Generation and reactivity of simple chloro(aryl)carbenes within the cavities of nonacidic zeolites

Article abstract of DOI:10.1021/ja064779+

A number of para-substituted chloro(aryl)carbenes are generated within the cavities of a series of dry alkali metal cation-exchanged zeolites (LiY, NaY, KY, RbY, and CsY) upon laser flash photolysis of the corresponding diazirine precursor. The absolute reactivity of the chloro(aryl)carbene is found to be strongly dependent on both the nature of the electron-donating and -withdrawing properties of the aryl substituent and the nature of the zeolite charge-balancing cations. The results strongly suggest that two opposing mechanisms for capture of the carbene can occur depending on whether the zeolite framework behaves as a nucleophilic reagent or an electrophilic reagent in its reaction with the carbene center. Hammett relationships for the decay of the carbene as a function of aryl substituent and zeolite counterion versus the σ⁺ substituent parameter support a change in mechanism as the carbene center toggles between being electron poor and electron rich. For the electron-poor chloro(4-nitrophenyl)carbene, a framework adduct is proposed upon reaction of the nucleophilic [Si-O-Al]^- bridge with the carbene center, and for the electron-rich chloro(4-methoxyphenyl)carbene, an adduct with the tight Li⁺ cation is proposed.

Full text of DOI:10.1021/ja064779+

Published on Web 10/31/2006
Generation and Reactivity of Simple Chloro(aryl)carbenes
within the Cavities of Nonacidic Zeolites
Reinaldo Moya-Barrios and Frances L. Cozens*
Contribution from the Department of Chemistry, Dalhousie UniVersity,
Halifax, NoVa Scotia B3H 4J3, Canada
Received July 5, 2006; E-mail: fcozens@dal.ca
Abstract: A number of para-substituted chloro(aryl)carbenes are generated within the cavities of a series
of dry alkali metal cation-exchanged zeolites (LiY, NaY, KY, RbY, and CsY) upon laser flash photolysis of
the corresponding diazirine precursor. The absolute reactivity of the chloro(aryl)carbene is found to be
strongly dependent on both the nature of the electron-donating and -withdrawing properties of the aryl
substituent and the nature of the zeolite charge-balancing cations. The results strongly suggest that two
opposing mechanisms for capture of the carbene can occur depending on whether the zeolite framework
behaves as a nucleophilic reagent or an electrophilic reagent in its reaction with the carbene center. Hammett
relationships for the decay of the carbene as a function of aryl substituent and zeolite counterion versus
the σ+ substituent parameter support a change in mechanism as the carbene center toggles between being
electron poor and electron rich. For the electron-poor chloro(4-nitrophenyl)carbene, a framework adduct is
proposed upon reaction of the nucleophilic [Si-O-Al]- bridge with the carbene center, and for the electron-
rich chloro(4-methoxyphenyl)carbene, an adduct with the tight Li+ cation is proposed.
Introduction
pair accepting or donating guest molecules located within the
framework void space of these zeolites.6-9 An additional and
Zeolites are well-known crystalline aluminosilicate materials
with an open framework structure constructed of corner-linked
[SiO4]4- and [AlO4]5- tetrahedra where every aluminum present
in the framework results in a net negative charge, which must
be charge-balanced by a cation.1,2 The assembly of the
tetrahedral units results in a variety of interesting topologies
each involving a periodic array of molecular-sized pores,
channels, or cavities. For the faujasite-type zeolites, the large
void space associated with the 13 Å cavities is connected to
four neighboring cavities in a tetrahedral array via 7.4 Å pores.
This periodic arrangement of pores and cavities allows the
efficient incorporation of many organic guest molecules within
the interior spaces of the zeolite framework.
Because of the presence of the negative charge on the
framework, which results from the tetracoordinated aluminum,
and the positive charge of the charge-balancing cations, the
zeolite framework can be considered amphoteric, possessing
both Lewis basic and Lewis acidic sites. The strength of these
acidic and basic sites can readily be manipulated by changing
the nature of the charge-balancing cation, with Lewis base
strength increasing and Lewis acid strength decreasing as the
cation size becomes larger.3-5
equally interesting method to investigate the interior properties
of zeolites involves examining the reactivity of strongly
electrophilic or nucleophilic species within these zeolite cavities.
