H
+
H
+
ZH +
+ Z –
+ Z –
R
R
R
R
R
R
H
+
R
R
Z –
+
+ ZH
R
R
Scheme 4
radical cation of the alkene.4 Consistent with this assignment,
EPR signals were observed with the sample. The EPR spectra in
all cases were broad and structureless. The radical cations
generated spontaneously within Ca Y were stable for several
days. Although we rule out Brønsted acid sites as possible
electron acceptor sites, we have no knowledge of the nature of
the electron acceptor at this time.5
We have shown that both carbocations and cation radicals
from alkenes can be generated within Ca Y. Further, we have
established that by carefully controlling the conditions of
activation, one can selectively generate long-lived organic
carbocations and cation radicals within such zeolites. We are in
the process of identifying the electron acceptor sites within
zeolites and the source of hydride ion during the reduction
process.
100
80
60
40
20
0
360
600
λ / nm
800
900
The authors thank the Division of Chemical Sciences, Office
of Basic Energy Sciences, Office of Energy Research, U.S.
Department of Energy and NSF-EPSCOR-LBR Center for
Photoinduced Processes, Tulane University for support of this
program.
Fig. 1 Diffuse reflectance spectrum of p,pA-dimethoxystilbene included
within Ca Y activated at 450 °C under vacuum (1024 Torr)
Y activated by method A was different from the ones activated
by methods B and C.
Footnote
The proposed mechanism for the geometric isomerization of
1 is shown in Scheme 4. The acid–base equilibria represented in
Schemes 2 and 4, we propose, are controlled by the number of
Brønsted acid sites present in Ca Y. When the number of
Brønsted acid sites is disproportionately large when compared
to the number of alkene molecules present in a zeolite, the acid–
base equilibrium favours the permanent formation of carboca-
tions. These carbocations have a long lifetime and abstract a
hydride ion from the solvent to yield 2. When the number of
Brønsted acid sites is relatively small, not enough carbocations
are generated to yield significant amounts of 2. However, when
the cis isomer is the guest, the protonation results in the
conversion of the cis isomer to the thermodynamically more
stable trans form. From the above studies it has become evident
that the key to effecting selective cis to trans isomerization
(without reduction) is to control the number of Brønsted acid
sites within a zeolite.
In addition to the carbocation, cation radicals were also
formed when stilbenes 1 were added to activated Ca Y. The
extent of cation radical formation dependent on the method of
activaton of Ca Y. While methods B and C gave relatively large
amount of radical cations, the method A gave only minor
amounts. The diffuse reflectance spectrum displayed in Fig. 1
for p,pA-dimethoxystilbene can clearly be identified with the
† Ca Y was prepared by exchanging Na Y with calcium nitrate for 12 h at
100 °C. The exchange was repeated four times, the solid filtrate was washed
thoroughly with water and dried at 90 °C for ca. 16 h. ICP analysis indicated
the exchanged zeolite to have a composition of Si138.7Al53.3Na7.5
Ca23.3O884
-
.
References
1 J. W. Ward, in Zeolite Chemistry and Catalysis, ed. J. A. Rabo, American
Chemical Society, Washington, DC, 1976, p. 118; M. L. Poutsma, in
Zeolite Chemistry and Catalysis, ed. J. A. Rabo, American Chemical
Society, Washington DC, 1976, p. 437.
2 N. Prevost, P. H. Lakshminarasimhan, V. J. Rao and K. Pitchumani,
unpublished results.
3 R. A. McClelland, C. Chan, F. Cozens, A. Modro and S. Steenken,
Angew. Chem., Int. Ed. Engl., 1991, 30, 1337.
4 V. Ramamurthy, J. V. Caspar and D. R. Corbin, J. Am. Chem. Soc., 1991,
113, 594.
5 D. N. Stamires and J. Turkevich, J. Am. Chem. Soc., 1964, 86, 749;
F. R. Chen and J. J. Fripiat, J. Phys. Chem., 1992, 96, 819; X. Liu,
K. K. Iu, J. K. Thomas, H. He and J. Klinowski, J. Am. Chem. Soc., 1994,
116, 11 811; A. Corma, V. Fornes, H. Garcia, V. Marti and
M. A. Miranda, Chem. Mater., 1995, 7, 2136.
Received, 23rd September 1996; Paper 6/06578C
128
Chem. Commun., 1997