Ruthenium-Catalyzed Cross-Selective Asymmetric Oxidative Coupling of Arenols

Article abstract of DOI:10.1021/acs.orglett.0c00048

(Aqua)ruthenium(salen) complex 1c achieved good to high chemo- A nd enantioselective oxidative cross-coupling of arenols. The catalytic system can be used to selectively produce C₁-symmetric bis(arenol)s from the combination of C3- A nd C7-substituted 2-naphthols or phenols even when there is no significant difference in oxidation potential between the cross-coupling partners. This unique cross-selectivity is dominated by steric rather than electronic effects of the arenols and can be controlled by chemoselective single-electron oxidation and oxidative carbon-carbon bond formation.

Full text of DOI:10.1021/acs.orglett.0c00048

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Letter
Ruthenium-Catalyzed Cross-Selective Asymmetric Oxidative
Coupling of Arenols
Hiroki Hayashi,∥ Takamasa Ueno,∥ Chungsik Kim, and Tatsuya Uchida*
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ABSTRACT: (Aqua)ruthenium(salen) complex 1c achieved
good to high chemo- and enantioselective oxidative cross-coupling
of arenols. The catalytic system can be used to selectively produce
C1-symmetric bis(arenol)s from the combination of C3- and C7-
substituted 2-naphthols or phenols even when there is no
significant difference in oxidation potential between the cross-
coupling partners. This unique cross-selectivity is dominated by
steric rather than electronic effects of the arenols and can be
controlled by chemoselective single-electron oxidation and oxidative carbon−carbon bond formation.
xidative coupling of arenols as a powerful tool for the
construction of bis(arenol) skeletons, which are
O
important motifs in natural products and chiral auxiliaries,
has been extensively developed over the past several decades.1,2
In 1978, Feringa and Wynberg reported seminal work on
enantioselective oxidative coupling of arenols.3 Later, Nakaji-
ma and co-workers made an important breakthrough in this
field when they discovered that chiral copper complexes can be
used to catalyze the transformation with good to high
enantioselectivity.4 Subsequently, various chiral catalysts were
introduced for asymmetric oxidative coupling, leading to a
well-established substrate range with known enantioselectiv-
ities.5−9 Although this asymmetric transformation to construct
optically active C2-symmetric bis(arenol)s has been extensively
studied, access to the C1-symmetric analogues using this
method remains challenging because it is difficult to make
arenols that have similar electronic and structural characters
undergo oxidation and C−C bond formation in a chemo-
selective manner.9−12 To realize cross-selective oxidative
coupling, one arenol should be oxidized to a radical cation
intermediate and coupled nucleophilically to the other arenol
or aroxyl intermediate in a chemoselective manner to avoid the
undesired homocoupling reaction (Figure 1a). Although
catalytic cross-selective asymmetric oxidative coupling has
been described, the substrate scope with reasonable stereo- and
chemoselectivities was still limited. Habaue and co-workers
presented Lewis acid-mediated copper-catalyzed cross-selective
oxidative coupling using less oxidizable 3-alkoxycarbonyl-
substituted 2-naphthols as coupling partners.13 Very recently,
Tu et al. successfully developed a strategy that uses chiral
spirocyclic bidentate ligands and 2-naphthols with a directing
group at C3.14 Katsuki also reported the iron(salan)-catalyzed
highly chemoselective synthesis of C1-symmetric bis(arenol)s
using a 2-naphthol containing an electron-withdrawing carbon-
yl group at C6 with batchwise addition of the other 2-naphthol
Figure 1. Cross-selective oxidative coupling between two different
arenols.
