Zwitterion-induced organic-metal hybrid catalysis in aerobic oxidation

Article abstract of DOI:10.1021/acscatal.0c05684

In many metal catalyses, the traditional strategy of removing chloride ions is to add silver salts via anion exchange to obtain highly active catalysts. Herein, we reported an alternative strategy of removing chloride anions from ruthenium trichloride using an organic [P⁺-N^-] zwitterionic compound via multiple hydrogen bond interactions. The resultant organic-metal hybrid catalytic system has successfully been applied to the aerobic oxidation of alcohols, tetrahydroquinolines, and indolines under mild conditions. The performance of zwitterion is far superior to that of many other common Lewis bases or Br?nsted bases. Mechanistic studies revealed that the zwitterion triggers the dissociation of chloride from ruthenium trichloride via nonclassical hydrogen bond interaction. Preliminary studies show that the zwitterion is applicable to catalytic transfer semi-hydrogenation.

Full text of DOI:10.1021/acscatal.0c05684

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Research Article
Zwitterion-Induced Organic−Metal Hybrid Catalysis in Aerobic
Oxidation
Rong-Bin Hu, Ying-Pong Lam, Wing-Hin Ng, Chun-Yuen Wong, and Ying-Yeung Yeung*
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ABSTRACT: In many metal catalyses, the traditional strategy of
removing chloride ions is to add silver salts via anion exchange to
obtain highly active catalysts. Herein, we reported an alternative
strategy of removing chloride anions from ruthenium trichloride
using an organic [P+-N−] zwitterionic compound via multiple
hydrogen bond interactions. The resultant organic−metal hybrid
catalytic system has successfully been applied to the aerobic
oxidation of alcohols, tetrahydroquinolines, and indolines under
mild conditions. The performance of zwitterion is far superior to
that of many other common Lewis bases or Brønsted bases.
Mechanistic studies revealed that the zwitterion triggers the
dissociation of chloride from ruthenium trichloride via nonclassical
hydrogen bond interaction. Preliminary studies show that the
zwitterion is applicable to catalytic transfer semi-hydrogenation.
KEYWORDS: aerobic oxidation, nonclassical hydrogen bond, organocatalysis, ruthenium, zwitterion
amides, intermolecular bromoesterification of alkenes, and
isomerization of maleic acid esters.7 Mechanistic studies
suggest that zwitterion-catalyzed reactions involve a synergistic
activation mode involving the following: (1) the basic
sulfonamide anion interacts with the electrophilies (H or
Br); (2) the cationic moiety interacts with the carbonyl groups
via a nonclassical hydrogen bond (NCHB),8 which involves
the C−H···O noncovalent interaction between the α-C−H of
the cation of zwitterion and the carbonyl oxygen of the
substrate (Figure 1A). These structurally stable zwitterions
were prepared simply by treating suitable aziridines with
amine/phosphine bases.9 A library of zwitterionic catalysts can
be accessed quickly, which is highly beneficial to reaction
optimization. In addition, the structural rigidity of these
zwitterions allows for effective site-isolation of the ion pairs for
highly efficient catalysis. To our own quest to explore the
application of these zwitterions in other catalytic trans-
formations, herein we report the use of ZW2 in activating
ruthenium trichloride for aerobic oxidation.
1. INTRODUCTION
Zwitterionic compounds are a class of molecules that contain
site-isolated positive and negative ions while the molecules are
overall neutral.1 Zwitterions have emerged as a new class of
bifunctional organocatalysts that exhibit unusual reactivity and
selectivity in various reactions.2 The ion pairs in zwitterions
can operate synergistically without self-neutralization through
charge delocalization.3 Some important developments have
been made using zwitterions to enhance catalytic performance
in transition metal catalysis, although the cases are sporadic.
