Generation, entrapment and reactivity of long-lived organic carbocations and radical cations within a supramolecular assembly: Ca Y zeolite

Article abstract of DOI:10.1039/a607124d

Diarylethenes spontaneously form the corresponding radical cations and carbocations upon inclusion within activated Ca Y zeolite; oxygen plays an important role in the generation of the radical cations.

Full text of DOI:10.1039/a607124d

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Generation, entrapment and reactivity of long-lived organic carbocations and
radical cations within a supramolecular assembly: Ca Y zeolite
a
a
a
b
b
K. Pitchumani, P. H. Lakshminarasimhan, Nicolette Prevost, D. R. Corbin and V. Ramamurthy*
a
Department of Chemistry, Tulane University, New Orleans, LA 70118, USA
Central Research and Development, The Du Pont Company, Wilmington, DE 19880, USA
b
Diarylethenes spontaneously form the corresponding radical
cations and carbocations upon inclusion within activated Ca
Y zeolite; oxygen plays an important role in the generation of
the radical cations.
product from the zeolite. Results of the deuteriation experi-
ments are summarized in Scheme 2.
Of the various activation conditions we attempted, Ca Y
activated > 400 °C on a vacuum line under reduced pressure
2
3
(
< 10 Torr) was found to be ideal for the generation of radical
By virtue of their ability to generate carbocations and radical
cations, zeolites have been used in a number of catalytic
processes. Although the mechanism of carbocation formation
cations with minimum interference from carbocations. Two
remarkable observations made by us in the case of 1,1-diaryl-
ethenes lead us to conclude that oxygen plays an important role
in the generation of the radical cation within Ca Y. Inclusion of
diphenylethene 1a into Ca Y activated at 450 °C on a vacuum
line and handled under nitrogen atmosphere generated only a
very light green colour.‡ However, when oxygen was bubbled
into such a solution, a dark green colour resulted and the
benzophenone yield was enhanced from 1 to 10%. Enhance-
ment of the yield of the radical cation was also confirmed by the
diffuse reflectance spectral data. This observation suggested to
us that oxygen is essential for radical cation generation. The
following observation made with 4,4A-dimethylaminodiphenyl-
ethene 1c further strengthened our view that oxygen plays a
1
is fairly well understood, the potential of such intermediates in
routine organic transformations has not been fully realized. On
the other hand, no agreement on the mechanism of cation radical
2
generation within zeolites has been reached. This communica-
tion addresses these concerns.
When Ca Y (300 mg) activated at 500 °C under aerated
conditions for about 12 h, and cooled to room temperature, was
dropped into a cyclohexane solution of 1,1-diphenylethene 1a,
(
20 mg 3 ml) a bright green colour developed which, in the
slurry, persisted for several weeks.† Extraction of the cyclohex-
ane–zeolite slurry with dichloromethane gave 1,1-diphenyl-
ethane 2a as the single product (Scheme 1). The zeolite residue
after extraction was still coloured and remained so for at least
80
12 h under aerated conditions. When the zeolite turned
colourless, it was extracted for a second time with dichloro-
methane to yield benzophenone. The overall yield of the two
products was 90% diphenylethane and 10% benzophenone.
The diffuse reflectance spectra of the zeolite–1a complex as
formed in cyclohexane, showed two strong absorptions with
60
4
0
0
0
l
max at 432 and 615 nm (Fig. 1). The zeolite residue after the
first extraction with dichloromethane showed only a peak at 615
2
nm. On the basis of literature reports, we assign the 432 nm peak
+
to the Ph
2
C Me carbocation and the one at 615 nm to the radical
cation of 1a3 The presence of a radical cation in the Ca Y
sample is also indicated by EPR spectroscopy. A strong EPR
,4
5
360
400
500
600
700
λ / nm
signal with a g-value of 2.00235 was obtained.
Ca Y activated in the temperature range 300–600 °C under
aerated conditions was found to be ideal for carbocation
generation. It has been established in the literature that such
conditions of activation lead to the generation of Brønsted acid
Fig. 1 Diffuse reflectance spectra of diphenylethene included within
activated (500 °C aerated) Ca Y. (—) Soon after the addition of Ca Y to a
cyclohexane solution of diphenylethene and (---) after extracting the above
2 2
solution with CH Cl .
6
sites. Through extensive deuteriation experiments, we have
established that one of the hydrogens in diphenylethane 2a
(
Scheme 1) comes from the water present within the zeolite and
the other from dichloromethane which is used to extract the
CH2D
CH3
D
D
R
R
R
R
R
R
C6D12–CD2Cl2
Ca Y (H2O)
C6D12–CD2Cl2
Ca Y (D2O)
Ca Y
Me
+
O
C6H12–CH2Cl2
C6H12–CH2Cl2
H
1
2
CH3
H
CH D
2
a R = H
b R = Me
H
c R = NMe2
Scheme 1
Scheme 2
Chem. Commun., 1997
181

