Palladium-catalyzed double C-H activation directed by sulfoxides in the synthesis of dibenzothiophenes

Article abstract of DOI:10.1002/anie.201100775

S=O shows where to go: A novel double C-H activation of aromatic compounds with a sulfoxide as a directing group results in the highly regioselective synthesis of polysubstituted dibenzothiophenes (see scheme). The reaction cascade consists of palladium-catalyzed double C-H activation and a Pummerer rearrangement followed by palladium-catalyzed C-S bond formation. Copyright

Full text of DOI:10.1002/anie.201100775

DOI: 10.1002/anie.201100775
À
C H Functionalization
À
Palladium-Catalyzed Double C H Activation Directed by Sulfoxides in
the Synthesis of Dibenzothiophenes**
Rajarshi Samanta and Andrey P. Antonchick*
À
C H functionalization is a sustainable and straightforward
approach to complex substances.[1] Numerous practical meth-
the known poisoning effect of sulfur.[7,8] The development of
an efficient method to these heterocyclic compounds using
easily accessible chemicals is of great interest.
The reaction conditions were optimized based on the
model transformation of sulfoxide 1a (Table 1). Our initial
attempts using Pd(OAc)2 and AgOAc led to the stoichio-
À
À
À
ods for the formation of C C, C N, and C O bonds through
direct C H activation have been developed using transition-
metal catalysis. The activation of C(sp ) H bonds of aromatic
compounds provides access to key scaffolds of natural
À
2
À
À
products, drugs, and materials. Double C H activation is a
challenging, attractive, and highly economical method to
[2,3]
À
create C C biaryl bonds.
However, the previously devel-
Table 1: Optimization of reaction conditions.[a,b]
oped methods often suffer from selectivity problems in the
formation of substituted biaryls and require a great excess of
one coupling partner. For the efficient solution to this
problem a number of directing groups such as carbonyl-
based or nitrogen-containing groups have been used. The
development of efficient methods for the construction of
Entry Catalyst
Cat.
[mol%]
Additive
t [h] Yield [%]
À
complex molecules by multiple C H functionalization poses a
great challenge. Herein, we report our preliminary results on
1
2
3
4
5
6
7
8
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
[PdCl2(CH3CN)2]
[PdCl2(PPh3)2]
[Pd(PPh3)4]
PdCl2
25
25
25
25
25
25
25
25
25
25
25
15
10
–
NIS
PhI(OAc)2
4-MeOC6H4I
PhI
4-FC6H4I
4-FC6H4I
4-FC6H4I
4-FC6H4I
4-FC6H4I
4-FC6H4I
4-FC6H4I
4-FC6H4I
20
16
20
20
20
20
20
20
20
20
16
40
90
18
<20
46
55
65
77
62
45
<15
78
À
a palladium-catalyzed double C H activation using sulfoxide
as a new traceless directing group, and its application in a
highly regioselective synthesis of polysubstituted dibenzo-
thiophenes through a cascade reaction (Scheme 1).
[c]
9
10
11
12
13
PdI2
PdCl2
PdCl2
67
74
66
[a] Conditions: 1a (0.2 mmol), Pd catalyst (0.03 mmol), AgOAc
(0.4 mmol), additive (0.4 mmol) in AcOH (1 mL) at 1108C. [b] Yield of
isolated product. [c] 0.2 mmol additive. NIS=N-iodosuccinimide.
Scheme 1. Sulfoxide-directed synthesis of dibenzothiophenes.
Dibenzothiophene is a key scaffold for pharmaceutically
active compounds. Besides, its derivatives have numerous
applications as dyes, agrochemicals, liquid crystals, photo-
active compounds, and conducting polymers.[4,5] Several
straightforward methods for the synthesis have been
reported.[6] These methods include multistep procedures,
require halogenated or metalated compounds, and are neither
atom- nor time-economical nor environmentally friendly.
