An Unexpected Construction of 2-Arylquinolines from N-Cinnamylanilines through sp3 Ci-H Aerobic Oxidation Induced by a Catalytic Radical Cation Salt

Article abstract of DOI:10.1002/adsc.201500574

An unexpected reaction of cinnamylanilines was achieved through the radical cation salt-induced aerobic oxidation of sp³ C-H bonds, providing a series of 2-arylquinolines. The mechanistic study shows that the cinnamylaniline was oxidized to an imine, which was attacked by the aniline generated through decomposition of the corresponding imine. After further intramolecular cyclization and aromatization, 2-arylquinolines were obtained. This reaction provides a new method to construct 2-arylquinolines from readily accessible starting materials.

Full text of DOI:10.1002/adsc.201500574

UPDATES
DOI: 10.1002/adsc.201500574
An Unexpected Construction of 2-Arylquinolines from
3
À
N-Cinnamylanilines through sp C H Aerobic Oxidation Induced
by a Catalytic Radical Cation Salt
Fang Liu,a Liangliang Yu,a Shiwei Lv,a Junjun Yao,a Jing Liu,a and Xiaodong Jiaa,*
a
Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education, China. Gansu Key Laboratory of
Polymer Materials, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou, Gansu
730070, Peopleꢀs Republic of China
E-mail: jiaxd1975@163.com
Received: June 14, 2015; Revised: October 8, 2015; Published online: January 18, 2016
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500574.
Abstract: An unexpected reaction of cinnamylani-
anilines,[10a] in which a leaving group (such as a hy-
droxy group) is necessary to achieve the terminal aro-
matizition. The combination of O-alkynylisocyano-
benzene with an appropriate nucleophile has also
been achieved both with and without a basic promo-
ter.[10c,d] Cyclization of propargyl-substituted aniline
derivatives through Friedel–Crafts-type reactions
could also be used to forge the quinoline skeleton.[8b]
In these elegant synthetic strategies, the substrates
must be pre-functionalized in order to construct the
quinoline motif.
lines was achieved through the radical cation salt-
3
À
induced aerobic oxidation of sp C H bonds, pro-
viding a series of 2-arylquinolines. The mechanistic
study shows that the cinnamylaniline was oxidized
to an imine, which was attacked by the aniline gen-
erated through decomposition of the corresponding
imine. After further intramolecular cyclization and
aromatization, 2-arylquinolines were obtained. This
reaction provides a new method to construct 2-aryl-
quinolines from readily accessible starting materi-
als.
Recently, MacMillan provided an artful direct b-al-
kylation of aldehydes via photoredox organocatalysis,
À
in which the allyl C H bond of an enamine was oxi-
Keywords: aerobic oxidation; 2-arylquinolines; N-
cinnamylaniline; radical cation salts
dized to an allyl free radical, followed by radical addi-
tion to the C=C double bond.[11] This methodology in-
spired us to investigate if similar free radicals could
be generated by oxidation of N-allylaniline, it could
then undergo intramolecular or intermolecular radical
À
The quinoline skeleton, which is widespread in natu- addition to the C=C double bond to achieve C C
ral products and drugs, plays important roles in organ- bond formation. If our hypothesis is feasible, the
ic and pharmaceutical chemistry. Their derivatives ex- quinoline skeleton could be formed through an intra-
hibit a wide range of biological activities, such as anti- molecular radical addition pathway [Figure 1, Eq.
malarial,[1] anti-inflammatory,[2] antibacterial[3] and (4)].
