Visible-Light Cascade Photooxygenation of Tetrahydrocarbazoles and Cyclohepta[b]indoles: Access to C,N-Diacyliminium Ions

Article abstract of DOI:10.1002/anie.202007549

Tetrahydrocarbazoles and perhydrocyclohepta[b]indoles undergo a catalytic cascade singlet oxygenation in alkaline medium, which leads to chiral tricyclic perhydropyrido- and perhydroazepino[1,2-a]indoles in a single operation. These photooxygenation products are new synthetic equivalents of uncommon C,N-diacyliminium ions and can be functionalized with the aid of phosphoric acid organocatalysis.

Full text of DOI:10.1002/anie.202007549

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Accepted Article
Title: Visible light cascade photooxygenation of tetrahydrocarbazoles
and cyclohepta[b]indoles: access to rare C,N-diacyliminium ions
Authors: Mario Frahm, Thorsten von Drathen, Lisa Marie Gronbach,
Alice Voss, Felix Lorenz, Jonas Bresien, Alexander Villinger,
Frank Hoffmann, and Malte Brasholz
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10.1002/anie.202007549
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COMMUNICATION
Visible light cascade photooxygenation of tetrahydrocarbazoles
and cyclohepta[b]indoles: access to C,N-diacyliminium ions
Mario Frahm,[a],‡ Thorsten von Drathen,[b],‡ Lisa Marie Gronbach,[a],‡ Alice Voss,[a] Felix Lorenz,[a]
Jonas Bresien,[a] Alexander Villinger,[a] Frank Hoffmann[b] and Malte Brasholz[a],*
Abstract: Tetrahydrocarbazoles and perhydrocyclohepta[b]indoles
undergo a catalytic cascade singlet oxygenation in alkaline medium,
which leads to chiral tricyclic perhydropyrido- and perhydro-
azepino[1,2-a]indoles in a single operation. These photooxygenation
products are new synthetic equivalents of uncommon C,N-
diacyliminium ions and can be functionalized with the aid of
phosphoric acid organocatalysis.
sations with anilines, to furnish aminated dihydropyrrolo[3,4‐
b]quinolines.[9]
We report here the cascade photooxygenation of tetrahydro-
carbazoles and perhydrocyclohepta[b]indoles in alkaline medium,
which leads to perhydropyrido- and perhydroazepino[1,2-
a]indoles in a single operation (Scheme 1b). Contrary to the
previous methods, the initially introduced oxygen is not expelled
during a sequence leading back to aromatic products, but three
oxygen atoms are permanently fixated in a chiral product
structure. This multistep cascade can be regarded as a
telescoped photochemical Witkop-Winterfeldt/C,C-cleavage
reaction, and the perhydropyrido- and perhydroazepino[1,2-
a]indole products are highly useful synthetic equivalents of new
and uncommon C,N-diacyliminium ions.
The selective oxidation of organic compounds by catalytic aerobic
photooxidation has long been a key objective in sustainable
synthetic and industrial chemistry.[1] The rapid recent develop-
ment of chemical photocatalysis with visible light consequently
has fueled a growing interest in the development of new
photooxygenation reactions with dioxygen, particularly by way of
photoelectron transfer-induced and radical C-H oxygenations.[2]
Photochemically produced organic peroxides have not only been
utilized as key high-energy intermediates in the synthesis of
natural products and functional carbo- and heterocyclic target
structures,[3] but they have also gained much attention as potential
new antiparasitics, especially for combating malaria.[4]
a) Previous works: photooxygenation / amination of H4-carbazoles & H4-β-carbolines
i. Klussmann et al., 2013
nucleophilic trapping of C-1 carbocation intermediate
X = CH2
As part of our investigations in the photooxidation of N-
heterocyclic compounds,[5] we reexamined the dye-sensitized
photooxygenation of tetrahydrocarbazoles and tetrahydro-β-
carbolines, which initially generates the corresponding benzylic
hydroperoxides by way of the singlet oxygen ene reaction
(Scheme 1a).[6] In case of tetrahydrocarbazole, the C-4a
hydroperoxide is relatively long-lived in pH-neutral solution, but
exposure to Brønsted acids causes the rapid elimination of H2O2.