Application of transition-metal catalysts is limited because of
metric formation of dibenzothiophene 2a. However, no
catalytic activity of PdII was observed (see the Supporting
Information, Table S1). Therefore, we screened different
additives in subsequent experiments. We found that the
reaction can be promoted by using iodoarenes as additives
(Table 1, entries 3–6). Application of p-fluoroiodobenzene
led to the target product in 77% yield. We then screened
various catalysts and oxidation reagents. We were able to
reduce the loading of the PdII catalyst and showed that a
variety of Ag+ sources can be applied in the reaction (see
Table S1).
These optimized reaction conditions were applied to
investigate the scope of the reaction (Table 2). Awide array of
sulfoxides 1 were subjected to the reaction, and the corre-
sponding products were isolated in moderate to good yields
with excellent functional-group tolerance.
[*] Dr. R. Samanta, Dr. A. P. Antonchick
Max-Planck-Institut fꢀr Molekulare Physiologie
Abteilung Chemische Biologie
Otto-Hahn-Strasse 11, 44227 Dortmund (Germany)
Fax: (+49)231-133-2499
E-mail: andrey.antonchick@mpi-dortmund.mpg.de
[**] We thank Prof. Dr. H. Waldmann for his generous support.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201100775.
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Communications
Table 2: Scope of the reaction.[a,b]
-poor phenyl and benzyl groups provided access to the desired
dibenzothiophenes.
In addition, when we extended the scope with substrates
with substituents in both parts of the sulfoxide, the corre-
sponding dibenzothiophene products were isolated in good
yields (compounds 2q–t in Table 2). Finally, we performed the
reaction on a larger scale with 5 mmol of the sulfoxide.
Dibenzothiophene 2l was obtained in 67% yield.
To further explore the applicability of the newly devel-
oped method we examined transformations of the obtained
dibenzothiophene. The carbonyl group of 2l was removed
using Wilkinsonꢀs catalyst (Scheme 2). 2-Methylbiphenyl (4)
was obtained by reduction of the carbonyl group to methyl
group and desulfurization. In addition dibenzothiophene 2l
was converted in two steps to the polycyclic aromatic
compounds 5 and 6, which are used for preparation of organic
transistors.[9c]
Scheme 2. Transformations of dibenzo[b,d]thiophene-1-carbaldehyde
(2l). Reagents and conditions: a) [RhCl(PPh3)3], toluene, reflux, 36 h,
68%; b) NaBH4, MeOH, 08C, 0.5 h, 90%; c) TsCl, Et3N, DMAP,
CH2Cl2, RT, 3 h; d) LiAlH4, THF, RT, 3 h, 65% for two steps; e) [Ni-
(cod)2], bipy, LiAlH4, THF, 48 h, reflux, 92%; f) see Ref. [9]. DMAP=4-
dimethylaminopyridine, bipy=2,2’-bipyridine.
[a] Conditions: 2 (0.1 mmol), PdCl2 (0.015 mmol), AgOAc (0.2 mmol),
para-fluoroiodobenzene (0.2 mmol) in AcOH at 1108C for the given
time. [b] Yields of isolated products. [c] Combined yield (mixture of two
diastereomers in 10:1 ratio according to H NMR analysis; the major
isomer is shown).
Mechanistically we assume that the first step in the
cascade synthesis of dibenzothiophenes is the sulfoxide-
1
À
group-directed double C H activation of 1a to give the
cyclic sulfoxide 7 (Scheme 3). A subsequent Pummerer[10]
À
À
reaction leads to mercaptoaldehyde 8. S H and C H
À
activation, followed by formation of a new C S bond,
In general, it was observed that sulfoxides in which the
thiophenol part contains electron-withdrawing or -donating
groups reacted smoothly to give the corresponding dibenzo-
thiophene products in good yields (compounds 2i–p in
Table 2). These transformations occurred smoothly when
the thiophenol part of the sulfoxide was substituted in meta
and para position with a variety of substituents including
halide, alkyl, trifluoromethyl, and methoxy groups. The ortho-
substituted derivatives led to desired products only in trace
amounts, owing to steric interactions between the sulfoxide
group and the ortho substituent. Substrates with an alkyl
substituent in the benzyl part of the sulfoxide afforded good
yields. A number of other substituents in meta- and para- are
tolerated giving products in yields of 45–74% (compounds
2a–h in Table 2). We also found that ortho substitution in the
benzylic part of the sulfoxide did not create problems for
product formation (see example 2c in Table 2). In general,
phenyl benzyl sulfoxides having substituted electron-rich and
provides the desired product 2a.
