Steady-State and Time-Resolved Studies on Oxetane Cycloreversion Using (Thia)pyrylium Salts as Electron-Transfer Photosensitizers

Article abstract of DOI:10.1021/ol0158516

(matrix presented) λ max=470 nm Cycloreversion of oxetane 1 is achieved using (thia)pyrylium salts as 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.

Full text of DOI:10.1021/ol0158516

ORGANIC
LETTERS
2001
Vol. 3, No. 13
1965-1967
Steady-State and Time-Resolved Studies
on Oxetane Cycloreversion Using
(Thia)pyrylium Salts as Electron-Transfer
Photosensitizers
Miguel A. Miranda,* M. Angeles Izquierdo, and Francisco Galindo
Departamento de Qu´ımica/Instituto de Tecnolog´ıa Qu´ımica UPV-CSIC, UniVersidad
Polite´cnica de Valencia, Camino Vera s/n, Apdo. 22012, 46022 Valencia, Spain
mmiranda@qim.upV.es
Received March 15, 2001
ABSTRACT
Cycloreversion of oxetane 1 is achieved using (thia)pyrylium salts as photosensitizers. RadicaI 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.
Cycloreversion (CR) of oxetanes by photoinduced electron
transfer (PET) has recently attracted considerable interest,
as this process appears to be involved in the photoenzymatic
repair of the (6-4) photoproducts of the DNA dipyrimidine
sites by photolyase.1 Both radical anions and radical cations
of oxetanes undergo fragmentation, though reaction with the
former is more exothermic.2
anism was found to operate when electron-donating substit-
uents are attached to the aryl groups.3
Pyrylium salts are a potentially useful group of photosen-
sitizers.4 They are good oxidizing agents and can be
selectively excited because of their absorption in the visible.5
On the other hand, depending on the substitution pattern and
the reaction conditions, they can generate radical ion pairs
of different multiplicity (singlet or triplet).6,7 Thus it may
be of interest to study the influence of this factor on the
reaction, for instance, regarding the splitting mode and the
stereoselectivity of the products. In the present work, we have
chosen the diphenyl-substituted oxetane 18 and a series of
pyrylium and thiapyrylium salts 2a-d as model compounds
In a preliminary study using 2,2-diaryloxetanes and
cyanoaromatic compounds as electron-transfer photosensi-
tizers, the CR reaction was found to occur through the excited
singlet state of the photosensitizer.3 Although the process
was explained via formation of radical cations, no intermedi-
ate of this type was detected. In addition, cycloreversion
occurred with cleavage of the same C-C bonds formed in
the Paterno-Bu¨chi photocycloaddition employed to synthe-
size the starting 2,2-diaryloxetanes. A radical chain mech-
(4) Miranda, M. A.; Garc´ıa, H. Chem. ReV. 1994, 94, 1063-1089.
(5) Martiny, M.; Steckhan, E.; Esch, T. Chem. Ber. 1993, 126, 1671-
1682.
(6) Akaba, R.; Arai, S. J. Chem. Soc. Chem. Commun. 1987, 1262-
1263.
(7) (a) Akaba, R.; Sakuragi, H.; Tokumaru, K. Chem. Phys. Lett. 1990,
174, 80-84. (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.
(8) Fleming, S. A.; Gao, J. J. Tetrahedron Lett. 1997, 38, 5407-5410.
(1) Prakash, G.; Falvey D. E. J. Am. Chem. Soc. 1995, 117, 11375-
11376.
(2) Wang, Y.; Gaspar, P. P.; Taylor, J. S. J. Am. Chem. Soc. 2000, 122,
5510-5519.
(3) Nakabayashi, K.; Kojima, J. I.; Tanabe, K.; Yasuda, M.; Shima, K.
Bull. Chem. Soc. Jpn. 1989, 62, 96-101.
10.1021/ol0158516 CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/31/2001