Received: January 6, 2020
https://dx.doi.org/10.1021/acs.orglett.0c00048
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
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Letter
as the coupling partner (Figure 1b, middle).9b Kozlowski and
Pappo have shown that the steric and/or electronic properties
of the coupling partner, such as strength of binding to the
metal catalyst, oxidation potential, and nucleophilicity, are
crucial factors for establishing cross-selective coupling.11,15,16
Further development of the methodologies for cross-selective
oxidative coupling has remained a current topic in these fields
and is intensively studied.17 As described in the previous report
on iron-catalyzed oxidative cross-coupling (Figure 1b, right),
chemoselective synthesis of chiral C1-symmetric bis(arenol)s
via enantioselective oxidative coupling with electronically
similar arenols without a directing group remains a challenge.11
In the present work, we found that (aqua)ruthenium(salen)
complexes 1 show efficient and unique catalysis in the cross-
selective oxidative coupling of arenols (Figure 1c). Ruthenium-
catalyzed oxidative coupling between two arenols with little
difference in their electronic characters (e.g., ΔEox = 0.01 V),
produces the corresponding C1-symmetric bis(arenol)s with
good to high cross- and enantioselectivies.
Scheme 1. Preliminary Studies of Asymmetric Oxidative
Coupling of 2-Naphthols Using Ruthenium(salen) Catalysts
to high enantioselectivity.19,20 Under 1c-catalyzed conditions,
we further investigated the homocoupling in the presence of
brominated 2-naphthols that have similar oxidation poten-
tials.19 6-Bromo-2-naphthol exhibited higher reactivity than 3-
or 7-bromo-2-naphthol despite numerous reports indicating
high reactivities for oxidative coupling of C3- or C7-substituted
2-naphthols, including 3- or 7-bromo-2-naphthol.5−9 The
reason for this substituent effect for these 2-naphthols in
ruthenium-catalyzed reactions presently remains unclear.
However, the substituents at C3 or C7 of the 2-naphthol
might individually suppress homocouplings because of the
steric repulsion with complex 1c and/or the radical cation
intermediates.
We recently found that ruthenium(salen) complexes are
efficient catalysts for aerobic asymmetric oxidations.6,18
Mechanistic studies showed that under aerobic conditions,
radical cation intermediate B was formed via single electron
transfer (SET) and radical delocalization (Figure 2a).18c,e This
Intrigued by these considerations, we began to study the
asymmetric oxidative cross-coupling between C7- and C3-
substituted 2-naphthols A and B with complex 1c as the
catalyst (Table 1). A 1:1 mixture of 7- and 3-bromo-2-
naphthol with a 5.0 mol % loading of 1c gave C1-symmetric
bis(arenol) 3a with high chemo- and enantioselectivity (entry
1). Despite the axiom proposed in previous reports,11,15,16 we
achieved excellent results for the cross-coupling with only a
slight difference in the oxidation potential between the two
coupling partners (ΔEox = 0.10 V). The chemical yield and
chemoselectivity could be improved by using a slight excess of
the coupling partner (entry 2). Encouraged by these results, we
further conducted the reaction with a combination of C3- and
C7-substituted 2-naphthols using complex 1c as the catalyst.
The desired cross-coupling products 3 were produced as the
major products in every combination with good to high
enantioselectivities regardless of the magnitudes of their
oxidation potentials (entries 3−14).19 Higher cross-selectivity
was observed when a relatively bulky phenyl group was
introduced at C3 of the 2-naphthol compared with the other
C3-substituted 2-naphthols. To our delight, the combinations
using electron-rich arenols, even if they were easily oxidizable
and preferred homocoupling,21 could be applied to our
catalytic conditions, yielding good to high cross-selectivity
(entries 15−19). The cross-selectivity and yield were improved
without reducing the enantioselectivies when the temperature
was increased to 40 °C (entries 15−17, values in parentheses).
These results indicated that the steric factor of the substituent
groups had a stronger effect on the cross-selectivity than the
electronic characters of the coupling partners.
Figure 2. (a) Possible oxidation mechanism for the ruthenium-
catalyzed alcohol oxidation. (b) Working hypothesis for the cross-
selective oxidative coupling using Ru(salen) as the catalyst.
mechanism suggests that arenols could be oxidized to the
corresponding electrophilic radical intermediates and then
converted to the desired bis(arenol)s (Figure 2b). Moreover,
we envisioned that if the ruthenium catalyst promoted the C−
C bond-forming process for the cross-coupling reaction
preferentially while simultaneously suppressing the homocou-
pling reaction, the cross-coupling might proceed in spite of the
similar electronic natures of coupling partners.
