What Happens When the Terminal Aromatization is Blocked? Construction of 1,2-Dihydroquinoline Derivatives by sp3 C-H Bond Oxidation of N-Arylalaninates

Article abstract of DOI:10.1002/adsc.201500885

Using an aromatization-blocked strategy, the 1,2-dihydroquinoline skeleton was efficiently constructed by sp³ C-H bond oxidation of N-arylalaninates under catalytic radical cation salt-promoted conditions. Investigation of the reaction scope sh

Full text of DOI:10.1002/adsc.201500885

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DOI: 10.1002/adsc.201500885
What Happens When the Terminal Aromatization is Blocked?
3
À
Construction of 1,2-Dihydroquinoline Derivatives by sp C H
Bond Oxidation of N-Arylalaninates
Shiwei Lü,a Yingzu Zhu,a Xingge Ma,a and Xiaodong Jiaa,*
a
College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, Gansu 730070, Peopleꢀs
Republic of China
Fax: (+86)-931-797-1989; e-mail: jiaxd1975@163.com or jiaxd1975@nwnu.edu.cn
Received: September 22, 2015; Revised: November 18, 2015; Published online: March 3, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500885.
Abstract: Using an aromatization-blocked strategy,
the 1,2-dihydroquinoline skeleton was efficiently
dihydroquinolines has focused on a catalytic version.
Based on the transition metal (Pd,[12] Ru,[13] Ag[14] and
Au[15]) catalyzed reactions between anilines with al-
kynes, polysubstitiuted 1,2-dihydroquinolines can be
produced in good yields. You and co-workers report-
ed an elegant route to enantiomerically enriched 1,2-
dihydroquinoline derivatives by Ru-catalyzed ring-
closing metathesis (RCM) reaction of N-allyl-2-vinyl-
anilines.[16] Using an intramolecular allylic amination
strategy, the Chan and Sun groups achieved a synthesis
of substituted dihydroquinolines under AuCl3/AgSbF6
(5 mol%/15 mol%) and FeCl3·6H2O (2 mol%) cata-
lyzed conditions, respectively.[17] The reaction between
carbonyl-containing compounds is also an efficient
method to construct 1,2-dihydroquinolines, which was
generally prompted by acids.[18] Besides these general
substrates and their transformations, some other
3
À
constructed by sp C H bond oxidation of N-aryl-
alaninates under catalytic radical cation salt-pro-
moted conditions. Investigation of the reaction
scope shows broad generality and good functional
group tolerance. This method provides a new way
to synthesize 1,2-dihydroquinoline derivatives from
easily accessible starting materials.
Keywords: aromatization-blocked strategy; N-aryl-
alaninates; sp3 C H oxidation; 1,2-dihydroquino-
À
lines; radical cation salts
The partially hydrogenated quinoline skeleton is methods can also be used to build the dihydroquino-
a key structural motif in various natural products, line framework.[19] However, some remarkable limita-
pharmaceuticals, and synthetic intermediates,[1] in tions still exist in these procedures, such as low yields,
which dihydroquinolines are known to display harsh reaction conditions, narrow substrate scope, in-
a broad range of biological activities and have poten- accessible catalysts, and poor functional group toler-
tial pharmaceutical applications, such as antioxida- ance. Therefore, a simple and facile method for con-
tive,[2] anti-inflammatory,[3] psychotropic,[4] anti-aller- structing dihydroquinolines, especially highly func-
genic,[5] and estrogenic activities.[6] For example, tionalized ones, is still desirable and has practical ben-
ethoxyquin (6-ethoxy-1,2-dihydro-2,2,4-trimethylqui- efits.
À
noline) is an FDA-approved antioxidant commonly
used as a preservative in the food processing indus- mediates has attracted much attention, and a variety
Recently, C H activation mediated by radical inter-
try.[7] Furthermore, some derivatives of dihydroquino- of methodologies have been established.[20] Since the
line were found to display outstanding antitrypanoso- first example of aerobic C H oxidation of an sp C
3
À
À
mal activities.[8]
H bond prompted by catalytic radical cation salts in
Due to their great importance, considerable efforts the presence of O2,[21a] we have reported a series of
have been devoted to the synthesis of the dihydroqui- syntheses of heterocyclic compounds, such as quino-
noline skeleton and numerous methods have been de- lines, quinoline fused lactones and lactams, pyridines,
veloped to access to this family of compounds.[9] Since 1,4-dihydropyridines.[21] In these reactions, two aspects
the pioneering work of Skraup in 1880s,[10] the Skraup were focused: (i) use of triarylamine radical cation to
3
À
reaction and its variants have been commonly utilized achieve catalytic oxidation of sp C H bonds avoiding
in the synthesis of dihydroquinolines.[11] Recently, effi- over use of a stoichiometric oxidant and (ii) construc-
À
cient methodology directed toward the synthesis of tion of important heterocyclic skeletons based on C
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What Happens When the Terminal Aromatization is Blocked?