For instance, Gessner and co-workers reported a class of
phosphonium ylides as zwitterionic ligands for hydroamination
and Buchwald coupling reactions.4 In addition, Albrecht and
co-workers developed donor-flexible N,N′-bis(pyridylidene)-
oxalamide (bisPYA) zwitterionic ligands, which were found to
be highly effective in the ruthenium-catalyzed Lemieux−
Johnson-type oxidative cleavage of olefins.5 Compared with
traditional neutral ligands, the application of zwitterions in
transition metal catalysis remains largely under-exploited,
partly due to lacking of efficient methods for the preparation
of useful zwitterions. Therefore, it is important to explore the
applications of zwitterions in transition metal catalysis, which
could unveil new reactivities and bring new opportunities in
tuning catalysts’ properties.6
Received: December 28, 2020
Revised: February 19, 2021
Published: March 4, 2021
Recently, our group has developed a new class of aziridine-
derived [N+-N−] ZW1 and [P+-N−] ZW2 zwitterions, which
were found to be highly effective in catalyzing (trans)-
esterification reactions, deacylative dihalogenation of β-oxo
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Figure 1. (A, B) Applications of zwitterions in organo- and metal catalysis.
The oxygen positioned on top of C−Hb with C−Hb−O almost
linear (C−Hb−O = 162°). The crystal sample was also studied
by 1H NMR in a solution phase (CDCl3). Compared with pure
ZW2, the downfield shifts of Hb (α-C−H of phosphonium,
3.46 ppm) and the existence of N−Hc (9.24 ppm) indicate
that the ZW2·HCl complex remains stable in a solution phase.
Besides the formation of ZW2·HCl complex, we believe that
ruthenium oxide species (see Figure S4 in the Supporting
Information) might be formed concurrently in this system
because they were commonly synthesized under basic
conditions.10 When compared with pure ZW2·HCl, the N−
H NMR signal in the inorganic/organic catalyst mixture is
more downfield shifted, indicating that there might be a
hydrogen bond interaction between the inorganic and organic
catalyst components (see Figure S5 in the Supporting
Information). Generally, Ru−O species could be deposited
on solid materials such as Al2O3 and Fe3O4 and further applied
to heterogeneous catalysis.10 Thus, our method can be
2. RESULTS AND DISCUSSION
2.1. Generation of Active Catalyst Species. Initially, we
attempted to use zwitterion ( )-ZW2 as a bifunctional ligand
to explore different metal-catalyzed reactions. After some
screenings, interestingly, a white solid was obtained when ZW2
was mixed with hydrated ruthenium trichloride (RuCl3·xH2O)
in wet 1,2-dichloroethane (DCE) at room temperature. A
single crystal was successfully obtained and studied by X-ray
crystallography (Figure 1B). It was found that ZW2 cocrystal-
lized with a molecule of HCl and water (i.e., ZW2·HCl·H2O)
potentially via multiple hydrogen bonds. Based on the position
of the chlorine atom, it appears that the Brønsted basic amide
anion interacts with a molecule of HCl via a hydrogen bond
(Cl−Hc = 2.398 Å, assuming N−Hc = 0.861 Å). The chloride
also appears interacting weakly with the α-proton of the
phosphonium cation and the proton of water. In addition, an
NCHB interaction (O−Hb = 2.565 Å) was observed between
H2O and the α-proton of the phosphonium cation in ZW2.
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Figure 2. Oxidation with RuCl3 and various ligands.
considered as a new approach to generate Ru−O species via
homogeneous systems.
ZW2 can capture the chloride from ruthenium trichloride via
noncovalent interactions under mild conditions, the protocol
can potentially be used to activate the inert ruthenium
trichloride for catalytic reactions.
Ruthenium trichloride is known to be relatively inert
compared with other ruthenium complexes due to the strongly
coordinating chloride anion.11 Aerobic oxidation of benzyl
alcohol catalyzed by RuCl3 under 1 atm oxygen resulted in very
low conversion.12 Harsh reaction conditions are typically
required for the oxidation reactions catalyzed by RuCl3. For
instance, it was reported that RuCl3 catalyzed the oxidation of
alcohols under high pressure (2−20 atm oxygen) at high
temperature (100 °C) in the presence of amine ligands (e.g.,
Et3N).13a,b Similar oxidation could be proceeded at room
temperature, but stoichiometric amount of dicyclohexylamine
and much longer reaction time (2−4 days) were needed.13c On
the other hand, silver salts have been applied in order to
remove the strongly coordinating chloride anion and provide
more active ruthenium species.14 However, this process is
expensive and AgCl is generated irreversibly as waste. Since
2.2. Reaction Studies. Oxidation of alcohols to carbonyl
compounds is a ubiquitous and pivotal reaction in both
academic research and chemical industry, and the resulting
compounds are important building blocks for medicines,
agricultural chemicals, and fragrances.15 Thus, this trans-
formation was chosen as a benchmark reaction to examine the
performance of the catalytic protocol. Molecular oxygen (1
atm) was used as the terminal oxidant, which is highly desired
in industrial processes because water molecules are the only
byproduct. Alcohol 2a was used as the model substrate with
RuCl3·xH2O as the metal source, and the reaction was carried
out under 1 atm oxygen at 23 °C. The background reaction
was sluggish in the absence of an external additive (Figure 2).