The resulting benzylic cation rearranges to a C-1 cation,[7] which
can be intercepted with nucleophiles like anilines to give 1-
aminotetrahydrocarbazoles, as reported by Klussmann et al.[8] In
the case of tetrahydro-β-carbolines, however, the benzylic
hydroperoxide instantly undergoes C,C-bond cleavage via its
unstable 1,2-dioxetane congener. Chen and coworkers engaged
the so-produced 2-acyl anilides in acid-mediated cycloconden-
X = N-Pg
X = CH2, N-Pg
ii. Chen et al., 2017
cyclocondensation of ketoamide with amine
b) This work: cascade photooxygenation of H4-carbazoles and cyclohepta[b]indoles
N-acyliminium ion
precursor
Scheme 1. Photooxygenation / functionalization of [b]-annulated indoles.
[a]
Prof. Dr. Malte Brasholz
In our initial experiment, we attempted the photooxidation of
hexahydrocyclohepta[b]indole (1a) under basic conditions, and
found that alongside the 4-quinolone 2a, the valuable
Institute of Chemistry
University of Rostock
Albert-Einstein-Str. 3A, 18059 Rostock, Germany
E-mail: malte.brasholz@uni-rostock.de
Web: https://www.brasholz.chemie.uni-rostock.de
ORCID: 0000-0001-8184-646X
Department of Chemistry
University of Hamburg
Martin-Luther-King-Platz 6, 20146 Hamburg, Germany
These authors have contributed equally to this work.
perhydroazepino[1,2-a]indole 3a was produced in
a small
amount. Consequently, we aimed to maximize the yield of 3a. We
evaluated various photocatalysts and photosensitizers in the
photooxygenation of 1a, and the reaction mixtures were analyzed
by quantitative 1H-NMR spectroscopy (Table 1). 460 nm blue LED
irradiation of 1a alone, in CD3OD solution in the presence of
NaOD and under O2 atmosphere, led to 15% conversion after
14 h, only generating quinolone 2a in 14% yield. In the presence
[b]
‡
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.202007549
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COMMUNICATION
of 2 mol-% of triphenylpyrylium cation (TPP+), a comparable result
was obtained (entries 1-2). Using ruthenium(II)-tris-bipyrazine
and 9-mesityl-10-methylacridinium (Mes+-Acr) improved con-
version of 1a to 60-77%, to give compound 2a in 24% and 59%
yield respectively, while the desired hemiaminal 3a was detected
in small quantities of ca. 5%, exclusively as its ring-closed
tautomer. However, 3a was also accompanied by the anthranilic
dicarboxylate 4a in the form of its disodium salt (entries 3-4).
Utilizing the xanthene dyes Eosin Y and Rose Bengal at 530 nm
(entries 5-6), full conversion of 1a was achieved within 14 h. Only
in the latter case, also the intermediate quinolone 2a was fully
consumed. However, dicarboxylate 4a was the major product,
formed in 53% yield and in 2.7:1 ratio with respect to hemiaminal
3a (20% yield). The yellow dye 1,8-dihydroxyanthraquinone (1,8-
HOAQ; chrysazine) has an exceptionally high 1O2 sensitizing
efficiency among all anthraquinones (ΦΔ = 0.69, SΔ = 0.96 in
MeCN),[10],[11] and in alkaline medium, it exists as a red dianion
(λmax = 505 nm, Figure S1). Its use (entry 7) led to a slightly
improved overall selectivity, with a higher yield of 3a (35%), but
similarly to Rose Bengal, undesired compound 4a still was the
major product (52%).
