J. Am. Chem. Soc. 1997, 119, 7583-7584
Direct Observation of Cumyl Cations in Nonacidic
7583
Zeolites. Absolute Lifetime and Reactivity with
Coabsorbed Alcohols
Frances L. Cozens,* Melanie O’Neill, and
Norman P. Schepp
Department of Chemistry, Dalhousie UniVersity
Halifax, NoVa Scotia, Canada B3H 4J3
ReceiVed March 4, 1997
1
An important property of acid zeolites used as industrial
Figure 1. Transient reflectance spectra obtained 0.5 µs after 266-nm
excitation of bicumene (closed circles) and 4,4′-dimethoxybicumene
catalysts is their ability to promote and support the formation
of unstable and highly reactive carbocation intermediates. As
a result, significant effort is currently being directed toward
understanding the mechanisms by which zeolites influence the
(open circles) encapsulated within the cavities of NaY, and 0.5 µs after
3
08-nm excitation of chloranil co-incorporated with 4,4′-dimethoxy-
bicumene (squares) in NaY. The insert shows the decay of cumyl cation
at 330 nm (closed circles) and the 4-methoxycumyl cation at 360 nm
reactivity of incorporated carbocations, especially with respect
to the stabilizing influence of zeolite cavities.2
-6
Several of
(open circles) generated within the cavities of NaY.
these studies have shown that various carbocations are suf-
ficiently long-lived in acid zeolites to be observed by diffuse
reflectance spectroscopy and, in some cases, solid-state NMR.
On the other hand, little information is currently available
concerning the formation and behavior of reactive carbocations
within zeolites containing no Br o¨ nsted acid sites.7 It is therefore
difficult to establish if the long lifetimes of carbocations in
proton exchanged zeolites are due primarily to the presence of
transient species is therefore identified as the 4-methoxycumyl
cation formed upon laser photolysis of the incorporated neutral
bicumene derivative, eq 1 (R1 ) R2 ) OCH3).
6
the strong Br o¨ nsted acid sites, or if other factors are also
important, such as electrostatic interactions8 in which the
negatively charged zeolite framework acts as a non-nucleophilic
6
counteranion, or the confined environment that protects the
A reasonable mechanism for the formation of the 4-meth-
oxycumyl cation is rapid fragmentation of the radical cation1
produced by photoionization of the precursor, eq 1. This agrees
with formation of the 4-methoxycumyl cation, together with
1,12
carbocation from nucleophilic quenchers. In the present work,
we describe results concerning the generation and absolute
reactivity of cumyl cations within cavities of the nonacidic
zeolite NaY. These results represent the first direct observation
of reactive carbocations in NaY, and provide novel information
about the reactivity of the carbocations in the absence and
presence of coadsorbed alcohol nucleophiles.
13
the chloranil radical anion (λmax ) 420 and 450 nm) upon
14
photoinduced electron transfer with co-incorporated chloranil
1
5-17
as the sensitizer, Figure 1 (squares).
Our inability to
observe the intermediate radical cation, which should have an
absorption band similar to that of the 4-methoxytoluene radical
The transient diffuse reflectance spectrum obtained upon 266-
nm laser irradiation (Nd:YAG laser, e10 mJ/pulse, e8 ns pulse
1
8
8
-1
cation at 420 nm, is consistent with the rapid (ca. 10 s )
9
width) of 4,4′-dimethoxybicumene incorporated into NaY,
fragmentation of the 4,4′-dimethoxybicumene radical cation in
1
9
Figure 1 (open circles), reveals the formation of a transient
species with strong absorption centered at 360 nm. The transient
was not affected by the addition of oxygen into the zeolite
sample, but its decay did increase in the presence of nucleophiles
solution, and indicates that fragmentation is also rapid in
2
0
NaY.
Photoionization of the unsymmetrical 4-methoxybicumene (eq
1, R ) OCH ; R ) H), which might have resulted in the
1
3
2
(
Vide infra). In addition, the maximum at 360 nm and the nice
formation of either the 4-methoxycumyl cation or the parent
cumyl cation, led only to the formation of the 4-methoxycumyl
cation. The parent cumyl cation was generated upon 266-nm
irradiation of bicumene (eq 1, R1 ) R2 ) H) in NaY as indicated
by the formation of a strong absorption band centered at 330
symmetrical shape of the band match closely those for the
4
10
-methoxycumyl cation previously observed in solution. The
(
1) Corma, A. Chem. ReV. 1995, 95, 559-614.
