UV-vis and IR spectral characterization of persistent carbenium ions, generated upon incorporation of cinnamyl alcohols in the acid zeolites HZSM-5 and HMor

Article abstract of DOI:10.1021/jo991801r

Cinnamyl alcohol (1) and two derivatives 2 and 3 have been incorporated in dehydrated HMor and HZSM-5 zeolites with the aim to characterize spectroscopically the corresponding carbocations generated within the solids. Product studies of the supernatant liquid phase combined with diffuse reflectance UV-vis and IR spectroscopy provide unequivocal evidence for the carbocations. Thus, cinnamyl alcohol (1) affords the 1,5-diphenylpentadienyl cation in HMor and HZSM-5 as a persistent species. In the case of HMor with larger pore dimensions the bulkier 1-(2′-cinnamyl)-3-phenylpropenyl cation was also spectroscopically detected. No persistent carbocation was observed when the α-methylcinnamyl alcohol (2) was incorporated in the acid zeolites, wherein a complete cyclization to 2-methylindene takes place. Finally, incorporation of 2-methyl-4-tolyl-3-buten-2-ol (3) in HZSM-5 allowed detection of the gem-dimethyl-subsituted p-methylcinnamyl cation, with a lifetime of hours. This cation is not persistent enough in HMor to be characterized. The present study illustrates how structurally related allylic substrates may give distinct carbenium ions whose persistence depends on the host-guest fit in the interior of the acid zeolites.

Full text of DOI:10.1021/jo991801r

J . Org. Chem. 2000, 65, 3947-3951
3947
UV-vis a n d IR Sp ectr a l Ch a r a cter iza tion of P er sisten t Ca r ben iu m
Ion s, Gen er a ted u p on In cor p or a tion of Cin n a m yl Alcoh ols in th e
Acid Zeolites HZSM-5 a n d HMor
Waldemar Adam,*,† Isabel Casades,‡ Vicente Forne´s,‡ Hermenegildo Garc´ıa,*,‡ and
Oliver Weichold†
Institute fu¨r Organische Chemie, University of Wu¨rzburg, Am Hubland, 97074 Wu¨rzburg, Germany, and
Instituto de Tecnologı´a Quı´mica CSIC-UPV, Universidad Polite´cnica de Valencia, Apartado 22012,
46071 Valencia, Spain
Received November 22, 1999
Cinnamyl alcohol (1) and two derivatives 2 and 3 have been incorporated in dehydrated HMor and
HZSM-5 zeolites with the aim to characterize spectroscopically the corresponding carbocations
generated within the solids. Product studies of the supernatant liquid phase combined with diffuse
reflectance UV-vis and IR spectroscopy provide unequivocal evidence for the carbocations. Thus,
cinnamyl alcohol (1) affords the 1,5-diphenylpentadienyl cation in HMor and HZSM-5 as a persistent
species. In the case of HMor with larger pore dimensions the bulkier 1-(2′-cinnamyl)-3-phenylpro-
penyl cation was also spectroscopically detected. No persistent carbocation was observed when the
R-methylcinnamyl alcohol (2) was incorporated in the acid zeolites, wherein a complete cyclization
to 2-methylindene takes place. Finally, incorporation of 2-methyl-4-tolyl-3-buten-2-ol (3) in HZSM-5
allowed detection of the gem-dimethyl-subsituted p-methylcinnamyl cation, with a lifetime of hours.
This cation is not persistent enough in HMor to be characterized. The present study illustrates
how structurally related allylic substrates may give distinct carbenium ions whose persistence
depends on the host-guest fit in the interior of the acid zeolites.
In tr od u ction
is related to the acid strength of the medium. By
attempting the generation of a series of carbocations of
different thermodynamic stability, it has been possible
to correlate the acid strength of solid acids with that of
homogeneous mixtures of acids and superacids. The
quintessence of this research is that the strongest acidic
zeolites compare either to the acidity of a 70% aqueous
solution of sulfuric acid13 or to pure sulfuric acid;15 thus,
zeolites cannot be considered as superacid solids.