the selectivity (entries 8-10), up to 5:1 in favor of 3a over 4a when
using Mn(IV)-oxide,[15] and hemiaminal 3a could be isolated in
54% yield on preparative scale (entry 10). Using 1,8-HOAQ as the
sensitizer, MnO2 largely decelerated the photooxidation resulting
in only 44% conversion of 1a (entry 13). Addition of DMTU or
MnSO4 again led to an increased selectivity towards 3a, with
yields of about 40% (entries 11-12). Ultimately, diluting the
reaction mixture to 0.03 M by using toluene as co-solvent and
changing the light source to blue CFL lamps, the reaction could
be much better controlled even without additives (entry 14). On
preparative scale, hemiaminal 3a could reproducibly be isolated
in 65% yield, yet, a total of 6 mol-% of 1,8-HOAQ, added in
portions, was required to achieve full conversion of the
intermediate 4-quinolone 2a.
Table 1. Cascade photooxygenation of cyclohepta[b]indole 1a.
conv. of 1a
yield (%)b
2a : 3a : 4a
#
cat./sens.
λex (nm)
(%)a
1
2
none
460
460
15
12
14 : 0 : 0
11 : 0 : 0
TPP·BF4
3
Ru(bpz)3(PF6)2
Mes+-Acr·ClO4
Eosin Y
460
60
24 : 5 : 2
4
460
77
59 : 6 : 2
5
530
100
100
100
100
100
100
100
100
44
13 : 31 : 21
0 : 20 : 53
0 : 35 : 52
0 : 45 : 10
0 : 35 : 7
6
Rose Bengal
530
7
1,8-HOAQ
460
8
Rose Bengal, DMTU
Rose Bengal, MnSO4
Rose Bengal, MnO2
1,8-HOAQ, DMTU
1,8-HOAQ, MnSO4
1,8-HOAQ, MnO2
1,8-HOAQ, PhCH3
530
9
530
10
11
12
13
14
530
0 : 55 (54)c : 11
0 : 40 : 12
0 : 38 : 10
29 : 2 : 2
460
Figure 1. Scope of photooxygenation products. Reactions performed on 0.25
mmol scale, yields after chromatography. a) 1,8-HOAQ (3 × 2 mol-%), O2, hv
400-450 nm CFL (36W), KOH aq., MeOH/PhCH3, r.t., 14-20 h. b) Rose Bengal
(2 mol-%), O2, hv 530 nm LED, MnO2 (1 equiv.), NaOH aq., MeOH, r.t., 14-20 h.
460
460
400-450
98
7 : 66 (65)c,d : 10
All reactions performed at 0.05 M concentration and irradiated with 10 W LED. a)
Determined by 1H-NMR. b) 1H-NMR yield against 1,3,5-trimethoxybenzene standard.
c) Isolated yield from preparative run on 0.25 mmol scale and using KOH aq. / MeOH.
d) PhCH3 cosolvent, c (1a) = 0.03 M, 2 mol-% catalyst addition at 0, 5, 10 h, total
duration 15 h, 36 W blue CFL lamps.
A variety of functionalized cyclohepta[b]indoles 1a-1l were
subjected to the optimized reaction conditions and the scope of
the cascade photooxygenation is depicted in Figure 1. Substrates
1b-d, 1i and 1l were readily prepared by Fischer indolization of
cycloheptanone with the corresponding aryl hydrazines.
Cycloheptaindoles with C2- and C3-bromine substitution at the
aromatic ring were further derivatized by Suzuki coupling
reactions to give the C2- and C3-alkylated and arylated
derivatives 1f-h and 1j,k (see SI section for details). All
photooxygenation reactions were performed using 1,8-HOAQ
(conditions a) as well Rose Bengal/MnO2 (conditions b). 1,8-
HOAQ appeared to be the superior sensitizer in most cases (best
The anthranilic amide 4a was obviously formed by oxidative C,C-
cleavage of hemiaminal 3a,[12] and we hypothesized that
hydrogen peroxide generated during the reaction sequence may
be the main cause for this undesired degradation reaction.