(
2) (a) Cano, M. L.; Cozens, F. L.; Garcia, H.; Vicente, M.; Scaiano, J.
C. J. Phys. Chem. 1996, 18152-18157. (b) Cano, M. L.; Cozens, F. L.;
Vicente, F.; Garcia, H.; Scaiano, J. C. J. Phys. Chem. 1996, 18145-18151.
(11) Okamato, A.; Snow, M. S.; Arnold, D. R. Tetrahedron 1986, 42,
6175-6178.
(
c) Cozens, F. L.; Garcia, H.; Scaiano, J. C. Langmuir 1994, 10, 2246-
2
249. (d) Cozens, F. L.; Garcia, H.; Scaiano, J. C. J. Am. Chem. Soc. 1993,
(12) Maslak, P. Top. Curr. Chem. 1993, 168, 1-65.
(13) Andre, J. J.; Weill, G. Mol. Phys. 1968, 15, 97-99.
(14) These samples were prepared by first incorporating 10 mg of
chloranil into 800 mg of NaY, followed by co-incorporation of the bicumene.
(15) Maslak, P.; Chapman, W. H. J. Org. Chem. 1996, 61, 2647-2656.
(16) Sankararaman, S.; Yoon, K. B.; Yabe, T.; Kochi, J. K. J. Am. Chem.
Soc. 1991, 113, 1419-1421.
1
15, 11134-11140.
(3) (a) Xu, T.; Zhang, J.; Munson, E. J.; Haw, J. F. J. Chem. Soc., Chem.
Commun. 1994, 2733-2735. (b) Xu, T.; Haw, J. F. J. Am. Chem. Soc.
994, 116, 10188-10195. (c) Haw, J. F.; Nicholas, J. B.; Xu, T.; Beck, L.
W.; Ferguson, D. B. Acc. Chem. Res. 1995, 29, 259-267.
1
(
4) Pitchumani, K.; Corbin, D. R.; Ramamurthy, V. J. Am. Chem. Soc.
1
996, 118, 8152-8153.
(17) Yoon, K. B.; Hubig, S. M.; Kochi, J. K. J. Phys. Chem. 1994, 98,
3865-3871.
(18) Liu, A.; Sauer, M. C. J.; Loffredo, D. M.; Trifunac, A. D. J.
Photochem. Photobiol. A: Chem. 1992, 67, 197-208.
(19) Maslak, P.; Chapman, W. H. J. Chem. Soc., Chem. Commun. 1989,
1809-1811.
(20) The radical is a necessary product upon fragmentation of the radical
cation. Under ideal conditions where the strong absorption due to the
4-methoxycumyl cation was removed by quenching with methanol, a weak
transient with an absorption maximum at wavelengths <320 nm and that
is quenched by oxygen was observed and is tentatively assigned to the
4-methoxycumyl radical.21
(
5) Tao, T.; Maciel, G. E. J. Am. Chem. Soc. 1995, 117, 12889-12890.
(
6) Cano, M. L.; Corma, A.; Forn e´ s, V.; Garc ´ı a, H. J. Phys. Chem. 1995,
9
9, 4241-4246.
(
G.; Ramamurthy, V. Tetrahedron Lett. 1997, 38, 371-374.
7) Pitchumani, K.; Lakshminarasimhan, P. H.; Turner, G.; Bakker, M.
(
8) Spackman, M. A.; Weber, H. P. J. Phys. Chem. 1988, 92, 794-796.
(
9) Incorporation of the bicumenes was carried out by mixing 10 mg of
the organic compound dissolved in 20 mL of hexane and 800 mg of activated
(
500 °C) NaY (Aldrich, Si/Al ) 2.4) for 1 h. The suspension was centrifuged
and the isolated zeolite was then washed with hexane and dried under
vacuum.
(10) McClelland, R. A.; Chan, C.; Cozens, F. L.; Modro, A.; Steenken,
(21) Tokumura, K.; Ozaki, T.; Nosaka, H.; Saigusa, Y.; Itoh, M. J. Am.
Chem. Soc. 1991, 113, 4974-4980.
S. Angew. Chem., Int. Ed. Engl. 1991, 30, 1337-1339.
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