Searching for New Polymorphs by Epitaxial Growth

Article abstract of DOI:10.1021/acscatal.0c03689

The formation of unknown polymorphs due to the crystallization at a substrate surface is frequently observed. This phenomenon is much less studied for epitaxially grown molecular crystals since the unambiguous proof of a new polymorph is a challenging tas

Full text of DOI:10.1021/acscatal.0c03689

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Conformational Control in Dirhodium(II) Paddlewheel Catalysts
Supported by Chalcogen-Bonding Interactions for Stereoselective
Intramolecular C−H Insertion Reactions
Takuya Murai, Wenjie Lu, Toshifumi Kuribayashi, Kazuhiro Morisaki, Yoshihiro Ueda, Shohei Hamada,
Yusuke Kobayashi, Takahiro Sasamori, Norihiro Tokitoh, Takeo Kawabata, and Takumi Furuta*
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ABSTRACT: D2-symmetric dirhodium(II) carboxylate catalysts
that bear axially chiral binaphthothiophene δ-amino acid
derivatives have been developed. Conformational control is
supported through chalcogen-bonding interactions between sulfur
and oxygen atoms in each ligand, providing well-defined and
uniform asymmetric environments around the catalytically active
Rh(II) centers. These structural properties make such complexes
asymmetric catalysts for the stereoselective intramolecular C−H
insertion into α-aryl-α-diazoacetates to yield a variety of cis-α,β-
diaryl γ-lactones, as well as the corresponding trans-isomers
through epimerization, in high diastereo- and enantioselectivities. Short total syntheses of the naturally occurring γ-lactones,
cinnamomumolide, cinncassin A7, and cinnamomulactone were also accomplished using this conformationally controlled catalyst.
KEYWORDS: paddlewheel complex, chalcogen bond, intramolecular C−H insertion, axially chiral amino acid, γ-Lactone
arrangements relative to the plane of the rhodium carboxylate
moieties, respectively, can be formed (Figure 1B).2 The
presence of uniform chiral environments around the Rh1 and
Rh2 centers is particularly important in C2 (up-up-down-
down)- and D2 (up-down-up-down)-symmetric catalysts, as
inequivalent chiral environments will produce products with
nonuniform optical purity. Therefore, an important issue in the
development of potential dirhodium(II) carboxylate catalysts is
establishing control over the conformational flexibility of the
carboxylate ligands around the C−C single bond.
We envisioned that the introduction of an interaction that
fixes one conformation of the rotatable carboxylate C−C bond
could provide a dirhodium(II) complex with relatively rigid
and uniform chiral environments around the Rh1 and Rh2
centers.
INTRODUCTION
■
Chiral dirhodium(II) paddlewheel complexes are highly
valuable catalysts for stereoselective transformations including
insertion reactions in X−H bonds (X = C, N, O, etc.) (Figure
1A) as well as the stereoselective cyclopropanation of
unsaturated C−C bonds through the decomposition of diazo
compounds.1 Some of the most frequently employed chiral
dirhodium(II) paddlewheel catalysts are dirhodium(II)
carboxylate complexes that bear chiral carboxylate groups as
paddlewheel ligands.
The construction of a defined asymmetric environment
around the catalytically active center is crucial for the
development of potential asymmetric catalysts. However, the
construction of defined asymmetric environments can be very
difficult, especially in the case of catalysts with rotatable bonds
near the catalytically active center. Dirhodium(II) carboxylate
complexes represent typical examples of catalysts that suffer
from undesirable conformational flexibility around the
rotatable C−C bond of the carboxylate groups tethered to
Rh(II) (Figure 1B). Their conformational flexibility often
complicates the construction of defined chiral environments
around the upper (Rh1) and lower (Rh2) rhodium atoms,
both of which act as catalytically active centers; however, it also
enables the construction of diverse and interchangeable
structures with different molecular symmetries. Specifically,
C1-, C2-, C4-, and D2-symmetric structures with up-up-up-
down, up-up-down-down, all-up, and up-down-up-down ligand
We have previously reported the synthesis of axially chiral
binaphthyl δ-amino acid (S)-1 and its N-protected derivatives
(Figure 1C),3 as well as their application as chiral carboxylate
ligands in dirhodium(II) carboxylate catalyst 2a (Figure 2B).4
Later, we extended these studies to prepare the binaphtho-
Received: August 24, 2020
Revised: December 9, 2020
Published: December 28, 2020
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Figure 2. Stereoselective intramolecular C−H insertion reactions of
α-aryl α-diazoesters catalyzed by dirhodium(II) carboxylate com-
plexes.
