Photoinduced rearrangements of 3,3′-bis(arylbenzofurans)

Article abstract of DOI:10.1021/ol102816a

Complex tetracyclic ring systems were assembled by a photoinduced rearrangement of 3,3′-bis(arylbenzofurans). Irradiation of 1 under N ₂ atmosphere yielded the benzonaphthofurans 2 in 75-90% yield. When the reaction was conducted under an O

Full text of DOI:10.1021/ol102816a

ORGANIC
LETTERS
2011
Vol. 13, No. 3
474-477
Photoinduced Rearrangements of
3,3′-Bis(arylbenzofurans)
Mathieu Auzias, Daniel Ha¨ussinger, Markus Neuburger, and Hermann A. Wegner*
Department of Chemistry, UniVersity of Basel, St. Johanns-Ring 19,
4056 Basel, Switzerland
hermann.wegner@unibas.ch
Received November 19, 2010
ABSTRACT
Complex tetracyclic ring systems were assembled by a photoinduced rearrangement of 3,3′-bis(arylbenzofurans). Irradiation of 1 under N2
atmosphere yielded the benzonaphthofurans 2 in 75-90% yield. When the reaction was conducted under an O2 atmosphere in the presence
of tetracyanoethylene (TCNE) a different reaction pathway was observed leading to the isolation of 3 as the major product. Also, the photochemical
properties of these novel structures were investigated.
Light has always been regarded as a powerful energy source
for chemists to perform reactions.1 Electronic excitation
changes the distribution of electrons in the molecules
affecting their chemical properties, which can lead sometimes
to novel and unexpected reactivities. Typical compounds
obtained from UV or visible light irradiation are polycyclic
or highly functionalized molecules. Such new products
formed by irradiation would be difficult to access with
standard chemistry reactions of ground state substrates.
Nowadays, the field of organic photochemical reactions
experiences a veritable revival of activity in both academics
and industry,2 in particular with its use in the context of green
chemistry or total synthesis.3
Among the polycyclic aromatic molecules, which are sub-
strates of interest for photochemistry, benzofuran derivatives
have attracted considerable attention on their photoluminescent
properties due to their conjugated π-system.4 Hence, the use
of benzofurans for functional materials became a rich field of
investigations. Applications have been reported as photochromic
compounds and dyes.5 Noteworthy is the recent use of benzo-
furan trimers as materials for organic electroluminescence.6 We
lately developed a quick and easy way to access dimeric
derivatives of benzofurans via a gold-catalyzed domino process.7
The new 3,3′-bisbenzofurans obtained following this procedure
are interesting substrates for photochemical reactions. During
our investigations on their photoluminescent properties we
(3) Ciana, C.-L.; Bochet, C. G. Chimia 2007, 61, 650. For recent selected
examples see: (a) Ravelli, D.; Zema, M.; Mella, M.; Fagnoni, M.; Albini,
A. Org. Biomol. Chem. 2010, 8, 4158. (b) Fasani, E.; Manet, I.; Capobianco,
M. L.; Monti, S.; Pretali, L.; Albini, A. Org. Biomol. Chem. 2010, 8, 3621.
(c) Fleck, M.; Bach, T. Chem.sEur. J. 2010, 16, 6015. (d) Jahjah, R.;
Gassama, A.; Bulach, V.; Suzuki, C.; Abe, M.; Hoffmann, N.; Martinez,
A.; Nuzillard, J.-M. Chem.sEur. J. 2010, 16, 3341. (e) Dichiarante, V.;
Fagnoni, M.; Albini, A. Green Chem. 2009, 11, 942. (f) Selig, P.;
Herdtweck, E.; Bach, T. Chem.sEur. J. 2009, 15, 3509. (g) Griesbeck,
A. G.; Neudo¨rfl, J.; Ho¨rauf, A.; Specht, S.; Raabe, A. J. Med. Chem. 2009,
52, 3420. (h) Vogt, F.; Jo¨dicke, K.; Schro¨der, J.; Bach, T. Synthesis 2009,
4268. (i) De´bieux, J. L.; Bochet, C. G. J. Org. Chem. 2009, 74, 4519. (j)
Zimmerman, H. E.; Shorunov, S. J. Org. Chem. 2009, 74, 5411. (k) Fleck,
M.; Bach, T. Angew. Chem., Int. Ed. 2008, 47, 6189. (l) Guthmann, H.;
Ko¨nemann, M.; Bach, T. Eur. J. Org. Chem. 2007, 632. (m) Bauer, A.;
Westka¨mper, F.; Grimme, S.; Bach, T. Nature 2005, 436, 1139.
(4) (a) Yamaguchi, T.; Takami, S.; Irie, M. J. Photochem. Photobiol.,
A 2008, 193, 146. (b) Sakamoto, M.; Kinbara, A.; Yagi, T.; Mino, T.;
Yamaguchi, K.; Fujita, T. Chem. Commun. 2000, 1201.
ˇ
ˇ
(5) (a) Skoriæ, I.; Ciorba, S.; Spalletti, A.; Sindler-Kulyk, M. J.
Photochem. Photobiol., A 2009, 202, 136. (b) Piloto, A. M.; Fonseca,
A. S. C.; Costa, S. P. G.; Gonc¸alves, M. S. T. Tetrahedron 2006, 62, 9258.
(c) Piloto, A. M.; Costa, S. P. G.; Gonc¸alves, M. S. T. Tetrahedron Lett.
2005, 46, 4757.
(6) Anderson, S.; Taylor, P. N.; Verschoor, G. L. B. Chem.sEur. J.
2004, 10, 518.
(1) Roth, H. D. Angew. Chem., Int. Ed. Engl. 1989, 28, 1193.
(2) Hoffmann, N. Chem. ReV. 2008, 108, 1052.
(7) Auzias, M. G.; Neuburger, M.; Wegner, H. A. Synlett 2010, 16, 2443.
10.1021/ol102816a  2011 American Chemical Society
Published on Web 12/20/2010