For example, zeolite encapsulated reactive carbocations are
known to have much longer lifetimes and significantly reduced
reactivity in Li+-exchanged zeolites as compared to Cs+-
exchanged zeolites.10,11 This reactivity trend nicely reflects the
Lewis acid-base properties of the cation-exchanged zeolites,
where Cs+-zeolites are basic in nature and Li+-zeolites are acidic
in nature, and is consistent with the notion that electrophilic
species are more reactive toward the [Si-O-Al]- active sites
as the Lewis basicity of the zeolite increases upon going from
LiY to CsY.5,6,10-12
The behavior of singlet carbenes within the cavities of
zeolites13,14 may also be of interest, because these reactive
species can, in principle, display both electrophilic and nucleo-
(6) Barthomeuf, D. In Acidity and Basicity of Solids: Theory, Assessment, and
Utility; Fraissard, J., Petrakis, L., Eds.; Kluwer Academic Publishers:
Netherlands, 1994; p 181-197.
(7) Ward, J. W. In Zeolite Chemistry and Catalysis; Rabo, J. A., Ed.; American
Chemical Society: Washington, DC, 1976; p 118-284.
(8) Grey, C. P. In Handbook of Zeolite Science and Technology; Auerbach, S.
M., Carrado, K. A., Dutta, P. K., Eds.; Marcel Dekker: New York, 2003;
p 205-255.
Much of the data concerning the nature of Lewis acid and
base sites within a range of zeolite types have been derived
from spectroscopic studies, such as IR and NMR, of electron-
(9) Pfeifer, H.; Ernst, H. Annu. Rep. NMR Spectrosc. 1994, 28, 91-187.
(10) O’Neill, M. A.; Cozens, F. L.; Schepp, N. P. Tetrahedron 2000, 56, 6969-
6977.
(1) Breck, D. W. Zeolite Molecular SieVes: Structure, Chemistry and Use;
John Wiley & Sons: New York, 1974.
(11) O’Neill, M. A.; Cozens, F. L.; Schepp, N. P. J. Am. Chem. Soc. 2000,
122, 6017-6027.
(2) Zeolite Chemistry and Catalysis; Rabo, J. A., Ed.; ACS Monograph;
American Chemical Society: Washington, DC, 1976.
(12) Heidler, R.; Janssens, G. O. A.; Mortier, W. J.; Schoonheydt, R. A. J. Phys.
Chem. 1996, 100, 19728-19734.
(3) Huang, M.; Adnot, A.; Kaliaguine, S. J. Catal. 1992, 137, 322-332.
(13) Kupfer, R.; Brinker, U. H. Liebigs Ann. 1995, 1721-1725.
(14) Kupfer, R.; Poliks, M. D.; Brinker, U. H. J. Am. Chem. Soc. 1994, 116,
7393-7398.
(4) Barthomeuf, D. Catal. ReV. 1996, 52, 521-612.
(5) Barthomeuf, D. J. Phys. Chem. 1984, 88, 42-45.
9
14836
J. AM. CHEM. SOC. 2006, 128, 14836-14844
10.1021/ja064779+ CCC: $33.50 © 2006 American Chemical Society

Reactivity of Chloro(aryl)carbenes within Zeolites
A R T I C L E S
Figure 1. Transient diffuse reflectance spectra obtained upon 355 nm laser excitation of (a) 3-chloro-3-(phenyl)diazirine 1a ( s ) 0.2), (b) 3-chloro-3-
(4-methylphenyl)diazirine 1b ( s ) 0.5), (c) (3-chloro-3-(4-chlorophenyl)diazirine) 1c ( s ) 0.5), and (d) 3-chloro-3-(4-trifluoromethylphenyl)diazirine 1d
( s ) 0.5) incorporated into the cavities of dry RbY. Insets show the transient decay kinetics at (a) 320 nm, (b) 330 nm, (c) 330 nm, and (d) 310 nm. All
spectra were obtained under vacuum conditions, and the spectral windows were taken at (a) 1.5 µs (b), 2.7 µs (O), 4.2 µs (9), and 13.0 µs (0), (b) 1.40 µs
(b), 2.44 µs (O), 3.80 µs (9), and 14.1 µs (0), (c) 1.40 µs (b), 2.52 µs (O), 4.36 µs (9), and 14.2 µs (0), and (d) 1.90 µs (b), 3.60 µs (O), 9.60 µs (9),
and 34.3 µs (0) after laser irradiation.
philic character.15 Thus, by examining the reactivity of a series
of singlet carbenes within different zeolites, it ought to be
possible to obtain information such as the relative importance
of the Lewis acid and the Lewis base properties of zeolites in
determining the chemistry of these incorporated guest molecules.