To investigate the roles of the reagents, a number of
control experiments were performed. Interestingly, under our
reaction conditions only the cyclic sulfoxide 7 undergoes the
Pummerer rearrangement, the starting sulfoxide 1a does not.
Addition of a known activator of the Pummerer rearrange-
ment to the reaction mixture, such as acetic anhydride, led to
selective formation of thiophenol and tolualdehyde and the
desired dibenzothiophene 2a formed in only trace amounts.
Under thermal conditions and with added acid, sulfoxide 7l
reacted to provide a mixture of compounds 9 and 10 (see
Scheme S1); however, the desired product 2l was obtained
from cyclic sulfoxide 7l in the presence of PdII and AgOAc in
acetic acid. This transformation proceeded smoothly and no
intermediates were detected. Both PdCl2 and AgOAc are
required for the successful transformation, and in the absence
of either the desired product 2l was not obtained. In the
reaction with AgOAc, in addition to products 9 and 10,
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Based on these results, we believe that the reaction
proceeds as follows (Scheme 3): First, electrophilic attack of
PdII, directed by the sulfoxide function, leads to the formation
of bimetallic species 13.[11] Subsequent oxidative addition of
aryl iodide provides complex 14, which undergoes first
À
reductive elimination and then C H activation to produce
palladacycle 15 and aryl acetate. It is interesting that aryl
iodides, which are common arylation reagents, promoted the
formation of species 15. The formation of arylated sulfoxides
by reductive elimination of PdII from complex 14 with
À
formation of a C C bond was not observed. However, the
desired dibenzothiophenes can be obtained without added
aryl iodides through the use of stoichiometric amounts of PdII
(see the Supporting Information). The ensuing reductive
elimination leads to the formation of cyclic sulfoxide 7 and
Pd0, which is reoxidized to PdII by AgOAc. In the next cycle
sulfoxide 7 undergoes a Pummerer rearrangement, which is
promoted by AgOAc and acetic acid, to give mercaptoalde-
hyde 8. Then, the coordination of PdII at the sulfur atom and
À
subsequent C H activation leads to palladacycle 16. Reduc-
tive elimination of 16 produces the desired dibenzothiophene
0
À
2a by formation of a C S bond and Pd , which can be
reoxidized by AgOAc.
In conclusion, we have developed a highly efficient double
À
C H activation directed by the sulfoxide group. The method
was applied to the straightforward synthesis of dibenzothio-
phenes from simple benzyl phenyl sulfoxides. The products
are formed in a cascade reaction by the abstraction of four
hydrogen atoms. The method is highly regioselective as a
result of the strictly defined sequence of reactions. An
impressive role for aryl iodide additives has been proposed.
Further work is currently underway.
Received: January 30, 2011
Published online: April 19, 2011
Scheme 3. Possible mechanistic pathways.
À
Keywords: arylation · C H activation · dibenzothiophenes ·
palladium · Pummerer rearrangement
.
disulfide was obtained as the result of the oxidation of the
Pummerer rearrangement product. In addition, the applica-
tion of PdCl2 and AgOAc in DMF led to dramatically reduced
yield of 2l from the cyclic sulfoxide 7l. Addition of aryl
iodides to the reaction mixture was not required for the
catalytic activity of PdCl2.
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À
À
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in acetic acid (see Scheme S1). Addition of aryl iodides to the
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formation of by-products resulting from S-arylation of 11.
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obtained by oxidation of 11 reacted to give a trace amount of
12.
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