others.[4] Because of their great importance, consider-
Recently, we reported, for the first time, that the
able efforts have been devoted to the synthesis of catalytic radical cation salt, [tris(4-bromophenyl)ami-
quinoline derivatives, including the Conrad–Limpach– nium hexachloroantimonate, TBPA+·], could efficient-
3
Knorr synthesis,[5] the Skraup–Doebner–Von Miller ly initiate C H oxidation of sp C H bonds adjacent
synthesis,[6] the Friedländer synthesis,[7] and other to nitrogen to build heterocyclic skeletons.[12] This
methods.[8] It is worth noting that the [4+2] cycloaddi- method represented a new approach to CDCs (cross-
tion of N-arylaldimines with dienophiles (Povarov re- dehydrogenative couplings), avoiding use of excess
action) is also a powerful approach to construct the quantities of the oxidants (such as DDQ, peroxides
À
À
quinoline skeleton.[9]
and so on). The catalytic oxidation of N-benzylani-
Besides these intermolecular approaches, the intra- lines was also achieved under radical cation salt-in-
molecular cyclization of aniline derivatives has also duced conditions, providing a series of 2,4-diarylqui-
been attempted to construct the quinoline skeleton.[10] nolines in high yield [Figure 1, Eq. (1)].[12e] Based on
For example, Wangꢀs group reported an iron-cata- these results, we questioned if the analogues of N-
lyzed annulation of homoallylic alcohol substituted benzylanilines, the N-allylanilines, couzld also be oxi-
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Fang Liu et al.
3
À
Figure 1. Radical cation salt-induced sp C H oxidation.
dized to the corresponding free radical (Figure 1, in- phenylquinoline instead of 4-phenylquinoline, which
À
termediate A) which is stabilized by the adjacent ni- means that the C N bond of 1b was broken during
trogen and C=C double bond. Since the radical inter- the process of the cyclization.
mediate A can resonate to B, two different reactions
To maximize the yield of the desired product, an
would be possible. In the presence of a radical accept- optimization of reaction conditions was performed,
or, such as alkenes, intermolecular addition might and the results are compiled in Table 1. From the sol-
occur via [4+2] cycloaddition to yield 2-vinyl-1,2,3,4- vent screen (entries 1–9), acetonitrile was the best sol-
tetrahydroqinolines [Figure 1, Eq. (3)]. If the radical vent, and the quinoline product was isolated in 72%
B was more stable, intramolecular cycloaddition yield (entry 9). Evaluation of the reaction tempera-
might be dominant and 1,4-dihyroquinolines could be ture shows that lower temperatures drastically de-
generated [Figure 1, Eq. (4)]. Since the aromatization creased the yields (entries 10 and 11), and slightly
of 1,2,3,4-tetrahydroqinolines and 1,4-DHPs can also lower yields could be obtained under elevated tem-
be achieved under radical cation salt-induced aerobic peratures (entries 12 and 13). Screens of catalyst load-
oxidation, it is predictable that quinoline derivatives ing show that in the presence of 5 mol% TBPA+C,
would be provided through tandem aromatization.[13]
a 73% yield of the desired product was isolated with
With these ideas in mind, we used the N-cinnamyl- an elongated reaction time (entry 14). Under an
aniline 1b as a model substrate to test the possibility argon atmosphere, only a trace of product was detect-
of the intermolecular and intramolecular reactions ed, so it is obvious that dioxygen is crucial to achieve
[Figure 1, Eq. (2)]. The reactions occurred with low full conversion of the starting material (entry 18). In
conversion of 1b, providing the quinoline products in the absence of TBPA+C, no reaction occurred, which
low yield. Even in the presence of 1 to 5 equivalents implied that dioxygen could not oxidize the substrate
of styrenes (such as 4-H, 4-Br and 4-MeO substituted and that the radical cation salt is vital to trigger the
3
À
styrenes), the intramolecular product was the exclu- sp C H bond oxidation (entry 17). To further accel-
sive product. However, to our great surprise, the erate the cyclization, a catalytic amount of Lewis acid
1H NMR shows that the structure of the product is 2- was added, but the reaction efficiency was decreased
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An Unexpected Construction of 2-Arylquinolines from N-Cinnamylanilines
Table 1. Optimization of the reaction conditions
Entry
TBPA+C (mol%)
Solvent
Temperature [oC]
Time [h ]
Yield [%][a]
1
2
3
4
5
6
7
8
10
10
10
10
10
10
10
10
10
10
10
10
10
5
CH3CN
CH2Cl2
CHCl3
CH3CO2Et
EtOH
PhCH3
THF
DMF
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
40
40
40
40
40
40
40
40
40
10
20
60
80
40
40
40
40
40
40
72
32
32
32
32
32
18
18
32
48
32
24
20
48
24
16
48
72
32
33[b,d]
trace[b]
trace[c]
trace[c]
trace[c]
trace[c]
35
45
72
trace
39
65
9
10
11
12
13
14
15
16
17
18
19
63
73
45
37
15
20
0
10
10
0
trace[c,e]
55[f]
[a]
Isolated yield.