To our delight, the oxidation proceeded smoothly with
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Figure 3. Scope of oxidation of alcohols, hydroquinones, indolines, and 1,2,3,4-tetrahydroquinoxaline.
catalytic amount of zwitterion ( )-ZW1. The phosphonium
zwitterion ( )-ZW2 showed better performance; the desired
aldehyde 3a was obtained in 97% yield, and no side product 4a
was detected. Fifty-five percent of 3a was obtained when the
loading of RuCl3 was reduced to 2.5 mol %. Optimum results
were obtained when ruthenium trichloride and ZW2 were used
in a 1:3 ratio. Generally, Lewis bases were used to enhance the
activity or selectivity of metal catalysts. Therefore, typical
neutral Lewis bases such as phosphines PL1−PL4 (PPh3,
PCy3, DPPE, and DPPF), amines NL1−NL4 (TEA, TMEDA,
DMAP, and Bpy), and N-heterocyclic carbene IPr were also
evaluated but poor yields of 3a were observed. In particular,
PL1 and PL2 gave significant amount of side product 4a.
Other zwitterions with different substituents or skeletons
were also investigated. For ( )-ZW3 that is derived from an
acyclic skeleton, the yield of 3a was unsatisfactory. Zwitterions
Figure 4. Catalyst recycling and chemoselective oxidation.
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Figure 5. (A−D) Mechanistic studies.
derived from DMAP, DABCO, Me3N, or PMe3 [( )-ZW4−
( )-ZW7] were examined but their catalytic performances
were inferior to ZW2. Zwitterions derived from phenyl rings
instead of cyclohexane were also studied; poor yields of 3a
were observed with zwitterions ZW8−ZW10. These results
indicate that both the cationic partner and the distance
between the ion pairs are crucial for superior catalytic effects.
2.3. Reaction Scope. Catalytic performance was examined
with various alcohol substrates (Figure 3). Benzyl alcohols with
electron-donating (methyl, methoxy, and dimethylamine) or
electron-withdrawing (halogen, nitro, methyl ester, and
sulfonamide) substituents were successfully oxidized to the
corresponding aldehydes in good yields (3a−3i). Alcohols
with naphthyl, pyridinyl, and furanyl substituents were also
found to be compatible with the catalytic protocol (3j−3l).
Various secondary alcohols 2m−2r were also studied. Similar
to the cases in the aldehyde synthesis, the desired aryl- and
alkyl-substituted ketones 3m−3r were obtained smoothly.
Gratefully, the catalytic system was also compatible with the
oxidation of allylic alcohols 2s−2v to give the corresponding
unsaturated carbonyl compounds 3s−3v. Notably, the olefin
functionality in the substrates remained intact under the mild
reaction conditions. The stereochemistry of the Z-olefin in Z-
3s was also retained. Aliphatic and cyclic alcohols were
subjected to investigation, and the desired carbonyl products
were obtained smoothly (3w−3z and 3aa−3ad). The reaction
temperature was increased to compensate the slower reaction
as a result of the less reactive alcohol substrates. Oxidation of
tropine that contains an amine substituent gave no reaction
and the substrate was recovered quantitatively, potentially due
to the poisoning of the metal center or neutralization of ZW2·
HCl by the amine.
We also explored the application of the catalytic protocol to
other class of substrates. Delightedly, oxidative dehydrogen-
ation of hydroquinone 5a gave 1,4-benzoquinone 6a in 86%
yield using 2.5 mol % of RuCl3 catalyst. In addition, substituted
hydroquinones 5b−5d were converted into 1,4-benzoquinones
6b−6d in excellent yields. Moreover, 1,4-naphthoquinone 6e
was furnished successfully starting from 1,4-dihydroxylnaph-
thalene 5e. A brief examination on other oxidation reactions
was conducted. Interestingly, the reaction could also be applied
to the oxidative dehydrogenation of indolines 7 to indoles 8.