to study the photosensitized electron-transfer cycloreversion
of oxetanes and to gain further insight into the mechanistic
aspects of this reaction.
nometer.9 The results are also given in Table 1. These results
clearly show that (a) the CR is photosensitized by pyrylium
salts, (b) it produces carbonyl and olefin units different from
the reagents employed in the Paterno-Bu¨chi process, and
(c) the most efficient photosensitizer is the thiapyrylium salt
2b. Moreover, the fact that the quantum yields were much
less than unity suggest that a chain reaction is not involved.
To elucidate the reaction mechanism actually operating,
a laser flash photolysis (LFP) study was performed. In a
typical experiment, LFP (355 nm) of a mixture of 1 and 2b
in CH3CN gave a transient with an absorption maximum at
470 nm (Figure 1), which can be ascribed to the radical cation
of trans-stilbene on the basis of literature data.7,10
When oxetane 1 was irradiated (λ > 340 nm) using
catalytic amounts of pyrylium salts 2a-d, CR was observed,
resulting in the production of trans-stilbene and acetaldehyde
as the only photoproducts (Scheme 1).
Scheme 1
Figure 1. Transient spectra obtained from LFP (λ ) 355 nm) of
1 (1.2 × 10-4 M) and 2b (10-4 M) in acetonitrile, under argon.
The reaction yields are given in Table 1. Control experi-
ments showed that no photocycloreversion takes place in the
dark or in the absence of the photosensitizer. Further control
This confirmed that photocycloreversion of oxetane 1
follows an electron-transfer mechanism and indicated that
splitting of the four-membered ring gives rise to the radical
cation of trans-stilbene instead of the radical cation of
1-phenylpropene. In principle the PET process between 1
and 2 might involve either the excited singlet or triplet of
the pyrylium salt. To check the feasibility of the two
pathways, the free energy change associated with electron
transfer was calculated using the Weller11 equation (eq 1)
for both excited states.
Table 1. Cycloreversion of Oxetane 1 Photosensitized by
Pyrylium Salts 2a-d
sensitizer
CR (%)a
CR (φ)b,c
2a
2b
2c
2d
8d
100
33d
0.02
0.10
0.04
∆G (kcal/mol) ) 23.06 × E
- E
- E* (1)
]
A
[
+•
-•
)
A/A
(
)
/D
(
D
a Oxetane 1, 4.7 × 10-2 M; sensitizer, 10-3 M; solvent, CDCl3; filter, λ
> 340 nm; Luzchem multilamp photoreactor, 8 W lamps (4×) with emission
maximum at 300 nm. b Oxetane 1, 10-4 M; sensitizer, 2.5 × 10-5 M;
solvent, CH3CN; lamp, Xenon 450 W, monochromatic light of λ ) 420
nm; actinometer, potassium ferrioxalate. The solutions were placed in sealed
cuvettes and bubbled with argon for 15 min to achieve deoxygenation.c No
significant CR reaction was observed in the presence of oxygen. d The rest
was unreacted starting material.
+•
The ED
for 1 was measured by cyclic voltammetry in
/D
acetonitrile and found to be 1.42 V vs SCE. On the other
hand, the singlet and triplet energies of the sensitizers had
been previously measured.4 Using these values, the PET
reaction was found to be exothermic in all cases (Table 2).
(9) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. London, Ser. A 1956,
235, 518-536. Braun, A. M.; Maurette, M. T.; Oliveros, E. Technologie
Photochimique; Presses Polytechniques Romandes: Lausanne, 1986; p 62.
(10) (a) Shida, T. Electronic Absortion Spectra of Radical Ions;
Elsevier: Amsterdam, 1988; p 113. (b) Hamil, W. H. Radical Ions; Kaiser,
E. T., Kevan, L., Ed.; Interscience: New York, 1968; p 399. (c) 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.
experiments (data not given in Table 1) clearly showed that
neither the photosensitizer concentration (up to 10-2 M) nor
the nature of the counteranion (BF4 or ClO4 ) produced
significant effects on the obtained results.
To obtain more reliable quantitative data, the CR quantum
yield was measured using potassium ferrioxalate as acti-
-
-
(11) Rehm, D.; Weller, A. Isr. J. Chem. 1970, 8, 259-271.
1966
Org. Lett., Vol. 3, No. 13, 2001