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crucial role in the generation of radical cations within zeolites.
When 4,4A-dimethylaminodiphenylethene was included within
activated Ca Y the corresponding radical cation was generated.
The diffuse reflectance spectrum of 1c within Ca Y is shown in
Fig. 2. The blue coloured solid, when degassed on a vacuum line
work, in progress, with a series of alkenes with oxidation
potential varying between 0.6 and 2.0 V is expected to clarify
this aspect of the oxidation of alkenes within Ca Y.
The conditions under which radical cations and carbocations
can be generated, independent of each other, within Ca Y zeolite
have been established. Using this technique we have been able
to generate long-lived radical cations and carbocations from a
number of alkenes. We are in the process of characterizing the
persistent carbocations and radical cations by CP MAS NMR
and by EPR respectively. The mechanism of radical cation
formation within activated zeolites is also being pursued.
We thank the Division of Chemical Sciences, Office of Basic
Energy Sciences, Office of Energy Research, US Department of
Energy and NSF-EPSCOR-LBR Center for Photoinduced
Processes, Tulane University for support of this program.
2
3
(
< 10 Torr) turned colourless and the diffuse reflectance
spectrum no longer contained the peak due to the radical cation
at 620 nm. Introduction of air into this sample, most
surprisingly, regenerated the blue colour and the peak at 620 nm
reappeared in the diffuse reflectance spectrum. This process
was reversible for at least six cycles. This remarkable
observation lead us to suggest that oxygen is most likely the
electron acceptor at least in the case of 4,4A-dimethylaminodi-
phenylethene. The alkene 1c was unique in its behaviour in
several ways: (a) no other alkenes including 1a and 1b above
showed the reversible behaviour exhibited by 1c; (b) the radical
cation from 1c was generated even in monovalent cation-
exchanged (Li, Na, K, Rb and Cs) Y zeolite. All other alkenes
gave radical cations only in divalent cation-exchanged Y and in
ZSM-5 zeolites.
Footnotes
†
ICP analysis indicated the zeolite used had the composition Si138.7-
384
Al53.3Na7.5Ca23.3O .
‡
Ca Y was placed in a quartz tube and heated at 450 °C under reduced
Recently, Frei and coworkers have exploited the unique
pressure (5 3 1024 Torr). Nitrogen was adsorbed onto this sample at room
feature of divalent cation-exchanged zeolites to stabilize
temp. and degassed. This process was repeated four times and finally the
sample was left under nitrogen overnight. Further handling was done under
nitrogen atmosphere. Very light colour formation is consistent with a report
charge-transfer complexes between hydrocarbons and oxygen.7
In the case of the alkenes used by Frei and coworkers, the
permanent charge separation occurs only upon activation by
light. However, the alkenes we have investigated, all possessing
[ref. 2(b)] that it is difficult to remove strongly adsorbed oxygen from
Ca Y.
8
much lower oxidation potentials than those studied by Frei and
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Fig. 2 Diffuse reflectance spectra of 4,4A-dimethylaminodiphenylethene 1c
included in Ca Y. (a) (---) Sample after extraction with dichloromethane
under aerated conditions. (b) (—) The above sample degassed on a vacuum
line and under reduced pressure (102 Torr). (c) (···) Sample (b) exposed to
air.
4
080.
3
Received, 18th October 1996; Com. 6/07124D
182
Chem. Commun., 1997