Quinoxaline 10 was also obtained through the double
oxidation of 1,2,3,4-tetrahydroquinoxaline 9.
Practicality of the oxidation was demonstrated by conduct-
ing the reaction under ambient air at a gram scale (Figure 4).
Gratefully, the oxidation reaction of 2a proceeded smoothly to
give aldehyde 3a in 95% yield under the optimal conditions. In
addition, the catalyst could be recovered simply by
precipitation and the recycled catalyst could promote the
same reaction without observable loss of catalytic efficiency.
The catalytic protocol could also be applied to the chemo-
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suggesting that the reaction might not undergo via redox of the
Ru center (see Figure S6 in the Supporting Information). Next,
the di-deuterated substrate 2a-D2 together with the unsatu-
rated ketone 13 was subjected to the reaction conditions under
N2 (Figure 5C). Interestingly, appreciable amount of
deuterated compound 14 was detected. Thus, we speculated
that the deuterium in substrate 2a-D2 might be captured to
form the Ru-D species, which could reduce unsaturated ketone
13 through Michael addition.16 We also conducted a control
experiment by carefully measuring the amount of oxygen used
in the reaction in a sealed tube (Figure 5D). It was found that
0.5 equiv of molecular oxygen (i.e., 1 equiv of oxygen atom)
was sufficient to drive the reaction to completion.
In order to understand the role of chloride anions, various
silver salts including AgSbF6, AgBF4, and AgOTf were added in
order to displace the chloride counteranion (Figure 6A).17 It
was found that the reaction efficiency was affected by the
basicity of the counteranions and chloride showed the better
catalytic activity than other less coordinating anions. In the
absence of ZW2, the silver salts alone were also tested to
displace the chloride from RuCl3·xH2O and only trace amount
of oxidized product was observed. These results suggest that
the catalytic efficiency of the protocol was not done by simply
removing the strongly coordinating chloride counterion from
the ruthenium center.
Next, we attempted to probe the role of ZW2 in the
reaction. Phosphonium salt PBr and tosyl amide NLi, which
are the segmented catalysts of ZW2, were also evaluated
(Figure 6B). A sluggish reaction was observed when either of
these components or their 1:1 mixture was used, indicating
that the site-isolated cation and anion in ZW2 might work
synergistically. The ZW2·HCl complex was used instead of
ZW2 in the reaction, and the oxidation gave no conversion,
suggesting that the basicity of ZW2 is pivotal to activate
ruthenium trichloride at the initial stage in order to achieve
high catalytic performance. Another set of experiment was
carried out with ZW2 or ZW2·HCl alone as the catalyst, and
no reaction was observed. This result reveals that the organic
(ZW2) and inorganic (Ru−O species) components are both
necessary in the catalytic system. Potassium carbonate was
added to the reaction in order to capture the acidic proton on
ZW2·HCl that was in situ generated in the reaction. Reaction
yield of the desired product dropped dramatically, implying
that the acidic proton is also playing a crucial role. When 4 Å
molecule sieves were added to remove moisture, the catalytic
performance diminished significantly, which indicates the
important role of water.
We suspected that the N−H−Cl moiety in the ZW2·HCl
complex might interact with the alcohol substrate via a
hydrogen bond.18 Indeed, after adding 4-methylbenzyl alcohol
(2b) to a solution of ZW2·HCl in CDCl3, it was found that
protons of the TsN−H moiety of ZW2·HCl (both the N−H
and the aromatic region) shifted upfield significantly while the
α-H of phosphonium (Ha) shifted downfield (Figure 6C). At
the same time, protons at the benzyl group (Hc and Hd) of the
substrate shifted upfield. These chemical shifts could be
corresponding to the putative complex A that contains
hydrogen bonds between O−H in the substrate and H−Cl
in ZW2·HCl.
Figure 6. (A−C) Studies on the role of zwitterion.
selective oxidation of diol 2ae, and the mono-oxidized product
3ae was obtained exclusively.
