Involvement of triplet excited states and olefin radical cations in electron-transfer cycloreversion of four-membered ring compounds photosensitized by (thia)pyrylium salts

Article abstract of DOI:10.1021/jo011103i

Cycloreversion of 1,2,3,4-tetraphenylcyclobutanes 1a,b and oxetane 2 is achieved using (thia)pyrylium salts as electron-transfer photosensitizers. Radical cation intermediates involved in the electron-transfer process have been detected using laser flash

Full text of DOI:10.1021/jo011103i

4138
J . Org. Chem. 2002, 67, 4138-4142
In volvem en t of Tr ip let Excited Sta tes a n d Olefin Ra d ica l Ca tion s
in Electr on -Tr a n sfer Cyclor ever sion of F ou r -Mem ber ed Rin g
Com p ou n d s P h otosen sitized by (Th ia )p yr yliu m Sa lts
Miguel A. Miranda,* M. Angeles Izquierdo, and Francisco Galindo
Departamento de Quı´mica, Instituto de Tecnologı´a Quı´mica UPV-CSIC, Universidad Polite´cnica
de Valencia, Camino Vera s/ n, Apartado 22012, 46022 Valencia, Spain
mmiranda@qim.upv.es
Received November 26, 2001
Cycloreversion of 1,2,3,4-tetraphenylcyclobutanes 1a ,b and oxetane 2 is achieved using (thia)-
pyrylium salts as electron-transfer photosensitizers. Radical cation intermediates involved in the
electron-transfer process have been detected using laser flash photolysis. The experimental results
are consistent with the reaction taking place from the triplet excited state of the sensitizer.
In tr od u ction
ceeds in a concerted fashion.2 However, substituents on
the cyclobutane ring may alter the mechanism by stabi-
lization of an acyclic intermediate on a very flat energy
hypersurface.3 In the case of oxetanes, semiempirical
calculations have shown that CR of their radical cations
is an exothermic and stepwise reaction.4
Stereoelectronic effects are of significance for exploring
the reactivity of these rigid systems.5,6 One of the most
important features is the cis effect of aryl substituents,
which can be subjected to electronic coupling through the
ring σ orbitals.7 This results in weaker σ bonds and
directs the splitting mode.
Cycloreversion (CR) of four-membered ring compounds
is one of the most important electron-transfer (ET)-
catalyzed pericyclic reactions.1 It has attracted much
attention in recent years, mainly focused on theoretical
calculations about the reaction pathways, stereoelectronic
effects, and biological implications in DNA repair.1-11
Recent theoretical studies on the CR of cyclobutane
radical cations have established that the reaction pro-
* To whom correspondence should be addressed. Phone: 963877340.
Fax: 963879349.
(1) (a) Mizuno, K.; Pac, C. In CRC Handbook of Organic Photo-
chemistry and Photobiology; Horspool, W. M., Song, P.-S., Eds.; CRC
Press: Boca Raton, FL, 1995; pp 359-365. (b) Fox, M. A.; Chanon, M.
Photoinduced Electron Transfer; Elsevier: Amsterdam, Oxford, New
York, Tokyo, 1988; Part A, pp 546-549.
(2) (a) Wiest, O. J . Phys. Chem. A 1999, 103, 7907-7911. (b)
J ungwirth, P.; Carsky, P.; Bally, T. J . Am. Chem. Soc. 1993, 115, 5776-
5782. (c) J ungwirth, P.; Bally, T. J . Am. Chem. Soc. 1993, 115, 5783-
5789.
(3) (a) Aida, M.; Inoue, F.; Kaneko, M.; Dupuis, M. J . Am. Chem.
Soc. 1997, 119, 12274-12279. (b) Rak, J .; Voityuk, A. A.; Ro¨sch, N. J .
Phys. Chem. A 1998, 102, 7168-7175.