The previous studies on the acid strength of zeolites
have not paid attention to the fact that geometrical
factors can also contribute to the stabilization of embed-
ded carbenium ions. Thus, when one deals with micro-
porous solids with definitely restricted reaction cavities,
besides the intrinsic nucleophilicity of the framework,
spatial considerations have to be taken into account.11,12
Of particular importance are the geometrical restrictions
imposed on the approaching external nucleophiles to the
carbocationic center such that an effective overlap be-
tween the HOMO of the nucleophile and the LUMO on
the carbenium ion center can occur. When the transition
states or products are larger in size than the available
space provided by the pores, the rigid zeolite framework
may impede the reaction for steric reasons.16
Generation and characterization of carbenium ions
inside zeolites has attracted renewed attention in recent
years.1-11 This interest arises from two different points
of view: first, zeolites are a convenient heterogeneous
medium to stabilize and protect elusive positively charged
reaction intermediates such as carbenium ions and
radical cations, which allows their spectroscopic charac-
terization and study of their chemical reactivity;12 and
second, generation of persistent carbenium ions has made
possible the calibration of the acid strength of the H+
form of zeolites, a pertinent property since zeolites are
the most used solid acids for catalytic hydrocarbon
cracking.13-15 The leading idea behind the latter studies
is that the reactivity and persistence of carbenium ions
† University of Wu¨rzburg.
‡ Universidad Polite´cnica de Valencia.
(1) Haw, J . F.; Richardson, B. R.; Oshiro, I. S.; Lazo, N. D.; Speed,
J . D. J . Am. Chem. Soc. 1989, 111, 2052-2058.
(2) Xu, T.; Haw, J . F. J . Am. Chem. Soc. 1994, 116, 10188-10195.
(3) Xu, T.; Haw, J . F. J . Am. Chem. Soc. 1994, 116, 7753-7759.
(4) Corma, A.; Forne´s, V.; Garc´ıa, H.; Miranda, M. A.; Primo, J .;
Sabater, M. J . J . Am. Chem. Soc. 1994, 116, 2276-2280.
(5) Xu, T.; Torres, P. D.; Beck, L. W.; Haw, J . F. J . Am. Chem. Soc.
1995, 117, 8027-8028.
While attempts to generate the parent allyl cation upon
inclusion of allyl alcohol in pentasil zeolites have met
with failure,17-19 we have shown that capped R,ω-diphen-
yl allylic cations persist inside monodirectional mordenite
(6) Tao, T.; Maciel, G. E. J . Am. Chem. Soc. 1995, 117, 12889-12890.
(7) Cano, M. L.; Cozens, F. L.; Forne´s, V.; Garc´ıa, H.; Scaiano, J . C.
J . Phys. Chem. 1996, 100, 18145-18151.
(8) Cano, M. L.; Corma, A.; Forne´s, V.; Garc´ıa, H.; Miranda, M.;
Baerlocher, C.; Lengauer, C. J . Am. Chem. Soc. 1996, 118, 11006-
11013.
(9) Forne´s, V.; Garc´ıa, H.; J ovanovic, S.; Martı´, V. Tetrahedron 1997,
53, 4715-4726.
(15) Umansky, B.; Engelhardt, J .; Hall, W. K. J . Catal. 1991, 127,
128-140.
(10) Rao, V. J .; Prevost, N.; Ramamurthy, V.; Kojima, M.; J ohnston,
L. J . Chem. Soc., Chem. Commun. 1997, 2209-2210.
(11) Corma, A.; Garc´ıa, H. J . Chem. Soc., Dalton Trans. in press.
(12) Corma, A.; Garc´ıa, H. Top. Catal. 1998, 6, 127-140.
(13) Haw, J . F.; Nicholas, J . B.; Xu, T.; Beck, L. W.; Ferguson, D. B.
Acc. Chem. Res. 1996, 29, 259-267.