observed photorearrangements on 3,3′-bis(arylbenzofurans)
under UV irradiation leading to the formation of different
benzonaphthofuran derivatives depending on the reaction condi-
tions used.
Table 1. Substrate Scope of the Photorearrangement on
3,3′-Bis(arylbenzofurans) 1a-ea
First observations revealed the relative reactivity of such
compounds when submitted to a UV light source: irradiation
of derivative 1a gave rise to the formation of benzonaph-
thofuran 2a mainly (Scheme 1). The structure of compound
Scheme 1. Photorearrangement of Dimer 1a
2a has been confirmed by a single-crystal X-ray analysis (The
crystallographic coordinates have been deposited with the
Cambridge Crystallographic Data Centre; deposition no.
798480. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre, 12 Union Rd.,
Cambridge CB2 1EZ, UK or via www.ccdc.cam.ac.uk/conts/
retrieving.html).
Optimization of the process was performed with 1a as
substrate. Different sources of ultraviolet irradiation were used:
UV TLC lamp,8 UV monochromator,9 Rayonet UV reactor,10
in combination with different wavelengths (from 300 to 365
nm); several solvents were also tested (chloroform, acetonitrile,
benzene, dimethylsulfoxide, acetone). Screening of conditions
on NMR scale experiments showed that the appropriate
atmosphere of the reaction is crucial since aerobic conditions
led to lower formation of 2a. Finally, running the reaction under
inert atmosphere in anhydrous acetone within a Rayonet reactor
(300 nm) at 40 °C led to the isolation of the photorearrangement
compound 2a in 88% yield.
a Reaction conditions: 1, anhydrous acetone (0.02-0.03 mol·L-1), 300
nm irradiation (Rayonet), N2 atmosphere, 18 h, 40 °C. b Isolated yields.
pounds 2b-e (Table 1). The reaction is not affected by the
nature of the starting material since the different compounds
have been obtained with high yields between 75% and 90%.
The presence of substituents at the para position of the 3,3′-
bis(arylbenzofurans) was tolerated for the photorearrange-
ment independently of their electron-donating or electron-
withdrawingeffect(entries2-4, Table1). Thephotoreaction
is also not prevented by the presence of an electron-
donating group (methoxy) on the benzofuran moiety (entry
5, Table 1).11
During the course of improving the formation of 2a-e,
we tested the introduction of photosensitizers to see whether
or not their use would increase the reaction yields. Several
sensitizers were employed: rose Bengal, benzophenone,
tetracyanoethylene, and maleic anhydride. Surprisingly, the
presence of tetracyanoethylene (TCNE) in the acetone
solution of dimer 1a led under ultraviolet irradiation (300
nm) to the formation of a new compound 3 (Scheme 3).
Unambiguous assignment of all 1H and 13C resonances of
3 was achieved by one- and two-dimensional NMR experi-
ments (selective TOCSY, COSY, HMQC, and HMBC
We prepared the 3,3′-bis(arylbenzofurans) 1b-e according
to the previously published method7(Scheme 2) and subjected
Scheme 2
.
Domino Process Toward the Formation of
3,3′-Bis(arylbenzofurans) 1
them to the optimized reaction conditions, which led to the
formation of the corresponding photorearrangement com-
(8) UV lamp 254/302/365 nm, UVProducts.
(9) UV Monochromator, Polychrome V, Till Photonics.
(10) Rayonet reactor RPR-100, Southern New England Ultra Violet
Company, 270/300/350 nm.
(11) Similar benzonaphthofurans have been synthesized recently by the
halopalladation/decarboxylation/carbon-carbon bond forming domino pro-
cess in moderate to good yields: Huang, X.-C.; Wang, F.; Liang, Y.; Li,
J.-H. Org. Lett. 2009, 11, 1139.
Org. Lett., Vol. 13, No. 3, 2011
475