into the corresponding amide derivative 5.6 Single-crystal X-ray
diffraction analyses of (S)-4 and 5 revealed chalcogen bonds
(S···O interactions) between the oxygen atom of the carbonyl
group and the sulfur atom of the naphthothiophene ring, which
resulted in a coplanar conformation.
Chalcogen bonding has been recognized as a noncovalent
interaction that plays a role in determining the conformation of
pharmaceuticals and organic materials.7 Recently, this
interaction has also received attention as a driving force in
organocatalysis8 as well as an attractive interaction in
supramolecular assembly.9 We envisaged that the chalcogen-
bonding interactions found in (S)-4 and 5 could be used to
lock the conformation of the carboxylate C−C bonds to fix the
stereostructure of dirhodium(II) complexes (Figure 1D(a)).10
Furthermore, this conformational restriction would result in a
tilt of the other aromatic ring system of the biaryl ligand
toward the inside of the complex to give a narrow and well-
Figure 1. (A) C−X bond insertion reaction catalyzed by a
dirhodium(II) carboxylate complex. (B) Conformational flexibility
and ligand arrangement in dirhodium(II) carboxylate complexes. (C)
Axially chiral binaphthyl and binaphthothiophene δ-amino acids and
their chalcogen-bond-containing derivatives. (D) Schematic illustra-
tion of the concept for the design of the catalyst used in this study.
thiophene δ-amino acid (S)-3, which contains sulfur atoms in
its fused π-system, from binaphthothiophene dicarboxylic acid
(S)-4 (Figure 1C).5,6 Furthermore, racemic 3 was transformed
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defined chiral space near the catalytically active Rh centers
(Figure 1D(b)). Such chiral environments can be expected to
result in good asymmetric induction.
Scheme 1. Preparation of Biaryl-Type Dirhodium(II)
Carboxylate Complexes 2b, 10a, and (S)-10b
Asymmetric intramolecular C−H insertion of α-diazoace-
tates has been investigated as a powerful synthetic tool for
constructing a variety of chiral cyclic compounds. Although a
variety of stereoselective intramolecular C−H insertions have
been reported,11 some of the reactions are still retained to
achieve high stereoselectivity. The stereoselective intramolec-
ular C−H insertion of α-aryl-α-diazoesters into α-aryl-β-
substituted lactones is one of the examples of the reactions that
are still difficult to access.12,13 As an exceptional example, the
highly enantio- and diastereoselective C−H insertion reaction
of 6 to give α,β-diaryl β-lactone 7 has been reported by Davies
and co-workers; however, a trifluoromethyl or halogen group
at the ortho-position of the α-aryl group was required to ensure
satisfactory levels of stereoselectivity (Figure 2A).13 We have
also examined the stereoselective C−H insertion reaction of 3-
butenyl α-aryl-α-diazoacetates 8 catalyzed by 2a to give α-aryl-
β-substituted γ-lactones 9 (Figure 2B).4 However, the obtained
levels of enantio- and diastereoselectivity were unsatisfactory.
We envisioned that catalysts that are conformationally
locked via chalcogen-bonding interactions could potentially
provide a solution for these problematic intramolecular C−H
insertion reactions.
Even though the initial attempts using 2a were unsatisfactory
(Figure 2B), the asymmetric construction of α-aryl β-
substituted γ-lactones continued to attract us because this
type of lactone is frequently found as the core structure of
natural products14 such as cinnamomumolide (13),15
cinncassin A7 (ent-13),16 and cinnamomulactone (14)17
(Figure 2C). Therefore, we developed in this study a novel
type of dirhodium(II) carboxylate catalyst and revealed that
10b, whose conformation can be controlled via chalcogen-
bonding interactions, is a highly valuable catalyst for
intramolecular C−H insertion reactions.
the structure of 2b was found to be pseudosymmetric, i.e., the
chiral environments around the two rhodium centers (Rh1 and
Rh2) are not identical, as indicated by the different geometries
of the blue naphthyl groups located at the “up” sides relative to
the Rh1 and Rh2 centers (cf. top views from Rh1 and Rh2 in
Figure 3A). These inequivalent chiral environments should
lead to transition states with different geometries on both Rh
centers in the stereo-determining C−C-bond-forming step to
give a mixture of products with different enantioselectivities.