(4) Wang, Y.; Gaspar, P. P.; Taylor, J . S. J . Am. Chem. Soc. 2000,
122, 5510-5519.
(5) (a) Moriarty, R. M. Top. Stereochem. 1974, 8, 271-421. (b)
Hoffmann, R.; Davidson, R. B. J . Am. Chem. Soc. 1971, 93, 5699-
5705. (c) Shima, K.; Kimura, J .; Yoshida, K.; Yasuda, M.; Imada, K.;
Pac, C. Bull. Chem. Soc. J pn. 1989, 62, 1934-1942. (d) Pac, C.; Go-
An, K.; Yanagida, S. Bull. Chem. Soc. J pn. 1989, 62, 1951-1959. (e)
Majima, T.; Pac, C.; Sakurai, H. J . Am. Chem. Soc. 1980, 102, 5265-
5273. (f) Pac, C. Pure Appl. Chem. 1986, 58, 1249-1256. (g) Yamashita,
Y.; Yaegashi, H.; Mukai, T. Tetrahedron Lett. 1985, 30, 3579-3582.
(6) (a) Yamashita, Y.; Ikeda, H.; Mukai, T. J . Am. Chem. Soc. 1987,
109, 6682-6687. (b) Okada, K.; Hisamitsu, K.; Miyashi, T.; Mukai, T.
J . Chem. Soc., Chem. Commun. 1982, 974-976. (c) Okada, K.;
Hisamitsu, K.; Mukai, T. Tetrahedron Lett. 1981, 22, 1251-1254.
(7) (a) Kaupp, G. Angew. Chem. 1974, 84, 741-742. (b) Kaupp, G.;
Stark, M. Chem. Ber. 1977, 110, 3084-3110.
(8) (a) Heelis, P. F.; Hartman, R. F.; Rose, S. D. Chem. Soc. Rev.
1995, 24, 289-297. (b) Sancar, A. Mutat. Res. 1990, 236, 147-160. (c)
Cadet. J .; Vigny, P. In Bioorganic Photochemistry; Morrison, H., Ed.;
Wiley: New York, 1990; pp 1-272. (d) Fisher, G. J .; J ohns, H. E. In
Photochemistry and Photobiology of Nucleic Acids; Wang, S. Y., Ed.;
Academic Press: New York, 1976; pp 225-294.
The ET-photosensitized CR of cyclobutanes and oxe-
tanes is of high biological interest because of its involve-
ment in DNA repair. Thus, CR of cyclobutane pyrimidine
dimers should be achieved in the repair of UV-damaged
DNA.8 Also, CR of oxetanes appears to be involved in the
photoenzymatic repair of the (6-4) photoproducts of the
DNA dipyrimidine sites by photolyase.9 Although pho-
toreducing sensitization is the mode usually operating
in biological systems,10 the photooxidative approach is
considered to be relevant for DNA repair therapies.11
Despite the considerable interest of ET-photosensitized
oxidative CR of cyclobutanes and oxetanes, some funda-
mental aspects require further studies. These include (i)
detection of transient intermediates by means of time-
resolved techniques and (ii) influence of the excited-state
multiplicity (singlet vs triplet) on the fate of the reaction.
For instance, 1,2,3,4-tetraphenylcyclobutanes have
been previously shown to produce two stilbene units upon
direct irradiation12 and γ radiolysis via radical cations;13
however, there is no report on their CR by irradiation in
the presence of ET photosensitizers. This may be due to
the high redox potentials of these cyclobutanes (ca. 2 V
vs SCE; see below), which prevent their oxidation using
most of the commonly employed photosensitizers.
(9) (a) Prakash, G.; Falvey D. E. J . Am. Chem. Soc. 1995, 117,
11375-11376. (b) Kim, S. T.; Malhotra, K.; Smith, C. A.; Taylor, J . S.;
Sancar, A. J . Biol. Chem. 1994, 269, 8535-8540. (c) Zhao, X.; Liu, J .;
Hsu, D. S.; Zhao, S.; Taylor, J . S.; Sancar, A. J . Biol. Chem. 1997, 272,
32580-32590.
