Reductive PET Cycloreversion of Oxetanes: Singlet Multiplicity, Regioselectivity, and Detection of Olefin Radical Anion

Article abstract of DOI:10.1021/jo034890n

Cycloreversion of 2-(p-cyanophenyl)-4-methyl-3-phenyloxetane (1) is achieved using 1-methoxynaphthalene (2) as electron-transfer photosensitizer. The experimental results are consistent with the reaction taking place from the singlet excited state of the

Full text of DOI:10.1021/jo034890n

Red u ctive P ET Cyclor ever sion of Oxeta n es: Sin glet Mu ltip licity,
Regioselectivity, a n d Detection of Olefin Ra d ica l An ion
Ra u´ l P e´ rez-Ruiz, M. Angeles Izquierdo, and Miguel A. Miranda*
Departamento de Qu ´ı mica/ Instituto de Tecnolog ı´ a Qu ı´ mica UPV-CSIC, Universidad Polit e´ cnica de
Valencia, Camino de Vera s/ n, Apdo. 22012, 46022 Valencia, Spain
mmiranda@qim.upv.es
Received J une 24, 2003
Cycloreversion of 2-(p-cyanophenyl)-4-methyl-3-phenyloxetane (1) is achieved using 1-methox-
ynaphthalene (2) as electron-transfer photosensitizer. The experimental results are consistent with
the reaction taking place from the singlet excited state of the sensitizer. Ring splitting of the radical
•
-
anion 1 occurs with cleavage of O-C
2
and C
3
4
-C bonds, leading to products (acetaldehyde and
p-cyanostilbene) different from the reagents used in the Paterno-B u¨ chi synthesis of 1. The olefin
radical anion involved in the electron-transfer process has been detected by means of laser flash
photolysis.
In tr od u ction
etanes. In addition, radical cation intermediates have
been detected by laser flash photolysis (LFP).
During the past few decades, photoinduced electron
transfer (PET) processes have attracted considerable
interest. They have been intensively studied as funda-
Only a few reports have appeared on the CR of oxetane
2
radical anions despite their higher biological interest.
1
12-14
Kim et al.
have pointed out that the repair of the
mental steps in mechanistic and synthetic organic pho-
tochemistry and appear to be involved in key biological
processes.
(
6-4) photoproducts of DNA follows a mechanism analo-
gous to the reductive electron-transfer established for
cyclobutane pyrimidine DNA dimers. Falvey et al.
2
,15-17
Cycloreversion (CR) of oxetanes by PET is important
have proposed that reversal of the (6-4) photoproduct
to the oxetane intermediate is followed by the photo-
chemical step involving transfer of one electron from a
FADH cofactor to the resulting oxetane. The oxetane
radical anion then cleaves to provide two pyrimidine
units (one of them as radical anion).
for the photoenzymatic repair of the (6-4) photoproducts
of the DNA dipyrimidine sites by photolyase.2
-5
The
-
oxidative version of this reaction has been achieved using
cyanoaromatics, chloranil or (thia)pyrylium salts as
electron-transfer photosensitizers.6
-11
In the case of the
To evaluate the proposed mechanism, cycloadducts of
(
thia)pyrylium salts, CR takes place from the triplet
1
,3-dimethylthymine with benzaldehyde and benzophe-
excited state of the photosensitizer and may lead to
products different from the reagents employed in the
Paterno-B u¨ chi photocycloaddition to synthesize the ox-
none have been used as model systems and reacted with
a variety of electron-donor photosensitizers. This has led
to LFP detection of the radical anion of the carbonyl
fragment, a clear proof in support of a PET mechanism.
On the basis of the fact that fluorescence of the photo-
sensitizer is quenched by the oxetanes, it has been
proposed that the reaction takes place from the singlet
excited state of the former.
In connection with the mechanistic aspects of the
reductive PET CR of oxetanes, the following questions
remain still open and deserve further investigations: (i)
the multiplicity of the involved excited state, as the
*
To whom correspondence should be addressed. Phone: 963877340.
