Catalyst-Free Synthesis of O-Heteroacenes by Ladderization of Fluorinated Oligophenylenes

Article abstract of DOI:10.1002/anie.202007427

A novel catalyst-free approach to benzoannulated oxygen-containing heterocycles from fluorinated oligophenylenes is reported. Unlike existing methods, the presented reaction does not require an oxygen-containing precursor and relies on an external oxygen source, potassium tert-butoxide, which serves as an O^2? synthon. The radical nature of the reaction facilitates nucleophilic substitution even in the presence of strong electron-donating groups and enables de-tert-butylation required for the complete annulation. Also demonstrated is the applicability of the method to introduce five-, six-, and seven-membered rings containing oxygen, whereas multiple annulations also open up a short synthetic path to ladder-type O-heteroacenes and oligodibenzofurans.

Full text of DOI:10.1002/anie.202007427

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Accepted Article
Title: Catalyst-Free Synthesis of O-Heteroacenes via Ladderization of
Fluorinated Oligophenylenes
Authors: Mikhail Feofanov, Vladimir Akhmetov, Ryo Takayama, and
Konstantin Amsharov
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.202007427
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Catalyst-Free Synthesis of O-Heteroacenes via Ladderization of
Fluorinated Oligophenylenes
Mikhail Feofanov*[a, b], Vladimir Akhmetov[a, b], Ryo Takayama[a] and Konstantin Amsharov*[a, b, c]
[a]
Friedrich-Alexander University Erlangen-Nuernberg, Department of Chemistry and Pharmacy, Organic Chemistry II, Nikolaus-Fiebiger Str. 10, 91058
Erlangen, Germany.
[b]
[c]
Institute of Chemistry, Organic Chemistry, Martin-Luther-University Halle-Wittenberg, Kurt-Mothes-Strasse 2, D-06120 Halle, Germany
South Ural State University, pr. Lenina 76, 454080 Chelyabinsk, Russia
E-mail: konstantin.amsharov@chemie.uni-halle.de
Supporting information for this article is given via a link at the end of the document.
Abstract: We report
a
novel catalyst-free approach to
conventional Pd-catalyzed C(aryl)-C(aryl) couplings. Therefore
the direct transformation of C-F bond into C-O functionality
appears to be a highly attractive approach to O-heteroacenes.
benzoannulated oxygen-containing heterocycles from fluorinated
oligophenylenes. Unlike other existing methods, the presented
reaction does not require oxygen-containing precursor and relies on
outer oxygen source i.e. potassium tert-butoxide, which serves as an
O2- synthon. The radical nature of the reaction facilitates nucleophilic
substitution even in the presence of strong electron-donating groups
and enables de-tert-butylation required for the complete annulation.
We demonstrate the applicability of the method to introduce five-,
six- and seven-membered rings containing oxygen, whereas multiple
annulations also open up a short synthetic path to ladder-type O-
heteroacenes and oligodibenzofurans.
Organic π-conjugated materials have been under constant focus
in academia and industry during recent years. Due to their
outstanding optical and charge properties, they play a key role in
the development of organic light-emitting diodes (OLEDs)[1],
organic field-effect transistors (OFETs)[2,3] and organic
photovoltaics (OPVs).[4]
Figure 1. Synthetic approaches to dibenzofuran and scope of this work.
The incorporation of heteroatoms into the carbon skeleton is an
excellent tool to adjust and improve the electronic properties of
[5,6]
the compounds.
In contrast to nitrogen- and sulphur-
analogs,[7–9]oxygen heterocycles, e.g. containing furan’s moiety,
have drawn much less attention and became an object of
interest only recently. [10–19]
Herein, we report a new catalyst-free approach to a wide range
of
O-heteroacenes
via “ladderization”
of
fluorinated
oligophenylenes (LooP).
Activated, i.e. containing strong electron-withdrawing groups,
fluorobenzene derivatives are known to undergo substitution
Among the main reasons for such a late splash of interest are
synthetic difficulties connected to lability issues of furans.
Meanwhile, a fusion of furan ring(s) to an all-carbon aromatic
system appears to be a viable strategy for obtaining stable furan
derivatives (also called ladder-type O-bridged heteroacenes[20]).
[21] Dibenzo[b,d]furan (1) serves as a model target compound for
the development of new synthetic approaches. Despite a large
number of works aimed to develop facile incorporation of the
furan ring, the principal synthetic strategies are not so numerous
and break down into few categories depicted in Figure 1. The
with O-nucleophiles[42–48]
,
whereas such intermolecular
substitution of fluorine in unactivated aromatics is less
common[49]
.
We started our investigations with the reactions of 2,2'-difluoro-
1,1'-biphenyl 1a with potassium hydroxide. We have carried the
reaction out in HMPA (hexamethylphosphoramide)[50,51] and
under microwave irradiation, which greatly accelerates SNAr
reactions [52,53]. Although the reaction does not proceed in the
presence of KOH, the substitution of the latter with t-BuOK
enables the formation of 1b in 10% yield (table 1, entry 2).
