Palladium-catalyzed carbopalladation and carbocyclization of arynes with aryl halides: A highly efficient route to functionalized triphenylenes

Article abstract of DOI:10.1039/b515846j

Highly substituted triphenylene derivatives were prepared in good yields via the palladium-catalyzed carbocyclization of arynes with aryl iodides. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b515846j

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Palladium-catalyzed carbopalladation and carbocyclization of arynes
with aryl halides: a highly efficient route to functionalized
triphenylenes{
Thiruvellore Thatai Jayanth and Chien-Hong Cheng*
Received (in Cambridge, UK) 9th November 2005, Accepted 14th December 2005
First published as an Advance Article on the web 19th January 2006
DOI: 10.1039/b515846j
of aryne into an arylpalladium intermediate A and subsequent
cyclopalladation. Its is also understood that a judicious selection of
reaction conditions is required to activate the C–H bond while at
the same time maintaining the conditions necessary for a steady
generation of aryne.10 Treatment of 2-(trimethylsilyl)-4,5-dimethyl-
phenyl triflate (1b) (2 equiv.) with 4-iodoacetophenone (2a,
0.30 mmol) in the presence of 5 mol% Pd(dba)2 and CsF (4 equiv.)
in CH3CN at 85 uC for 8 h gave the expected unsymmetrical
triphenylene product (Scheme 2) in 61% yield.11 We also found
that arynes 1b reacted with various aryl iodides bearing an
electron-withdrawing group to give moderate yields (50 y 60%) of
the expected triphenylene products along with the cyclotrimeriza-
tion product of aryne 1b. To our surprise, when benzyne precursor
1a was used instead of 1b, the reaction gave only a trace of the
expected triphenylene product 3a (Scheme 2). Similarly, the
reaction of other aryl iodides with 1a also afforded only a trace
of the expected products. The cyclotrimerization product
triphenylene from 1a was instead the major species observed.
Thus, under the reaction conditions used, it appears that the
reaction is extremely sensitive to the aryne substrates. To obtain
more reliable reaction conditions which can be used for various
arynes, we chose to find a suitable additive rather than undertake
an ordinary manipulation of the reaction conditions. No desired
product was obtained when silver salts (AgBF4, AgNO3, AgOAc
or Ag(O2CCF3)) were added to the reaction solution. The use of
Et3N gave the expected product 3a in only 6% yield. Interestingly,
the use of 1.2 equiv. of Tl(OAc) remarkably improved the yield of
3a up to 72%. Moreover, 3d was obtained from 1b and 2a in 83%
yield (Table 1, entries 1 and 5).12
Highly substituted triphenylene derivatives were prepared in
good yields via the palladium-catalyzed carbocyclization of
arynes with aryl iodides.
Triphenylenes are the most widely-synthesized and studied discotic
mesogens.1 Recently, these materials have been discussed as
potential candidates for opto-electronic devices.2 Several research
groups have worked on the synthesis of symmetrical, unsymme-
trical and functionalized triphenylene for discotic liquid crystals.3
Moreover, the interest shown by many research groups over the
years clearly signifies the importance of substituted triphenylenes
in materials chemistry.4 Among the recent developments in the
synthesis of triphenylenes, the use of highly reactive arynes in
palladium-catalyzed trimerization is a very notable method.5
However, the method was useful only for the synthesis of
symmetrical triphenylenes. In addition to the preparation of
triphenylenes, arynes have been used to prepare a variety of fused
polyaromatics. Palladium-catalyzed cocyclotrimerization of
alkynes and benzynes or allenes and benzynes (alkyne– or
allene–benzyne–benzyne), leading to phenanthrene derivatives,
have been reported by Castedo’s, Yamamoto’s and our groups.6
Castedo et al. described a synthesis of benzotriphenylenes.7
Furthermore, they also reported an intramolecular [2 + 2 + 2]
cyclotrimerization of benzynes with suitably functionalized benzo-
diynes to give benzo[b]fluorenones.8 Recently, we also reported a
synthesis of benzo[b]triphenylenes via deoxyaromatization of the
product obtained from [2 + 2 + 2] cocyclotrimerization of arynes
with bicyclic alkenes.9
It is well known that all of the above palladium-catalyzed
cocyclotrimerization reactions proceed mechanistically via
a
Table 1 summarizes the results of the reactions of various aryl
iodides with arynes under reaction conditions similar to those of
5-membered ring palladacycle. We envisaged that a similar
palladacycle C (Scheme 1) could be generated from an insertion
Scheme 1
Department of Chemistry, National Tsing Hua University, Hsinchu,
30013, Taiwan. E-mail: chcheng@mx.nthu.edu.tw;
Fax: (+886) 3-5724698; Tel: (+886) 3-5721454
{ Electronic Supplementary Information (ESI) available: General experi-
mental procedures, spectral data for all compounds, 1H and 13C NMR
spectra for all compounds. See DOI: 10.1039/b515846j
Scheme 2
894 | Chem. Commun., 2006, 894–896
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product in nearly the same yield. 4-Iodonitrobenzene (2d) afforded