Fax: 963879349.
1) (a) Schmittel, M.; Burghart, A. Angew. Chem., Int. Ed. Engl.
(
1
9
4
997, 36, 2550-2589. (b) M u¨ ller, F.; Mattay, J . Chem. Rev. 1993, 93,
9-117. (c) Kavarnos, G. J .; Turro, N. J . Chem. Rev. 1986, 86, 401-
49. (d) Mattay, J . Angew. Chem., Int. Ed. Engl. 1987, 26, 825-845.
(
2) Prakash, G.; Falvey, D. E. J . Am. Chem. Soc. 1995, 117, 11375-
1
1376.
(3) Wang, Y.; Gaspar, P. P.; Taylor, J . S. J . Am. Chem. Soc. 2000,
1
22, 5510-5519.
(
4) Sancar, A. Chem. Rev. 2003, 42, 6747-6753.
(5) Cichon, M. K.; Arnold, S.; Carell, T. Angew. Chem., Int. Ed. 2002,
4
1, 767-770.
6) Nakabayashi, K.; Kojima, J . I.; Tanabe, K.; Yasuda, M.; Shima,
K. Bull. Chem. Soc. J pn. 1989, 62, 96-101.
7) Miranda, M. A.; Izquierdo, M. A.; Galindo, F. Org. Lett. 2001, 3,
965-1967.
8) Miranda, M. A.; Izquierdo, M. A.; Galindo, F. J . Org. Chem. 2002,
7, 4138-4142.
9) Miranda, M. A.; Izquierdo, M. A. J . Am. Chem. Soc. 2002, 124,
532-6533.
10) Miranda, M. A.; Izquierdo, M. A. Chem. Commun. 2003, 3, 364-
65.
11) Miranda, M. A.; Izquierdo, M. A.; P e´ rez-Ruiz R. J . Phys. Chem.
A 2003, 107, 2478-2482.
(
(12) Kim, S.-T.; Malhotra, K.; Smith, C. A.; Taylor, J . S.; Sancar, A.
J . Biol. Chem. 1994, 269, 8535-8540.
(
(13) Kim, S.-T.; Malhotra, K.; C. A.; Taylor, J . S.; Sancar, A.
Photochem. Photobiol. 1996, 63, 292-295.
1
6
6
3
(
(14) Hitomi, K.; Nakamura, H.; Kim, S.-T.; Mizukoshi, T.; Ishikawa,
T.; Iwai, S.; Todo, T. J . Biol. Chem. 2001, 276, 10103-10109.
(15) J oseph, A.; Prakash, G.; Falvey, D. E. J . Am. Chem. Soc. 2000,
122, 11219-11225.
(16) J oseph, A.; Falvey, D. E. J . Am. Chem. Soc. 2001, 123, 3145-
3146.
(
(
(
(17) J oseph, A.; Falvey, D. E. Photochem. Photobiol. Sci. 2002, 1,
632-635.
1
0.1021/jo034890n CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/03/2003
J . Org. Chem. 2003, 68, 10103-10108
10103

P e´ rez-Ruiz et al.
SCHEME 1
occurrence of fluorescence quenching may or may not be
associated with the actual ring splitting reaction, (ii) the
location of spin and charge in the intermediates, as in
principle the radical anion of the olefin fragment could
also be formed, and (iii) the regioselectivity of CR, with
special attention to the possibility of cleavage leading to
products different from the reagents used for the Pa-
terno-B u¨ chi synthesis of oxetanes.
Therefore, in the present work, the behavior of 2-(p-
cyanophenyl)-4-methyl-3-phenyloxetane (1) under electron-
transfer conditions has been examined. By contrast with
previous results on 2-phenyl and 2,2-diphenyl systems,1
the 2,3-diphenyl substitution could result in the forma-
tion of stilbenes, as it has been observed for the oxidative
CR of oxetanes.7-9 On the other hand, the electron-
withdrawing substituent at the 2-phenyl group could
stabilize the olefin radical anion relative to the carbonyl-
derived species. 1-Methoxynaphthalene (2) has been
chosen as photosensitizer in view of its good electron-
5-17
F IGURE 1. Quenching of the fluorescence of 2 in the presence
of increasing amounts of 1. Inset: Stern-Volmer plot to obtain
k (S ).
q
1
donor properties18 (E* •+/D ) - 2.49 V).
