Metal-Free Visible-Light-Mediated Aromatization of 1,2–Dihydronaphthalenes

Article abstract of DOI:10.1002/ejoc.201901410

A series of polyaromatic naphthalenes have been synthesized through the dehydrogenation of the corresponding 1,2-dihydroarylnaphthalenes by using 9-mesityl-10-methylacridinium perchlorate as a photocatalyst and diphenyliodonium triflate as an external oxidant under visible light irradiation. The reaction proceeds smoothly under metal-free conditions and tolerates some functionalities. Interestingly, the reaction is also amenable to the aromatization of tetrahydronaphthalenes and fair conversions were obtained. Preliminary mechanistic investigations have been conducted and a reasonable mechanism is proposed.

Full text of DOI:10.1002/ejoc.201901410

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Accepted Article
Title: Metal-Free Visible-Light-Mediated Aromatization of 1,2–
Dihydronaphthalenes
Authors: Fatima Rammal, Annie-Claude Gaumont, and Sami Lakhdar
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10.1002/ejoc.201901410
European Journal of Organic Chemistry
COMMUNICATION
Metal-Free Visible-Light-Mediated Aromatization of 1,2–
Dihydronaphthalenes
Fatima Rammal, Annie–Claude Gaumont, and Sami Lakhdar*
Abstract:
A series of polyaromatic naphthalenes have been
synthesized through the dehydrogenation of the corresponding 1,2–
dihydroarylnaphthalenes by using 9-mesityl-10-methylacridinium
perchlorate as a photocatalyst and diphenyliodonium triflate as an
external oxidant under visible light irradiation. The reaction proceeds
smoothly under metal–free conditions and tolerates some
functionalities. Interestingly, the reaction is also amenable to the
aromatization of tetrahydronaphthalenes and fair conversions were
obtained. Preliminary mechanistic investigations have been
conducted and a reasonable mechanism is proposed.
Introduction
Polyaromatic naphthalenes are privileged scaffolds that can be
found in numerous biologically active molecules and functional
materials.[1] In this regard, the development of straightforward and
practically simple protocols to access those molecules has been
and still is the subject of numerous studies.[2] Typically, most
synthesis of those molecules relay on the formation of one or two
six–membered rings by classical methods (Diels–Alder or
Friedel–Crafts) or via metal-catalyzed cyclizations.[3] They can
also be accessed through the aromatization of 1,2–
dihydronaphthalenes. The latter approach has gained remarkable
interest obviously due to the development of straightforward
Scheme 1. Aromatization of 1,2–dihydronaphthalenes.
Results and discussion
methods for the synthesis of
dihydronaphthalenes.[4]
a large variety of 1,2–
To test this hypothesis, we first investigated the aromatization of
1a in the presence of a photocatalyst, and oxidant under visible
light irradiation (Table 1). While the use of different photocatalysts
For instance, the group of Gaunt has recently reported an elegant
Cu-catalyzed alkyne carboarylation that enabled efficient
synthesis of a large variety of a dihydronaphthalenes 1 (Scheme
1).[5] To obtain the corresponding aromatic derivatives 2, a
stoichiometric amount of DDQ has been used as an oxidant at
high temperature (reflux in toluene).[5]
In view of the importance of these aromatic molecules, in
particular in material science, the development of a practically
simple, mild and efficient method is naturally appealing. Because
the dehydrogenation of 1 involves two proton-two electron
process, we reasoned that this could be accomplished
2+
such as Eosin Y (entry 2, 41%), Ru(bpy)3 (entry 3, 45%) and
Rose Bengal (entry 4, 6%) in ethyl acetate gave moderate to low
conversion, the use of 9-Mesityl-10-methylacridinium (Mes–Acr),
commonly known as Fukuzumi photocatalyst,[7] gave almost
quantitative conversion (99%, entry 1). With the aim to use
environmentally begin oxidant for our photoreaction, we have first
tested O2. Although good yield was isolated (81%, entry 5), the
reaction was found to be less efficient with other
dihydronaphthalenes. The use of phenyldiazonium salt as an
oxidant yielded the desired aromatic compound but in only 72%
(entry 6). Interestingly, decreasing the catalyst loading to 2.5
mol% didn’t affect the efficiency of the reaction (entry 7). Different
solvents have also been tested and even though fair to good
conversions have been obtained (42–72%, entries 8–11), they
remained lower than that obtained in EtOAc (entry 7). Finally, the
importance of the photocatalyst (Mes–Acr) has been elucidated
as only 16% of conversion has been observed when it was
removed from the reaction (entry 14).
photochemically by the association of an oxidant and
a
photocatalyst under visible light irradiation.[6] Indeed, the
generation of a highly reactive radical through monoelectronic
reduction of an oxidant would enable a hydrogen atom transfer
from 1 to form a benzylic radical. The latter can then be oxidized
by the oxidized form of the photocatalyst to give rise to the
corresponding carbocation that yields 2 after fast deprotonation
step.
