(ꢀ254 ) 29 000) and the photochemically inert product 16a
(ꢀ254 ) 17 000).
species 13; (iv) in LiY, and in more extent in NaY,
instead, species 13 can be forced (by steric constriction)
to rearrange into 14 and give the final 1,3,4-oxadiazole
16 in good yields.
To assess wether the role of zeolite in promoting the
observed phototransposition was due to its highly polar
environment or also to some steric interaction, we
considered irradiations of the substrate on the polar silica
surface. The irradiation of 12a adsorbed on silica and
suspended in perfluorohexane left the starting material
for the most part unchanged with formation of only 2%
of 16a (Table 1). This indicates that the highly polar silica
surface has only a slight effect in promoting the rear-
rangement and this supports the hypothesis that the
observed reactivity of 12a@NaY is a function of its
residence inside the zeolite supercages and not on its
silica-like surface.
Finally, since the formation of 16a does not follow the
order of acidity (LiY >NaY > CsY) or basicity (CsX >
CsY > NaY > LiY) of the zeolite2,12 the electronic effect
of the cation or of the zeolite wall in promoting the
reaction may be excluded [the only case where such
effects could be claimed is during irradiation in CsY,
where the formation of a few percent of the quinazolin-
4-one (6 R2 ) Ph) can be ascribed to a photoinduced
electron transfer13 between the zeolite framework and the
oxadiazole]. Interestingly, irradiation in the better donor
more basic CsX left the substrate practically unchanged.
There appears to be no correlation between the zeolite
electronic properties and the reaction outcome. Although
a complete study that considers steric and electronic
substituent effects on this intrazeolitic transformation is
needed, an interesting first-glance hypothesis considers
what follows: (i) polar media favors the formation of
photolytic species such as 13, that in methanol will
immediately undergo a nucleophilic attack by the solvent
leading to the formation of solvolysis product; (ii) in the
absence of a nucleophilic or hydrogen donor solvent, the
photolytic intermediate 13 can choose between recombi-
nation to the original oxadiazole, recyclization into a
quinazolin-4-one, or formation of a diazirine-like inter-
mediate 14 evolving to 1,3,4-oxadiazole 16; (iii) on the
highly polar silica surface, the formation of 14 (and hence
16) is observed only in a limited extent since it requires
a large deformation of the spatial assembly of photolytic
These observations, and in particular the lack of
reactivity in CsX which has a significantly reduced
supercage void volume in comparison to NaY,14 suggests
that size and shape selectivity15 might be playing an
important role in directing the intrazeolite photochemical
behavior of 12 preventing or allowing the necessary
spatial reorganizations of the intermediates. In particu-
lar, it is tempting to suggest that the preferential
intrazeolite formation of 16 is a direct result of the
spatially compact structure of intermediate 14 enforced
by the ring-strain prohibition on a planar amide nitrogen.
In contrast, the elongated structures of intermediates 5
and 15 spatially discriminate against formation of 6, and
17.
In summary, the polar solvent cage in methanol and
supercage in the zeolite both support formation of zwit-
terionic intermediate 13. In solution the high concentra-
tion of methanol promotes formation of the solvolysis
product 18. In the zeolite the lifetime of intermediate 13
could be extended and either slower formation of inter-
mediates 14 and 15 allowed to compete or, more gener-
ally, the relative rate of the branch point for either are
inhibited or accelerated by the supercage. In addition,
the smaller molecular volume of intermediate 14 in
comparison to 15 leads to 16 as the major and 17 as the
minor product of this novel intrazeolite photorearrange-
ment.
The possibility of size and shape selectivity in other
heterocyclic photorearrangements is currently under
investigation.
Acknowledgment. We are grateful to Professor
Edward L. Clennan of the University of Wyoming for
useful discussions. We thank the University of Palermo
and the Italian MIUR for their support of this research.
Supporting Information Available: Plots of loading %
of 12a,b in zeolite, excitation and emission spectra of 12a in
methanol and in NaY, and full experimental details. This
material is available free of charge via the Internet at
JO047910+
(12) Ono, Y.; Baba, T. Catal. Today 1997, 38, 321-337.
(13) Formation of the quinazolin-4-one system from 5-aryl-1,2,4-
oxadiazoles irradiated in the presence of sensitizers or electron-donor
reagents has been systematized through an electron-transfer process.
Buscemi, S.; Pace, A.; Vivona, N.; Caronna, T.; Galia, A. J. Org. Chem.
1999, 64, 7028-7033.
(14) The vacant space in the zeolite supercage is as follows: LiY,
834 Å3; NaY, 827 Å3; CsY, -781 Å3; CsX, 732 Å3. Ramamurthy, V.;
Eaton, D. F.; Caspar, J. V. Acc. Chem. Res. 1992, 25, 299-307.
(15) Ramamurthy, V. J. Photochem. Photobiol. C 2000, 1, 145-166.
2324 J. Org. Chem., Vol. 70, No. 6, 2005