(16) Ramamurthy, V.; Eaton, D. F.; Caspar, J . V. Acc. Chem. Res.
1992, 25, 299-306.
(17) Biaglow, A. I.; Gorte, R. J .; White, D. J . Chem. Soc., Chem.
Commun. 1993, 1164-1166.
(18) Fa´rcasiu, D. J . Chem. Soc., Chem. Commun. 1994, 1801-1802.
(19) Xu, T.; Zhang, J .; Munson, E. J .; Haw, J . F. J . Chem. Soc.,
Chem. Commun. 1994, 2733-2735.
(14) Corma, A. Chem. Rev. 1995, 95, 559-614.
10.1021/jo991801r CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/02/2000

3948 J . Org. Chem., Vol. 65, No. 13, 2000
Adam et al.
F igu r e 1. Pictorial representation of the impeded attack of
water on the allylic functionality of the R,ω-diphenylpropenyl
cation included inside the zeolite channel, whose dimensions
closely match that of a phenyl ring.
Ch a r t 1
F igu r e 2. DR UV-vis spectra (plotted as the inverse of the
reflectance, R) of HZSM-5 (plot a) and HMor (plot b) zeolites
after adsorption of the alcohol cinnamyl (1).
ing Friedel-Crafts alkylation products, presumably
through the intermediacy of the 1-phenylpropenylium
cation (1a ).26-28 However in these studies, no attention
was paid to the characterization of this intermediate in
the solid. It is known that the cinnamyl cation can only
persist as the dominant species in a strongly acidic
medium.29
and medium-pore ZSM-5 by protection from the attack
by external nucleophiles.20,21 In the case of the 1,5-
diphenylpentadienylium incorporated in ZSM-5, the cat-
ion survives indefinitely even after suspending the solids
in water.21 It was established by Olah and Spear, by
working in liquid SO2, that phenyl substitution on the
allylic system produces only a relatively minor extra
stabilization of these allylic ions,22 which in solution may
readily undergo intramolecular cyclization to indenyl
cations.23,24 The contrasting persistence of these substi-
tuted R,ω-diphenyl allylic cations within zeolites was
attributed to spatial constraints impossed by the tight
fit of the phenyl substituents inside the zeolite channels
of similar dimensions. In these cases, the end-capping
phenyl rings act like stoppers in the channel, which on
one hand impede the access of external nucleophiles to
the inner positive charge of the allyl bridge and on the
other hand impose a linear arrangement preventing
cyclization (Figure 1).
Herein we present a product study, combined with
spectroscopic data that provide evidence that the incor-
poration of cinnamyl alcohols within medium-pore and
monodirectional large-pore, acidic zeolites also give rise
to characterizable carbocations. The molecular structures
of the cinnamyl alcohols under study here are depicted
in Chart 1. All of them encompass a single aryl ring at
one of the termini of the allyl cation bridge. These
substrates fill the gap between the as yet not observed
allyl cation18,19 and the remarkably long-lived capped R,ω-
diphenyl derivatives in pentasil zeolites.20,21,25
Incorporation of alcohol 1 inside thermally dehydrated
HMor (a parallel channel system of 7.4 Å diameter) or
HZSM-5 (two perpendicular channel systems: straight
5.2 × 5.7 Å, sinusoidal 5.3 × 5.6 Å), by stirring at the
reflux temperature a solution in CH2Cl2 or CCl4, gave
rise to an intense purple coloration of the solid. Diffuse
reflectance UV-vis (DR) spectrum of the HZSM-5 zeolite
(Figure 2) showed the presence of a new band at 580 nm,
with a shoulder at 520 nm. This band is absent in the
starting cinnamyl alcohol or any of its simple neutral or
charged derivatives. In particular, the cinnamyl cation
has an absorption band at 380 nm and, therefore, can be
unequivocally ruled out as the species responsible for the
purple color of the solid. We noticed that the DR spectrum
of HMor has an extra absorption band at 515 nm, which
suggests that a second species is present in this zeolite.