Indeed, 2b provided only moderate levels of enantioselectivity
(67% ee) in the intramolecular C−H insertion of 11a to give
trans-lactone 12a (Table 1, entry 10). Another noteworthy
property of 2b is the hydrogen bond formed between the
carboxylate oxygen atom and the amide NH group (O···N
distance: 2.71 (1) Å) in ligand II, which is shown by the dotted
line in Figure 3A.
Herein, we report the stereostructures of the sulfur-
containing catalyst 10b and related derivatives, together with
its successful application in the stereoselective C−H insertion
of 11 to give α,β-diaryl γ-lactones 12, as well as its extension to
the total syntheses of 13, ent-13, and 14 (Figure 2C).
We have previously reported the solid-state stereostructure
of the methoxycarbonyl-functionalized binaphthyl-type catalyst
2a (Figure 3B).4 This complex forms a C2-symmetric
conformation in which the carbamate-substituted naphthyl
rings of the ligands are oriented in an “up-up-down-down”
conformation. However, this complex also contains unsym-
metric chiral environments around Rh1 and Rh2, as shown by
the different geometries of the blue naphthyl groups around
the Rh1 and Rh2 centers. In this complex, two hydrogen bonds
were found between the carboxylate oxygen atoms and the
carbamate NH groups of the carboxylate ligands I and II
located at the upper side of the complex relative to Rh1 (O···N
distances: 2.974 (7) Å (ligand I); 2.962(7) Å (ligand II);
dotted lines in Figure 3B). This complex also furnished trans-
12a with unsatisfactory levels of enantioselectivity (55% ee)
(Table 1, entry 9).
This was attributed to the unsymmetric chiral environments
around the rhodium centers, similar to the case of 2b. The
crystal structures suggested that the unsymmetric stereo-
structures of 2a and 2b originate from the irregular angle of the
naphthyl rings relative to the π-face of the carboxylate group
via the rotation of the carboxylate C−C single bonds, as shown
by the variation in their dihedral angles, i.e., ϕ C−C′−C″−O:
RESULTS AND DISCUSSION
■
Preparation and Stereostructure of Dirhodium(II)
Carboxylate Complexes. Complex (S)-10b, which bears a
trifluoroacetamide group on its binaphthothiophene frame-
work, was prepared from benzyl ester (S)-156 by installing a
trifluoroacetamide moiety at the aniline-type amino group and
successive deprotection of the benzyl group (Scheme 1).
Treatment of carboxylic acid (S)-16b with Rh2(OAc)4 in
refluxing chlorobenzene led to a smooth ligand exchange from
AcOH to (S)-16b on the rhodium(II) centers to afford (S)-
10b. We also prepared enantiomer (R)-10b, the methox-
ycarbonyl-functionalized analogue 10a, and the binaphthyl-
type 2b, which contains trifluoroacetamide groups, to compare
their catalytic potential to that of 10b.
The success of the catalyst-design concept presented in
Figure 1D was confirmed by single-crystal X-ray diffraction
analyses of (S)-10b and 2b. The crystal structure of 2b is
shown in Figure 3A.
