Chemical and transient spectroscopic evidence for C2-C3 cleavage of 2,3-diaryloxetane radical cations

Article abstract of DOI:10.1039/b209500a

Electron transfer cycloreversion of the methoxy substituted oxetane 1b results in the production of trans-anethole and benzaldehyde through C₂-C₃ bond cleavage. trans-Anethole radical cation has been detected as transient intermediat

Full text of DOI:10.1039/b209500a

Chemical and transient spectroscopic evidence for C –C cleavage of
2
3
2,3-diaryloxetane radical cations
Miguel A. Miranda* and M. Angeles Izquierdo
Dpto. de Química de la Universidad Politécnica de Valencia, Camino de Vera s/n, Apdo. 22012, 46022
Valencia, Spain. E-mail: mmiranda@qim.upv.es; Fax: 34 963877349; Tel: 34 963877807
Received (in Corvallis, OR, USA) 27th September 2002, Accepted 13th December 2002
First published as an Advance Article on the web 13th January 2003
Electron transfer cycloreversion of the methoxy substituted
A possible strategy to control the reaction pathway could be
the attachment of electron donor or electron acceptor sub-
stituents to the aryl group(s). This could favour location of the
charge and the spin at different positions of the oxetane radical
cation or the intermediates derived therefrom.
oxetane 1b results in the production of trans-anethole and
2 3
benzaldehyde through C –C bond cleavage. trans-Anethole
radical cation has been detected as transient intermediate by
laser flash photolysis.
Methoxy substituents are known to stabilize the positive
charge of aromatic radical cations.10 For example, in the
oxidative ring opening of 2,2-diaryloxetanes4 the electron
appears to be initially withdrawn from an electron-rich aryl
Oxetanes are easily available by Paterno-Büchi photoreaction
of carbonyl compounds with alkenes. Their cycloreversion
1
(
CR) entails cleavage of two bonds and may yield formal
metathesis products. It presents clear analogies with the
Chauvin mechanism operating in the metal catalyzed process
2 3
group. This decreases the strength of the C –C s bond through
s–p interaction.
(
Scheme 1), which involves the interconversion of an olefin and
The aim of the present work was to introduce a methoxy
2
a metal alkylidene via a metallacyclobutane intermediate.
3
substituent on the C -phenyl group of 1a, the only oxetane
known to undergo C–O cleavage upon oxidative ET. This could
favour C
ity.
2 3
–C cleavage and lead to the reverse regioselectiv-
Oxetane 1b was synthetized by a Paterno-Büchi photoreac-
1
1
tion, following the procedure described in the literature. In
addition to product studies, laser flash photolysis of 1b (Chart 1)
could allow for the detection of the radical cation of anethole or
Scheme 1
4
-methoxystilbene, providing a direct mechanistic evidence for
Ring opening of oxetanes can be achieved via chemical3
+
the cleavage of 1b ·. Such direct evidence is still missing for the
–C cleavage of oxetane radical cations.
To generate the radical cation 1b ·, the pyrylium salts 2a,b
(
thermolysis, hydrogenolysis or nucleophilic attack) or photo-
C
2
3
4,5
chemical activation. The latter has been little explored and
involves the use of electron transfer (ET) photosensitizers.
Theoretical calculations on the oxidative ET cycloreversion
of oxetanes point to an initial C–C bond breaking, according to
free energy changes estimated for the gas-phase reaction. By
contrast, the anionic ET cycloreversion appears to start with C–
O cleavage.6
+
have been the photosensitizers of choice, in view of their well
established photophysical, photochemical and redox proper-
ties.12 According to simple calculations based on the Weller
equation,13 the ET process between the ground state of 1b and
the triplet excited states of 2a and 2b should be thermodynam-
2
1
ically favourable (DG ca. 213 and 27 kcal mol , re-
Control of the regioselectivity in the CR of oxetanes by
photoinduced ET (Scheme 2), besides being synthetically
interesting (see above), is relevant to key biological processes.
Thus, it is involved in the enzymatic repair of the (6-4)
spectively).†
Preparative irradiation of oxetane 1b was carried out in the
presence of catalytic amounts of photosensitizers 2a,b (Chart
1
).‡ By contrast with the behaviour reported for 1a, trans-
7
photoproducts of the DNA dipyrimidine sites by photolyase.
anethole and benzaldehyde were detected as the only photo-
products by direct H NMR analysis of the reaction mixtures
8,9
Prior to our work, only two reports have appeared on the
CR of oxetane radical cations.4 Preparative studies on the ET
reaction of 2,2-diaryloxetanes photosensitized by cyanoaro-
matics suggest that CR proceeds through a stepwise mechanism
1
,5
(Scheme 3, pathway b).
4
with initial C–C bond breaking. The products of these reactions
are the same as the reagents used to make the oxetanes.
The opposite regioselectivity has been found in the irradia-
tion of 2,3-diphenyloxetane 1a using 2,4,6-triaryl(thia)pyr-
8
ylium salts 2a,b as ET photosensitizers (Chart 1). The radical
cation of 1a undergoes stepwise splitting via initial O–C
2
9
cleavage; this results in the production of trans-stilbene and
acetaldehyde (Scheme 3, pathway a). This mechanism is
supported by laser flash photolysis (LFP) detection of the
involved transient intermediates (trans-stilbene radical cation
and pyranyl radical).
Chart 1
Scheme 2
Scheme 3
3
64
CHEM. COMMUN., 2003, 364–365
This journal is © The Royal Society of Chemistry 2003