F. Rammal, Prof. Dr. A.–C. Gaumont, Dr. S. Lakhdar
Normandie Univ., LCMT, ENSICAEN, UNICAEN, CNRS,
6, Boulevard Maréchal Juin, Caen 14000 France
E-mail : sami.lakhdar@ensicaen.fr
URL: http://www.lcmt.ensicaen.fr/
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10.1002/ejoc.201901410
European Journal of Organic Chemistry
COMMUNICATION
Table 1. Survey of reaction conditions for the photoredox-catalyzed
dehydrogenation of 1a.
PC
loading
(mol %)
Yield
[%][a]
Oxidant
PC
Solvent
1
2
Ph2I+ [b]
Ph2I+ [b]
Mes-Acr
Eosin Y
5
5
AcOEt
AcOEt
99
41
2+
3
4
Ph2I+ [b]
Ph2I+ [b]
O2
Ru(bpy)3
Rose Bengal
Mes-Acr
Mes-Acr
Mes-Acr
Mes-Acr
Mes-Acr
Mes-Acr
Mes-Acr
Mes-Acr
Mes-Acr
–
5
AcOEt
AcOEt
AcOEt
AcOEt
AcOEt
DCM
45
6
5
5
5
81
72
99
42
48
70
72
57
23
16
+ [c]
6
PhN2
5
7
Ph2I+ [b]
Ph2I+ [b]
Ph2I+ [b]
Ph2I+ [b]
Ph2I+ [b]
Ph2I+ [b]
Ph2I+ [b]
Ph2I+ [b]
2.5
2.5
2.5
2.5
2.5
2.5
2.5
–
8
9
MeCN
Toluene
DCE
10
11
12
13
14
DMF
THF
AcOEt
[a] NMR yields have determined by 1H NMR spectroscopy by refereeing
to methoxy group signal of 1. [b] Triflate was used as a couterion.
With the optimization conditions in hand, we have next examined
the scope of the reaction by testing the compatibility of the
developed methodology with various 1,2–diarylnaphthalenes 1
bearing different substituents. As depicted in Figure 1, good to
excellent yields (67–99%) have been obtained with
diarylnaphthalenes bearing electron–acceptors (2b–f) or –donor
2g at the aromatic ring at the C2–position. More importantly, the
method tolerates some functionalities such as halogens (2b–d)
and carbonyls (2e,f). Similarly, the reaction works well with
aromatic bearing a trifluoromethyl group at the meta position (2h,
99%). We have next examined the effect of the aromatic ring of
the dihydronaphthalene on the photoredox aromatization reaction.
As shown in Figure 1, the electronic nature of the substituent does
not affect the performance of the reaction and naphthalenes (2i–
l) have been isolated in good yields (55–86%). Finally, the
Figure 1. Scope of the dehydrogenation of 1,2–dihydronaphthalenes. General
Conditions: 1,2–dihydronaphtalene 1 (1.0 equiv.), diphenyliodonium triflate (1.0
equiv.), Mes–Acr (2.5 mol%) in AcOEt.
We have furthermore tested the efficiency of our methodology in
the aromatization of the challenging tetrahydronaphthalenes 3,
where even rhodium[8] or iron[9] catalysts gave traces of
naphthalenes. Interestingly, when two equivalent of
diphenyliodonium triflate has been employed along with 2.5 mol%
of Mes–Acr in AcOEt under blue irradiation, fair to good
conversions of 2p–s have been attained (Figure 2). These results,
albeit modest, are encouraging and even comparable to those
recently reported by Kanai et al.[10] when using a binary hybrid
catalyst system comprising Mes–Acr and a palladium metal
catalyst.
efficiency
of
the
method
has
been
tested
with
dihydronaphthalenes bearing other substituents than the para-
methoxyphenyl at the R position (Figure 1). Delightfully, the
polyaromatic naphthalenes have been isolated in good yields
(2m–o; 56–99%).