In the crystalline solid state, 2b adopts a D2-symmetric
conformation in which the amide-substituted naphthyl groups
reside in an “up-down-up-down” conformation.18,19 However,
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Table 1. Survey of Dirhodium(II) Carboxylate Catalysts
Tested in the Asymmetric Intramolecular C−H Insertion of
a h
,
11a
d
trans-12a
ee (%) of trans-
b
c
entry
Rh catalyst
cis-12a (%)
71
64
80
(%)
12a
1
2
3
Rh2(S-DOSP)4
Rh2(S-PTTL)4
Rh2(S-
TCPTTL)4
68
64
80
27
−41
−37
4
5
6
7
Rh2(S-PTAD)4
Rh2(S-NTTL)4
Rh2(R-BTPCP)4 97
61
69
60
68
96
20
−39
−54
40
Rh2(5R-
MEPY)4
41
47
e
8
9
10
11
12
13
17
2a
2b
10a
(R)-10b
(R)-10b
83
65
84
81
96
85
60
84
81
29
55
67
70
95
f
95
f
95 (95%
ee)
g
a
Standard reaction conditions: 11a (33.5 mg, 0.075 mmol, 1.0 equiv)
in degassed CH2Cl2 (0.75 mL) was added over 1.5 h to 4.5 mL of a
refluxing CH2Cl2 solution of the dirhodium(II) catalyst (1.5 μmol, 2.0
b
mol %); then, refluxing was continued for 1 h. Determined by
integrating the 1H NMR signals in the presence of the internal
c
standard 1,3,5-trimethoxybenzene. NMR yield over two steps.
d
e
Determined by chiral HPLC analysis. 5 mol % of Rh2(5R-
f
g
h
MEPY)4 was used. Isolated yield. Ee (%) of isolated cis-12a.
Figure 3. (A) Pseudo D2-symmetric structure of 2b. (B) Pseudo C2-
symmetric structure of 2a. (C) Twist of the naphthyl rings relative to
the π-face of the carboxylate group in 2a and 2b expressed as the
dihedral angles around the carboxylate C−C bonds of ligands I−IV.
In the crystal structures of 2a and 2b, the hydrogen atoms and solvent
molecules coordinated at the axial positions are omitted for clarity. In
the top views of the complexes, the pairs of amide-substituted
naphthyl rings located at the upper and lower side relative to the
indicated Rh center are shown in blue and orange, respectively.
in which the chiral environments around the Rh1 and Rh2
centers are almost identical (cf. the blue naphthothiophene
rings in Figure 4A).20−22 Unlike in 2a and 2b, hydrogen
bonding was not observed between the carboxylate oxygen
atoms and the amide NH groups of the ligands. Instead, as
expected, four chalcogen bonds, one between the sulfur atom
of the naphthothiophene ring and the oxygen atom of the
carboxylate group of each ligand, were observed in ligands I−
IV and are shown as dotted lines in Figure 4A and exemplified
by ligand III in Figure 4B. It should be noted here that all S···O
−39.0(9)° to 49 (1)° for ligands I−IV of complex 2a and − 49
(1)° to 44(2)° for those of complex 2b (Figure 3C).
On the other hand, in sharp contrast to 2a and 2b, complex
(S)-10b exhibits a well-ordered D2-symmetric stereostructure
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flexibility around the carboxylate C−C bonds, thus resulting
in a coplanar alignment of the carboxylate groups and the
naphthothiophene π-systems in all the ligands (ϕ S−C−C−O
≤
16(2)°) (Figure 4B).
The defined conformations of the carboxylate moieties are
crucial for the nearly perfect symmetric structure of 10b.
Moreover, as expected, the coplanar geometry of the
carboxylate and the naphthothiophene π-systems force the
alternating naphthothiophene rings (blue) to tilt toward the
Rh centers (cf. side view of the complex in Figure 4C). Due to
the tilted aromatic rings in this D2-symmetric arrangement,
trapezoidal chiral spaces are formed near each Rh center (the
chiral space around Rh2 is indicated by a dotted line).
Furthermore, chalcogen bonds between the carbonyl oxygen
atom of the trifluoroacetamide moiety and a sulfur atom are
also observed in all these ligands (S′···O′ distances: 2.82(2)−
2.97(2) Å) (Figure 4B). These chalcogen-bonding interactions
make the trifluoroacetamide moieties form a five-membered
‘heterocycle’ parallel to the naphthothiophene rings (Figure
4B). The space-filling model of 10b revealed that a wide chiral
cleft is formed by the fused cyclic systems of the
naphthothiophene and trifluoroacetamide moieties (Figure
4D).