Product studies gave an important clue on the nature
of the intermediate formed upon incorporation of sub-
strate 1 in the zeolites. Solid-liquid extraction allowed
recovery, together with unreacted alcohol 1, of 1,5-
diphenyl-1,4-pentadiene (4) and 2-cinnamylindene (5).
The structure of both compounds was confirmed by
comparison with authentic samples, obtained by alterna-
tive synthesis as indicated in the Experimental Section.
The 4:5 product ratio in the organic phase changed from
60:40 for HMor to 20:80 for HZSM-5. The variation in
the 4:5 ratio indicates that the dimensions of the zeolite
micropores play a role in the product distribution found
in the liquid phase and presumably also in the material
retained inside the voids. In this regard, molecular
modeling at the semiempirical level has indicated that
indene derivative 5 is too bulky to fit inside the ZSM-5
pores. Therefore, this product is formed presumably on
the external surface of the ZSM-5 particles, but not in
their interior.
Resu lts a n d Discu ssion
The reaction of cinnamyl alcohol (1) with a large excess
of aromatic hydrocarbons in the presence of HZSM-5 and
HMor zeolites has been reported to form the correspond-
(20) Cano, M. L.; Forne´s, V.; Garc´ıa, H.; Miranda, M. A.; Pe´rez-
Prieto, J . J . Chem. Soc., Chem. Commun. 1995, 2477-2478.
(21) Garc´ıa, H.; Garc´ıa, S.; Pe´rez-Prieto, J .; Scaiano, J . C. J . Phys.
Chem. 1996, 100, 18158-18164.
(22) Olah, G. A.; Spear, R. J . J . Am. Chem. Soc. 1975, 97, 1539-
1546.
(23) Miller, W. G.; Pittman, C. U., J r. J . Org. Chem. 1974, 39, 1955-
1956.
(24) Pittman, C. U., J r.; Miller, W. G. J . Am. Chem. Soc. 1973, 95,
2947-2956.
(25) Ramamurthy, V.; Caspar, J . V.; Corbin, D. R. J . Am. Chem.
Soc. 1991, 113, 594-600.
(26) Armengol, E.; Corma, A.; Garc´ıa, H.; Primo, J . Appl. Catal., A
1995, 126, 391-399.
(27) Armengol, E.; Cano, M. L.; Corma, A.; Garc´ıa, H.; Navarro, M.
T. J . Chem. Soc., Chem. Commun. 1995, 519-520.
(28) Algarra, F.; Corma, A.; Garc´ıa, H.; Primo, J . Appl. Catal., A
1995, 122, 125-134.
(29) Olah, G. A.; Schleyer, P. V. R. Carbenium Ions; Wiley: New
York, 1968-1976; Vol. I-V.

Characterization of Persistent Carbenium Ions
J . Org. Chem., Vol. 65, No. 13, 2000 3949
Sch em e 1
F igu r e 3. Aromatic region of the IR spectra of two HZSM-5
wafers in sealed cells after degassing at 200 °C for 1 h under
10-2 Pa, recorded at room temperature: (a) after incorporation
of 1,5-diphenyl-1,5-diacetoxypentane as described in ref 20,
and (b) after reaction of cinnamyl alcohol (1) at reflux of CH2-
Cl2. Spectrum a corresponds to the R,ω-diphenyl allylic cation
7; the shaded band corresponds to an overtone of the alumi-
nosilicate framework.
Sch em e 2
(3300-1300 cm-1), has been found useful to characterize
the organic species embedded within zeolite matrixes.12,32
Indeed such spectroscopy serves as a fingerprint, in
which a good match between the IR spectrum of the
embedded organic guest and of the same species in
another medium is considered as one of the safest criteria
for identification. IR spectroscopy can also be used to rule
out a charge-transfer complex between diphenylpenta-
diene and the zeolite as responsible for the 580 nm band
in the DR spectra.