In the present work, we have studied the reactivity of a variety
of 4-substituted chloro(aryl)carbenes 2a-f formed by laser
irradiation of the appropriate diazirines 1a-f, eq 1, and have
examined the reactivity of these carbenes as a function of the
aryl substituent and the composition of zeolite Y. Our results
show that both variables not only have a distinct influence on
the reactivity of the carbenes, but also on the mechanism of
the reactions of the carbenes within the zeolite cavities.
diazirines 1a-1c incorporated at loading levels (molecules of
diazirine per supercage) of s ) 0.2-0.5 in RbY and CsY are
dominated by short-lived transients with absorption maxima at
320 or 330 nm, as well as a long-lived absorption band near
310 nm and permanent bleaching between 360 and 430 nm,
Figure 1a-c. The short-lived transients can be identified as the
corresponding chloro(aryl)carbenes 2a-2c on the basis of the
following observations. In each case, the absorption maximum
of the transient matches well with the known absorption
maximum of the same carbene previously generated in solu-
tion.16 In addition, the bleaching between 360 and 430 nm
corresponds nicely with the absorption of the diazirines in this
spectral region. Thus, the zeolite-incorporated diazirines are
depleted upon laser irradiation, with the carbenes being the
expected products upon photodecomposition of the diazirines.
The insets in Figure 1a-c show the time-resolved decays of
carbenes 2a-2c in RbY at room temperature. These decays, as
well as those obtained in CsY, fit well to a simple, first-order
rate expression. Rate constants, calculated by nonlinear least-
squares fitting of the decay profiles to a single-exponential rate
expression, are summarized in Table 1. The decays remained
unchanged in the presence of oxygen, but were accelerated upon
addition of methanol or 2-chloroacrylonitrile to the diazirine/
zeolite composite. This behavior provides further evidence for
the identification of the transients as the chloro(aryl)carbenes,
Results
Generation of Chloro(aryl)carbenes in Dry Alkali Metal
Cation-Exchanged Zeolites. Chloro(phenyl)-, Chloro(4-methyl-
phenyl)-, and Chloro(4-chlorophenyl)carbenes, 2a-2c. The
diffuse reflectance spectra obtained upon laser irradiation of
(15) Moss, R. A. In Carbene Chemistry; Bertrand, G., Ed.; Marcel Dekker: New
York, 2002; p 57-101.
(16) Gould, I. R.; Turro, N. J.; Butcher, J., Jr.; Doubleday, C., Jr.; Hacker, N.
P.; Lehr, G. F.; Moss, R. A.; Cox, D. P.; Guo, W.; Munjal, L. A.; Perez,
L. A.; Fedorynski, M. Tetrahedron 1985, 41, 1587-1600.
9
J. AM. CHEM. SOC. VOL. 128, NO. 46, 2006 14837

A R T I C L E S
Moya-Barrios and Cozens
Table 1. First-Order Rate Constants (kobs) for the Decay of the Chloro(aryl)carbenes in the Alkali Metal Cation-Exchanged Y Zeolites under
Dry Vacuum Conditions at 24 ( 1 °C
1
k
obs (105 s-
)
4NO2PhClC:
4CF3PhClC:
4ClPhClC:
PhClC:
4CH3PhClC:
4CH3OPhClC:
zeolite
2f
2d
2c
2a
2b
2e
LiY
NaY
KY
RbY
CsY
too fast
too fast
too fast
too fast
too fast
2.1 ( 0.2
0.65 ( 0.05
0.84 ( 0.06
1.6 ( 0.1
too fast
too fast
too fast
3.6 ( 0.4
3.1 ( 0.3
3.4 ( 0.3
too fast
too fast
5.2 ( 0.5
3.6 ( 0.4
3.1 ( 0.3
too fast
too fast
5.5 ( 0.5
4.5 ( 0.4
3.9 ( 0.4
too fast
too fast
too fast
too fast
too fast
which, as singlet carbenes,16 are known to be insensitive to
oxygen and to be reactive toward alcohols and electron-poor
alkenes.16,17
The long-lived absorption band at 300 nm observed upon
laser irradiation of 1a-1c does not match the absorption maxima
of carbenes 2a-2c and must be due to the formation of a
different photogenerated species. In previous work,18 this species
was identified as R,R′-dichlorobenzaldazine produced by addi-
tion of chloro(phenyl)carbene 2a to its diazirine precursor, eq
2 (Ar ) Ph), which is a well-known reaction of carbenes
generated from diazirines in solution.19-21 This assignment is
consistent with the absorption maximum near 300 nm for the
azine in solution,20 and with the isolation upon thermolysis of
1a in CsY of 2,5-diphenyl-1,3,5-oxadiazole, which is produced
by hydrolysis of the azine and subsequent cyclization, eq 2.22
Figure 2. Transient diffuse reflectance spectra obtained upon 355 nm laser
excitation of 3-chloro-3-(phenyl)diazirine 1a ( s ) 0.5) in LiY at -26 °C
under dry vacuum conditions. The inset shows the decay kinetics at 320
nm. The spectral windows were taken at 1.48 µs (b), 3.60 µs (O), 5.12 µs
(9), and 14.8 µs (0) after laser irradiation.