Scheme 3
.
Photorearrangement of Dimer 1a in the Presence of
Tetracyanoethylene (TCNE)
spectra). The structure of the new compound could be
determined by using NOESY techniques. According to the
analytical data, only one diastereomer of 3 was formed during
the reaction installing two stereocenters stereospecifically.
Although only one diastereomer could be isolated the yield
of 32% does not exclude the formation of the other
diastereomer. Interestingly, no formation of 3 was observed
when other photosensitizers were used. Moreover, no reaction
occurred between tetracyanoethylene and 1a in the absence
of UV irradiation. The atmosphere of the reaction seemed
to be a crucial point since the formation of 3 was improved
under oxygen atmosphere. Optimization for the formation
of compound 3 led to a yield of 32% (Table 2).
Figure 1. Proposed mechanism for the formation of 2a and 3.
the UV spectrum). Under N2 atmosphere the cleavage of one
furan ring via a syn-elimination occurs and releases 2a. On
the other hand, in the presence of TCNE an electron tranfer
can be envisioned leading to the radical cation of A, which
then deprotonates to form a radical intermediate B. This
reacts with O2 from the less hindered convex face to peroxo-
compound C, which finally turns into 3 after trapping of the
peroxo by tetracyanoethylene (tetracyanoethylene epoxide
formation was osberved on crude NMR).14 Absorption and
fluorescence spectra have been measured for derivatives
2a-e and 3. The measurements have been performed on
degassed solutions of 2a-e and 3 (1 × 10-5 M in dichlo-
romethane). Compounds 2a-e exhibited an UV absorption
maximum centered around 270 nm for all the compounds
showing a weak impact of the nature of the substituents
(Figure 2). Despite the more extended π-system in the
Table 2. Screening of the Conditions Toward the Formation of
3a
entry
irradiation
atmosphere time, h 3 yield,b
%
1
2
3
UV lamp, 302 nm
Rayonet, 300 nm
Rayonet, 300 nm
air
O2
O2
2
4
18
8
19
32
a Reaction conditions: 1, TCNE (10 equiv), anhydrous acetone
(0.02-0.03 mol·L-1). b Isolated yields.
Taking into account that the formation of 3 was done in
the presence of a sensitizer (tetracyanoethylene) and im-
proved by oxygen atmosphere, it would be reasonable to
assume a photo-oxygenation of 1a and the involvement of
singlet oxygen.12,13 However, in this case, another sensitizer
producing singlet oxygen should also lead to product 3
formation, which is not observed.
A proposed mechanism for the formation of 2a and 3 is
depicted in Figure 1. A common step for the two pathways
is the conrotatory 6π-photocyclization of 1a leading to
intermediate A with the creation of a new six-membered ring
(after irradiation at 302 nm for 2 min a new transient species
with an absorption maximum at 510 nm was observed in
Figure 2. UV spectra of compounds 2a-e in dichloromethane.
(12) (a) Clennan, E. L. Tetrahedron 2000, 56, 9151. (b) Prein, M.; Adam,
W. Angew. Chem., Int. Ed. Engl. 1996, 35, 477. (c) Prein, M.; Adam, W.
Acc. Chem. Res. 1996, 29, 275. (d) Schweitzer, C.; Schmidt, R. Chem. ReV.
2003, 103, 1685. (e) For examples of organic chemistry of singlet oxygen
products compared to the starting material, an hypsochromic
effect is observed for compounds 2a-e by comparison with
the 3,3′-bis(arylbenzofurans) 1a-e, which presented a maxi-
see: Tetrahedron 2006, 62, 46
.
(13) (a) Clenann, E. L. In Molecular and Supramolecular Photochem-
istry; Ramamurthy, V., Schanze, K. S., Eds.; Vol. 12, Synthetic Organic
Photochemistry; Griesbeck, A. G., Mattay, J., Eds.; Marcel Dekker: New
York, 2005; p 365. (b) Stratakis, M.; Orfanopoulos, M. Tetrahedron 2000,
(14) (a) Miller, J. S. Angew. Chem., Int. Ed. 2006, 45, 2508. (b) Linn,
W. J. Org. Synth. 1969, 49, 103.
56, 1595. (c) Adam, W.; Wirth, T. Acc. Chem. Res. 1999, 32, 703
.
476
Org. Lett., Vol. 13, No. 3, 2011