(10) (a) Heelis, P. F.; Hartman, R. F.; Rose, S. D. J . Photochem.
Photobiol., A 1996, 95, 89-98. (b) Scannell, M. P.; Fenick, D. J .; Yeh,
S. R.; Falvey, D. E. J . Am. Chem. Soc. 1997, 119, 1971-1977. (c) Yeh,
S. R.; Falvey, D. E. J . Am. Chem. Soc. 1991, 113, 8557-8558.
(11) (a) Dandliker, P. J .; Holmlin, R. E.; Barton, J . K. Science 1997,
275, 1465-1468. (b) Pac, C.; Miyamoto, I.; Masaki, Y.; Ferusho, S.;
Yanagida, S.; Ohno, T.; Yoshimura, A. Photochem. Photobiol. 1990,
52, 973-979.
(12) (a) Takamuku, S.; Beck, G.; Schnabel, W. J . Photochem. 1979,
11, 49-52. (b) Shizuka, H.; Seki, I.; Morita, T.; Iizuka, T. Bull. Chem.
Soc. J pn. 1979, 52, 2074-2078.
(13) Tojo, S.; Morishima, K.; Ishida, A.; Majima, T.; Takamuku, S.
Bull. Chem. Soc. J pn. 1995, 68, 958-966.
10.1021/jo011103i CCC: $22.00 © 2002 American Chemical Society
Published on Web 05/11/2002

Electron-Transfer Cycloreversion of Ring Compounds
J . Org. Chem., Vol. 67, No. 12, 2002 4139
Ch a r t 1
Sch em e 1
Ch a r t 2
Ta ble 1. Cyclor ever sion Qu a n tu m Yield sa
φCR(1a )
φCR(1b)
φCR(2)
Sb
argon
oxygen
argon
oxygen
argon
oxygen
3a
3b
3c
3d
0.10
0.50
0.40
-
0.06
0.11
0.10
-
0.02
0.09
0.13
-
0.003
0.02
0.01
-
0.02
0.10
0.04
-
-
-
-
-
a
Experimental conditions: 1a , 1b , 2, 10-4 M; sensitizers
3a -d , 2.5 × 10-5 M; solvent, acetonitrile; 450 W xenon lamp,
monochromatic light of λ ) 420 nm; actinometer, potassium
ferrioxalate. The solutions were placed in sealed cuvettes and
b
bubbled with argon/oxygen for 15 min. S ) sensitizer.
of sensitizers 3a -d . In all cases except for 3d , trans-
stilbene 4 was detected as the only photoproduct by direct
1
GC-MS and H NMR analysis of the reaction mixtures.
When oxetane 2 was irradiated under the same condi-
tions, CR led to formation of trans-stilbene and acetal-
dehyde as the only photoproducts (Scheme 1).
To obtain reliable quantitative data, the CR quantum
yields (φCR) were measured using potassium ferrioxalate
as actinometer, under both oxygen and argon. The results
are shown in Table 1.
These results clearly show that (a) the CR reaction is
photosensitized by pyrylium salts, and the extent of the
reaction clearly depends on the nature of the employed
salt, (b) in the case of oxetane 2, CR produces carbonyl
and olefin units different from the reagents employed in
the Paterno-Bu¨chi cycloaddition, (c) the most efficient
photosensitizer is the thiapyrylium salt 3b, while the
methoxy derivative 3d is unreactive, and (d) the reaction
is clearly quenched by oxygen. Moreover, the fact that
the obtained quantum yields were much less than unity
appears to rule out the possibility of a chain reaction.
La ser F la sh P h otolysis Stu d ies of Cyclobu ta n es
1a ,b. To gain further insight into the mechanism of the
observed cycloreversion, laser flash photolysis (LFP)
experiments were conducted under different experimen-
tal conditions.