Therefore, we experimented with the addition of various H2O2
scavengers and disproportionation catalysts.[13] In the case of
Rose Bengal, equimolar
amounts of DMTU (1,3-
dimethylthiourea)[14] or manganese salts significantly improved
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10.1002/anie.202007549
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results are shown in Figure 1). The cascade reaction was found
to be sensitive with regard to the aromatic substituent. Compared
to the reaction of 1a, both, the halide-substituted perhydro-
azepino[1,2-a]indoles 3b-d and the donor-substituted derivative
3e were isolated with yields of 30-40%.[16] The alkylated
compounds 3f-i were again accessible in good yields between 50-
61%, including the sensitive cyclopropane derivative 3h. The C2-
arylated compound 3j was isolated in 48%, while its C3-
regioisomer 3k was obtained in 40%, interestingly accompanied
by 12% of product 3a, apparently as the result of an unexpected
C3-dearylation. In all examples 3, the intermediate 4-quinolones
2 were generally fully consumed when utilizing Rose Bengal;
using 1,8-HOAQ, intermediates 2 were present in up to 15% after
the reaction in several cases. Reacting the highly electron-
deficient 2-cyanocyclohepta[b]indole 1l gave only low conversion
and generated a mixture of products where compound 3l could
be detected in trace amounts only.
and 3o (60%) in Figure 1. Hence, in these examples, the gem-
dimethyl group could stabilize the hemiaminals 3n and 3o against
oxidative degradation.
Under the conditions established for cyclohepta[b]indoles,
1,2,3,4-tetrahydrocarbazole (1m) reacted much faster and the
corresponding anthranilic amide 4m was formed quantitatively
after 14 h reaction time, regardless which sensitizer was used. To
gain insight into this effect of ring size, we investigated the
photooxygenation of the intermediate quinolones 2a and 2m by in
Scheme 3. Proposed mechanism.
1
The proposed mechanism of the cascade photooxygenation, in
strongly alkaline medium, is depicted in Scheme 3. Using both
sensitizers, Rose Bengal or 1,8-HOAQ, singlet oxygen is the
dominant reactive oxygen species (ROS) in the reaction
sequence. This was verified by studying the influence of various
ROS scavengers on the reactions 1a → 3a + 4a and 2a → 3a +
4a (Tables S1 and S2), and the 1O2-quencher sodium azide
(NaN3) showed the most pronounced effect on the overall reaction
rate and product distribution. The ene reaction of substrate 1 with
1O2 intially gives hydroperoxide 5, which under basic conditions
undergoes rapid C,C-cleavage to keto amide 7 via the unstable
1,2-dioxetane 6. The enolate 8 cyclizes by aldol condensation
(‘Camps’-cyclization[17]) to give 4-quinolone 2 as product of a
photochemical Witkop-Winterfeldt[18]-type reaction. The singlet
oxygenation of 2 followed by ring cleavage of a second dioxetane
9, via a charge-transfer-induced decomposition,[19] generates
product 3. The oxidation of quinolones 2 is fast for strained
cyclopentane derivatives (n = 1), while for cyclohexane
derivatives (n = 2), it becomes the rate-determining step. A
conceivable second product-forming pathway involving addition
of O2 to the enolate 8, to give the hydroperoxide 10 as a precursor
to 3, can be ruled out as none of 10 or related intermediates could
be observed.[20] The C,C-cleavage of hemiaminal 3 to dicarbo-
xylate 4 commences from the ring-opened 1,2-diketone tautomer
3’ and can occur by photooxidation as well as by the attack of in
situ-generated hydrogen peroxide (see SI section).
situ H-NMR spectroscopy, in oxygen-saturated CD3OD / NaOD
solution with 445 nm blue laser photoexcitation and using 2 mol-
% 1,8-HOAQ (Scheme 2).