A natural bond orbital analysis of the crystal structure
corroborated attractive S···O and S′···O′ interactions due to
the delocalization of lone pairs of electrons on the oxygen
atoms into adjacent C−S σ* orbitals in each ligand. The
corresponding second order perturbation energies ES···O (0.71
kcal/mol) and ES’···O′ (2.42 kcal/mol) in ligand III were
determined by DFT calculations at the M06/6-31G*-
[LANL2DZ] level of theory (Figure 5).23
Figure 5. Natural bond orbital (NBO) overlap between the oxygen
lone pair (nO) and the antibonding orbital of the C−S bond (σ*C−S),
together with the corresponding second order perturbation energies
for ligand III in the crystal structure of (S)-10b.
Figure 4. (A) D2-symmetric structure of (S)-10b. Hydrogen atoms
and solvent molecules coordinated at the axial positions are omitted
for clarity. In the top views of 10b, the pairs of amide-substituted
naphthothiophene rings located at the upper and lower sides relative
to the indicated Rh centers are shown in blue and orange,
respectively. (B) Dihedral angles of S−C−C−O and S′−C′−N′−
C″, as well as the S···O and S′···O′ distances of each ligand; the
locations of these angles and distances are illustrated using ligand III.
(C) Side view of 10b. The blue amide-substituted naphthothiophene
rings are located on the Rh(2) side. (D) Space-filling model of the top
view from Rh(2).
These chalcogen-bonding interactions can be expected to
contribute to the symmetric and defined structural properties
of complex 10b. Having identified these promising structural
features for asymmetric induction, we then moved on to
examine intramolecular C−H insertion reactions using 10b.
Intramolecular C−H Insertion Reactions and Total
Synthesis of Natural Products. The potential for
asymmetric induction of each Rh complex was evaluated in
the intramolecular C−H insertion reaction of 11a, as the
product of this reaction provides ready access to cinnamomu-
molide (13) (Table 1). After finding appropriate conditions by
distances (2.84 (1)−2.95 (1) Å) are shorter than the sum of
the van der Waals radii (3.32 Å) of these atoms (Figure 4B).
These chalcogen-bonding interactions could restrict the
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a b c d
, , ,
Table 2. Intramolecular C−H Insertion to Give a Variety of Cis- and Trans-γ-Lactones
a
Standard reaction conditions: 11 (0.075 mmol, 1.0 equiv) in degassed CH2Cl2 (0.75 mL) was added over 1.5 h to 4.5 mL of a refluxing CH2Cl2
b
1
solution of 10b (3.43 mg, 1.5 μmol, 2.0 mol %); then, refluxing was continued for 1 h. Determined by integrating the H NMR signals in the
presence of the internal standard 1,3,5-trimethoxybenzene, except in the cases of 12u and 12v. Determined by chiral HPLC analysis. Catalyst
(S)-10b was used.
c
d
screening solvents and temperature (for details, see the
Supporting Information), the intramolecular C−H insertion
was examined in the presence of 2.0 mol % of the
dirhodium(II) carboxylate complex in refluxing CH2Cl2. In
all cases, cis-12a was obtained as the initial product in perfect
diastereoselectivity (cis:trans > 99:1). However, although cis-
12a was isolable, isomerization of cis-12a to trans-12a was
sometimes observed during the isolation. Therefore, epimeri-
zation to trans-12a was carried out by treatment with DBU
after work up of the C−H insertion reaction. The chemical
yield of cis-12a was determined using an internal standard, and
the asymmetric induction was evaluated using the enantio-
meric excess (ee) of trans-12a. The same ee value (95% ee)
was measured for the isolated cis-12a and trans-12a,
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demonstrating that no change in the optical purity occurred
during the epimerization (entry 12 vs 13).
unsubstituted 11b and of 11c−11e, which contain halogen
groups at the para- or meta-positions, gave the corresponding
products (cis- and trans-12b−12e) in good yield with excellent
enantioselectivity (90−95% ee). The presence of a t-Bu group
at the para-position of 11f was still acceptable. On the other
hand, a remarkable decrease in the chemical yield was observed
in the reactions of para-methoxy-substituted 11g, with 34%
yield of the trans-isomer, although the enantioselectivity
remained high. This could be due to the decreased
electrophilicity of the rhodium-carbenoid intermediate formed
by the α-phenyl moiety with an electron-donating para-
methoxy group.