In the case of oxetanes, a preliminary study using 2,2-
diaryl derivatives and cyanoaromatic compounds as ET
partners has shown that the CR reaction occurs through
the excited singlet state of the photosensitizer.14 Although
the process was explained via formation of radical cat-
ions, no intermediate of this type was detected. Besides,
CR ensues with cleavage of the same C-C bonds formed
in the Paterno-Bu¨chi photocycloaddition employed to
synthesize the starting 2,2-diaryloxetanes. A radical
chain mechanism has been found to operate when
electron-donating substituents are attached to the aryl
groups.
In the present work, the above points i and ii will be
addressed by using 1,2,3,4-tetraphenylcyclobutanes 1a ,b
and the diphenyl-substituted oxetane 2 as electron donors
(Chart 1), together with a series of (thia)pyrylium salts
3a -d as acceptors (Chart 2), to gain further insight into
the mechanistic aspects of this reaction.
Pyrylium salts are a potentially useful group of pho-
tosensitizers.15 They are extremely good photooxidizing
agents and can be selectively excited because of their
absorption in the visible. On the other hand, depending
on the substitution pattern and the reaction conditions,
they can generate radical ion pairs of different multiplici-
ties (singlet or triplet)16 and hence may be of interest to
study the influence of this factor on the reaction, for
instance regarding the splitting mode and the stereose-
lectivity of the products.
Working with low cyclobutane concentrations (ca. 10-4
M), irradiation of deoxygenated acetonitrile solutions of
cyclobutane 1a in the presence of 3a -c (excitation at λ
) 355 nm) gave rise to an intense signal centered around
470 nm, which was assigned to the radical cation 4•+ on
the basis of literature data16,17 (Figure 1). It was formed
“instantaneously” (i.e., its growth was not observable
within our time scale) and exhibited a monoexponential
decay with τ ) 11.53 µs (see the inset of Figure 1).
Resu lts a n d Discu ssion
Stea d y-Sta te P h otolysis. Irradiation of cyclobutanes
1a ,b was carried out in the presence of catalytic amounts
(16) (a) Miranda, M. A.; Izquierdo, M. A.; Galindo, F. Org. Lett. 2001,
3, 1965-1967. (b) Akaba, R.; Oshima, K.; Kawai, Y.; Obuchi, Y.;
Negishi, A.; Sakuragi, H.; Tokumaru, K. Tetrahedron Lett. 1991, 32,
109-112. (c) Kuriyama, Y.; Arai, T.; Sakuragi, H.; Tokumaru, K. Chem.
Lett. 1988, 1193-1196. (d) Akaba, R.; Kamata, M.; Koike, A.; Mogi,
K. E.; Kuriyama, Y.; Sakuragi, H. J . Phys. Org. Chem. 1997, 10, 861-
869.
(17) (a) Shida, T. Electronic Absortion Spectra of Radical Ions;
Elsevier: Amsterdam, 1988; p 113. (b) Hamil, W. H. In Radical Ions;
Kaiser, E. T., Kevan, L., Eds.; Interscience: New York, 1968; p 399.
(14) Nakabayashi, K.; Kojima, J . I.; Tanabe, K.; Yasuda, M.; Shima,
K. Bull. Chem. Soc. J pn. 1989, 62, 96-101.
(15) (a) Miranda, M. A.; Garcı´a, H. Chem. Rev. 1994, 94, 1063-
1089. (b) Corma, A.; Forne´s, V.; Garc´ıa, H.; Miranda, M. A.; Primo, J .;
Sabater, M. J . J . Am. Chem. Soc. 1994, 116, 9767-9768. (c) Galindo,
F.; Miranda, M. A. J . Photochem. Photobiol., A 1998, 113, 155-161.
(d) Martiny, M.; Steckhan, E.; Esch, T. Chem. Ber. 1993, 126, 1671-
1682. (e) Akaba, R.; Arai, S., Sakuragi, H.; Tokumaru, K. J . Chem.