In order to elucidate the mechanism of the observed
cycloreversion, LFP experiments were conducted under differ-
ent conditions. In a typical experiment, LFP (355 nm) of 2a in
the presence of 1b, using acetonitrile as solvent, gave a transient
with absorption maxima at ca. 380 and 600 nm (Fig. 1). These
bands correspond to the same intermediate, and decay with the
same rate constant. They were assigned to the radical cation of
trans-anethole on the basis of literature data.14 To confirm the
assignment, LFP of trans-anethole was performed in the
presence of 2a, giving rise to the same intermediate.
the oxetane radical cation. Electron transfer from triplet 2a to 1b
is much faster, as evidenced by the quenching rate constant (4.0
9
21
3 10 s ). This means that the initial oxetane radical cation
fragments in the submicrosecond time domain, to generate
trans-anethole radical cation and neutral benzaldehyde.
The behaviour of 1b under ET conditions can be justified
according to Scheme 4. Ionization would probably occur at the
+
electron rich aromatic ring, to give 1b ·. Subsequent cleavage of
2 3
the C –C bond would generate a distonic 1,4 radical cation,
whose C–O splitting would lead to the observed intermediate.
In summary, methoxy substitution at the 3-aryl group induces
2 3
C –C cleavage of 2,3-diaryloxetane radical cations. The
Another band peaking at 550 nm was also clearly observed in
the LFP experiments performed using 2b as photosensitizer
(
data not shown). This band is ascribed to the pyranyl radical
reaction mechanism is supported by detection of anethole
radical cations, instead of 4-methoxystilbene radical cations.
Thus, the regioselectivity of ET mediated CR of oxetanes can be
controlled by modifying the nature of the substituents.
Financial support by the Spanish MCYT (grant BQU2001-
2725) is gratefully acknowledged.
15
resulting from the reduction of 2b. As the thiapyranyl radical
does not absorb in the range 500–600 nm, it does not appear in
Fig. 1. Moreover, no band related to 4-methoxystilbene radical
cation (lmax = 500 nm)16 was detected; this is consistent with
cleavage along pathway b rather than pathway a (Scheme 3).
The insert of Fig. 1 shows the growth of the 600 nm band. It
6
21
occurs with a rate constant of 2.5 3 10 s (independently
from the concentration of 1b) and must be related to splitting of
Notes and references
†
The calculation of the free energy change was performed according to
+ +
21
Weller equation: DG (kcal mol ) = 23.06 3 [E(D ·/D) 2E(A /A·)] 2
E(T A*The redox potentials were measured by cyclic voltammetry in
acetonitrile vs SCE: E(D ·/D) (1b) = 1.48 V, E(A /A·) = 20.21 V (2a) and
0.29 V (2b). The triplet energy for 2a and 2b is 52 and 53 kcal mol
1
)
+
+
21
2
,
12
respectively.
Irradiation conditions: oxetanes 1a,b: 4 3 102 M; 2a: 2 3 10 M;
2
23
‡
3
solvent: CD CN (0.8 ml); atmosphere: argon; time: 15 min. Irradiation in
Luzchem multilamp photoreactor, using 8 W lamps (43) with emission
maxima at 350 nm. There was no reaction in the dark or in the absence of
photosensitizer.
1
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1
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Scheme 4
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