In the case considered here, the IR spectrum of cation
7 in the HMor zeolite had been already reported.20 As it
can be seen in Figure 3, the spectra of the zeolite wafers
after incorporation of cinnamyl alcohol (1) coincide with
that of carbenium ion 7, and therefore, on the basis of
the DR UV-vis and IR spectra, we may conclude that
they are the same species.
As mentioned earlier, the DR spectrum of HMor does
not coincide exactly with that of cation 7, because a
second species may also be present. On the basis of the
reaction products, we speculate that the indene derivative
5 could also serve as the precursor for the stabilized
carbenium ion 8 by hydride abstraction (Scheme 2). The
alternative hydride abstraction of an indenyl hydride to
afford the benzocyclopentenyl cation seems unlikely in
view of the 4π-antiaromatic nature of the latter. In fact,
the incorporation of the authentic indene 5 in HMor gives
rise to the DR spectrum shown in Figure 4, characterized
by two unresolved bands with λmax at 515 and 580 nm.
Thus, the 515 nm band in this spectrum corresponds to
the allyl cation 8, generated from indene 5 through
hydride abstraction (Scheme 2). Comparison of Figures
2 and 4 manifests that the DR spectra of HMor with the
incorporated alcohol 1 is a superposition of individual
spectra corresponding to the cations 7 and 8.
The formation of the indene product 5 may be readily
understood as arising from the intramolecular cyclization
of an intermediary 2-hydroxymethyl-1,5-diphenyl-1,4-
pentadiene (6), the dimer that arises from the electro-
philic attack of a cinnamyl cation on the CdC bond of
the cinnamyl alcohol (1) as shown in Scheme 1. Alter-
natively, the same dimer 6 could be formed through a
concerted acid-catalyzed mechanism in which cinnamyl
cation (1a ) is not fully developed. This would agree better
with the lower acid strength of the zeolites compared to
superacid media and with the previous failure to generate
the parent allyl cation in ZSM-5.18,19 Although the mech-
anism of formation of diene 4 is uncertain at the moment,
there are precedents in the literature in which arylalk-
enes such as 4 are formed by acid-catalyzed cleavage of
methylols.30,31 This provides support that the diene 4 and
its precursor 6 are generated upon incorporation of the
cinnamyl alcohol (1) in the acid zeolites with the cin-
namyl cation (1a ) as the primary common intermediate
(Scheme 1).
With the presence of diene 4 in the organic phase
established, the most likely species responsible for the
purple color in the acid zeolites should be 1,5-diphenyl-
pentadienylium cation (7), formed from diene 4 through
hydride abstraction (Scheme 2). The hydride acceptor
could be any other more reactive carbenium ion. The DR
spectrum of HZSM-5 perfectly matches that previously
reported by us for the cation 7 in the same zeolite.20,21
Unambiguous confirmation of the assignment for the
purple species as pentadienylium cation 7 was obtained
by IR spectroscopy. The spectral window, in which the
zeolite framework is transparent for IR spectroscopy
The DR shown in Figure 2 does not exhibit any
significant decay in the intensity of the 580 nm band over
days. Thermogravimetric analysis of these ZSM-5 and
HMor samples incorporating cation 7 and 8 show the
presence of ca. 8 wt % of coadsorbed water simulta-
(30) Kabalka, G. K.; Li, N.-S.; Malladi, R. R.; Tejedor, D.; Trotman,
S. J . Org. Chem. 1999, 64, 3157-3161.
(31) Kabalka, G. K.; Li, N.-S.; Malladi, R. R.; Tejedor, D.; Trotman,
S. J . Org. Chem. 1998, 63, 6438-6439.
(32) Little, L. H. Infrared Spectra of Adsorbed Molecules; Willmer
Brothers Ltd: New York, 1966.

3950 J . Org. Chem., Vol. 65, No. 13, 2000
Adam et al.