Soc., Chem. Commun. 1987, 1262-1263.

4140 J . Org. Chem., Vol. 67, No. 12, 2002
Miranda et al.
F igu r e 1. Transient spectrum obtained from LFP (λ ) 355
nm) of 1a (1.4 × 10-4 M) and 3a (1.2 × 10-4 M) in acetonitrile,
under nitrogen. Spectrum recorded 10 µs after the laser pulse.
Inset: rise time and decay of the signal peaking at 470 nm.
Besides the intense band at 470 nm, a smaller one at
550 nm was also clearly distinguished; the latter was
safely assigned to 2,4,6-triphenylpyranyl radical, the
reduced species of 3a .18 Analogous results were obtained
using 3b and 3c as photosensitizers; by contrast, no
transient spectrum was obtained in the case of 3d . On
the other hand, LFP of solutions of the all-trans dimer
1b in the presence of 3a -c also gave rise to a band at
470 nm assigned to radical cation 4•+. Again, pyrylium
salt 3d gave no measurable signal. On the basis of the
results obtained in the above time-resolved experiments,
CR of 1a ,b follows an ET mechanism where ring splitting
at low substrate concentrations gives rise to the radical
cation of trans-stilbene as the only detectable intermedi-
ate. Although in principle the isomeric radical cation of
cis-stilbene could also be formed, the absence of its
characteristic transient absorption at 515 nm allows this
possibility to be ruled out.16c
F igu r e 2. Transient spectra obtained from LFP (λ ) 355 nm)
of 1a ,b (10-2 M) and 3b (1.2 × 10-4 M) in acetonitrile, under
nitrogen, recorded at different times after the laser pulse.
Top: obtained for 1b. Bottom: obtained for 1a .
ruled out. Probably the lifetime of this intermediate is
too short, and/or steric interactions lead rapidly to a
complete bond rupture.5
La ser F la sh P h otolysis Stu d ies of Oxeta n e 2. In
a typical experiment, LFP (355 nm) of a mixture of 2 and
3b in acetonitrile gave a transient with an absorption
maximum at 470 nm (Figure 3), which can be ascribed
to the radical cation of trans-stilbene on the basis of
literature data.16,17
This confirmed that photocycloreversion of 2 follows
an electron-transfer mechanism and indicated that split-
ting of the oxetane gives rise to the radical cation of
trans-stilbene instead of the radical cation of 1-phenyl-
propene.
Th er m od yn a m ic An a lysis of th e Electr on -Tr a n s-
fer Rea ction . In principle the PET process might involve
either the excited singlet or the excited triplet of the
pyrylium salt. To check the feasibility of the two path-
ways, the free energy change associated with electron
Slightly different shapes and positions of the bands
were observed at higher concentrations of cyclobutane
1b (ca. 10-2 M), especially considering the evolution with
time. While in the case of 1a decay of the 470 nm band
was uniform (Figure 2, bottom), the absorption band
generated from 1b changed its shape as it decayed
(Figure 2, top). This is indicative of the presence of at
least two species: 2 µs after the pulse the maximum is
located at 470 nm, which could be associated with 4•+
,
but 3 µs later the maximum is clearly shifted to 460 nm.
As a matter of fact, analysis of the decay at 470 nm shows
two distinct components. The species with a longer
lifetime could be attributed to a complex between 4 and
4
•+. Actually, when concentrated (10-2 M) acetonitrile
solutions of 4 were submitted to LFP, the peaks at 470
and 460 nm were clearly observed. By contrast, only the
peak at 470 nm was detected when more diluted solutions
of 4 were employed.
Similar dimeric complexes have been previously
postulated19a in the LFP of stilbenes in the presence of
pyrylium salts. Likewise, pulse radiolysis experiments
with stilbene19b-d and 1,2,3,4-tetraphenylcyclobutanes13
have led to detection of related species assigned to
dimeric radical cations.
