Pyrene-benzoylthiophene exciplexes as selective catalysts for the [2+2] cycloaddition between cyclohexadiene and styrenes

Article abstract of DOI:10.1021/ol070404x

Efficient intramolecular fluorescence quenching in pyrene-benzoylthiophene systems leads to formation of exciplexes. These species interact with 1,3-cyclohexadiene (or styrenes), leading to reactive excited triplexes. The overall process affords [2+2] cro

Full text of DOI:10.1021/ol070404x

ORGANIC
LETTERS
2
007
Vol. 9, No. 11
067-2070
Pyrene
−
Benzoylthiophene Exciplexes as
2
Selective Catalysts for the [2
+
2]
Cycloaddition between Cyclohexadiene
and Styrenes
Maria Gonz a´ lez-B e´ jar, Abdeslem Bentama, Miguel Angel Miranda,
Salah-Eddine Stiriba,* and Julia P e´ rez-Prieto*
Departmento Q. Org a´ nica, Instituto de Ciencia Molecular/ICMOL, UniVersidad de
Valencia, Pol ´ı gono La Coma, sn, 46980 Paterna, Spain, and Departmento Qu ´ı mica,
Instituto de Tecnolog ´ı a Qu ´ı mica UPV-CSIC, UniVersidad Polit e´ cnica,
46022 Valencia, Spain
julia.perez@uV.es
Received February 16, 2007
ABSTRACT
Efficient intramolecular fluorescence quenching in pyrene−benzoylthiophene systems leads to formation of exciplexes. These species interact
with 1,3-cyclohexadiene (or styrenes), leading to reactive excited triplexes. The overall process affords [2
average yield of 57%.
+2] cross-cycloadducts with an
Cross-cycloaddition is a well-established synthetic methodol-
ogy for creation of two new carbon-carbon bonds, leading
to the formation of four- ([2+2]) or six-membered ([4+2])
rings. In general, these reactions are inefficient when using
electron-rich dienes and olefins.1
cycloaddition, as well as olefin and diene dimerization, are
usually competitive processes. The use of singlet photocata-
lysts ( C*), such as cyanoarenes, leads mainly to the [4+2]
1
4
cross-cycloadducts; their formation has been postulated to
-3
1
occur via (C-Sty-CHD)* ternary excited-state complexes
In the reaction between cyclohexadienes (CHDs) and
styrene (Sty) derivatives, both [2+2] and [4+2] cross-
(singlet triplexes). Alternatively, electron-transfer photocata-
lysts such as triarylpyrylium salts have proved selective
toward [2+2] cross-cycloaddition, upon restricted solvent
election (THF), albeit with low yield (18%) and accompanied
by a considerable degree of dimerization (24%).4b By
contrast, both benzophenone triplet photosensitization and
4
(1) Interesting exceptions of [2+2] cross-cycloadditions are: (a) photo-
cycloaddition involving O-, N-, or S-substituted ethenes: Akbulut, N.;
Schuster, G. B. Tetrahedron Lett. 1988, 29, 5125. Mattay, J.; Vondenhof,
M.; Denig, R. Chem. Ber. 1989, 122, 951. Pabon, R. A.; Bellvile, D. J.;
Bauld, N. L. J. Am. Chem. Soc. 1984, 106, 2730. (b) The photochemical
cycloaddition of C60 to alkyl-substituted 1,3-butadienes: Vassilikogiannakis,
G.; Chronakis, N.; Orfanopoulos, M. J. Am. Chem. Soc. 1998, 120, 9911.
4a
thermal treatment lead exclusively to CHD dimerization.
The pyrene (Py) chromophore is characterized by a high
(
c) The 2-acetonaphthanone photosensitized formation of [2+2] cyclo-
adducts of cyclohexadiene with ethylene at pressures above 10 bar: Mirbach,
M. F.; Mirbach, M. J.; Saus, A. Z. Naturforsch. B. Anorg. Chem. Org. Chem.
fluorescence quantum yield and a long singlet lifetime (ca.
5
0
.65 and 650 ns, respectively, in nonpolar solvents). Its
1
977, 32B, 47.
2) Interesting exceptions of [4+2] cross-cycloadditions are the direct
triplet is long-lived and has a very low energy (E
T
) 2.1
(
eV), but it is generated with low quantum yield (Φ
Τ
ca. 0.2),6
or photosensitized irradiation using alkoxy-substituted 1,3-butadienes or
olefins: (a) Mikami, K.; Matsumoto, S.; Okubo, Y.; Fujitsuka, M.; Ito, O.;
Suenobu, T.; Fukuzumi, S. J. Am. Chem. Soc. 2000, 122, 2236. (b) Akbulut,
N.; Hartsough, D.; Kim, J.-I.; Schuster, G. B. J. Org. Chem. 1989, 54, 2549.
(4) (a) Hartsough, D.; Schuster, G. B. J. Org. Chem. 1989, 54, 3. (b)
Martiny, M.; Steckhan, E.; Esch, T. Chem. Ber. 1993, 126, 1671.
(5) Murov, S.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry;
Marcel Dekker: New York, 1993.
(
3) Some thermal Diels-Alder reactions are metal-catalyzed processes:
Hilt, G.; Smolko, K. I. Synthesis 2002, 686.
1
0.1021/ol070404x CCC: $37.00
© 2007 American Chemical Society
Published on Web 04/26/2007

