobe 657-8501 (Japan)
E-mail: amori@kobe-u.ac.jp
Dr. S. Arae, Dr. W.-Y. Wu, Prof. Dr. T. Takahashi,
Prof. Dr. M. Ogasawara
[9]
alkylidene complex is employed as a metathesis catalyst.
Although an asymmetric metathesis reaction itself does not
form a stereogenic carbon atom directly, it has been
demonstrated to control peripheral chirality by kinetic
resolution of racemic chiral molecules or desymmetrization
of prochiral substrates by the use of a chiral metathesis
Catalysis Research Center and Graduate School of Life Science
Hokkaido University
Kita21, Nishi10, Kita-ku, Sapporo 001-0021 (Japan)
E-mail: ogasawar@cat.hokudai.ac.jp
[
**] This work was supported by the Cooperative Research Program
from Catalysis Research Center, Hokkaido University (Grant no.
catalyst. However, less studies have shown how to control
1
3A1001) and a Grant-in-aid for Scientific Research by MEXT
[10]
noncarbon centrostereogenicities
and non-centrochiral-
(
Japan) (Challenging Exploratory Research no. 24655083). We thank
[
11–13]
ity.
Although we have shown how to successfully control
Prof. Makoto Fujita of the University of Tokyo for fruitful discussions
on the absolute configuration of compound 1a. We also thank Kei
Miyamura for the morning glory illustration.
[
12]
the planar chirality in ferrocenes as well as in (p-arene)-
[13]
chromium complexes by asymmetric metathesis reactions,
we were challenged to apply the metathesis protocol to the
enantioselective synthesis of the winding-vine-shaped com-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201500459.
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pounds. Herein, we report the results of our studies for
preparing bisimidazole derivatives 1 in high enantiomeric
purity by the molybdenum-catalyzed asymmetric ring-closing
metathesis (ARCM).
chiral catalysts were generated in situ by mixing molybde-
num–pyrrolide C and an appropriate chiral bishydroxy
[17]
ligand. The structures of the chiral ligands used are listed
in Figure 1.
At the outset, we examined the catalytic activity of achiral
molybdenum–alkylidene complex B toward the ring-closing
metathesis reaction of bisbenzimidazole 2a and the reaction
was shown to proceed smoothly at room temperature afford-
ing the metathesis product 1a in 89% yield (Scheme 1). The
result markedly contrasts with our previous results, in which
much higher temperatures 80–1408C were required to
promote the reaction in the presence of the ruthenium-
based Grubbs II catalyst (A). It was also observed that no
Lewis acid additive was required for the molybdenum-
catalyzed reaction, whereas the related metathesis reaction
with Grubbs II was accelerated by addition of ZnCl or
Ti(OR)4.
Figure 1. Chiral binaphthol/biphenol ligands for Mo-alkylidene com-
plexes.
2
[5a,14]
The obtained racemic macrocyclic bisazole 1a was
subjected to preparative HPLC with a chiral stationary
phase column (Daicel Chiralpak-IF, 20 mm i.d. ” 200 mmL)
using hexane/EtOH (50:50) as an eluent to achieve the
baseline separation of its two enantiomers. The separated
The ARCM reaction of 2a was conducted with the
molybdenum catalyst (10 mol%) in benzene and the results
are summarized in Table 1. The use of substituted biphenol
derivatives (R)-L1-3 bearing tert-butyl groups adjacent to the
hydroxy groups resulted in giving (+)-1a in 68–85% ee with
reasonably high conversions after stirring for 12 h at 408C
(entries 1–3), whereas the reaction with (R)-L4, which con-
tains less bulky diphenylmethyl groups, brought about
inferior enantioselectivity (37% ee; entry 4). The molybde-
num catalysts generated from ligands (R)-L5–(R)-10, which
are composed of 3,3’-diarylbinaphthyl structures, were found
to be less reactive. Little or no reactions took place when L5
and L6 were employed at 408C (entries 5 and 6). Elevated
enantiomers exhibited [a] values of + 1368 (the fast-eluting
D
enantiomer: (+)-1a) and [a] = À1458 (the slow-eluting
D
enantiomer: (À)-1a), respectively. The racemization behavior
of the resolved enantiomer was studied at various temper-
atures. It was shown that significant racemization hardly
occurred in a 1,2-dichloroethane solution of (+)- or (À)-1a at
and below 808C. Whereas less than 5% diminution of the
enantiomeric purity in 1a was detected at 808C for 24 h,
nearly complete racemization was observed at 1408C in
chlorobenzene within 3 h. The Gibbs free energy for the
racemization in 1a at 1008C was thus calculated to be
°
À1
DG (1008C) = 130 [kJmol ] based on the treatment of
(
[
15,16]
+)-1a in chlorobenzene for 24 h (98% ee!45% ee).