D
The PET process between 1 and 2 has been studied in
detail in order to gain further insight into the mechanistic
aspects of the reductive CR of oxetanes. Although both
singlet and triplet quenching have been observed, the
calculated free energy changes, together with product
studies in the presence of triplet quenchers, clearly
support involvement of the singlet excited state in the
electron-transfer reaction. These studies have provided
clear evidence for the involvement of olefin radical anion
intermediates.
uct, together with small amounts of trans-p-cyanostilbene
3a ). The 3b/3a ratio after 30 min of irradiation was
(
found to be ca. 4/1. Acetaldehyde (4) was also detected.
To check whether 3b could be formed from 3a by a rapid
trans-cis isomeration, solutions of both isomers were
separately irradiated in the presence of 2 under the same
conditions. In the two cases, photoequilibrium mixtures
strongly shifted toward 3b were obtained. Thus, the
reductive PET reaction of oxetane 1 follows pathways b
+
c of Scheme 1. These results contrast with previous
2
,15-17
Finally, instead of leading to the retro-Paterno-B u¨ chi
process (Scheme 1, pathway a), the reaction proceeds
with the opposite regioselectivity with cleavage of the
reports,
where CR of oxetane radical anions with
different substitution exhibits the opposite regioselectiv-
ity, leading to the Paterno-B u¨ chi precursors (Scheme 1,
pathway a).
O-C
2
and C
3
4
-C bonds (Scheme 1, pathway b).
F lu or escen ce Qu en ch in g. To disclose the nature of
the excited state involved in the PET process, the
fluorescence of 1-methoxynaphthalene (2) was studied in
the presence of increasing amounts of 1.
The fluorescence intensity gradually decreased (see
Figure 1), indicating significant quenching by the oxe-
tane. The quenching rate constant k (S ) was obtained
q 1
from the Stern-Volmer analysis (eqs 1 and 2, inset of
Figure 1):
Resu lts a n d Discu ssion
Stea d y-Sta te Ir r a d ia tion s. Oxetane 1 was prepared
by Paterno-B u¨ chi photocycloaddition of â-methylstyrene
to p-cyanobenzaldehyde.11 It was irradiated (λ ) 300 nm)
in acetonitrile, under argon, in the presence of catalytic
amounts of 1-methoxynaphthalene (2). The reaction was
1
followed by H NMR and GC-MS and resulted in the
production of cis-p-cyanostilbene (3b) as main photoprod-
Φ /Φ ) 1 + K [oxetane]
(1)
(2)
0
SV
(
18) (a) Maroulis, A. J .; Arnold, D. R. Synthesis. 1979, 10, 819-820.
b) Maroulis, A. J .; Arnold, D. R. J . Am. Chem. Soc. 1977, 99, 7355-
356.
(
7
KSV ) τ k (S )
s
q
1
1
0104 J . Org. Chem., Vol. 68, No. 26, 2003

Reductive PET Cycloreversion of Oxetanes
The value of the Stern-Volmer constant (KSV) was
-1
found to be 101 M . Taking into account the fluorescence
lifetime (τ
s
) reported for 2 in acetonitrile (13.6 ns), 1 the
8a
rate constant for singlet quenching can be estimated as
9
-1 -1
k
q
(S
1
) ) 7.4 × 10 M
s . From these data, oxetane 1
quenches the fluorescence of 2 at a nearly diffusion
controlled rate.
Based on the k (S ) value, the relative contributions
q 1
of the competing pathways resulting in the deactivation
of the singlet excited state of 2 in the presence of different
concentrations of 1 were determined: fluorescence emis-
sion, fluorescence quenching, and intersystem crossing.