Scheme 2
Table 2. Thermodynamics of the PET Reaction between 1 and
2a-d
a
E(S)b,c
E(T)b,c
∆G(S)b
∆G(T)b
-•
sensitizer
E(A/A )
.
2a
2b
2c
2d
2.53
2.59
2.49
1.98
66
66
63
58
53
52
-24
-26
-23
-11
-11
-12
nad
-4
nad
51
a Given in V (vs SCE). b Given in kcal/mol. c Taken from the literature.4
d na: not available.
the different singlet deactivation pathways under the em-
ployed reaction conditions (Table 4).
To gain further insight into the reaction mechanism,
quenching of the pyrylium salts fluorescence by the oxetane
was investigated. Pyrylium salt concentration was fixed by
adjusting the absorbance of the solutions at the arbitrary value
of 0.19. Then, the fluorescence intensities (I) observed upon
addition of different concentrations of 1 ([Q]) were used to
calculate the Stern-Volmer constants (K). A linear fit was
observed in all cases according to the equation I0/I ) 1 +
K[Q]. These values together with the fluorescence lifetimes
(measured by time-resolved emission spectroscopy) were
used to calculate the quenching rate constants (kq). The results
are shown in Table 3. From these data it is clear that
fluorescence quenching occurs at a nearly difusion-controlled
rate, except in the case of the methoxy derivative 2d (Table
3).
Table 4. Relative Contribution of the Different Pathways
Resulting in Deactivation of the Excited Singlet of 2a-d at
Two Concentrations of 1
[1] (M)
sensitizer
Q (%)a
F (%)b
ISC (%)c
10-2
2a
2b
2c
2d
2a
2b
2c
2d
23
23
12
36
5
29
97
46
6
41
72
59
3
53
93
66
3
10-4
11
1
1
33
97
a Singlet quenching. b Fluorescence. c Intersystem crossing.
Table 3. Deactivation (Fluorescence and Intersystem Crossing)
of the Excited Singlet of 2a-d
It is interesting that thiapyrylium salt 2b produces the
highest amount of triplets at the two oxetane concentrations,
followed by the tribromo derivative 2c and the parent
compound 2a. This correlates well with the relative CR
efficiencies given in Table 1. In the case of the trimethoxy
analogue 2d, the almost exclusive deactivation pathway is
fluorescence; neither singlet quenching nor intersystem
crossing occur to a significant extent.
a,b
b
c
sensitizer
10-8 × kfa,b
10-8 × kisc
φisc
10-9 × kq
2a
2b
2c
2d
1.1
0.1
2.5
1.8
1.2
2.1
5.1
0.53
0.94
0.67
0.03
6.8
6.9
10.6
<0.1
a Given in s-1 and taken from the literature.4 b In the absence of
quencher. c Fluorescence quenching by oxetane 1, given in s-1 M-1
.
In summary, the electron-transfer cycloreversion of the
model oxetane1 can be achieved by photosensitization with
triaryl(thia)pyrylium salts. The reaction takes place from the
excited triplet state of the photosensitizers and ensues with
splitting of 1 into acetaldehyde plus the trans-stilbene radical
cation, which can be detected by laser flash photolysis.
Further studies are in progress to check the scope, limitations,
and potential synthetic interest of this reaction.
Thus, the cycloreversion reaction might in principle occur
from both excited states; however, the fact that the pyrylium
salt 2b, with the highest intersystem crossing quantum yield,
is the most efficient sensitizer points clearly to triplet
involvement (Scheme 2).
This is also supported by the fact that the reaction does
not occur in the presence of oxygen, a good triplet quencher.
To make sure that the triplets of 2a-c are formed in
sufficient amount, even at high oxetane concentrations, it
appeared necessary to calculate the relative contributions of
Acknowledgment. Financial support by the Spanish
DGES(GrantPB97-0339andFP98)isgratefullyacknowledged.
OL0158516
Org. Lett., Vol. 3, No. 13, 2001
1967