Sch em e 3
F igu r e 4. DR spectrum of a HMor sample after incorporation
of the indene derivative 5.
ured CdC bond predestined for intramolecular cycliza-
tion, a process that is favored in substrate 2 compared
to the intermolecular trapping required for the cinnamyl
alcohol (1). Thus, the E-configured R-methylcinnamyl
alcohol (2) first undergoes acid-catalyzed cis-trans isomer-
ization to provide a stationary concentration of Z-2, which
subsequently intramolecularly cyclizes to the indene
derivative (Scheme 3). In this regard, GC-MS of the
supernatant liquid allowed detection of a minor peak
(<5%) during the incorporation process with a MS
spectrum identical to that of E-2 that may be attributed
to its Z isomer.
The influence of the substituents on the allyl lateral
chain and the pore size of the acid zeolite on the
persistence of allylic cations is nicely exemplified in the
case of cinnamyl alcohol 3. Geminal dimethyl substitution
increases the steric encumbrance at one of the termini
of the allyl system and, as suggested by Figure 1, should
exert spatial restrictions toward the approach of nucleo-
philes onto the positive allylic core. At the same time,
dimethyl substitution intrinsically stabilizes the resulting
allylic carbocation. These two factors should favor the
persistence of the gem-dimethyl cinnamyl cation derived
from substrate 3 with respect to that of the parent
cinnamyl alcohol (1). The question is, then, whether gem-
dimethyl subtitution could have the same steric effect as
a phenyl cap inside the channels of zeolites.
Indeed, upon incorporation of alcohol 3 in HZSM-5 the
zeolite develops a yellow color. The DR spectrum showed
an intense sharp band at 430 nm, which decays within
hours and indicates the generation of the corresponding
allyl cation (Figure 5A). The same optical spectrum was
recorded when substrate 3 is dissolved in trifluoroacetic
acid, although in this solution the band was hypsochro-
mically shifted by 30 nm (Figure 6).
The 430 nm band is, however, not observed upon
incorporation of alcohol 3 in HMor (Figure 5B). In this
case, the larger pore size of the mordenite compared to
the pentasil zeolites provides a more loosely held 1,1-
dimethylcinnamyl cation inside the mordenite channels;
thus, this cation may be trapped by coadsorbed water or
nucleophilic framework oxygens of HMor. The actual
situation is, however, more complex since a weak absorp-
tion band is observed at 630 nm for mordenite (see inset
of Figure 5B), which decays with time and presumably
corresponds to a different carbocation. This band is not
observed in the case of ZSM-5. A transient species with
such long wavelength absorption may arise from inter-
molecular dimerization of the substrate 3, analogous to
that shown in Scheme 1 for the cinnamyl alcohol (1).
In conclusion, the persistent carbocationic species
generated from cinnamyl alcohols by incorporation in acid
neously with these carbocations. The amount coadsorbed
is less than that adsorbed on the original zeolites prior
to the adsorption (over 15%). This reflects a certain
hydrophobization of the zeolite by incorporation of the
organic material, but the residual amount of water is still
high enougth to quench totally any cation in solution.
Thus, a reasonable explanation to justify how simulta-
neously water and cations 7 and 8 can coexist is based
on the steric constraints imposed by the lattice to the
attachment of water on the positive carbocation center
as depicted in Figure 1.
Unfortunately, the authentic cation 8 could not be
generated inside the ZSM-5 zeolite from the indene 5,
because this compound is too large to penetrate into the
interior of the pores. The fact that the cation 8 is also
not detected inside the medium-pore ZSM-5 zeolite when
the cinnamyl alcohol (1) is incorporated may be under-
stood on the basis of shape selectivity.33-35 Compared to
large-pore sized Mor (7.4 Å), the ZSM-5 channels (∼ 5.5
Å) rigidly hold the linear diene 4 or its derived cation 7
in an extended conformation, which precludes the con-
formational folding necessary for intramolecular cycliza-
tion. This interpretation is in agreement with the very
weak and not persistent coloration observed in a control
experiment in which the authentic indene derivative 5
was exposed to the ZSM-5 zeolite and also in agreement
with molecular modeling calculations. In contrast, indene
5 does diffuse inside the larger channels of Mor and
therewith serves as the precursor of the cation 8 (see
Figure 4).