order rate expression, giving the rate constants summarized in
Table 1. The other component comes from carbenes generated
in a more reactive microenvironment, where the decay is too
fast and therefore undetectable, and takes place within the time
duration of the laser system with rate constants >5 × 106 s-1
.
Because the absorption maxima of the long-lived absorption
bands observed upon irradiation of 1b and 1c in RbY and CsY
are similar to that for the azine generated upon irradiation of
1a, it is reasonable to suggest that the corresponding substituted
R,R′-dichlorobenzaldazines (eq 2, Ar ) 4-CH3Ph from 1b and
4-ClPh from 1c) are responsible, at least in part, for the
absorption near 300 nm.
Results obtained upon laser irradiation of 1a-1c in KY were
somewhat similar to those in RbY and CsY, with the spectra
showing absorption due to carbenes 2a-2c near 320 nm,
bleaching at 360-430 nm, and the long-lived absorption band
at 300 nm. However, the absorption by the carbenes near 320
nm in this cation-exchanged zeolite was much weaker than that
observed in RbY and CsY, and appeared as a small shoulder
on the long-wavelength side of 300 nm bands instead of distinct
maxima. The presence of substantial bleaching between 360
and 430 nm, but only weak carbene absorption, suggests that
the carbenes decay with two distinct components in KY. One
decay component corresponds to carbenes generated in an
environment where they react sufficiently slowly to be observed.
This time-resolved decay component fit well to a simple first-
The relatively small transient signal due to the carbene in KY
indicates that the majority of the initially formed carbenes are
too reactive to be directly observed within this cation-exchanged
zeolite.
In NaY and LiY, only bleaching between 360 and 430 nm
and the long-lived absorption at 300 nm were observed, with
no direct spectral evidence for the presence of carbenes 2a-
2c. Because bleaching indicates that carbene formation had
occurred upon irradiation, the inability to detect the carbenes
suggests that they decay too rapidly within NaY and LiY to be
observed. In previous work, the presence of carbene 2a within
NaY and LiY was established by detection of the 4,4′-
bipyridine-2a ylide produced after laser irradiation of 1a in NaY
and LiY with co-incorporated 4,4′-bipyridine.18 Similar results
were obtained in the present work, whereby ylides with
absorption maxima at 450 nm were observed from the reaction
of 4,4′-bipyridine with carbenes 2b and 2c in NaY. Thus, it is
reasonable to conclude that our inability to observe carbenes
2a-2c resulted from these carbenes having short-lifetimes
within the cavities of NaY and LiY.
In the present work, further information concerning the
behavior of carbene 2a in LiY was obtained by carrying out
experiments at reduced temperatures. As shown in Figure 2,
when laser irradiation of diazirine 1a was carried out in LiY at
a reduced temperature of -26 °C, an absorption band at 320
nm was observed that is closely similar to that for 2a in
(17) Moss, R. A.; Turro, N. J. In Kinetics and Spectroscopy of Carbenes and
Biradicals; Platz, M. S., Ed.; Plenum: New York, 1990; pp 213-238.
(18) Moya-Barrios, R.; Cozens, F. L. Org. Lett. 2004, 6, 881-884.
(19) Liu, M. T. H.; Toruyama, K. J. Phys. Chem. 1972, 76, 797-801.
(20) Bonneau, R.; Liu, M. T. H. J. Phys. Chem. A 2000, 104, 4115-4120.
(21) Rosenberg, M. G.; Brinker, U. H. J. Org. Chem. 2003, 68, 4819-4832.