Under air atmosphere.
Starting material was recovered.
Conversion of the starting material is about 50%.
Under argon.
10 mol% InCl3 added.
[b]
[c]
[d]
[e]
[f]
(entry 19), which implied that the cyclization process
was not the rate-determining step and the reaction ef-
ficiency could not be increased by a Lewis acid.
To determine the generality of this protocol, we
turned our attention toward the construction of qui-
nolines using the optimized conditions (Scheme 1).
All tested N-cinnamylanilines exhibited good reactivi-
ty, affording the quinoline products in moderate to
good yields. Electron-donating groups show higher re-
activity towards intramolecular annulations, giving
better yields of the desired quinoline products (2c, 2d,
2e and 2g). Electron-withdrawing groups could also
be tolerated. The reaction of N-cinnamyl-4-nitroani-
line was also tested, but no reaction occurred (not
shown in Scheme 1). The starting material remained
unchanged and was recovered after reaction. This is
À
due to the fact that the strong EWG makes the C H
bonds adjacent to nitrogen inactive and hard to be
oxidized by TBPA+C/O2. In accord with our previous
results,[12] in the absence of a para-substituent to the
aniline nitrogen, the quinolines were isolated in lower
yields (2h–2k). This is due to the formation of a diaryl-
methane derivative which is more favored than quino-
Scheme 1. Reaction of N-cinnamylanilines. Reaction condi-
tions: 1 (0.5 mmol), TBPA+C, MeCN (2 mL), 408C under O2,
lines.[14] N-Cinnamyl-a- and -b-naphthylamines also isolated yields given.
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Fang Liu et al.
Scheme 2. Reaction of substituted N-cinnamylanilines. Reac-
tion conditions: 1 (0.5 mmol), TBPA+C (10 mol%), MeCN
(2 mL), 408C under O2, isolated yields given.
Scheme 4. Proposed mechanism for the oxidation of N-allyl-
4-methylaniline.
tion on the b-position would be possible, providing
a phenanthridine derivative. However, 2,8-diphenyl-
quinoline 2t was isolated exclusively in 56% yield,
which was fully identified by single crystal X-ray
structure analysis.
We also tried the reaction of N-allyl-4-methylani-
line, and a novel amide product 4 was isolated in 23%
yield, together with some unidentified products. Al-
though the exact mechanism to product 4 remains un-
known,
a
plausible pathway was proposed
(Scheme 4). N-Allyl-4-methylaniline was oxidized to
radical intermediate A, which was trapped by dioxy-
gen, providing a peroxide radical B. After 5-endo-trig
cyclization,[17] an intermediate C was generated, which
Scheme 3. Reaction of N-cinnamylbiphenyl-2-amine.
show good reactivity to provide the benzoquinolines was further transformed to product 4. This result im-
in good yields (2l and 2m).
plied that the phenyl group Ar2 is crucial to stabilize
To evaluate the effect of Ar2, reactions of several the radical intermediate to achieve the cyclization.