To generalize the rather surprising behavior of the
cinnamyl alcohol (1) upon incorporation into acid zeolites,
the related allylic alcohol R-methylcinnamyl alcohol (2)
was also examined in HMor and HZSM-5. In this case,
the observed chemistry was notably different from that
of the cinnamyl alcohol (1), which helped to clarify the
importance of the CdC bond configuration. Thus, incor-
poration of alcohol 2 into the HMor and HZSM-5 zeolites
resulted in 2-methylindene as the only detectable prod-
uct. No persistent carbocationic intermediate could be
observed in the solid, as judged by the absence of any
optical absorption at wavelengths longer than 350 nm.
This result implies that the immediate carbocation
precursor of the 2-methylindene possesses a cis-config-
(33) Gessner, F.; J ohnston, L. J .; Scaiano, J . C.; Olea, A.; Lobaugh,
J . H. J . Org. Chem. 1989, 54, 259-261.
(34) Gessner, F.; Scaiano, J . C. J . Photochem. Photobiol. A: Chem.
1992, 67, 91-100.
(35) Baldov´ı, M. V.; Corma, A.; Garc´ıa, H.; Mart´ı, V. Tetrahedron
Lett. 1994, 35, 9447-9450.

Characterization of Persistent Carbenium Ions
J . Org. Chem., Vol. 65, No. 13, 2000 3951
F igu r e 5. (A) DR spectrum (plotted as the inverse of the reflectance, R) of a HZSM-5 sample after incorporation of alcohol 3
from a CH2Cl2 solution, recorded at different time intervals; the inset displays the decay of the intensity measured at 390 nm as
a function of time. (B) DR UV-vis spectrum of a HMor sample (as in spectrum A); the inset shows the decay of a weak absorption
band at 630 nm, probably due to minor amounts of a highly delocalized carbenium ion.
decomposition of Et4N-ZSM-5 by following the reported
procedure.38 The Si/Al ratio of the resulting HZSM-5, measured
by chemical analysis, was 27. The incorporations were carried
out at reflux temperature by stirring magnetically a suspen-
sion of the corresponding cinnamyl alcohol (20 mg) in CH2Cl2
or CCl4 (15 mL) in the presence of thermally dehydrated (500
°C, overnight) zeolite (1.00 g) for 2 h. After this time, the solid
was filtered and Soxhlet extracted with CH2Cl2. The combined
organic phases were analyzed by GC-MS quadrupolar mass
spectrometer equipped with a 25 m capillary column of cross-
linked 5% phenylmethylsilicone and GC-FTIR gas chromato-
graph with the same column as the GC-MS and coupled to a
FTIR detector. The solvent was then removed under reduced
pressure, and the residue was purified by thin-layer chroma-
tography with mixtures of hexanes-CH2Cl2 as eluent. 1H and
13C NMR spectra of the isolated compounds were recorded on
a 300-MHz NMR instrument with CDCl3 as solvent and TMS
as internal standard. DR UV-vis spectra of the solids were
F igu r e 6. UV-vis absorption spectra of alcohol 3 dissolved
recorded on a spectrophotometer by using a praying mantis
in CF3CO2H (ca. 10-3 M) recorded at 1 h intervals.