Increasing the amount of t-BuOK up to 6 equiv., we have
achieved an excellent (93%) yield of 1b (table 1, entry 6).
Despite the anticipated stability of tert-butyl ether under basic
conditions[61], intermediates containing tert-butoxy substituents
were not found in the mixture. The reaction also proceeds in
other aprotic solvents such as DMA and DMSO, although in
slightly lower yields (table 1, entry 7-8). Interestingly, 1b was not
detected in the experiment with MeOK (table 1, entry 9). In the
case of EtOK the yields do not exceed 20 % (table 1, entry 10;
more detailed information is in SI).
final step includes either transition metal-catalyzed C-C
[27–36]
coupling[22–26] or C-O bond creation,
which is usually
implemented via a two-step deprotection/dehydration procedure.
[10,14,37–40]
With rare exceptions[41], the existing synthetic approaches to the
dibenzofuran core require an oxygen atom in the precursor’s
structure. Meanwhile, a technique enabling the incorporation of
oxygen and simultaneous formation of a heterocycle could
simplify overall synthesis. In the ideal case, the technique should
exploit a functionality that stays unreactive otherwise. In this
context, C(aryl)-F bond appears to be a potential functionality as
it, in contrast to other C-Hal bonds, remains intact during
1
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10.1002/anie.202007427
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between fluorines facilitate the formation of aryne, which leads
to the formation of inseparable mixture of compounds. The
reaction poorly tolerates CF3 groups. The furan 12b was isolated
only in 4% yield. Moreover, the conditions are also suitable for
the synthesis of π-extended homologs of dibenzofuran. Thus,
dinaphtofuran 13b was isolated in excellent 93% yield.
Table 1. Reaction of 2,2'-difluoro-1,1'-biphenyl with different oxygen sourses.
Entry
Oxygen Source
KOH (2 equiv.)
Solvent
HMPA
HMPA
HMPA
HMPA
HMPA
HMPA
DMSO
DMA
Yield of 1b
Table 2. Synthesis of substituted dibenzofurans from 2,2'-difluoro-1,1'-
biphenyls.
1
2
0
t-BuOK (2 equiv.)
t-BuOK (3 equiv.)
t-BuOK (4 equiv.)
t-BuOK (5 equiv.)
t-BuOK (6 equiv.)
t-BuOK (6 equiv.)
t-BuOK (6 equiv.)
MeOK (6 equiv.)
EtOK (6 equiv.)
KOH (6 equiv.)
10
25
41
82
93
78
72
0
3
4
5
6
7
8
9
HMPA
HMPA
HMPA
10
11
20
0
Based on the screening, t-BuOK can be considered as an
optimal oxygen source and the O2- synthon, which enables one-
pot substitution of two fluorines yielding benzoannulated furan
derivatives. To test the viability of this claim and to investigate
the scope and limitation of this reaction, we have studied other
fluorinated precursors 2a-11a bearing halogen-, methyl-, and
methoxy- substituents (Table 2).
Dibromodibenzofurans 2b and 3b were synthesized in 70% and
92% yields, respectively. Although the method seems to be not
compatible with C-I, the tolerance towards C-Br functionality can
be achieved by decreasing the reaction temperature to 80 ºC. As
of methylated representatives 6b-8b, the yields vary from 60 to
70%, The yields are slightly decreased due to partial
demethylation, the mechanism of which will be discussed later.
Notably, the reaction conditions are suitable even for the
synthesis of dibenzofurans with strong donor substituents such
as OMe group. Thus, 9b bearing methoxy groups in 4- and 8-
positions was isolated in 44% yield. It is also interesting, that the
reaction reveals
a pronounced regioselectivity to fluorine
a) 80 °C. b) 12 eq. of t-BuOK from 2, 2’, 6, 6’-tetrafluorobiphenyl.
substituents. We have found that the nucleophilic substitution
process favors fluorine atoms in 2,2’- positions since 4,8-
difluorodibenzofuran (10b) was isolated in 89% yield. However,
if biphenyl bears fluorine atoms at 2,2’,6,6’- positions, it is not
possible to stop the reaction at the formation of 1,9-
difluorodibenzofuran, whereas a complex mixture of different
oxygen-containing compounds is observed. Meanwhile, the
addition of 12 equiv. of potassium tert-butoxide allows
oxodefluorination and full hydrolysis of the two remaining C-F
bonds yielding 11b in 84% yield. We assume that the observed
phenomena are connected to mutual activation of C-F
functionalities in close proximity, while the second annulation in
the case of 11a does not occur due to strain energies needed to
be overcome [54]. Surprisingly, we did not observe and isolate
any reasonable products in the case of 2,2',4,4'-tetrafluoro-1,1'-
biphenyl. We assume that increased acidity of the hydrogen
Another aspect that we find extremely, if not the most, important
is the possibility to implement multiple annulations in one step
(Figure 2). Having the established synthetic protocol, we have
turned our attention to benzobisbenzofurans 14b-16b containing
two oxygen atoms. To our pleasure, excellent efficiency of the
double-annulation enables target benzobisbenzofurans in high
(up to 91%) yields. Furthermore, the consequential formation of
six C-O bonds enables a triple annulation of 17a in 92% yield
making the new ladder-type heteroacene 17b readily available
through
fluorinated meta-oligophenylene 18a transforms solely into
trisdibenzofuran 18b instead of complicated mixture.