2.4. Mechanistic Studies. Several control experiments
were conducted to shed light on the mechanism. Kinetic
isotopic effect (KIE) experiments were performed to reveal the
process of C−H cleavage (Figure 5A). Mono-deuterated
substrate 2a-D was subjected under the standard oxidation
conditions, and the ratio of kH/kD of product 3a was found to
be 5.75, indicating that the C−H abstraction of substrate 2a
might be the rate-determining step.12 Radical clock experi-
ments were conducted with substrate 2af under the standard
conditions (Figure 5B). The desired ketone product 3af was
obtained quantitatively and no cyclopropane ring-opening
product 12 was detected, suggesting that the reaction is
unlikely to go through the putative benzylic radical
intermediate 11. Furthermore, no signal relevant to the
redox of Ru was observed in the cyclic voltammetry study,
A plausible mechanism was established by piecing the
abovementioned experimental results (Figure 7). First, the
amide anion in ZW2 might coordinate with the ruthenium
center of ruthenium trichloride, causing the dissociation of
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Figure 7. Plausible mechanism.
Figure 8. Catalytic transfer semi-hydrogenation.
chloride to give ZW2·HCl. We speculate that the NCHB
originating from the phosphonium cation might assist the
dissociation of the strongly coordinating chloride anion from
the ruthenium center (Figure 1B) and the cooperative effect of
the amide anion and phosphonium cation appears to be crucial
(Figure 6B). Hydrolysis might take place concurrently, giving
Ru−O species as the metal catalyst component in the reaction
system. This degradation of ruthenium trichloride could be
suppressed when the basicity of ZW2 was neutralized or the
water was removed (Figure 6B). Then, the alcohol substrates
might be activated by the ZW2·HCl complex via hydrogen
bonds to give species A (Figure 6C). We believe that such
interaction might be interrupted by removing the chloride
anion (Figure 6A) and the acidic proton (Figure 6B).
Subsequently, the Ru−O species might oxidize the alcohol
together with the generation of Ru−H species (Figure 5C) via
β-H elimination. In the presence of molecular oxygen, the
ruthenium hydride species might give the Ru−OOH species
via insertion (Figure 5D). Since 0.5 equiv of molecular oxygen
is sufficient for the reaction, we suspect that Ru−OOH species
might eliminate 1/2O2 to give Ru−O species, which could be
used in the next catalytic cycle.12,13a,19
2.5. Transfer Hydrogenation. Since ruthenium hydride
intermediates might be involved in the catalytic cycle, we
speculated that the zwitterion might be applicable to the
transfer hydrogenation reaction of alkyne 15 (Figure 8).
Preliminary studies showed that the RuCl3 (4 mol %)/ZW2 (8
mol %) catalytic protocol gave small amount (c.a. 5%) of trans-
1,2-diphenylethylene (16). Further optimization revealed that
using [Ru(cymene)Cl2]2 together with ZW2 effectively
catalyzed the transfer semi-hydrogenation of diphenylacetylene
(15) into trans-1,2-diphenylethylene (16) in 80% yield. More
importantly, excellent stereoselectivity (16:17 = 40:1) was
observed. In contrast, Lewis bases such as triphenylphosphine
PL1 and NHC carbene IPr gave only moderate yield and
stereoselectivity.
3. CONCLUSIONS
In summary, we report a new strategy of capturing chloride
anions from RuCl3 catalysts using a novel organic [P+-N−]
zwitterion compound via multiple hydrogen bond interaction
(classical and nonclassical hydrogen bonds). The organic
(ZW2·HCl) and inorganic (Ru−O species) catalytic compo-
nents were in situ generated, which were found to be useful in
catalytic aerobic oxidation of alcohols, hydroquinolines, and
indolines under mild conditions. Mechanism studies revealed
that the chloride anion was pivotal for catalytic efficiency. A
more detailed mechanistic study is currently underway to
elucidate a clearer picture on role of the zwitterionic
compounds in the reactions. This proof-of-concept study
opens a new avenue for catalyst design in metal catalysis
chemistry.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.0c05684.