especially in nonpolar solvents. However, the covalent
linking of pyrene and benzoylthiophene (BT) units leads to
Py-BT bichromophoric compounds such as 3 (Figure 1).
Table 1. [2+2] Cross-Cycloaddition between Styrenes 4a-c
and 1,3-Cyclohexadiene, Photocatalyzed by Bichromophore 3
styrene
solvent
time (h)
[2+2] cycloadduct
yield (%)
4
4
a
a
dioxane
dioxane
dioxane
dioxane
dioxane
CH2Cl2
CH2Cl2
CH2Cl2
6
15
6
15
15
15
6
5a
5a
5b
5b
5c
5a
5b
5c
45
45
55
60
40
61
68
64
4
4
b
b
4
4
c
a
4b
4
c
6
-
4
amount of 3 (5 × 10 M) led to [2+2] cycloadducts 5a-c
(exo/endo ratio close to 2:1) as the only observable products
(
see Scheme 1 and Table 1 and Figure S2 in Supporting
Figure 1. Photocatalysts.
7
Information). However, when 3 was replaced by the model
compounds 1 or 2, polymerization was extensive and only
trace amounts of [2+2] cycloadducts were detected by GC.
Likewise, cycloadduct formation was not observed in the
absence of the photocatalyst.
Their lowest-lying triplet excited state, located in the pyrene,
is generated with an enhanced quantum yield (Φ
Τ
) 0.91 in
1,4-dioxane). This has been explained by exciplex formation
between the pyrene singlet excited state and the benzoyl-
thiophene ground state.
With this background, it seemed of interest to explore
whether (Py-BT)* intramolecular exciplexes could be
quenched by unsaturated compounds, such as 1,3-cyclohexa-
diene (CHD) and styrenes (4a-c), and, if so, whether they
can be used as catalysts in cycloaddition processes. We wish
to report here on the preliminary results obtained with
bichromophore 3 in solvents of different polarity, such as
dioxane and dichloromethane (Scheme 1, Table 1). Interest-
Scheme 2. Processes Involved in the
Photoexcitation-Deactivation of Py-BT in the Presence of
CHD and 4a
Scheme 1. Photocatalyzed [2+2] Cross-Cycloaddition
between 1,3-Cyclohexadiene and Styrenes
Scheme 2 summarizes the major pathways that can in
principle be involved in the reaction mechanism. The relative
contributions of the different processes were determined by
means of steady-state fluorescence and transient absorption
measurements. For this purpose, the main deactivation
pathways considered for the singlet excited state were
fluorescence emission, intermolecular quenching, and intra-
molecular deactivation by the BT moiety (Scheme 2, ii-v).
Competition among them should be governed by the respec-
tive rate constants and by the quencher concentration.
As previously observed in dioxane, the emission spectrum
of 3 in dichloromethane after excitation at 355 nm (Figure
ingly, it will be shown that 3 photoinduces formation of the
[2+2] cross-cycloadducts without competitive formation of
dimers. Conversely, model compounds containing the iso-
lated chromophores, N-acetyl-1-pyrenylmethylamine (1) and
2-benzoylthiophene (2) (Figure 1), lead exclusively to
polymeric mixtures.
2) was remarkably weaker than that of the parent compound,
The UV-visible absorption spectra of 3, in dichloro-
methane (Figure S1 in Supporting Information) and dioxane,
indicated that at a wavelength longer than 320 nm more than
although the absorption of the samples at the excitation
(6) P e´ rez-Prieto, J.; Pastor-P e´ rez, L.; Gonz a´ lez-B e´ jar, M.; Miranda, M.
9
9% of the excitation energy is absorbed by the pyrene
moiety.
Irradiation (320 nm > λ > 360 nm) of deaerated solutions
containing CHD (0.04 M), 4a-c (0.1 M), and a catalytic
A.; Stiriba, S. E. Chem. Commun. 2005, 5569.
(7) Standard procedure: A deaerated dichloromethane solution of 4a
-
4
(
0.10 M), CHD (0.04 M), and Py-BT (5 × 10 M) was irradiated under
a nitrogen atmosphere. After solvent evaporation, the product was purified
by column chromatography (using hexane as eluent).
2068
Org. Lett., Vol. 9, No. 11, 2007