(18) Pragst, F. Electrochim. Acta 1976, 21, 497-500.
(19) (a) Spada, L. T.; Lewis, F. D.; Bedell, A. M.; Dykstra, R. E.;
Elbert, J . E., Gould, I. R.; Farid, S. J . Am. Chem. Soc. 1990, 112, 8055-
8064. (b) Akaba, R.; Sakuragi, H.; Tokumaru, K. Chem. Phys. Lett.
1990, 174, 80-84. (c) Majima, T.; Tojo, S.; Ishida, A.; Takamuku, S.
J . Phys. Chem. 1996, 100, 13615-13625. (d) Majima, T.; Tojo, S.;
Ishida, A.; Takamuku, S. J . Org. Chem. 1996, 61, 7793-7800. (e)
Kuriyama, Y.; Sakuragi, H.; Tokumaru, K.; Yoshida, Y.; Tagawa, S.
Bull. Chem. Soc. J pn. 1993, 66, 1852-1855.
The lack of such a signal at 460 nm in the irradiations
of 1a does not allow formation of 4/4•+ complexes to be

Electron-Transfer Cycloreversion of Ring Compounds
J . Org. Chem., Vol. 67, No. 12, 2002 4141
Ta ble 3. Dea ctiva tion P a th w a ys for th e Excited Sin glet
Sta tes of 3a -d
d
10-10 kq
b
c
Sa
10-8kfb
τf (ns)
10-8kISC
φISC
1a
1b
2
3a
3b
3c
3d
1.1
0.1
2.5
1.8
4.45
4.41
1.31
5.52
1.2
2.1
5.1
0.53
0.94
0.67
0.03
1.5
1.2
1.7
1.4
0.8
1.9
0.7
0.7
1.1
<0.1
-
-
-
a
b
S ) sensitizer. Rate constants for fluorescence (kf) or inter-
system crossing (kISC) given in s-1 (taken from the literature)15a,21
c
in the absence of quencher. φISC ) intersystem crossing quantum
d
yield, in acetonitrile as solvent. Rate constant for fluorescence
quenching (kq) by 1a ,b and 2 given in s-1 M-1
.
Sch em e 2
F igu r e 3. Transient spectra obtained from LFP (λ ) 355 nm)
of 2 (1.2 × 10-4 M) and 3b (10-4 M) in acetonitrile, under
argon.
Ta ble 2. Th er m od yn a m ics of th e P h otosen sitized
ET-r ea ction
c
c
∆GS
∆GT
b,c
b,c
Sa ES
ET
1a
1b
2
1a
1b
2
3a
3b
3c
3d
66
66
63
58
53d
52d
nae
51d
-13
-15
-13
-13
-15
-13
-24 -0.7 -0.2 -11
-26 -1.5 -1.0 -12
-25
11 +6.7 +7.1
nae
nae
nae
+0.1
+0.1
-4
a
b
S ) sensitizer. ES ) singlet energy, ET ) triplet energy.
c Given in kcal/mol. Taken from the literature.21 e na ) not
available.
d
transfer was calculated using the Weller20 equation (eq
1) for both excited states.
posed mechanism for the electron-transfer process is
shown in Scheme 2.
∆G (kcal/mol) ) 23.06[ED•+/D - EA/A•-] - E*A (1)
This is also supported by the fact that the reaction is
much less efficient in the presence of oxygen, a good
triplet quencher. Also, the φCR lower than unity suggests
that the reaction does not proceed through a radical chain
mechanism as has been reported for analogous com-
pounds.6
The last equation of Scheme 2 is back electron transfer
from the reduced species of sensitizer (3•-) to 4•+, leading
to the neutral trans-stilbene.