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10.1002/ejoc.201901410
European Journal of Organic Chemistry
COMMUNICATION
the benzylic radical 4. At this stage, an oxidation of the latter can
take place either with the oxidative form of the photocatalyst or
with the diphenyliodonium salt present in the media.To
discriminate between both pathways, we determined quantum
yield (QY) of the aromatization reaction of 1a by following a
protocol previously reported by Yoon.[12] With a QY of 11.5, it is
likely that a radical chain mechanism is operative in this reaction
(Figure 3). Because the aromatization of the carbocation must be
a driving force of the reaction, a fast deprotonation take place with
triflate to give rise to 2.
In order to confirm the formation of the carbocation as
intermediate in the reaction, we carried out the reaction of 1a with
diphenyliodonium triflate in the presence of different nucleophiles
(amides, thiol, etc…). Unfortunately, all these attempts to trap the
carbocation with nucleophiles have failed, obviously due to the
rapid deprotonation of the cation 4 as stated above. However,
Figure 2. Aromatization of tetrahydronaphthalenes 3. General Conditions:
tetrahydronaphthalenes 3 (1.0 equiv.), diphenyliodonium triflate (2.0 equiv.),
Mes–Acr (2.5 mol%) in AcOEt. Conversions are determined by 1H NMR
spectroscopy by using tetrachloroethane as internal standard.
when
1,3,5-cycloheptatriene
or
4,4'-methylene-bis(N,N-
In order to gain insights into the reaction mechanism of the
photooxidation, we first followed kinetics of the formation of 2a
with Ph2I+ and in the presence of different photocatalysts (see
Figure S3, supporting information, Page S7). Interestingly, while
almost 40% of conversion have been attained after 5 hours with
both Eosin Y and Ru(bpy)32+, a quantitative formation of 2a within
less than 5h has been observed when Mes–Acr has been
employed as a photocatalyst. This shows clearly the performance
of the latter in the aromatization reaction, which we attribute the
high oxidizing ability of [Mes–Acr]•+, which is higher than those of
the other photocatalysts (Table 1, entries 1–3).[11]
dimethylaniline) have been submitted to our photooxidative
conditions, the formation of the corresponding stabilized
carbocations have been unambiguously elucidated by both
1HNMR and UV–visible spectroscopy (see supporting
information).[13]
Conclusion
In conclusion, we have developed a photoredox-catalyzed
aromatization of 1,2–dhydronaphthalene. This process is efficient,
tolerates functionalities and takes place under mild conditions.
Furthermore, the aromatization of four tetrahydronaphthalenes
have been disclosed under our optimized conditions. Mechanistic
investigations have been performed and allowed the suggestion
of a reasonable reaction mechanism, where the formation of a
benzylic carbocation as a key reactive intermediate is proposed.
This new approach for the generation of high reactive cation may
open new avenues for the formation of carbon–carbon and
carbon–heteroatom bonds. This work is underway in our
laboratory.
Experiments using either TEMPO as additive resulted in complete
inhibition of the aromatization reaction, suggesting that radical
intermediates were involved.
Experimental Section
General procedure:
dihydronaphthalene
A
schlenk tube was charged with the
2
(1.0 equiv.) Mes-Acr (2.5 mol%) and
diphenyliodonium triflate (1.0 equiv.) in ethyl acetate (0.1 M). The reaction
mixture was stirred at room temperature for two hours under blue LED
irradiation (5W). The solvent was removed in vacuo and the crude residue
was purified by flash chromatography 99:1 (n-pentane/ AcOEt) to yield the
desired product.
Acknowledgments
Figure 3. Proposed reaction mechanism.
The authors thank the LABEX SynOrg (ANR-11-LABX-0029)
for financial support. F. R. is i indebted to the French Ministry of
Research for a doctoral fellowship.
Based on our previous work [11a] and recent contributions by Wu
and Lei, respectively [11b,c] we hypothesize that the excited form
of (Mes-Acr)* reduce Ph2I+ to generate the phenyl radical that
abstracts a hydrogen from the 1,2–dihydronaphthalene to form
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Keywords: aromatization • mechanisms• photoredox catalysis •
radicals
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Entry for the Table of Contents (Please choose one layout)
Layout 2:
COMMUNICATION
Aromatization
Fatima Rammal,[a] Annie–Claude
Gaumont,[a] and Sami Lakhdar*[a]
Page No. – Page No.
Metal-Free Visible-Light-Mediated
Aromatization of 1,2–
Dihydronaphtalenes
Aromatization of dihydronaphthalenes have been accomplished under mild
conditions using an organic photocatalyst. Scope and limitations of the approach
have been evaluated along with the reaction mechanism.
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