Surprisingly high 90% yield indicates that the process has
a
three-step procedure. Rather unexpectedly,
a
2
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10.1002/anie.202007427
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certain preferences over C-F bond substitutions’ sequence and,
presumably, is not connected with self-correction (see SI)[55]
.
Figure 3. Synthesis of benzoannulated pyrans and oxepines.
To gain additional data, we have exposed the model substrate
23a to the following reaction conditions. Thus, we have found
that in the presence of 3 equiv. of t-BuOK, the reaction
terminates at the formation of the 2-(tert-butoxy)-1,1'-biphenyl
23b, which was isolated in 60% yield. Meanwhile, using 6 equiv.
of t-BuOK, we isolated [1,1'-biphenyl]-2-ol in 93% yield (Scheme
1, A). These facts confirm that the transformation occurs via tert-
butyl ethers. However, the mechanism of de-tert-butylation
remained uncertain. Capturing the volatile products with bromine
water, we have observed exclusively brominated acetone’s
derivatives. Since acetone is a typical product of tert-butoxyl
radical degradation (Scheme 1, B)[64] and t-BuOK is a suitable
reagent for single electron transfer (SET) reactions[65], we
conclude the radical nature of de-tert-butylation. Presumably,
tert-butoxyl radical abstracts hydrogen from the -C(CH3)3. In the
absence of hydrogen donors the formed radical 1f undergoes
β‑scission releasing isobutylene and radical 1g which is reduced
by the following SET. Isobutylene was not detected apparently
due to the fast radical polymerization, which leads to the
increased viscosity of the obtained reaction mixture.
The initiation step including the substitution of fluorine with tert-
butoxide anion is the most crucial and at the same time intricate
step. Despite low nucleophilicity of t-BuOK and its high basicity,
aryne mechanism of the substitution is excluded due to the
successful transformations of ortho-substituted substrates e.g.
3b and 7b. As mentioned earlier, t-BuOK can undergo SET and
initiate SRN1 type of reactions[66]. However, the absence of
defluorinated products and dimers evidences against this
mechanism. Meanwhile, deiodination and partial demethylation
(in the cases of 4b-8b) indicate the radical character of the
Figure 2. a) Multiple ladderization of fluorooligophenylenes.
The incorporation of benzoannulated oxygen-containing
heterocycles other than five-membered is less-known
transformations [56–61]. However, the compounds containing six-
and seven-membered rings have some promising applications
as OLED materials[62,63]. To test whether our method is capable
of tackling such synthesis, we have obtained fluorinated
precursors 19a and 20a, which under reactionary conditions
transformed into 19b containing pyrane ring and 20b containing
oxepine ring. Extrapolation of the approach to other oxepines
has been proved successful as 21b and 22b were obtained in
excellent 90% and 85% yields.
From the mechanistic perspective, the reaction can be formally
broken down into three steps: substitution, deprotection, and
second substitution (Figure 4). While the last step represents a
well-established SNAr process exploited for the synthesis of
dibenzofurans[30], the rest of the mechanistic route is less trivial
and requires thorough consideration.
reaction. The possibility of the alternative radical SRN
2
mechanism has been discussed elsewhere and is considered as
3
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an unlikely event due to charge repulsion and orbital type oxygen heteroacenes and dibenzofurans oligomers.
arguments[67]
.
Besides oxygen-containing pentagons, the possibility to obtain
xanthene’s and oxepine’s derivatives has also been shown.
Acknowledgements.
Funded by the Deutsche Forschungsgemeinschaft (DFG)
–
Projektnummer 182849149 – SFB 953, AM407 and by Act 211
Government of the Russian Federation, Contract No.
02.A03.21.0011.
Keywords: Ladderization• O-heteroacenes • dibenzofurans•
xathenes • oxepins
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10.1002/anie.202007427
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O-heteroacenes are now available via one-step “LooP,” i.e. Ladderization Of OligoPhenylenes bearing fluorine atoms in the key
positions. An elegant sequence of SET-induced SNAr-radical de-tert-butylation-SNAr does not require electron-poor π-system and
occurs even in the presence of strong electron donors. During LOOP t-BuOK serves as a reliable oxygen-atom source and O2-
synthon enabling synthesis of dibenzofuran, benzoxanthenes and benzooxepines, and a range of longer ladder-type O-heteroacenes
from fluorinated oligophenylenes.
@AmsharovG, @AkhmetovVl, @UniHalle, @FAU_Germany.
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