In contrast, 11h, which contains two methoxy groups at its
meta-positions, afforded cis-12h in high yield and 94% ee. The
electron-withdrawing inductive effect of the meta-substituted
methoxy groups may have contributed to this high yield.
Naphthyl groups were also accepted and furnished cis- and
trans-12i and 12j in excellent yield, whereby 12j showed the
highest enantioselectivity of all products (96% ee).
The absolute configuration of trans-12b (3S,4S) was
determined by comparison of its optical rotation to literature
values33 (cf. Supporting Information), and the absolute
configuration of the other products was assigned in analogy.34
Subsequently, we tested the effect of the ester moiety (Table
2B). The reactions of the tested substrates generally proceeded
with high enantioselectivity (87−93% ee for trans-12k−12r),
although their chemical yield varies depending on the
electronic nature of the aromatic rings. While the methoxy-
functionalized substrates 11o and 11p afforded the products in
excellent yield, substrates 11k−11n, which bear electron-
withdrawing groups, furnished the corresponding γ-lactones in
moderate yield. This may have been due to the poorer
stabilization of the positive charge at the benzylic position
generated in the transition state for C−H insertion.
Unfortunately, the C−H insertion of thiophene derivative 11
s and our previous substrate 8 did not produce promising
results, providing the corresponding products trans-12s and
trans-9 in 46% yield and 77% ee as well as 28% yield and 75%
ee, respectively.
Substrates bearing bromo, alkoxy, or 2-naphthyl groups on
the α-aryl groups and electron-abundant aromatic groups in
ester moieties generally furnished the corresponding products
in high yield and enantioselectivity (Table 2C). It is
noteworthy that substrate 11t was reasonably converted on
the gram scale at a reduced catalyst loading (0.5 mol %) to give
trans-12t in 90% yield and 93% ee (cf. dashed box). In the case
of 12u and 12v, the cis-γ-lactones were isolated to demonstrate
the utility of this reaction for producing the cis-isomer.
Having established the substrate scope, we then applied the
reaction to the total syntheses of naturally occurring γ-lactones.
Cinnamomumolide (13), which exhibits a 3S,4S configuration,
and its enantiomer, cinncassin A7 (ent-13), which exhibits a
3R,4R-configuration, were readily prepared by removal of the
benzyl group of the enantiomers trans-12a and ent-trans-12a
(Scheme 2). Substrate 11a was prepared from the
commercially available compounds 18 and 19 in two steps.
Therefore, cat. (R)- and (S)-10b enable the five-step
stereoselective total syntheses of 13 and ent-13 in 59 and
51% total yield, respectively. The absolute configurations of the
prepared compounds were confirmed by comparison of their
optical rotation with literature values.15b,16
Initial trials of this reaction using several established
catalysts, including Rh2(S-DOSP)4,24 Rh2(S-PTTL)4,25
Rh2(S-TCPTTL)4,26 Rh2(S-PTAD)4,11c Rh2(S-NTTL)4,27
Rh2(R-BTPCP)4,28 and Rh2(5R-MEPY)4,29 resulted in only
low to medium levels of enantioselectivity (Table 1, entries 1−
7). Moreover 2a, 2b, and binaphthyl-type catalyst 1730 were
also tested. The use of 17 did not improve the ee (29%; entry
8). On the other hand, the pseudo C2-symmetric catalyst 2a
(Figure 3B) furnished trans-12a in 55% ee (entry 9), while
pseudo D2-symmetric 2b (Figure 3A) further improved the
enantioselectivity to give trans-12a in 84% yield with 67% ee
(entry 10). Although the stereoselectivity was still unsat-
isfactory, some improvement was observed using binaphthyl-
type catalysts 2a and 2b instead of 17 (entries 9 and 10 vs
entry 8).
While the different symmetries of the Rh complexes should
significantly affect their performance with respect to
enantioselectivity, the hydrogen bond formed between the
carboxylate oxygen atom and the NH group of the amide and
the carbamate substituents in 2a and 2b might also contribute
to their increased stereoselectivity by acting as a conforma-
tional lock. A plausible explanation for the limited
enantioselectivity of 2a and 2b might be the inequivalent
chiral environments around their rhodium centers (Rh (1) and
Rh(2)) (Figure 3A,B). These inequivalent chiral environments
might lead to a decrease in enantioselectivity through
transition states with different geometries in the stereo-
determining C−C-bond-forming step. Thus, we examined
the binaphthothiophene catalysts 10a and 10b.