Competition between them should be governed by the
respective rate constants and by the quencher concentra-
tion.
f
The rate constant of fluorescence (k ) and intersystem
crossing (kisc) are intrinsic properties of the photosensi-
tizer. The values for 2 were calculated from eqs 3 and 4,
F IGURE 2. Transient absorption spectra obtained upon LFP
-
4
(
λ ) 308 nm) of 2 (10 M), in acetonitrile, under argon: in
where the quantum yields of fluorescence (Φ
f
) and
-3
the absence of quencher (9) and in the presence of 1.43 × 10
intersystem crossing (Φisc) have been taken as 0.36 and
M of 1 (-). Spectra recorded 1 µs after the laser pulse.
1
9
0
.64, respectively. The resulting values for the rate
7
-1
7
-1
constants were k
respectively.
f
) 2.6 × 10 s and kisc ) 4.7 × 10 s
,
τ k ) Φ
f
(3)
(4)
s
f
τskisc ) Φisc
Hence, under the preparative irradiation conditions ([1]
-
2
)
10 M), quenching of the excited singlet of 2 by 1 is
the major deactivation pathway (50%). However, under
these conditions there is still some residual fluorescence
(
(
18%), and a significant amount of excited molecules
32%) intersystem cross to the excited triplet state.
Therefore, the observation of fluorescence quenching
alone does not allow to rule out the possibility that the
PET CR of oxetane 1 could take place from the excited
triplet state of the photosensitizer.
F IGURE 3. UV-vis absorption spectra of 1 (‚‚‚‚), 3a (-), and
Tim e-Resolved Stu d ies. The possible involvement of
the sensitizer triplet state in the electron-transfer reac-
tion depends on both the triplet yield and the rate
constant of triplet quenching. As the triplet of 2 is
significantly populated even in the presence of 1, it could
in principle be reactive excited state. The feasibility of
this hypothesis was checked by means of LFP. Excitation
of 2 at 308 nm resulted in an intense peak at 435 nm,
which was assigned to the characteristic triplet-triplet
absorption of 2.19 This experiment was also performed
upon addition of increasing amounts of 1 to identify the
possible transient reaction intermediates and to deter-
mine the triplet quenching rate constant. In the presence
of 1, a diminished absorption at 435 nm was observed
and a new intense signal appeared around 500 nm which
was tentatively assigned to the trans-p-cyanostilbene
-
5
3
b (- - -) in acetonitrile at 7.14 × 10 M.
Hence, 2,6-dimethoxynaphthalene (DMN) was chosen
as an alternative photosensitizer, as it absorbs at sig-
nificantly longer wavelengths and can be selectively
excited at 355 nm. As a matter of fact, the transient
species obtained upon 355 nm LFP of DMN in the
presence of 3a (Figure 4A) was similar to the intermedi-
ate formed upon the PET CR of oxetane 1. Both species
presented similar absorption bands with maxima around
5
4
00 nm and decayed with comparable kinetics (Figure
B). By contrast, when cis-p-cyanostilbene (3b) was
submitted to LFP in the presence of DMN, under identi-
cal conditions, a broader band with a higher absorption
beyond 540 nm was obtained, attributable to the corre-
•
-
sponding radical anion 3b (Figure 4A); its decay was
•
-
radical anion (3a ) (Figure 2).
•
-
slower than that of 3a (Figure 4B). From the spectral
To confirm the assignment, this species was indepen-
dently generated from its neutral precursor. For this
purpose, 1-methoxynaphthalene (2) was not a suitable
photosensitizer, due to the intense absorption band
exhibited by trans-p-cyanostilbene at the excitation
wavelength of 308 nm (Figure 3).
•
-
•-
evolution of 3a and 3b with time, it is clear that no
appreciable interconversion occurs; thus, trans-cis isomer-
ation does not appear to take place at the radical anion
stage.