Table 1: Enantioselective ring-closing metathesis of 2a with various
[a]
Interestingly, the racemization process was greatly acceler-
ated in the presence of a catalytic amount of the Grubbs II
complex. For example, a mixture of (+)-1a (> 99% ee) and
Grubbs II (10 mol%) in 1,2-dichloroethane was stirred at
chiral Mo catalysts.
8
08C, and 1a racemized completely within 24 h. The ruthe-
nium-catalyzed racemization is proposed to proceed through
a ring-opening/recyclization sequence as shown in Scheme 2.
Although molybdenum–alkylidene complex B showed the
better catalytic activity than ruthenium complex A in the
RCM reaction of 1a, the enantioselective synthesis of 1a was
examined using chiral molybdenum–alkylidene species. The
[b]
[c]
Entry
Ligand
T [8C]
Conv. [%]
ee [%]
1
2
3
4
5
6
7
8
9
(R)-L1
(R)-L2
(R)-L3
(R)-L4
(R)-L5
(R)-L6
(R)-L7
(R)-L8
(R)-L9
(R)-L1
(R)-L2
(R)-L3
40
40
40
40
40
40
60
40
40
23
23
23
86
88
96
44
7
NR
61
37
6
(P)-85
(P)-68
(P)-76
(P)-37
<1
–
<1
(P)-27
–
(P)-88
(P)-73
(P)-96
1
1
1
0
1
2
90
80
86
[a] The reaction of 2a (0.1 mmol) was carried out for 12 h in anhydrous
benzene (2 mL) in the presence of 10 mol% molybdenum precursor C
and 10 mol% chiral ligand. [b] The conversion was determined by
1
H NMR analysis of the crude mixture. [c] The enantiomeric excess was
determined by HPLC using a chiral stationary phase column (Daicel
Chiralpak IC).
Scheme 2. Proposed pathway for Ru-catalyzed racemization of 1a.
4
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reaction temperature at 608C resulted in virtually no selec-
tivity with L7 (entry 7). The use of L8 bearing a 1-naphthyl
substituent in the reaction led to moderate conversion and
Table 2: Mo-catalyzed asymmetric ring-closing metathesis of bisimida-
zole derivatives 2b–2d.
[
a]
2
7% ee (entry 8), whereas the ligand with the 9-anthryl group
(
L9) hardly effected the reaction at 408C even at a longer
reaction time of 48 h (entry 9). The highest enantioselectivity
was finally achieved at lower temperature (238C) with L3 to
result in 96% ee (entry 12).