TBPA+C [(tris(4-bromophenyl)aminium hexachloroan-
timonate] induced conditions.[22] To our surprise, the
intermolecular cyclization of 1a and styrene was total-
ly inhibited, and 2a was isolated in 40% yield, instead
(Figure 2). From retrosynthetic disconnections, the
synthesis of this 1,2-dihydroquinline is inaccessible
through the corresponding alkyl acrylates or alkyl
propiolate, for 2-azadienes are generally electron-
poor, and only react with electron-rich dienophiles.
However, the tautomer of the imine, an enamine, ex-
hibited good reactivity towards 2-azadienes, which
could be easily provided by oxidation of N-arylalani-
À
nates. Furthermore, the termination C N bond cleav-
age could be achieved under radical cation salt-in-
duced conditions.[23] Inspired by the above result, we
used the reaction of 1a as a model reaction to screen
the best reaction conditions (Table 1). Initially, in the
presence of 5 mol% TBPA+C, the desired product was
obtained in 46% yield (entry 1). On increasing the
amount of TBPA+C to 10 mol%, the yield was im-
proved to 64% (entry 2).
Figure 1. Reactions designed with the aromatization-blocked
strategy.
H functionalization. During this research, we found
If 20 mol% of TBPA+C was added, the reaction
that late stage aromatization was an important driving became complicated, and the expected dihydroquino-
force to achieve an efficient transformation. For ex- line was isolated in lower yield (entry 3). Next, the re-
ample, if a-methylstyrene was used to react with N-ar- action temperature was screened. At room tempera-
ylglycine esters and N-benzylanilines, respectively, in ture, the reaction became slow, and only trace of the
which the terminal aromatization was blocked by the product was detected by TLC after 13 h (entry 4).
methyl group, the reaction efficiency decreased and With use of higher temperatures, the yields were in-
none of the expected 1,2,3,4-tetrahydroquinoline creased (entries 5 and 6). Further optimization of the
(THQ) was detected [Figure 1, Eq. (1)]. Similarly, if solvents showed that DCE was the best solvent, and
a methyl group was installed on the nitrogen of N-ar- the desired product was isolated in 72% yield
ylglycine esters, no THQ was found, either [Figure 1, (entry 7). Addition of acids was tried to increase the
Eq. (2)]. So we questioned if blocking the terminal ar- yield, but no positive effect on the reaction was found
omatization could be a strategy to find new reactions (entries 10–13). To confirm the need for dioxygen, the
and make some new compounds. Herein, we report oxidative reaction was performed under an argon at-
an interesting construction of 1,2-dihydroquinolines mosphere; however, no reaction occurred (entry 14),
3
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with a tertiary center by aerobic oxidation of N-aryl- implying that O2 was crucial to sp C H bond oxida-
alaninates induced by TBPA+C [Figure 1, Eq. (3)].
tion. Under an air atmosphere, the reaction of 1a was
To investigate the aromatization-blocked strategy, also unsuccessful, and only a trace of the desired
the reaction between 1a and styrene was tried under product was detected by TLC (entry 15).
Figure 2. Reaction of 1a induced by the radical cation salt.
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Shiwei Lü et al.
Table 1. Optimization of the reaction conditions.[a]
Entry
TBPA+C (mol%)
Additive (mol%)
Solvent
Temperature [oC]
Time [h]
Yield [%][b]
1
2
3
4
5
6
7
8
5
no
no
no
no
no
no
no
no
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
DCE
CH3CN
CHCl3
DCE
60
60
60
r.t
40
80
60
60
60
60
60
60
60
60
60
8
8
8
13
10
6
8.5
8.5
8.5
15
12
12
12
24
48
46
64
50
tace
42
10
20
10
10
10
10
10
10
10
10
10
10
10
10
72
85 (72)[c]
57
9
no
66
trace
57
10
11
12
13
14
15
BF3·O(Et)2
TsOH
InCl3·4H2O
CuCl
no
no
DCE
DCE
DCE
DCE
58
54
N.R.[d]
trace[e]
DCE
[a]
Unless otherwise specified, the reaction was carried out with 1a (0.1 mmol) in the presence of TBPA+C, and anhydrous
solvent (1.5 mL).
[b]
1
Yield of crude product by H NMR using 1,3,5-trimethoxylbenzene as internal standard, and the yield was calculated
based on 0.05 mmol of 1a.
[c]
[d]
[e]
Yield in the parentheses is the isolated yield.
The reaction was performed under argon.
The reaction was performed under an air atmosphere.