To make sure that the triplets of 3a -c are formed in
sufficient amount, even at high oxetane and cyclobutane
concentrations, it appeared necessary to calculate the
relative contributions of the different singlet deactivation
pathways under the employed reaction conditions (Table
4). Calculations were performed taking into account the
competition among fluorescence emission, fluorescence
quenching, and intersystem crossing, governed by the
respective rate constants and by the quencher concentra-
tion.
It is interesting that (thia)pyrylium salts 3b and 3c
produce the highest amount of triplets at the two cy-
clobutane concentrations, followed by the parent com-
pound 3a . This correlates well with the relative CR
efficiencies given in Table 1.
+/D
The ED•
was measured by cyclic voltammetry in
acetonitrile and was found to be 1.98 V vs SCE for 1a ,
2.00 V vs SCE for 1b, and 1.42 V vs SCE for 2. On the
-
other hand, the EA• /A singlet and triplet energies of the
sensitizers had been previously measured.15a,d,21 The ∆G
values obtained by fitting the above data in eq 1 are given
in Table 2. According to them, the reaction would be
possible in all cases, although CR of cyclobutanes would
be less favorable starting from triplet 3d . Thus, the
cycloreversion reaction might in principle occur from both
excited states.
F lu or escen ce Qu en ch in g of th e P yr yliu m Sa lts.
To gain further insight into the reaction mechanism,
quenching of the pyrylium salt fluorescence by 1a ,b and
2 was investigated. The rate constants (kq) were calcu-
lated using a Stern-Volmer analysis according to the
equation Io/I ) 1 + Ksv[Q], were Ksv ) kqτ.
The results are shown in Table 3. From these data it
is clear that fluorescence quenching occurs at a nearly
diffusion controlled rate, except in the case of the meth-
oxy derivative 3d (Table 3).
Thus, according to the above photophysical data, the
cycloreversion reaction might take place both for the
singlet and for the triplet excited states; however, the
fact that the pyrylium salt 3b, with the highest inter-
system crossing quantum yield, is the most efficient
sensitizer points clearly to triplet involvement. A pro-
Exp er im en ta l Section
Ch em ica ls. Sensitizer 3a was commercially available. Its
analogue 3b was synthesized according to the procedure
(21) Searle, R.; Williams, J . L. R.; DeMeyer, D. E.; Doty, J . C. J .
Chem. Soc., Chem. Commun. 1967, 22, 1165.
(20) Weller, A. Z. Phys. Chem. (Wiesbaden) 1982, 133, 93.

4142 J . Org. Chem., Vol. 67, No. 12, 2002
Miranda et al.
Ta ble 4. Rela tive Con tr ibu tion of th e Differ en t
P a th w a ys Resu ltin g in Dea ctiva tion of th e Excited
Sin glet of 3a -d a t Tw o Con cen tr a tion s of Su bstr a tea
sensitizer was the only absorbing species. Potassium ferriox-
alate was employed as actinometer.25 The CR quantum yields
were calculated following the increase in the absorbance at
300 nm associated with the formation of trans-stilbene.
F (%)c
ISC (%)d
F lu or escen ce Sp ectr oscop y. The steady-state fluores-
cence spectra were obtained with a FS 900 spectrofluorimeter
equipped with a 450 W xenon lamp. The samples were placed
in quartz cells of 1 cm path length. The pyrylium salt con-
centration was fixed, adjusting the absorbance of the solutions
at arbitrary concentration between 0.1 and 0.3.
Tim e-Resolved Absor p tion Sp ectr oscop y. The laser
flash photolysis system was based on a pulsed Nd:YAG laser,
using 355 nm as the excitation wavelength. The single pulses
were of ca. 10 ns duration, and the energy was ca. 20 mJ /pulse.
A xenon lamp was employed as the detecting light source. The
laser flash photolysis apparatus consisted of the pulsed laser,
the xenon lamp, a monochromator, a photomultiplier (PMT)
system made up of a side-on PMT tube, PMT housing, and a
PMT power supply. The output signal from the oscilloscope
was transferred to a personal computer for study.