These results allow to conclude that the primary
process in the CR of 1 photosensitized by 2 is formation
of 3a via its radical anion. The extensive isomerization
to the cis isomer 3b is not a PET reaction; it simply
(
19) Yamaji, M.; Sekiguchi, T.; Hocino, M.; Shizuka, H. J . Phys.
Chem. 1992, 96, 9353-9359.
J . Org. Chem, Vol. 68, No. 26, 2003 10105

P e´ rez-Ruiz et al.
F IGURE 5. Decay traces of the T-T absorption of 2 (10- M)
4
measured at 435 nm in the presence of increasing amounts of
-
4
-3
-3
1
(
: 0 M (b), 5.7 × 10 M (O), 1.7 × 10 M (9), 2.3 × 10
M
-
3
-3
0), 3.4 × 10 M (2), 4.5 × 10 M (4). Inset: Plot of 1/τ
against concentration of 1 to obtain k (T ).
q
1
Sin glet vs Tr ip let Electr on Tr a n sfer . The free
energy changes associated with electron transfer from
both the singlet and the triplet excited state were
estimated using the Weller20 equation (eq 6):
•
+
•-
∆GET (kcal/mol) ) 23.06[ED
- EA /A ] -
/D
E*(S or T ) (6)
1
1
The reduction potential, EA/A•-, of 1 was measured by
F IGURE 4. (a) Transient absorption spectra obtained upon
cyclic voltammetry in acetonitrile, under argon. Its
-4
LFP (λ ) 355 nm) of DMN (8.5 × 10 M), in acetonitrile, under
experimental value after the first scan was -1.52 V vs
-
3
argon, in the presence of 1.43 × 10 M 3a (9) and 3b (O).
Spectra recorded at 0.4 µs after the laser pulse. (b) Decay
traces of the transient absorption at 510 nm in the presence
•+
SCE. The oxidation potential (E
D
/D) of 2 as well as its
or T ) have been
Their corresponding values are
.38 V vs SCE, 89.3 and 59.8 kcal/mol. Therefore, the
) and ∆GET (T ) values were estimated by fitting
singlet and triplet energies E* (S
1
1
previously measured.2
1,22
-
3
of 1.43 × 10 M 1 (0), 3a (O), and 3b (2).
1
∆
G
ET(S
1
1
reflects the fact that, at the wavelength employed for the
preparative irradiations (300 nm), the molar absorption
coefficient of 3a is much higher than that of 3b (see
Figure 3). As a consequence, photoequilibrium mixtures
overwhelmingly enriched in cis-p-cyanostilbene (3b) are
obtained.
the above data into eq 6; they were found to be -22.4
kcal/mol and +7 kcal/mol, respectively. Hence, PET from
the singlet excited state would be an exergonic process,
while the mechanism from the triplet excited state does
not appear to be thermodynamically feasible.
Stea d y-Sta te a n d Tim e-Resolved Stu d ies in th e
P r esen ce of Com p etin g Qu en ch er s. To establish in
an unambiguous way the nature of the excited state of 2
involved in the CR of 1, additional experiments were
performed in the presence of competing quenchers such
As stated above, significant amounts of triplet 2 are
formed in the presence of concentrated oxetane, despite
the efficient fluorescence quenching. Then, the question
arises whether the triplet could be involved in the
electron-transfer process, what would require as a first
condition the existence of observable quenching of triplet
2
as oxygen and 1,3-cyclohexadiene (CHD). Both O and
CHD were found to quench the fluorescence of 2, though
with different rate constants. Figure 6 shows the two
Stern-Volmer plots; from the linear relationships KSV
2
by 1. Indeed, such triplet quenching did actually occur.
The corresponding rate constant k (T ) was determined
from the decay traces obtained for the T-T absorption
of 2 in the presence of increasing amounts of 1 (Figure
q
1
-1
-1
2
were found to be 446 M (O ) and 11.9 M (CHD). Then,
1
0
-1 -1
the corresponding k
q
(S
1
) values were 3.3 × 10
M
s
5
). By plotting the reciprocal lifetimes (1/τ) against the
and 8.2 × 10 M s-1, respectively.