Although methoxycarbonyl-functionalized 10a furnished
approximately the same level of enantioselectivity as 2b
(entry 11), the stereoselectivity was dramatically improved
by the trifluoroacetamide-bearing (R)-10b, which gave trans-
12a in 95% yield with 95% ee (entry 12). The initial product of
the C−H insertion, cis-12a, was isolated in 95% yield with 95%
ee under the same conditions (entry 13). These results confirm
that 10b is valuable for preparing both the corresponding cis-
and trans-isomers in high diastereo- and enantioselectivity. As
described below, the absolute configuration of trans-12a was
determined to be 3S,4S by comparison of its optical rotation
after transformation into cinnamomumolide (13).
The remarkable improvement in enantioselectivity using
10b corroborates the importance of conformational control of
the paddlewheel catalyst to give uniform and defined chiral
environments around the catalytically active centers. Although
10a provided only 70% ee, this value represents a significant
increase compared to that achieved in the reaction using 2a
(55% ee), which contains the same substituents but no sulfur
atoms (entry 9 vs entry 11). This improvement could be
interpreted in terms of valuable control over the conformation
of the ligands via chalcogen-bonding interactions.31 Further-
more, not only the stereoselectivity, but also the chemical yield
of the product improved when 10b was used. In this reaction,
the dimerization of substrate 11a tends to compete with the
intramolecular C−H insertion to generate side products.32 The
well-defined and rigid pockets formed by the naphthothio-
phene rings around the Rh centers of 10b might also help to
prevent these side reactions.
Having achieved satisfactory results using 10b, we then
explored its substrate scope. First, we tested substrates with a
variety of α-aryl groups (Table 2A). The reactions of
Furthermore, the total synthesis of cinnamomulactone (14)
was achieved in five steps via the asymmetric intramolecular
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Research Article
Scheme 2. Asymmetric Syntheses of Cinnamommumolide
(13) and Its Enantiomer Cinnacassin A7 (ent-13)
Scheme 3. Asymmetric Synthesis of Cinnamomulactone
(14)
Figure 6. CD and UV spectra of 13, ent-13, and 14 in MeOH.
CONCLUSIONS
C−H insertion of 11z, which was prepared from commercially
available 20 and 21, in the presence of (S)-10b, followed by
deprotection of the benzyl groups of trans-12z (Scheme 3).
Compound 14 was synthesized in the naturally occurring form,
given that the optical rotation of synthesized 14 had the same
sign as the isolated one from the natural source.17 However,
the absolute configuration of 14 has not yet been determined,
although the 3R,4R configuration was elucidated based on the
■
Chiral dirhodium(II) carboxylate complex 10b, which contains
sulfur atoms in its fused cyclic system, was designed and
synthesized. A single-crystal X-ray diffraction analysis indicated
that the Rh centers in 10b are embedded in uniform and
defined chiral environments that might arise from the presence
of intramolecular chalcogen bonds. 10b demonstrated out-
standing catalytic performance in diastereo- and enantiose-
lective intramolecular C−H insertion reactions, as well as
versatile synthetic utility in the concise asymmetric syntheses
of the naturally occurring α,β-disubstituted γ-lactones such as
13, ent-13, and 14. These results demonstrate the importance
of establishing conformational control in chiral paddlewheel
complexes in order to achieve remarkable asymmetric
induction. Furthermore, these results suggest that noncovalent
interactions might play a significant role in controlling the
conformation of catalysts that bear a rotatable bond near the
catalytically active center. Further examination and applica-
tions of this concept of establishing conformational control via
chalcogen-bonding interaction in asymmetric catalysts are
currently in progress in our laboratories.
