Ruthenium(II)-Catalyzed Regioselective C-8 Hydroxylation of 1,2,3,4-Tetrahydroquinolines

Article abstract of DOI:10.1021/acs.orglett.8b02926

Ru(II)-catalyzed chelation-assisted highly regioselective C8-hydroxylation of 1,2,3,4-tretrahydroquinolines has been developed. Various 1,2,3,4-tetrahydroquinolines underwent smooth C8-H hydroxylation with cheap and safe K₂S₂O₈ as the oxidant and oxygen source to furnish the corresponding products in good to excellent yields with high tolerance of the functional groups. The choice of a readily installable and removable N-pyrimidyl directing group is the key to catalysis. Mechanistic studies suggest the involvement of a six-membered ruthenacycle intermediate in the catalytic cycle. The method can also be extended to the direct hydroxylation of other (hetero)arene C-H bonds.

Full text of DOI:10.1021/acs.orglett.8b02926

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Ruthenium(II)-Catalyzed Regioselective C‑8 Hydroxylation of 1,2,3,4-
Tetrahydroquinolines
Changjun Chen, Yixiao Pan, Haoqiang Zhao, Xin Xu, Zhenli Luo, Lei Cao, Siqi Xi, Huanrong Li,
and Lijin Xu*
Department of Chemistry, Renmin University of China, Beijing 100872, China
S
* Supporting Information
ABSTRACT: Ru(II)-catalyzed chelation-assisted highly regiose-
lective C8-hydroxylation of 1,2,3,4-tretrahydroquinolines has been
developed. Various 1,2,3,4-tetrahydroquinolines underwent
smooth C8−H hydroxylation with cheap and safe K2S2O8 as the
oxidant and oxygen source to furnish the corresponding products
in good to excellent yields with high tolerance of the functional
groups. The choice of a readily installable and removable N-
pyrimidyl directing group is the key to catalysis. Mechanistic
studies suggest the involvement of a six-membered ruthenacycle
intermediate in the catalytic cycle. The method can also be extended to the direct hydroxylation of other (hetero)arene C−H
bonds.
1,2,3,4-Tetrahydroquinolines are abundant in a number of
biologically and pharmacologically active natural products and
synthetic compounds.1 Representative examples include the
antibiotic alkaloid helquinoline,2a the bioactive natural product
(S)-angustureine,2b the highly selective D2 receptor full agonist
sumanirole,2c and the novel antiarrhythmic agent nicainoprol2d
(Figure 1). Consequently, the efficient construction of 1,2,3,4-
under palladium, ruthenium, or rhodium catalysis. Despite this
progress, exploration of simple and efficient methods is still
highly appealing.
Over the past decade, transition-metal catalyzed hydrox-
ylation of inert C−H bonds has evolved as a powerful tool for
phenol syntheses.10 Early in 1987, Fujiwara and co-workers
groundbreakingly reported Pd(OAc)2-catalyzed direct hydrox-
ylation of benzene with oxygen, but this method suffered from
low reaction efficiency and very harsh reaction conditions.11 In
order to overcome these limitations, Pd-catalyzed arene C−H
bond hydroxylation has been widely studied, and a number of
catalytic systems have been successfully developed by using
various directing groups.12 Meanwhile, Ru-catalyzed chelation-
assisted C−H hydroxylation of arenes has been independently
developed by the groups of Rao,13a,e,f,h,k Ackermann,13b−d,g,i
Hong,13j and Singh.13l In addition, the catalytic systems based
on copper,14 rhodium,15a,b,d and iridium15c complexes also
performed efficiently to catalyze C−H hydroxylation of arenes.
Inspired by these elegant studies, we became interested in the
selective hydroxylation of the C8-position of tetrahydroquino-
lines because of the potential utility of such products. Actually,
methods for obtaining 8-hydroxy-1,2,3,4-tetrahydroquinolines
have been less developed, and the known protocols including
Povarov reaction of N-arylimines with alkenes and selective
reduction of 8-hydroxyquinolones are generally plagued by
limited substrate scope, uneasily available starting materials, and
the requirement of prefunctionalization (Scheme 1a,b).16 In this
context, the development of an efficient and general synthetic
pathway to 8-hydroxy-1,2,3,4-tetrahydroquinolines from simple
1,2,3,4-tetrahydroquinolines via transition-metal catalyzed
Figure 1. Some examples of 1,2,3,4-tetrahydroquinoline-containing
pharmaceuticals and bioactive molecules.