■
sı
Experimental details and spectroscopic and analytical
data for new compounds (PDF)
Crystallographic data of ZW2·HCl (CIF)
AUTHOR INFORMATION
Corresponding Author
■
Ying-Yeung Yeung − Department of Chemistry and State Key
Laboratory of Synthetic Chemistry, The Chinese University of
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Organic Photoredox Catalyst. J. Org. Chem. 2018, 83, 3921−3927.
(i) Ohmatsu, K.; Suzuki, R.; Furukawa, Y.; Sato, M.; Ooi, T.
Zwitterionic 1,2,3-Triazolium Amidate as a Catalyst for Photoinduced
Hydrogen-Atom Transfer Radical Alkylation. ACS Catal. 2020, 10,
2627−2632.
Hong Kong, Shatin, Hong Kong; orcid.org/0000-0001-
9881-5758; Email: yyyeung@cuhk.edu.hk
Authors
Rong-Bin Hu − Department of Chemistry and State Key
Laboratory of Synthetic Chemistry, The Chinese University of
Hong Kong, Shatin, Hong Kong
Ying-Pong Lam − Department of Chemistry and State Key
Laboratory of Synthetic Chemistry, The Chinese University of
Hong Kong, Shatin, Hong Kong
Wing-Hin Ng − Department of Chemistry and State Key
Laboratory of Synthetic Chemistry, The Chinese University of
Hong Kong, Shatin, Hong Kong
̀
(3) (a) Briere, J.-F.; Oudeyer, S.; Dalla, V.; Levacher, V. Recent
advances in cooperative ion pairing in asymmetric organocatalysis.
Chem. Soc. Rev. 2012, 41, 1696. (b) Brak, K.; Jacobsen, E. N.
Asymmetric Ion-Pairing Catalysis. Angew. Chem., Int. Ed. 2013, 52,
534. (c) Qian, D.; Sun, J. Recent Progress in Asymmetric Ion-Pairing
Catalysis with Ammonium Salts. Chem. − Eur. J. 2019, 25, 3740.
(d) Legros, F.; Oudeyer, S.; Levacher, V. New Developments in
Chiral Cooperative Ion Pairing Organocatalysis by Means of
Ammonium Oxyanions and Fluorides: From Protonation to
Deprotonation Reactions. Chem. Rec. 2017, 17, 429.
(4) (a) Weber, P.; Scherpf, T.; Rodstein, I.; Lichte, D.; Scharf, L. T.;
Gooßen, L. J.; Gessner, V. H. A Highly Active Ylide-Functionalized
Phosphine for Palladium-Catalyzed Aminations of Aryl Chlorides.
Angew. Chem., Int. Ed. 2019, 58, 3203−3207. (b) Scharf, L. T.;
Rodstein, I.; Schmidt, M.; Scherpf, T.; Gessner, V. H. Unraveling the
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CyJohnPhos and PtBu3. ACS Catal. 2020, 10, 999−1009.
Chun-Yuen Wong − Department of Chemistry, City
University of Hong Kong, Kowloon, Hong Kong;
orcid.org/0000-0003-4780-480X
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.0c05684
Notes
The authors declare no competing financial interest.
(5) Salzmann, K.; Segarra, C.; Albrecht, M. Donor-Flexible
Bis(pyridylidene amide) Ligands for Highly Efficient Ruthenium-
Catalyzed Olefin Oxidation. Angew. Chem., Int. Ed. 2020, 59, 8932−
8936.
ACKNOWLEDGMENTS
■
We thank the financial support from the HKSAR University
Grant Council (grant no. CUHK 14305520), The Chinese
University of Hong Kong direct grant (project no. 4053394),
and Innovation and Technology Commission to the State Key
Laboratory of Synthetic Chemistry (GHP/004/16GD). R.-B.
H. thanks The Chinese University of Hong Kong Research
Fellowship Scheme for financial support. We also thank
Professor H. K. Lee (The Chinese University of Hong Kong)
for assisting the cyclic voltammetry experiment.
(6) Some zwitterionic metal complexs enhance catalytic perform-
ance: (a) Chauvin, R. Zwitterionic Organometallates. Eur. J. Inorg.
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J. 2014, 20, 7868−7877. (d) Ramakrishna, V.; Reddy, N. D. Synthesis
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in cross-coupling reactions of aryl, heteroaryl and benzyl bromides
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