On the basis of these considerations, we began our
investigation to find an efficient oxidative arenol coupling
catalyst using 2-naphthol as the ideal test substrate (Scheme
1). The reaction in the presence of 1a produced BINOL 2a
quantitatively but in nearly racemic form. The enantioselec-
tivity of the reaction could be improved by using a catalyst
bearing an aryl group at C2″ on the binaphthyl unit, such as 1b
or 1c. After extensive screening, we found that complex 1c led
We further conducted the cross-coupling between 2-
naphthols and phenols, which are typically less oxidizable
than naphthols; however, they act as good nucleophiles
(Scheme 2).15 To achieve this combination, we examined
the cross-coupling of m-xylenol with 3-methoxy-2-naphthol.
The reaction mixture gave the desired cross-coupling products
with good chemoselectivity and moderate regioselectivity
B
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Letter
Table 1. Asymmetric Oxidative Cross-Coupling between
C7- and C3-Substituted 2-Naphthols
Scheme 2. Oxidative Cross-Coupling between 3-Substituted
2-Naphthols and Phenols
a
a
a
Reaction conditions: A (0.1 mmol), B (0.2 mmol), 1c (5 mol %),
and MS 3 Å (40 mg) in toluene (1 mL) at 30 °C under air for 48 h.22
b
c
1
Determined by H NMR analysis. Isolated yields of products 4.
d
e
Determined by HPLC analysis. Run at 20 °C.
oxidative coupling system with other nucleophiles as coupling
substrates.
Using DFT23 calculations in the Gaussian 09 package,24 we
performed computational studies to elucidate the origin of the
cross-selectivity. The calculated radical cation intermediate
indicated that C3 of 2-naphthol ligand A at the ruthenium ion
was located at the aryl−binaphthyl pocket (Figure 3a). The
a
Reaction conditions: A (0.1 mmol), B (0.15 mmol), 1c (5 mol %),
and MS 3 Å (40 mg) in toluene (1 mL) at 30 °C under air for 48 h.22
b
c
ΔEox = |Eox(A) − Eox(B)|.19 Determined by H NMR analysis or
1
d
molar ratios of isolated products. Isolated yields of products 3.
e
f
Figure 3. Optimized structures of reaction intermediates of 1c.
Determined by HPLC analysis. Run with a 1:1 ratio of A and B.
Run at 40 °C. Run at 25 °C. TBSO: tert-butyldimethylsiloxy. Run
g
h
i
j
at 20 °C.
coupling partner B approached the 2-naphthol intermediate
from the top side (Figure 3b) to form the desired product. On
the basis of these considerations, in the oxidation process of
arenol A it was observed that 1c preferentially allowed access
of the C6- or C7-substituted naphthols compared with the C3-
substituted ones to the ruthenium ion because of the sterically
congested environment around the C3 substituent in the
pocket. On the other hand, in the C−C bond formation
process, the C7 substituent group on the nucleophilic arenol B
was close to the aryl group at C2″ on the naphthyl unit to gain
access to aryloxy intermediate A. This steric repulsion between
the C7-substituent group and the 3,5-difluoro-4-trimethylsilyl-
phenyl group was sufficient to affect the cross-selectivity.
(4aortho:4apara = 1:2.6). The cross-coupled products 4 with a
good enantiomeric ratio were also selectively obtained as a
regiomixture (4bortho:4bpara = 1:2.5) in the reaction of 3-
phenyl-2-naphthol with m-xylenol. Additionally, the C1-
symmetric bis(arenol)s 4c and 4d were selectively produced
with good to high enantioselectivities in the reactions in which
p-bromo-m-xylenol was used as a coupling partner. On the
other hand, homocoupling products were obtained as the
major products for 7-methoxy-2-naphthol. These results
showcased the potential application of the ruthenium-catalyzed
C
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Letter
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among different arenols in the presence of complex 1c as the
catalyst. Substituent groups on the 2-naphthols were found to
strongly affect their chemoselectivity, superseding their natural
electronic nature. C3-substituted 2-naphthols yielded the
corresponding C1-symmetric bis(arenol)s with good to high
cross- and enantioselectivity with an appropriate coupling
partner such as C7-substituted naphthols or phenols. We
anticipate that our ongoing mechanistic study will shed more
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reactions in various coupling reactants.