tetrahydroquinolines has been a topic of sustained interest
during the past several decades. Catalytic reduction of
quinolines represents one of the most reliable methods to
obtain tetrahydroquinolines, and diverse successful catalytic
systems have been developed.3 Extensive efforts have also been
made to synthesize 1,2,3,4-tetrahydroquinolines via intra-
molecular amination reactions,4 Povarov reactions,5a,b and
other cyclization reactions.5c Notably, with significant advances
in catalytic C−H bond functionalization,6 the application of
transition-metal-catalyzed C−H functionalization events to
access functionalized 1,2,3,4-tetrahydroquinolines in a step-
and atom-economic fashion from simple starting materials is of
much current research interest.7−9 In particular, by taking
advantage of N-directing groups, simple 1,2,3,4-tetrahydroqui-
nolines underwent smooth C−H bond chlorination,9a aryla-
tion,9b−f alkenylation,9e,j acylation,9h,i and amination9m at the
C8 position with high efficiency and functional group tolerance
Received: September 12, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02926
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Letter
a
Scheme 1. Synthetic Pathways for 8-Hydroxy-1,2,3,4-
tetrahydroquinolines
Table 1. Optimization of Reaction Conditions
b
entry
solvent
oxidant
K2S2O8
additive
yield (%)
1
2
3
4
5
6
7
8
DCE
TCE
none
none
none
trace
14
23
trace
40
28
75
33
90
76
34
46
40
trace
trace
NR
NR
83
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
K2S2O8
PhI(OAc)2
PhI(OTFA)2
O2 (1 atm)
BQ
acetone
DCE
HBF4
HBF4
HBF4
HBF4
HBF4
HBF4
HPF6
MeSO3H
AcOH
PivOH
HBF4
HBF4
HBF4
HBF4
HBF4
HBF4
HBF4
acetone
TCE
DCE/acetone (1:1)
TCE/acetone (1:1)
DCE/acetone (2:1)
DCE/acetone (2:1)
DCE/acetone (2:1)
DCE/acetone (2:1)
DCE/acetone (2:1)
DCE/acetone (2:1)
DCE/acetone (2:1)
DCE/acetone (2:1)
DCE/acetone (2:1)
DCE/acetone (2:1)
DCE/acetone (2:1)
DCE/acetone (2:1)
9
regioselective C−H hydroxylation is highly desirable. On the
basis of our previous studies on transition-metal catalyzed
(hetero)arene C−H bond functionalization,17 herein, we
describe such an approach where hydroxyl group is installed
to the C8 position of 1,2,3,4-tetrahydroquinoline substrates by
using a readily accessible and removable pyrimidyl group as the
directing group and cheap and safe K2S2O8 as the oxidant and
oxygen source under Ru(II) catalysis (Scheme 1c). This
protocol features high yields, a broad substrate scope, excellent
regioselectivity, and wide functional group tolerance.
10
11
12
13
14
15
16
17
18
19
Oxone
none
K2S2O8
NR
NR
c
20
Inspired by Rao’s reports on Ru-catalyzed direct hydrox-
ylation of anilides using the N-acylated amine directing
group,13e,k our studies were initiated by examining the
hydroxylation of various N-acylated 1,2,3,4-tetrahydroquino-
lines under Ru catalysis. However, a variety of Ru(II) catalytic
systems including those that have proven to be effective in
catalyzing direct hydroxylation of anilides, benzamides,
acetophenones, benzoates, and benzaldehydes turned out to
be totally ineffective (see Supporting Information). Recent
studies regarding transition-metal catalyzed C−H bond
functionalization have exhibited the power of the readily
installable and removable N-(2-pyrimidyl) directing group in
controlling the site selectivity and promoting the reactivity.18
Then, we envisioned that the installation of a 2-pyrimidyl group
on the tetrahydroquinoline N atom may facilitate the direct
hydroxylation of C8−H bond. With this in mind, our
investigations focused on the direct hydroxylation of 1-
(pyrimidin-2-yl)-1,2,3,4-tetrahydroquinoline (1a) under Ru
catalysis (Table 1 and Supporting Information). With [Ru(p-
cymene)Cl2]2 as the catalyst and K2S2O8 as the oxidant, we
attempted the hydroxylation of 1a in DCE at 100 °C for 24 h.