Figure 3. Stern-Volmer plots for fluorescence quenching of 3
-
5
(
2.5 × 10 M) in dichloromethane by (A) CHD and (B) 4a.
-
5
Figure 2. Fluorescence spectra of 1 (3.6 × 10 M) and 3 (×5)
-
5
(
2.5 × 10 M) (λexc ) 355 nm) in deaerated dichloromethane.
f
The fluorescence rate constant (k ) and the intramolecular
quenching rate constant (kintra) are intrinsic properties of 3.
wavelength was adjusted to the same value (0.3). In general,
fluorescence quantum yields were lower in dichloromethane
The value of the former was calculated with eq 3 and found
8
6
-1
to be k
f
) 8.3 × 10 s .
τ k ) Φ
f
than in dioxane (Table 2). The intramolecular quenching rate
8
-1
constant (kintra) was estimated as 8.3 × 10 s using eq 1
(3)
f
f
0
f
kintra ) (1/τ - 1/τ )
(1)
f
The fluorescence quenching of 3 by styrene and 1,3-
cyclohexadiene was examined in dichloromethane. Indeed,
the emission (λexc ) 350 nm, λem ) 360-650 nm) of 3 was
efficiently quenched by CHD and 4a with rate constants kq1
where τ
f
and τ 0
are the pyrene singlet lifetimes in the
f
bichromophoric compound 3 and in 1, respectively.
Because the 2-benzoylthiophene chromophore has a singlet
energy 0.43 eV higher than that of pyrene, such emission
quenching cannot be attributed to singlet-singlet energy
transfer from the pyrene to the BT chromophore.
The free energy change, ∆G , for the intramolecular
8
-1 -1
8
-1 -1
)
4.0 × 10 M
s
and kq2 ) 0.9 × 10 M s ,
9
respectively. These values were obtained from the Stern-
Volmer analysis (Figure 3A and B), according to eqs 4 and
0
5
, using the singlet lifetime measured for 3 (τ
f
) 1.2 ns).
0
eT
electron-transfer process in dichloromethane was estimated
0
f
0
f
Φ /Φ - 1 ) τ k [CHD]
(4)
(5)
using the known half-wave reduction potential of BT (around
f
q1
1
0
-
1.6 eV vs SCE) and the half-wave oxidation potential of
0
0
11
Φ /Φ - 1 ) τ k [4a]
f f f q2
pyrene (1.16 eV vs SCE) in the Rehm-Weller relationship
(eq 2).
From these data, using eqs 6-10, a very minor contribution
of the pyrene singlet excited state to the cycloaddition should
be expected under the employed reaction conditions.
0
eT
+
-
∆
G
) E (D/D ) - E (A /A) - E * +
ox
red
0,0
2.6 eV/ꢀ - 0.13 eV (2)
where E0,0* is the singlet energy of the pyrene chromophore
3.3 eV).
Contrary to the case of dioxane, the ∆G value (Table 2)
indicates that photoelectron transfer between pyrene and BT
in dichloromethane is now a thermodynamically feasible
process.
k ) k + k + k [CHD] + k [4a]
(6)
(7)
(8)
(9)
s
f
intra
q1
q2
(
0
eT
Residual fluorescence:
k
/k ) 0.01
s
f
Intramolecular fluorescence quenching:
) 0.96
Fluorescence quenching by CHD:
0
eT
0
exc
Table 2. Values of ∆G , ∆G , Singlet and Triplet Quantum
Yields, and Singlet Lifetimes for 1 and 3 in Dioxane and
Dichloromethanea
kintra/k
s
1
1
3
3
b
b
dioxane
CH2Cl2
dioxane
CH2Cl2
0
∆
∆
Φf
τf
ΦT
G
G
na
na
0.67
133.3
0.18
na
na
0.30
130.0
0.70
0.47
-0.22
0.02
2.4
0.91
-0.42
-
0.01
1.2
eT
0
exc
k
q1[CHD]/k
Fluorescence quenching by 4a:
k [4a]/k ) 0.02
) 0.01
s
0.99
a
Units: ns (τf), eV (∆G0). b From ref 6. c na: not applicable.
(10)
q2
s
Org. Lett., Vol. 9, No. 11, 2007
2069