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ASSOCIATED CONTENT
* Supporting Information
■
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sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c00048.
Optimization, oxidation potentials of arenols, exper-
imental procedures, characterization data, computational
data, HPLC conditions, and NMR spectra (PDF)
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Padilla-Salinas, R.; Kozlowski, M. C. Aerobic Copper-Catalyzed
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AUTHOR INFORMATION
Corresponding Author
■
Tatsuya Uchida − Faculty of Arts and Science and International
Institute of Carbon-Neutral Energy Research (WPI-I2CNER),
Kyushu University, Fukuoka 819-0395, Japan; orcid.org/
0000-0002-6511-5752; Email: uchida.tatsuya.045@
m.kyushu-u.ac.jp
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Coupling of 2-Naphthol Derivatives Catalyzed by Photo-Activated
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Li, C.-H.; Kuo, J.-H.; Barhate, N. B.; Liu, Y.-H.; Wang, Y.; Chen, C.-T.
Catalytic Asymmetric Coupling of 2-Naphthols by Chiral Tridentate
Oxovanadium(IV) Complexes. Org. Lett. 2001, 3, 869. (b) Chu, C.-
Y.; Hwang, D.-R.; Wang, S.-K.; Uang, B.-J. Chiral Oxovanadium
Complex Catalyzed Enantioselective Oxidative Coupling of 2-
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L.; Cui, X.; Mi, A.; Jiang, Y. The Rational Design of Novel Chiral
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Mi, A.; Jiang, Y. Novel Achiral Biphenol-Derived Diastereomeric
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Coupling of 2-Naphthols. Angew. Chem., Int. Ed. 2002, 41, 4532.
(f) Somei, H.; Asano, Y.; Yoshida, T.; Takizawa, S.; Yamataka, H.;
Sasai, H. Dual Activation in a Homolytic Coupling Reaction
Promoted by an Enantioselective Dinuclear Vanadium(IV) Catalyst.
Tetrahedron Lett. 2004, 45, 1841. (g) Tada, M.; Taniike, T.; Kantam,
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Gong, L.-Z. Highly Enantioselective Oxidative Couplings of 2-
Naphthols Catalyzed by Chiral Bimetallic Oxovanadium Complexes
with Either Oxygen or Air as Oxidant. J. Am. Chem. Soc. 2007, 129,
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Yamataka, H.; Jayaprakash, D.; Sasai, H. Dual Activation in Oxidative
Coupling of 2-Naphthols Catalyzed by Chiral Dinuclear Vanadium
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Yoshida, Y.; Sako, M.; Breukers, S.; Enders, D.; Sasai, H.
Enantioselective Oxidative-Coupling of Polycyclic Phenols. Tetrahe-
Authors
Hiroki Hayashi − Faculty of Arts and Science, Kyushu
University, Fukuoka 819-0395, Japan
Takamasa Ueno − Department of Chemistry, Graduate School
of Science, Kyushu University, Fukuoka 819-0395, Japan
Chungsik Kim − International Institute of Carbon-Neutral
Energy Research (WPI-I2CNER), Kyushu University, Fukuoka
819-0395, Japan
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c00048
Author Contributions
∥H.H. and T.U. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Support by JSPS KAKENHI Grant JP18H04264 in Precisely
Designed Catalysts with Customized Scaffolding and by the
International Institute for Carbon-Neutral Energy Research
(WPI-I2CNER) from MEXT, Japan, is gratefully acknowl-
edged.
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E
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Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
pubs.acs.org/OrgLett
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coupling products AB, AA, and BB in ratios of 1.3:1:1.2 and 2.4:1:2.2,
respectively.
(22) No other byproduct was observed, and unreacted starting
materials remained.
(23) Hohenberg, P.; Kohn, W. Inhomogeneous Electron Gas. Phys.
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Org. Lett. XXXX, XXX, XXX−XXX