However, only a trace amount of the desired product 2a was
observed (Table 1, entry 1). When CHCl2CHCl2 (TCE) was
used as the solvent, the yield of 2a was increased to 14% (Table
1, entry 2). Further improvement was seen by switching the
solvent to acetone, whereupon 2a was isolated in 23% yield
(Table 1, entry 3). Subsequent studies showed that introducing
HBF4 as the additive enabled the reactions in TCE and acetone
to deliver 2a in better yields of 28% and 40%, respectively, but is
ineffective in DCE (Table 1, entries 4−6). With HBF4 as the
additive, the yield of 2a was enhanced to 75% by using a mixed
solvent of DCE/acetone (1:1), but the solvent system of TCE/
acetone (1:1) only delivered 2a in 33% yield (Table 1, entries 7
and 8). Gratifyingly, performing the reaction in DCE/acetone
(2:1) resulted in the formation of 2a in 90% yield (Table 1, entry
9). Further optimization revealed that HBF4 outperformed
other commonly employed acids (Table 1, entries 10−13).
a
Reaction conditions: 1a (0.2 mmol), oxidant (0.4 mmol), [Ru(p-
cymene)Cl2]2 (2.5 mol %), additive (40.0 mol %), solvent (1.5 mL),
b
c
100 °C, 24 h, under air. NR: no reaction. Isolated yield. No [Ru(p-
cymene)Cl2]2.
Screening of oxidants proved that no other oxidant gave a better
outcome than K2S2O8 (Table 1, entries 14−18). For
comparison, the palladium catalytic systems that worked well
to catalyze hydroxylation of arenes with N-directing groups were
investigated, but none of them were effective (see Supporting
Information). Control experiments confirmed that both the
ruthenium catalyst and oxidant were essential for this reaction to
occur (Table 1, entries 19 and 20). We finally evaluated the
performance of other N-protected tetrahydroquinolines under
otherwise identical conditions (see Supporting Information). It
was found that the use of the N-(2-pyridyl) moiety only led to
the formation of a trace amount of the desired product, and the
reaction failed to provide any hydroxylation product when using
acyl, Me, or Ph as the N-directing group. No reaction was
detected in the case of free 1,2,3,4-tetrahydroquinoline. These
results obviously confirm the importance of the N-(2-pyrimidyl)
directing group for achieving high reactivity and selectivity in
this transformation.
After determining the optimized reaction conditions, we next
explored the scope of tetrahydroquinoline substrates (Scheme
2). The reaction proceeded smoothly for 2-alkyl substituted
tetrahydroquinolines (1b−1f) to exclusively deliver the 8-
hydroxylated products (2b−2f) in high yields, and the lower
yield of 2e is attributed to the steric hindrance of the isobutyl
group. When 2-phenyl-1-(pyrimidin-2-yl)-1,2,3,4-tetrahydro-
quinoline (1g) was employed, the reaction provided the
corresponding product 2g in 64% yield. The 3-, 4-, and 5-
substituted tetrahydroquinolines (1h−1l) performed well to
afford the hydroxylated products (2h−2l) in good to excellent
yields. Good to excellent yields were achieved in the
hydroxylation of a variety of 6-substituted tetrahydroquinolines
B
DOI: 10.1021/acs.orglett.8b02926
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Letter
of the dihydroxylated products was observed. Various N-aryl
acetamides (3g−3m) also underwent facile ortho-hydroxylation
to deliver the corresponding products (4g−4m) in good yields,
regardless of the nature of substituent on the aryl ring. However,
when using oxime ethers 3n and 3o, only traces of the
hydroxylated products were observed due to the decomposition
of the starting materials under acidic conditions. When replacing
K2S2O8 with Oxone and obviating the acid additive, the yields of
4n and 4o could be increased to 60% and 57%, respectively.
To show the synthetic utility of the current catalytic method,
we conducted some derivatizations of the C8-hydroxylated
tetrahydroquinoline products (see Supporting Information).
For example, treatment of product 2a with Tf2O easily resulted
in the formation of 1-(pyrimidin-2-yl)-1,2,3,4-tetrahydroquino-
lin-8-yl trifluoromethanesulfonate (5), which could be readily
functionalized via Pd-catalyzed cross-coupling reactions
(Scheme 4a). Furthermore, the pyrimidyl directing group
Scheme 2. Direct Hydroxylation of Various
Tetrahydroquinolines
a
a
Reaction conditions: 1a (0.2 mmol), K2S2O8 (0.4 mmol), [Ru(p-
Scheme 4. Synthetic Applications of the Products and
Removal of the N-Directing Group
cymene)Cl2]2 (2.5 mol %), HBF4 (40.0 mol %, ca. 40% w/w aq),
DCE/acetone (2/1) (1.5 mL), 100 °C, 24 h, under air, isolated yield.
b
Gram-scale reaction.
(1m−1u), and the electronic-rich substrates exhibited better
reactivity. Notably, the sensitive functional groups including F,
Cl, Br, NO2, alkene, and alkyne were well tolerated, thereby
providing the opportunity for further elaboration. The structure
of 2p was confirmed by single-crystal X-ray diffraction. Finally,
the hydroxylation of 1a could be carried out on a gram scale to
give the desired product 2a in 85% yield under the standard
reaction conditions, clearly demonstrating the scalability of the
current transformation.