[b]
[c]
The absolute configuration of the major enantiomer in the
Entry
Substrate
Ligand
T [8C]
Conv. [%]
ee [%]
[18]
ARCM products was determined to be (P) when the (R)-
1
2
3
4
5
6
7
8
9
2b
L1
L3
L4
L1
L2
L3
L8
L1
L1
L2
L2
L3
L3
60
60
60
60
60
60
60
60
40
60
40
60
40
89
55
19
4
6
30
0
0
[
19]
isomer of the chiral ligands were employed. The stereo-
chemistry of the preferential formation of (P)-(+)-1a can be
rationalized as in Scheme 3. The initial metathesis occurs
[b]
2c
2d
95
96
95
11
95
95
96
94
86
90
2
22
86
90
75
90
92
94
1
1
1
1
0
1
2
3
[a] The reaction of 2b–d (0.1 mmol) was carried out for 12 h in
anhydrous benzene (2 mL) in the presence of 10 mol% molybdenum
Scheme 3. Plausible stereochemical pathway of the Mo-catalyzed
ARCM of 2a.
precursor C and 10 mol% chiral ligand. [b] The conversion was
determined by H NMR analysis of the crude mixture. [c] Enantiomeric
excess was determined by HPLC using a chiral stationary phase column
1
(Daicel Chiralpak IC).
between the Mo-catalyst and 2a to form bisimidazole-bound
Mo–alkylidene intermediate D, in which the nonmetalated
imidazole moiety takes the opposite position with respect to
the Ar-imido ligand to avoid steric congestion. Subsequently,
the second alkenyl pendant approaches the molybdenum
center from the more open side (the side opposite to the
marked tBu group) to furnish (P)-1a predominantly.
In addition to bisbenzimidazole 1a, several other bisimi-
dazole derivatives 1b–d, whose metathesis precursors 2b–d
were synthesized in a similar manner to the preparation of
[
5a,20]
2
a,
were also subjected to the asymmetric metathesis
reaction (Table 2). Although 4,4’,5,5’-unsubstituted bisimida-
zole 2b also underwent the reaction to afford the ring-closed
product 1b, very low enantioselectivity was observed (4–6%
ee) with the molybdenum catalysts coordinated with L1 or L3
(
entries 1 and 2). The selectivity was slightly improved at
a low conversion when L4 was employed as a chiral ligand,
however, the selectivity was still insufficient (30% ee;
entry 3). Introduction of bromo substituents in the 5,5’-
positions of the bisimidazole (the reaction with 2c) showed
little effect in improving the enantioselectivity using L1–L3,
although the reaction proceeded smoothly to deliver 1c. The
use of binaphthyl-derived ligand L8 resulted in 22% ee, while
the conversion values were much inferior. It should be
pointed out that excellent selectivity was achieved in the
reaction of tetrabrominated derivative 2d. The reaction with
chiral ligands L1–L3 was carried out at 408C to 608C for 12 h.
It is remarkable to observe excellent enantioselectivities of up
to 94% ee (entry 13).
Scheme 4. Transformation reactions of (+)-1d.
various functional compounds by standard organic reactions
with retention of the helical structure (Scheme 4). For
example, 1d was subjected to the Suzuki–Miyaura cou-
[21]
pling. Treatment of (+)-1d (> 99% ee) with p-tolylboronic
acid 3x in the presence of a palladium catalyst (15 mol%)
smoothly afforded the corresponding tetra(p-tolyl)bisimida-
zole (+)-4x (66%). The mass spectrum (ESI +) showed the
[
16]
formation of 4x as a sole product and the enantiopurity of
(+)-4x was found to be 85% ee. Likewise, the palladium-
catalyzed reaction of (+)-1d with p-methoxycarbonylphenyl-
boronic acid (3y) gave the corresponding tetraarylbisimida-
zole (+)-4y (90% ee in 69% yield).
The obtained RCM product 1d possesses modifiable
substructures, such as carbon–bromine bonds and an olefinic
double bond. Thus, 1d is potentially transformable into
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[
22]
epoxide (À)-5 in 99% ee and 85% yield (Scheme 4).
[
The winding-vine-shaped molecular asymmetry is not
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Keywords: asymmetric catalysis · bisimidazoles ·
molybdenum catalyst · molecular asymmetry ·
ring-closing metathesis
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4
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Published online: February 23, 2015
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
4931