25
opposite sign of the optical rotation ([α]D = −193 in
25
CH2Cl2) relative to that of cinnamomumolide (13) ([α]D
=
+161.5 in CH2Cl2).17
To determine the absolute configuration of 14 unambigu-
ously, the CD spectra of synthesized cinnamomumolide
(3S,4S)-13, cinncassin A7 (3R,4R)-ent-13, and 14 were
compared (Figure 6). Both 14 and ent-13 show negative
Cotton effects at 270−300 nm, 230−250 nm, and 200−220
nm, which clearly indicate a 3R,4R-configuration of 14. The
mirror image patterns of the CD spectra for 14 and (3S,4S)-13
also support the 3R,4R-configuration of 14. These asymmetric
total syntheses aptly demonstrate the high applicability of cat.
10b for the preparation of optically active α,β-diaryl γ-lactones.
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ASSOCIATED CONTENT
pubs.acs.org/acscatalysis
Research Article
Notes
■
The authors declare no competing financial interest.
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.0c03689.
ACKNOWLEDGMENTS
■
This work was financially supported by a Grant-in-Aid for
Scientific Research (C) (26460007) and Scientific Research
(B) (18H02554).
General information; list of abbreviations; preparations
of Rh(II) catalysts; preparations of substrates; opti-
mization of reaction conditions for Rh(II)-catalyzed
intramolecular C−H insertion; substrate scope for
intramolecular C−H insertions; total syntheses of
cinnamomumolide (13) and cinncassin A7 (ent-13);
total synthesis of cinnamomulactone (14); model to
rationalize the stereoselectivity; and computational
details (PDF)
HPLC analysis results for determination of the
enantiomeric excess (PDF)
Spectroscopic data for all new compounds (PDF)
X-ray diffraction analysis results for 2a (CIF)
X-ray diffraction analysis results for 2b (CIF)
X-ray diffraction analysis results for (S)-10b (CIF)
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AUTHOR INFORMATION
Corresponding Author
■
Takumi Furuta − Department of Pharmaceutical Chemistry,
Kyoto Pharmaceutical University, Yamashina-ku, Kyoto 607-
8414, Japan; orcid.org/0000-0003-1037-9715;
Email: furuta@mb.kyoto-phu.ac.jp
Authors
Takuya Murai − Department of Pharmaceutical Chemistry,
Kyoto Pharmaceutical University, Yamashina-ku, Kyoto 607-
8414, Japan
Wenjie Lu − Institute for Chemical Research, Kyoto University,
Uji, Kyoto 611-0011, Japan
́
(k) Calo, F. P.; Furstner, A. A Heteroleptic Dirhodium Catalyst for
̈
Toshifumi Kuribayashi − Institute for Chemical Research,
Kyoto University, Uji, Kyoto 611-0011, Japan
Kazuhiro Morisaki − Institute for Chemical Research, Kyoto
University, Uji, Kyoto 611-0011, Japan
Yoshihiro Ueda − Institute for Chemical Research, Kyoto
University, Uji, Kyoto 611-0011, Japan; orcid.org/0000-
0002-5485-2722
Shohei Hamada − Department of Pharmaceutical Chemistry,
Kyoto Pharmaceutical University, Yamashina-ku, Kyoto 607-
8414, Japan
Yusuke Kobayashi − Department of Pharmaceutical
Chemistry, Kyoto Pharmaceutical University, Yamashina-ku,
Kyoto 607-8414, Japan; orcid.org/0000-0003-3074-
7378
Takahiro Sasamori − Graduate School of Natural Sciences,
Nagoya City University, Nagoya, Aichi 467-8501, Japan;
orcid.org/0000-0001-5410-8488
Norihiro Tokitoh − Institute for Chemical Research, Kyoto
University, Uji, Kyoto 611-0011, Japan; orcid.org/0000-
0003-1083-7245
Takeo Kawabata − Institute for Chemical Research, Kyoto
University, Uji, Kyoto 611-0011, Japan; orcid.org/0000-
0002-9959-0420
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Complete contact information is available at:
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Author Contributions
The manuscript was collated by contributions from all authors.
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(31) The structure of binapthofuran analog of cat. 10b was also
calculated by DFT method for evaluation of the contribution of the
chalcogen-bonding interactions in cat. 10b. The optimized structure
of the binapthofuran complex, which would lack the strong chalcogen
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ACS Catal. 2021, 11, 568−578