Photo-NMR study: effect of ring size of 2a and 2m
conc. [%]
100
conc. [%]
100
80
60
40
20
80
2m·Na
3a·Na
4m
2a·Na
60
40
20
0
3m·Na
4a
0
0
100
200
t [min]
300
0
100
200
300
400
t [min]
Scheme 2. Photo-NMR study of the photooxygenation of
quinolones 2a and 2m.
Using these model conditions, conversion of the cyclohexane-
fused quinolone 2a was comparably slow (65% after 5 h), the
seven-membered hemiaminal 3a being the main product,
accompanied by only ca. 10% of dicarboxylate 4a. Consistently,
a reaction time of 42 h was needed in a preparative run aimed at
converting quinolone 2a quantitatively into 4a (see SI section).
Photooxygenation of the five-membered ring quinolone 2m,
derived from tetrahydrocarbazol 1m, proceeded much faster, and
the 6-membered-ring hemiaminal 3m formed rapidly, but it was
further converted into dicarboxylate 4m almost instantaneously.
As a consequence, compound 3m could be isolated only in small
amounts in preparative experiments. However, introduction of a
gem-dimethyl group in the C3-position of tetrahydrocarbazole
allowed for the isolation of the corresponding perhydropyrido[1,2-
a]indoles with good yields, as exemplified by products 3n (52%)
The perhydroazepino- and pyrido[1,2-a]indoles 3 are new
precursors to N-acyl iminium ions,[21] and we attempted their
activation by phosphoric acid organocatalysis.[22] Our preliminary
results, shown in Scheme 4, indicated that strong Brønsted acids
like N-triflyl phosphoramides[23] are required to effectively convert
model compound 3a into the iminium ion 11. Moreover, a strong
effect of solvent was observed in the arylation reaction with 1H-
indole. In toluene solution, a reaction temperature of 120 °C was
necessary to achieve full conversion of hemiaminal 3a within 24 h,
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however, elimination to the corresponding enone was
predominant, and the 1,4-addition product 12 was isolated as the
major product in 44% yield. Using acetonitrile, the temperature
could be lowered to 65 °C, and pleasingly, the Friedel-Crafts
product 13a could be isolated in 96% yield. Various substituted
indoles including N-methylindole could be employed as
nucleophiles, leading to products 13b-13e, whose N-acyl-2-
(indol-3-yl)-3-oxoindoline core structure is encountered in several
natural products, and thus this class of compounds may be of
value in drug discovery research.[24] Iminium ion 11 is one of the
rare examples from the class of C,N-diacyliminium ions,[25] and a
systematic study of this chemotype’s reactivity has not been
undertaken so far. Further investigations are currently underway
in our laboratory and more results will be reported in due course.
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Scheme 4. Brønsted acid-catalyzed functionalization of 3a via
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[16] CCDC 1982106, 1982107, and 1982108 (3d , 12, and 13a, respectively)
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Data Centre.
Acknowledgement
[17] R. Camps, Ber. Dtsch. Chem. Ges. 1899, 32, 3228-3234.
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M.B. acknowledges financial support of this project by the
Deutsche Forschungsgemeinschaft (DFG, grant no. BR 3748/2-
2).
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Keywords: Photooxidation • Singlet oxygen • Cascade reactions
• Indoles • N-Acyliminium ions
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10.1002/anie.202007549
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Mario Frahm, Thorsten von Drathen,
Lisa Marie Gronbach, Alice Voss,
Felix Lorenz, Jonas Bresien,
Alexander Villinger, Frank Hoffmann,
Malte Brasholz*
Page No. – Page No.
Visible light cascade
photooxygenation of
tetrahydrocarbazoles and
cyclohepta[b]indoles: access to new
C,N-diacyliminium ions
Splashing down the ponds: the cascade photooxygenation of tetrahydrocarbazoles and
cyclohepta[b]indoles leads to chiral tricyclic perhydropyrido- and perhydroazepino[1,2-a]indoles
as precursors to uncommon C,N-diacyliminium ions.
This article is protected by copyright. All rights reserved.