substituted N-cinnamylanilines were performed under To probe the reaction mechanism, several control
the standard reaction conditions (Scheme 2). Elec- experiments were conducted (Scheme 5). In the pres-
tron-withdrawing groups on Ar2 decreased the yields ence of the radical inhibitor TEMPO (1 equivalent),
dramatically (2n–2q), which suggested the existence the reaction was inhibited and only trace of the de-
1
of an electron-deficient intermediate. Increasing the sired product was detected by H NMR on the crude
electron density of nitrogen would be beneficial to ox- mixture [Scheme 5, Eq. (1)], which suggested that
3
À
idation of the sp C H bond, probably due to a stron- radical intermediates might be involved. The reaction
ger conjugative effect (see 2r compared with 2n).[15] of cinnamaldehyde imine was conducted under the
We also installed a 4-methoxy group on the phenyl standard conditions, and only 15% of the quinoline
ring to test the reaction efficiency, but only a trace of was isolated [Scheme 5, Eq. (2) compared with
the desired product 2s was detected by TLC, together entry 9 in Table 1]. This result shows that Skraup re-
with decomposition of the starting material. The action via a diazetidinium cation intermediate was
reason probably lies in the fact that the strong EDG not the dominant reaction pathway.[18] Addition of
group, MeO, made the substrate too electron-rich, 1 eqivalent of water did not increase the yield of 2b
and easy to be oxidized by TBPA+C directly (single [Eq. (3)]. In the presence of 0.1 equivalent of 4-bro-
À
electron oxidation). So the C H oxidation process moaniline, the cyclization of the cinnamaldehyde
(by TBPA+C/O2) was suppressed.[16]
imine occurred smoothly, yielding the desired product
To test the other reaction possibility, N-cinnamylbi- 2b in 75% yield [Eq. (4)]. These results suggested
phenyl-2-amine 1t was synthesized, in which two posi- that the existence of aniline is crucial to trigger the ef-
tions are active (Scheme 3). We questioned if cycliza- ficient transformation. The reaction of 1b was con-
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An Unexpected Construction of 2-Arylquinolines from N-Cinnamylanilines
Scheme 6. Proposed mechanism for the cyclization of N-cin-
namylanilines.
elimination of the aniline. The released aniline partic-
ipates in the second reaction cycle.
In summary, we have developed an efficient synthe-
sis of 2-arylquinoline using a radical cation salt-
3
À
prompted sp C H aerobic oxidation. The study pro-
vides a new route for the direct formation of quino-
lines by using facile N-cinnamylanilines. Related stud-
ies including scope, mechanism and further applica-
tions are in progress in our laboratory.
Experimental Section
Scheme 5. Control experiments.
Typical Procedure
A solution of 1b (0.5 mmol) in CH3CN (2 mL) was mixed
fully and flushed with O2, then TBPA+C (10 mol%) was
added dropwise under an oxygen atmosphere. The reaction
solution was stirred at under 408C. After completion as
monitored by TLC (by UV visualization), the reaction was
quenched by addition of saturated Na2CO3 solution in
MeOH (10 mL). The mixture was poured into a separatory
funnel with the addition of excess DCM (10 mL), and then
the crude organic solution was extracted three times with
water to remove inorganic salts. The organic phase was then
dried over anhydrous magnesium sulfate, filtered, and the
solvent was removed under reduced pressure. The products
were separated by silica gel column chromatography eluted
with petroleum ether/acetone (v/v 80:1) to afford the prod-
ucts.
ducted in the presence of one equivalent of more nu-
cleophilic 4-methoxylaniline and the corresponding
quinoline products 2b and 2c were isolated in 7% and
47% yields, respectively. From these results we can
see that the more nucleophilic aniline gave a higher
À
yield, which supported that the C N bond was
broken during the reaction process. Furthermore,
since aniline in high concentration could decompose
the radical cation salt, the overall yield was lower.
Based on above results, a tentative mechanism to
rationalize this radical cation salt-induced oxidation
3
3
À
of sp C H bond is illustrated in Scheme 6. The sp
À
C H bond adjacent to the anilino group was oxidized
by TBPA+C in the presence of O2, yielding a radical in-
termediate A, which underwent further oxidation
generating an iminium intermediate. Then a Michael-
type addition occurred between cinnamaldehyde
imine and aniline,[19] yielding an intermediate B. After
an intramolecular Friedel–Crafts-type cyclization, a 4-
anilinotetrahydroquinoline was formed, which was
further oxidized to a dihydroquinoline intermediate.
Acknowledgements
The authors thank Natural Science Foundation of China
The desired quinoline product was obtained after (NSFC) (No. 21362030) for supporting our research.
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