8
-1
concentration of oxetane, a linear relationship was
obtained (see insert of Figure 5). The slopes of the
straight lines correspond to k
On the other hand, the triplet excited state of 2 was
2
also quenched by O and CHD with nearly diffusion-
q 1
(T ) according to eq 5.
1
/τ ) k + k (T )[oxetane]
(5)
(20) Weller, A. Z. Phys. Chem. (Wiesbaden) 1982, 133, 93-98.
21) Zweig, A.; Maurer, A. H.; Roberts, B. G. J . Org. Chem. 1967,
2, 1322-1329.
22) Murov, S. L., Carmichael, I., Hug G. L. Handbook of Photo-
chemistry, 2nd ed.; Marcel Dekker: New York, 1993; pp 33 and 115.
0
q
1
(
3
Operating in this way, the value obtained for the triplet
quenching rate constant was k
(
7
-1 -1
q
(T
1
) ) 4.9 × 10 M
s .
1
0106 J . Org. Chem., Vol. 68, No. 26, 2003

Reductive PET Cycloreversion of Oxetanes
TABLE 1. 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 Both th e Sin glet a n d Tr ip let
Excited Sta te of 2 by 1, O2 a n d CHD, Togeth er w ith th e Rela tive CR Efficien ciesa
singlet quenching
triplet quenching
Fb
Q(1)c
Q(O2)c
Q(CHD)c
ISCd
T(1)e
T(O2)e
T(CHD)e
CRf
1
1
1
18
17
6
50
48
17
32
30
10
28
100
90
33
+ CHD
+ O2
5
30
67
10
a
Experimental conditions: 1 and CHD, 10 M; O2, 9.1 × 10 M; 2, 10-3 M; solvent, acetonitrile. b Fluorescence. c Fluorescence
-2
-3
d
e
f
quenching. Intersystem crossing. Triplet quenching. Relative CR efficience. All values are given in percent.
F IGURE 7. (a) Decay traces of the T-T absorption of 2 (10-
4
M) measured at 435 nm, under argon, in the presence of
-
6
increasing amounts of CHD: 0 M (9), 2.85 × 10 M (0), 5.68
-
6
-5
×
10 M (b), 1.4 × 10 M (O). (b) Linear relationship to
obtain k (T ).
q
1
F IGURE 6. Stern-Volmer plot for the quenching of fluores-
cence of 2 in the presence of increasing amounts of (a) O and
b) CHD to obtain the k (S ) values.
2
(
q
1
f q 1 q 1
above values of k , kisc, k (S ), and k (T ), as well as the
quencher concentration. As shown in Table 1, addition
of 10 M CHD does not produce significant quenching
of the fluorescence of 2.
10
controlled rate (k
q
(T
1
) ca. 5 × 10 M-1 s-1 in the two
-2
cases). For example, the decay traces for the T-T
absorption in the presence of increasing amounts of CHD
are shown in Figure 7A, while the linear relationships
between the reciprocal triplet lifetimes and the CHD
concentrations are given in Figure 7B.
Therefore, the relative contributions of the different
pathways resulting in the deactivation of the singlet and
triplet excited state of 2 in the presence of 1, O
Under these conditions, reaction of the singlet excited
state of 2 with oxetane 1 still continues to be the major
singlet deactivation pathway. By contrast, quenching of
triplet 2 (which is formed in nearly the same amounts)
by CHD competes favorably with that by 1, providing the
major triplet deactivation pathway. Thus, inhibition of
the CR of 1 by addition of CHD would be an excellent
2
, and CHD
were determined. This was done taking into account the
J . Org. Chem, Vol. 68, No. 26, 2003 10107

P e´ rez-Ruiz et al.
anobenzaldehyde and trans-â-methylstyrene.11 Compounds 3a
and 3b were synthesized as previously described, by following
the modified Wittig-Horner reaction.23
probe for the involvement of the triplet excited state of
the photosensitizer in the process.