2-nitrotriphenylene (3c) in 79% yield (Table 1, entry 4). The
reaction of aryne precursor 1b with 4-fluoroiodobenzene (2e) gave
the corresponding fluorotriphenylene 3e in 82% yield (Table 1,
entry 6). The use of unsubstituted iodobenzene (2f) gave the
tetramethyltriphenylene 3f in 75% yield (Table 1, entry 7).
Similarly, indanyl-substituted aryne precursor 1c reacted with 2a
and 2b to give the corresponding triphenylenes 3g and 3h in 73 and
77% yields, respectively (Table 1, entries 8 and 9). The present
reaction conditions appear unsuitable for aryl iodides with an
electron-donating group and benzynes with electron-withdrawing
substituents. For example, no expected unsymmetrical triphenyl-
ene products were observed for the reaction of 4-iodoanisole with
1b and 2-(trimethylsilyl)-4,5-difluorophenyl triflate with 2a under
the standard reaction conditions.
Table 1 Results of the Pd-catalyzed carbocyclization of arynes 1 and
aryl halides 2a
Entry
1
1
2
Product 3
Yield (%)b
72
2
1a
81
3
4
1a
1a
3b
79
78
A plausible mechanism for the reaction of aryne 1 with aryl
iodide 2 is illustrated in Scheme 3 (neutral ligands are omitted).13
The oxidative addition of
2 to the Pd(0) species gives
arylpalladium(II) intermediate A. Coordination of benzyne to
intermediate A, followed by insertion into the Ar–Pd bond,
forms an ortho-biarylpalladium species B, in which the aryl group
is g2-bonded to the palladium center. It is well known that g2-
arene complexes are intermediates in the C–H activation of arene
systems.14 Cyclopalladation through an intramolecular C–H
activation affords a five membered palladacycle C. Further
reaction of this intermediate with another benzyne and subsequent
reductive elimination affords the corresponding annulated product
with regeneration of the Pd(0) species. The possibility of insertion
of two molecules of benzyne into A and the subsequent
cyclopalladation to form a seven-membered palladacycle cannot
be totally excluded. Although the role of thallium salts is not very
clear at this stage, they are known to assist the removal of the
halide from palladium complexes and enhance ortho-metallation.15
In summary, we have demonstrated that arynes can be
efficiently employed in the palladium-catalyzed carbopalladation
and cyclopalladation sequence directed towards the synthesis of
polyaromatic compounds. While the present method is useful for
the preparation of a variety of highly functionalized triphenylenes,
its catalytic mechanism is not yet clear. A more detailed study of
the effect on the reaction of substituents attached to the aryne
5
6
2a
83
82
1b
1b
7
8
75
73
2a
2b
9
1c
77
a
Reaction conditions: 0.60 mmol of benzyne precursors 1, 0.30 mmol
of arylhalide 2, 0.015 mmol of Pd(dba)2, 1.20 mmol of CsF,
0.36 mmol of Tl(OAc), under nitrogen in 3.0 mL of CH3CN at 85 uC
b
for 8 h. Isolated yields.
Table 1, entry 1. When 4-iodoethylbenzoate (2b) was treated with
1a, triphenylene-2-ethylcarboxylate (3b) was obtained in 81% yield
(Table 1, entry 2). The reaction of 3-iodoethylbenzoate (2c) gave
3b as the only isomer (Table 1, entry 3). It is interesting to note that
both 4-iodoethylbenzoate and 3-iodoethylbenzoate gave the same
Scheme 3
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7 D. Pen˜a, A. Cobas, D. Pe´rez, E. Guitia´n and L. Castedo, Org. Lett.,
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10 As a fluoride source, CsF in CH3CN is the most common and suitable
reagent used to generate arynes from their precursor 2-(trimethylsilyl)-
phenyl triflates.
moiety and the aryl iodide, and the isolation of reaction
intermediates should be beneficial to understanding the detailed
reaction mechanism. Further studies in these directions are
currently under way.
We thank the National Science Council of the Republic of
China (94-2113-M-007-011) for their support of this research.
11 For the palladium-catalyzed reaction of an aryl halide and two alkynes
leading to naphthalenes, see: (a) G. Wu, A. L. Rheingold, S. L. Geib
and R. F. Heck, Organometallics, 1987, 6, 1941; (b) S. Kawasaki,
T. Satoh, M. Miura and M. Nomura, J. Org. Chem., 2003, 68, 6836.
12 Since the completion of this work, an article has appeared which
describes a synthesis of triphenylene. According to the single example
presented in the paper, aryne 1b reacted with 2b to yield the
corresponding triphenylene in only 50% yield. It is noteworthy that
our method, utilizing a suitable additive and thereby milder,
reproducible conditions, afforded triphenylenes in very good yields.
Moreover, aryne 1a was also successfully employed. See: Z. Liu,
X. Zhang and R. C. Larock, J. Am. Chem. Soc., 2005, 127, 15716.
13 Even though there is much precedence for the generation of arynes from
precursors 1, we carried out a reaction with an unsymmetrical aryne
4-methyl-2-trimethylsilylaryltriflate with 2a. A mixture of regioisomers,
as expected for a reaction of 4-methylbenzyne, was obtained, thereby
confirming the involvement of arynes in this reaction.
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896 | Chem. Commun., 2006, 894–896
This journal is ß The Royal Society of Chemistry 2006