accessory and BaSO4 as calibration standard. FTIR spectra
of the zeolites with the organic material were recorded in a
zeolites may be observed quite generally. The structure
of the carbenium ion depends on the substitution pattern
of the cinnamyl alcohol precursor and on the pore
dimensions of the zeolite. The chemistry may be rational-
ized in terms of acid-catalyzed dehydration of the cin-
namyl alcohols by the acid sites of the zeolite. When the
primary cinnamyl cation is sufficiently bulky to persist
in the channels of the zeolite, then such cations may be
spectroscopically characterized. An unsubstituted cin-
namyl functionality dimerizes or intramolecularly cy-
clizes to extendedly conjugated cation intermediates. For
example, the unprecedented formation of the 1,5-diphe-
nylpentadienylium ion from cinnamyl alcohol has been
assessed by UV-vis and IR spectroscopy. Significantly,
a gem-dimethyl terminus is sufficient to stabilize and
protect the initial allylic carbocation within ZSM-5
against bimolecular reactions (although less efficiently
than a phenyl cap) but not in the larger channels of the
mordenite.
spectrophotometer by using a greaseless cell supplied with
CaF2 windows. Self-supported wafers (10 mg) were compressed
at 1 ton × cm-2. Prior to recording the IR spectra at room
temperature, the samples were successively degassed at room
temperature, 100, 200, and 300 °C under 10-2 Pa for periods
of 1 h.
2-Methylindene was characterized by comparison with a
commercial sample (Aldrich). The diene 4 was identified by
comparison with an authentic sample, which was obtained by
dehydrohalogenation of 1,5-dichloro-1,5-diphenylpentane39 with
sodium carbonate. 2-Cinnamylindene (5) was synthesized in
68% from 2-cinnamyl-1-indanone (1.00 g, 4.00 mmol) by
NaBH4 (160 mg, 4.00 mmol) reduction in EtOH (10 mL) and
subsequent dehydration with phosphoric acid (400 mg). 2-Cin-
namyl-1-indanone was obtained in 78% yield by reacting
1-indanone (1.00 g, 7.60 mmol) with NaH (365 mg of a 60%
suspension in mineral oil) in dry THF (5 mL) at -10 °C for 2
h and then adding a solution of cinnamyl chloride (1.20 g, 7.60
mmol) in dry THF (5 mL) at room temperature for 5 h.
1
2-Cin n a m ylin d en e (5). H NMR δ (ppm): 7.39-7.11 (m,
9H, Ar-H), 6.6 (s, 1H, H at 1), 6.5 (d, 1H, J ) 16 Hz, PhCHd
CH-), 6.3 (dt, 1H, J 1 ) 16 Hz, J 2 ) 7 Hz, PhCHdCH-), 3.4
(s, 2H, H at 3), 3.3 (d, 2H, J ) 7 Hz). 13C NMR δ (ppm): 131.4,
128.5, 127.9, 127.3, 127.2, 126.3, 126.1, 123.8, 123.5, 120.1,
41.1, 34.8. HR-MS (m/z): calcd for C18H16 232.1253, found
232.1342.
This work is relevant for the use of acid zeolites as
heterogeneous catalysts or supramolecular hosts for
processes that involve carbenium ion intermediates.
Exp er im en ta l Section
J O991801R
Cinnamyl alcohols 1 and 2 were commercial samples (Ald-
rich) and were used as received. 2-Methyl-4-(4-methylphenyl)-
3-buten-2-ol (3) was prepared by addition of 2 equiv of MeLi
at -78 °C to ethyl p-methylcinnamate, which was obtained in
a Reformatsky reaction from ethyl bromoacetate and 4-meth-
ylbenzaldehyde.37 HMor was obtained by ammonium decom-
position at 550 °C under air of a commercial NH4Mor (P.Q.
CBV 20A, Si/Al 14). HZSM-5 was prepared by template
(36) Parameswara Reddy, M. V.; Krishna Rao, G. S. Synthesis 1980,
10, 815-816.
(37) Fu¨rstner, A. Synthesis 1989, 9, 571-590.
(38) Flanigen, E. M.; Bennett, J . M.; Grose, R. W.; Cohen, J . P.;
Patton, R. L.; Kirchner, R. M.; Smith, J . V. Nature 1978, 271, 512.
(39) Pe´rez-Prieto, J .; Miranda, M. A.; Garc´ıa, H.; Ko´nya, K.; Scaiano,
J . C. J . Org. Chem. 1996, 61, 3773-3777.