On the other hand, dissolved O
2
appears to be a
-
2
Cyclor ever sion Rea ction . A solution of oxetane 1 (10
suitable quencher of both the singlet and triplet excited
states of 2. This is clear from the data of Table 1,
following the same reasoning as above. Thus, combined
-
3
3
M) with photosensitizer 2 (10 M) in CH CN (2 mL) was
placed in a quartz tube and bubbled with argon. Then, the
solution was irradiated during 30 min in a multilamp photo-
reactor, using 8 W lamps (4×) with emission maximum at λ
2
preparative experiments using both CHD and O as
max
1
quenchers should allow to draw clear conclusions con-
cerning the multiplicity of the photosensitizer excited-
state involved in the PET CR of oxetane 1.
) 300 nm. The reaction was followed by GC-MS and H NMR
spectroscopy. Control experiments showed that CR does not
take place in the dark or in the absence of photosensitizer.
Cyclic Volta m m etr y. The redox potentials were measured
by cyclic voltammetry. All measurements were made in
acetonitrile containing tetrabutylammonium perchlorate (0.1
M) as supporting electrolyte, using a glassy carbon working
electrode and ferrocene in acetonitrile (5 mg in 30 mL) as
standard, Epa ) 423 mV and Epc ) 345 mV. The scan rate was
400 mV/s. Potentials are reported with respect to the saturated
calomel electrode (SCE) as reference.
-
2
-3
Accordingly, mixtures of 1 (10 M) and 2 (10 M) were
-
2
irradiated in the presence of either CHD (10 M) or O
2
-
3
(
9.1 × 10 M) at the same concentrations used for the
quenching experiments. The results are also given in
Table 1. The small effect produced by CHD, together with
2
the clear inhibition produced by O , allows to conclude
that the reaction takes place from the singlet excited
state of 2.
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
into quartz cells of 1 cm path length. 1-Methoxynaphthalene
concentration was fixed adjusting the absorbance of the
solution at an arbitrary value between 0.2 and 0.3.
Tim e-Resolved Absor tion Sp ectr oscop y. The laser flash
photolysis system was based on a pulsed XeCl Excimer Laser,
using 308 nm as excitation wavelength. The single pulses were
ca. 17 ns duration and the energy was ca. 100 mJ /pulse. A
Lo255 Oriel xenon lamp was employed as detecting light
source. The laser flash photolysis apparatus consisted of the
pulsed laser, the Xe lamp, a monochromator, a photomultiplier
Con clu sion s
Unambiguous experimental evidence has been ob-
tained supporting that the reductive PET cycloreversion
of oxetane 1 mediated by 1-methoxynaphthalene takes
place from the singlet excited state of the photosensitizer.
By contrast with the regioselectivity previously observed
for the analogous reaction of thymine-derived oxetanes,
splitting of 1 occurs with cleavage of the O-C
4
2 3
and C -
C
bonds, leading to products different from the reagents
(PMT) system, and an oscilloscope. The output signal from the
used in the Paterno-B u¨ chi synthesis. Finally, the radical
anion of p-cyanostilbene is observed, instead of the
radical anion of the carbonyl fragment.
oscilloscope was transferred to a personal computer for study.
Ack n ow led gm en t. Financial support by the MCYT
(
Grant No. BQU2001-2725) is gratefully acknowledged.
Exp er im en ta l Section
J O034890N
Ch em ica ls. 1-Methoxynaphthalene (2) and 2,6-dimethoxy-
naphthalene (DMN) were commercially available. Oxetane 1
was prepared via Paterno-B u¨ chi photocycloaddition of 4-cy-
(23) (a) Singh, A. K., Kanvah, S. J . Chem. Soc., Perkin Trans. 2 2001,
395-401. (b) Seus, E. J ., Wilson, C. V. J . Org. Chem. 1961, 26, 5243.
1
0108 J . Org. Chem., Vol. 68, No. 26, 2003