mum between 302 and 327 nm. Compound 3, on the other
hand, showed a small bathochromic effect with a maximum
absorption at 320 nm (Figure 3).
Figure 4. Fluorescence spectra of compounds 2a-e and 3 in
dichloromethane.
Figure 3. Comparative UV spectra of 1a (green), 2a (red), and 3
nature of the reaction atmosphere. In the presence of O2 and
TCNE 3 was isolated as the mayor product in a side reaction,
which is formed via a different mechanism. The benzonaph-
thofuran derivatives obtained present a potentially interesting
backbone for bioactive molecules given the large range of
biological applications exhibited by benzofurans and diben-
zofurans of which they are closely related.18 In the future,
the reaction will be applied to synthesize derivatives with
interesting photoluminescent or biological properties.
(purple) in dichloromethane.
This could be explained by the cross-conjugated chro-
mophore nature of compounds 1a-e.15 This phenomena is
observed for certain oligomeric and polymeric systems16 and
notably for indigo dyes where the inherent cross-conjugation
has been shown responsible for the high bathochromicity of
these π-systems.17
The fluorescence responses of 2a-e exhibited a quite
similar curve shape meaning that the nature of the substit-
uents on the aryl moieties also has low impact on emission
properties of these compounds. Fluorescence of compound
3 was found to be very weak; this is explained most likely
by the smaller conjugated π-system present in the molecule
compared to its analogue 2a (Figure 4).
In summary, we have investigated the photochemical and
photoluminescent properties of 3,3′-bis(arylbenzofurans).
Compounds 1 can be efficiently converted to the benzonaph-
thofurans 2. We have shown that the photoreactivity is
dependent on the presence/absence of additive and on the
Acknowledgment. The authors thank Prof. Dr. Bernd
Giese, University of Fribourg (Switzerland), for stimulating
discussions. M.A. thanks the Roche Research Foundation
and the Novartis Foundation for financial support. H.A.W.
is indebted to the Fonds der Chemischen Industrie for a
Liebig Fellowship. Umicore and Johnson Matthey are
gratefully acknowledged for generously providing auric acid.
Supporting Information Available: Experimental pro-
cedures and full spectroscopic data for all new compounds.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OL102816A
(15) Phelan, N. F.; Orchim, M. J. Chem. Educ. 1968, 45, 633.
(16) Gholami, M.; Tykwinski, R. R. Chem. ReV. 2006, 106, 4997.
(17) (a) Naef, R. Dyes Pigm. 1991, 17, 113. (b) Klessinger, M. Dyes
Pigm. 1982, 3, 235.
(18) Francolini, I.; Norris, P.; Piozzi, A.; Donelli, G.; Stoodley, P.
Antimicrob. Agents Chemother. 2004, 48, 4360.
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