With the best conditions established, we then inves- ciency and the desired dihydroquinolines were afford-
tigated scope of synthesis of 1,2-dihydroquinolines. ed in good yields (2l–2n). Only the decarboxylation
Firstly, the substituent effect on anilines was exam- product was isolated when tert-butyl ester was used in
ined, and the results are compiled in Scheme 1. The the standard reaction conditions (3o). The phenyl
substrates with electron-donating groups gave better ester could also be tolerated, giving 73% yield (2p).
results than those with electron-withdrawing groups A lower yield was provided when the benzyl ester
(2a–2c compared with 2d and 2e). The reason is at- was involved, probably due to the existence of anoth-
3
À
tributed to their higher electron-donating ability to er active benzyl sp C H bond, which might disturb
stabilize the radical intermediate of alaninates. In our the oxidation process (2q). We were pleased to note
previous papers, we reported that a free phenolic hy- that carbon-carbon unsaturated bonds such as those
droxy group could be tolerated under the oxidative in allyl, 2-butenyl, cinnamyl and but-3-yn-1-yl groups
conditions, but in this case, no desired product was could be well tolerated, yielding the desired products
isolated when ethyl (4-hydroxyphenyl)alaninate was in 69–90% yields (2r–2u). Even the fragile cyclopro-
involved. Introduction of ortho-groups did not result pylmethyl group remained unchanged under the oxi-
in a decrease of the reaction efficiency, and the corre- dative conditions (2v), showing good functional group
sponding dihydroquinolines were obtained in good tolerance. The furfuryl ester was also tested, and the
yields (2f–2g). Interestingly, if meta-group-substituted product was obtained in a slightly lower yield (2w).
anilines were employed, the cyclization selectively oc-
To evaluate the practical application of our
curred on the para-position (2j to 2k). It is worth method, the reaction of N-arylalaninates has been
noting that, in some cases, decarboxylation products performed on a large scale. And to our delight, no
of dihydroquinoline, 2-methylquinoline-4-carboxy- yield loss was observed (Scheme 3).
lates, were isolated in certain amount (3h–3k). The
exact reason remains unknown.
To elucidate the reaction process, several control
experiments were conducted under the standard con-
Next, different esters were examined to evaluate ditions (Scheme 4). In the presence of the radical in-
the tolerance of various functional groups (Scheme 2). hibitor, TEMPO (1 equiv.) or BHT (2 equiv.), the re-
In general, alkyl esters did not affect the reaction effi- action was totally inhibited, which means that the re-
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Scheme 1. Reaction of ethyl N-arylalaninates. Reaction conditions: 1 (1 mmol), TBPA+C (10 mol%), DCE (5 mL), 608C
under O2, isolated yield (the yields were calculated based on 0. 5 mmol of 1).
Scheme 2. Reaction of alkyl N-arylalaninates. Reaction conditions: 1 (1 mmol), TBPA+C (10 mol%), DCE (5 mL), 608C
under O2, isolated yield (the yields were calculated based on 0. 5 mmol of 1).
action was mediated by a radical intermediate. Ac- alkyl pyruvate under Lewis acid-catalyzed conditions,
cording to literature reports, anilines can react with providing the corresponding dihydroquinolines.[19] So
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Shiwei Lü et al.
prompted Mannich-type reaction was not the domi-
nant reaction pathway. If alkyl pyruvate imine was
used instead of N-arylalaninate, the reaction efficien-
cy was also decreased, giving the corresponding prod-
uct 2a in 50% yield, which implied that the reaction
pathway might be different from the reported acid-
catalyzed process. To rule out other mechanisms, we
tested the reaction of 1a in the presence of methyl ac-
rylate, which has a different ester group, but after the
product analysis, none of the anticipated methyl acry-
late was found. It is understandable that methyl acry-
late is a poor substrate in 2-aza-[4+2] cycloaddition.
Based on the results described above, a possible
Scheme 3. Preparation of DHQs in a large scale.
mechanism is proposed to rationalize the formation
3
À
of the products (Scheme 5). The sp C H bond of an
N-arylalaninate was oxidized by the radical cation
salt, in the presence of dioxygen, yielding a radical in-
termediate A. Then an imine intermediate was pro-
vided whose tautomerization leads to enamine B,
a good radical acceptor. Attack on the enamine B by
radical A results in the formation of a new radical C.
After intramolecular radical cyclization and further
aromatization, the tetrahydroquinoline skeleton is as-
sembled. Finally, elimination of aniline leads to a 1,2-
dihydroquinoline. However, at this stage, the acid-
prompted process cannot be totally ruled out, al-
though the control experiments showed that it was
not the dominant reaction pathway.
In conclusion, based on this aromatization-blocked
3
À
strategy, an efficient sp C H oxidation of N-arylala-
ninate derivatives was achieved, synthesizing a series
of 1,2-dihydroquinolines. The scope examination
shows good substrate generality and functional group
tolerance. This method provides a new way to con-
struct the biologically relevant dihydroquinoline skel-
eton. Further applications of this reaction and studies
Scheme 4. Control experiments.
the reaction between 4- methoxyaniline and ethyl pyr- on the synthesis of partially hydrogenated quinolines
uvate was tried under these radical cation salt-in- are underway in our laboratory.
duced conditions, and the desired product was only
isolated in 42% yield. This result shows that the acid-
Scheme 5. Proposed mechanism for DHQ formation.
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À
and flushed with O2, then TBPA+C SbCl6 (10 mol%) was
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filtered, and the solvent was removed under reduced pres-
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matography eluted with petroleum ether/acetone (v/v 20:1)
to afford the products.
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Acknowledgements
The authors thank Natural Science Foundation of China
(NSFC, No. 21362030 and 21562038) for supporting our re-
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Adv. Synth. Catal. 2016, 358, 1004 – 1010