Transient absorption studies were performed to check
whether the pyrene triplet excited state may be involved in
the catalytic process. Laser excitation of 3 at 355 nm (Nd:
YAG, 10 ns laser pulse) of deaerated dichloromethane
solutions showed for 3 (as in dioxane) the typical transient
k ) k + k + k [Q]
(11)
(12)
(13)
s
f
intra
q
Residual fluorescence:
12
kf/k
s
absorption spectrum of the pyrene triplet with maxima at
20 and 520 nm (see Figure 4A).
4
Intramolecular quenching of fluorescence:
k /ks
q1
Quenching of fluorescence by Q:
k [Q]/k ) y
(14)
(15)
q
s
Acalcd ) A - yA
o
o
However, triplet formation could also be prevented via
quenching of the exciplex by CHD or 4a. In this case, the
difference Acalcd - Afound would be proportional to the
quencher concentration (eq 16).
Acalcd - Afound ) K[Q]
(16)
Indeed, a linear relationship was found for both quenchers
Figure 4B). This result confirms deactivation of the exciplex
Figure 4. (A) Transient absorption spectrum of 3 in deaerated
dichloromethane, recorded 200 ns after the 355 nm laser pulse.
(
by the unsaturated compounds (Scheme 2, vi and vii).
These observations, together with the chemoselectivity
toward [2+2] cross-cycloaddition, contrast with the results
obtained with pyrylium salts and benzophenone and therefore
constitute clear evidence against the involvement of either
(B) Decrease of the triplet absorbance (measured at 420 nm
inmediately after the laser pulse) in the presence of increasing
amounts of 4a and CHD.
3
The intersystem crossing quantum yield was even higher
the cyclohexadiene radical cation or CHD in the cyclo-
13
than in dioxane (Φ
T
0.99 vs 0.91 ). Therefore, in both
addition photocatalyzed by 3. Accordingly, only polymeri-
zation products were obtained when using either 1 or 2 as
the photocatalyst.
dioxane and dichloromethane a dramatic enhancement of
intersystem crossing to the pyrene triplet (necessarily arising
from the exciplex, route iii in Scheme 2) was observed in
the bichromophore. Thus, no evidence for electron transfer
within the Py-BT system was detected.
Interestingly, the triplet absorbance was markedly reduced
in the presence of increasing amounts of CHD or 4a. If the
action of these quenchers was only limited to their interaction
with the measured pyrene singlet excited state, the theoretical
In summary, pyrene-benzoylthiophene systems act as
selective photocatalysts in the cross-cycloaddition between
CHD and styrenes. The concept is based on the efficient
generation of intramolecular exciplexes, quenchable by
unsaturated compounds to form reactive excited triplexes.
The obtained [2+2] products are of synthetic interest, given
the wide distribution of bicyclo[4.2.0]octanes in nature and
the inherently difficult synthesis of these strained com-
A
calcd value in the presence of the quencher could be
1
4
calculated using eq 15, taking into account the expected
pyrene singlet deactivation (eqs 11-14) for each quencher
concentration.
pounds.
Acknowledgment. We thank the Spanish Government
(
CTQ2005-00569, M.G.B for a pre-doctoral grant) and the
(
8) The fluorescence quantum yield was determined by a comparative
Generalitat Valenciana (ACOMP06/134, GV06/234) for
generous support of this work.
method, using recrystallized quinine sulfate in 0.5 M H2SO4 (Φf ) 0.546)
as the reference standard: Crosby, G. A.; Demas, J. N. J. Phys. Chem.
1
971, 75, 991.
9) (a) Arnold, D. R.; Birtwell, R. J. J. Am. Chem. Soc. 1973, 95, 4599.
b) Gonz a´ lez-Luque, R.; Merch a´ n, M.; Rubio, M.; Serrano-Andr e´ s, L.; Roos,
B.-O.; Miranda, M. A. Mol. Phys. 2003, 101, 1977.
10) P e´ rez-Prieto, J.; Stiriba, S. E.; Bosc a´ , F.; Lahoz, A.; Domingo, L.
R.; Mourabit, F.; Monti, S.; Miranda, M. A. J. Org. Chem. 2004, 69, 8618.
(
Supporting Information Available: NMR spectra (crude
after steady-state irradiation, compounds 5 and 4c) and details
of the characterization of compounds. This material is
available free of charge via the Internet at http://pubs.acs.org.
(
(
(
(
(
11) Weller, A. Z. Isr. J. Chem. 1970, 259.
12) Ford, W. E.; Rodgers, M. A. J. J. Phys. Chem. 1992, 96, 2917.
13) The triplet quantum yield was determined for 3 by a comparative
OL070404X
method using a deaerated acetonitrile solution of pyrene (ΦT ) 0.38) with
the same absorbance at the excitation wavelength.
(14) Lee-Ruff, E.; Mladenova, G. Chem. ReV. 2003, 103, 1449.
Org. Lett., Vol. 9, No. 11, 2007
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