[1] or [2] (M)
Sb
1a
1b
2
1a
1b
2
10-2
3a
3b
3c
3d
3a
3b
3c
3d
29
3
27
97
48
5
30
4
26
97
46
5
36
5
29
97
46
6
31
63
55
3
50
94
67
3
33
70
54
3
52
95
67
3
41
72
59
3
53
93
66
3
10-4
33
97
33
97
33
97
a
The rest up to 100 corresponds to quenching of the fluorescence
of 3a -d by 1a ,b or 2. S ) sensitizer. c F ) fluorescence. ISC )
intersystem crossing.
b
d
described by Wizinger and Ulrich,22 while 3c,d were obtained
following the method described by Steckhan and co-workers.15d
Cyclobutanes 1a ,b were synthesized by photodimerization of
stilbene.23 Oxetane 2 was prepared via the Paterno-Bu¨chi
photocycloaddition of benzaldehyde and trans-â-methylsty-
rene.24
Con clu sion s
In summary, electron-transfer CR of cyclobutanes 1a ,b
and oxetane 2 can be achieved by photosensitization with
triaryl (thia)pyrylium salts. The reaction takes place from
the excited triplet state of the photosensitizer, as indi-
cated by quenching by molecular oxygen and by the
negative (or slightly positive) values of ∆G from this
state. It ensues with splitting into trans-stilbene radical
cation and neutral trans-stilbene in the case of cyclobu-
tanes 1a ,b or acetaldehyde plus trans-stilbene radical
cation for oxetane 2. trans-Stilbene radical cation has
been detected as an intermediate of the reaction by
means of laser flash photolysis. Further studies are in
progress to check the scope, limitations, and potential
synthetic interest of this reaction.
All compounds were characterized by 1H and 13C NMR
spectra, which were recorded on a 300 MHz spectrometer. Data
were consistent with those found in the literature.
Cyclor ever sion Rea ction s. Solutions of cyclobutane 1a ,b
and oxetane 2 (5 × 10-2 M) with sensitizer 3a -d (10-3 M) in
CDCl3 (1 mL) were placed in NMR tubes and bubbled with
argon. Then, cyclobutane solutions were irradiated for 1 h
while oxetane solutions were irradiated for 20 min in a
multilamp photoreactor, using 8 W lamps (4×) with emission
maximum at 300 nm and using a filter to select λ > 340 nm.
The reaction was followed by 1H NMR, which was recorded
before and after the irradiation. Control experiments showed
that photocycloreversion does not take place in the dark or in
the absence of photosensitizer. In the case of 1a ,b the reaction
was also followed by GC and GC-MS.
Cyclor ever sion Qu a n tu m Yield s. Acetonitrile solutions
of cyclobutane 1a ,b and oxetane 2 (10-4 M) with sensitizer
3a -d (2.5 × 10-5 M) were placed in sealed cuvettes and
bubbled with argon/oxygen for 15 min. Samples were irradi-
ated by means of a xenon 450 W lamp, isolating the λ ) 420
nm line by means of a monochromator, to ensure that the
Ack n ow led gm en t. Financial support by the Span-
ish Ministerio de Ciencia y Tecnologia, Grant PB 97
0339, is gratefully acknowledged.
J O011103I
(22) Wizinger, R.; Ulrich, P. Helv. Chim. Acta 1956, 39, 1-4.
(23) Shechter, H.; Link, W. J .; Tiers, G. V. D. J . Am. Chem. Soc.
1963, 85, 1601-1605.
(24) Fleming, S. A.; Gao, J . J . Tetrahedron Lett. 1997, 38, 5407-
541.
(25) (a) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. London, Ser. A
1956, 235, 518-536. (b) Braun, A. M.; Maurette, M. T.; Oliveros, E.
Technologie Photochimique; Presses Polytechniques Romandes: Lau-
sanne, Switzerland, 1986; p 62.