Notably, this catalytic protocol is not restricted to the
tetrahydroquinoline substrates but also enables direct hydrox-
ylation of other (hetero)arene C−H bonds (Scheme 3). It was
a
Scheme 3. Direct Hydroxylation of Other (Hetero)arenes
could be smoothly removed by treating with NaOtBu in
DMSO, and the NH-free 8-hydroxyl tetrahydroquinolines could
be produced in high yields (Scheme 4b).
Preliminary experiments were carried out to gain mechanistic
insight (see Supporting Information). The observed H/D
scrambling at the C8-position in the deuterium labeling
experiments between 1a and D2O under the standard conditions
in the presence or absence of K2S2O8 suggested that the C(8)−
H bond undergoes reversible activation in the present catalytic
system. A kinetic isotope effect (KIE) of kH/kD = 1.38 was
obtained from the parallel experiments, indicating that the C−H
bond cleavage may not be involved in the turnover-determining
step. Conducting the reaction of 1a under argon or 18O2
atmosphere instead of air did not affect the reaction efficiency,
and no formation of 18O labeled 1a was detected. The yield of 2a
decreased when introducing H218O into the reaction system, but
we still did not observe the generation of 18O labeled 1a.
Obviously, these results rule out the possibility that the oxygen
atom in the produced hydroxyl group originates from O2 in air or
water in the solvent, and K2S2O8 serves as the source of oxygen
atom. The reaction of 1a proceeded smoothly in the presence of
2 equiv of TEMPO or BHT, thereby excluding the possibility of
radical generation in this transformation. Further studies
a
Reaction conditions: 1a (0.2 mmol), K2S2O8 (0.4 mmol), [Ru(p-
cymene)Cl2]2 (2.5 mol %), HBF4 (40.0 mol %, ca. 40% w/w aq),
DCE/acetone (2/1) (1.5 mL), 100 °C, 24 h, under air, isolated yield.
b
Oxone (0.4 mmol) was used to replace K2S2O8 in the absence of
acid.
found that 4-(pyrimidin-2-yl)-3,4-dihydro-2H-benzo[b][1,4]-
oxazine (3a) and 1-(pyrimidin-2-yl)indoline (3b) were
competent participants in this transformation, affording the
ortho-hydroxylated products (4a, 4b) in 86% and 77% yields,
respectively. Moreover, the structure of 4a was confirmed by
single-crystal X-ray diffraction. The substrates (3c−3f) contain-
ing different N-directing groups were compatible in generating
the desired products (4c−4f) in good yields, and no formation
C
DOI: 10.1021/acs.orglett.8b02926
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Letter
showed that the thermodynamically stable six-membered
ruthenacycle C1 could be readily prepared from the reaction
of 1a with [Ru(p-cymene)Cl2]2 by employing NaOAc as the
base in DCE at 60 °C (Scheme 5a), and its structure was
preparing various C8-functionalized 1,2,3,4-tetrahydroquino-
lines, especially those biologically interesting compounds, such
as nicainoprol. The identical catalytic system was also applicable
to the direct hydroxylation of other (hetero)arene C−H bonds.
Further investigation focusing on the applications and
mechanism of this reaction is underway in our laboratory.
Scheme 5. Mechanistic Studies
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.8b02926.
Detailed experimental procedures, characterization data,
1
and copies of H and 13C NMR spectra of products
(PDF)
Accession Codes
CCDC 1858877−1858879 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336033.
confirmed by single crystal X-ray diffraction analysis. Notably,
complex C1 reacted stoichiometrically to give the product 2a in
a good yield in shorter time (Scheme 5b). Moreover, complex
C1 exhibited almost the same catalytic activity in the reaction of
1a as [Ru(p-cymene)Cl2]2 did (Scheme 5c), indicating that this
six-membered complex is indeed an active species in the catalytic
cycle.
AUTHOR INFORMATION
■
Although the exact mechanism of this coupling reaction is still
not clear, a plausible mechanism (Scheme 6) is suggested on the
Corresponding Author
*E-mail: 20050062@ruc.edu.cn
ORCID
Scheme 6. Plausible Reaction Mechanism
Lijin Xu: 0000-0003-4067-8898
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from National Natural Science Foundation of
China (21372258) and the Fundamental Research Funds for the
Central Universities and the Research Funds of Renmin
University of China (Program 16XNLQ04) is gratefully
acknowledged.
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DOI: 10.1021/acs.orglett.8b02926
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