(22) Cronin, J.; Hegarty, A. F.; Cashell, P. A.; Scott, F. L. J. Chem. Soc., Perkin
Trans. 2 1973, 1708-1719.
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Reactivity of Chloro(aryl)carbenes within Zeolites
A R T I C L E S
Figure 3. Transient diffuse reflectance spectra obtained upon 355 nm laser excitation of 3-chloro-3-(4-methoxyphenyl)diazirine 1e ( s ) 0.5) in (a) RbY
at 24 ( 1 °C and (b) CsY at -28 ( 1 °C. All spectra were obtained under dry vacuum conditions, and the spectral windows were taken at (a) 1.56 µs (b),
3.20 µs (O), 5.48 µs (9), and 15.4 µs (0) and (b) 3.08 µs (b), 4.52 µs (O), and 14.1 µs (9) after laser irradiation.
solution16 and in RbY and CsY. At this reduced temperature,
the carbene decayed in a simple first-order manner, Figure 2
(inset), with a rate constant of 1.8 × 105 s-1 that is similar to
the rate constant for the decay of 2a in CsY at room temperature.
As the sample temperature was increased, the observed rate
constant for the decay of the carbene at 320 nm increased, and
near 0 °C, the absorption at 320 nm was no longer present,
with the carbene becoming too reactive to be observed. These
results clearly indicate that carbene 2a is considerably more
reactive in LiY than in zeolites RbY and CsY where the carbene
is readily detectable at room temperature.
Chloro(4-trifluoromethylphenyl)carbene 2d. Laser flash
photolysis of 3-chloro-3-(4-trifluoromethylphenyl)diazirine 1d
incorporated into dry RbY under evacuated conditions was
dominated by bleaching between 360 and 430 nm and a strong
absorption band centered at 310 nm, Figure 1d. Within 20 µs
after the laser pulse, the 310 nm absorption band partially
decayed with a first-order rate constant of 1.6 × 105 s-1, leaving
a long-lived absorption centered at 310 nm. The chloro(4-
trifluoromethylphenyl)carbene 2d is known to have a strong
absorption band at approximately 300 nm.16 Thus, the time-
resolved diffuse reflectance spectra in Figure 1d show that
carbene 2d is generated upon laser irradiation of the diazirine
precursor, and then decays to reveal a long-lived species at 310
nm, which is likely the carbene-diazirine adduct (eq 2, Ar )
4-CF3Ph) similar to that previously described for carbene 2a.
Upon the addition of methanol to the diazirine/zeolite sample,
the rate constant for the decay at 310 nm steadily increased,
while the long-lived component continued to show no decay
over the time scale of the experiment. This provides further
evidence that the two components of the absorption at 310 nm
are due to two distinct species and is consistent with the
assignment of the short-lived component of the 310 nm
absorption as the carbene 2d.
upon going from CsY to NaY, which is opposite from the trends
observed with carbenes 2a-2c whose reactivities increase upon
going from CsY to NaY.
Chloro(4-methoxyphenyl)carbene 2e. Figure 3a shows the
transient diffuse reflectance spectra obtained upon laser irradia-
tion of 3-chloro-3-(4-methoxyphenyl)diazirine 1e in dry RbY
under vacuum conditions at room temperature. Long-lived
absorption in the 310 nm region and bleaching between 370
and 460 nm due to photodecomposition of the diazirine are
observed, but no absorption near 340 nm, the known absorption
maximum of the chloro(4-methoxyphenyl)carbene 2e,16 is
evident in these spectra. Identical results were obtained upon
laser irradiation in CsY and KY, with only long-lived absorption
at 310 nm and bleaching between 370 and 460 nm being
observed and no absorption near 340 nm.
At -28 °C, laser irradiation of diazirine 1e in CsY did lead
to the formation of an absorption band at 350 nm that
corresponds well with carbene 2e, Figure 3b. As the temperature
increased, the rate constant for the decay of 2e increased, and
near 0 °C, carbene 2e decayed too rapidly to be observed. In
addition, the transient diffuse reflectance spectrum generated
upon 355-nm laser irradiation of diazirine 1e co-incorporated
in CsY with 4,4′-bipyridine at room temperature is dominated
by a large broad absorption band centered at 540 nm. The
position of the absorption maximum at 540 nm and the shape
of the band are closely similar to the absorption spectrum of
the 4,4′-bipyridine-carbene 2e ylide in solution.23 Together, the
low-temperature and 4,4-bipyridine results indicate that carbene
2e is photogenerated in CsY at room temperature and is
sufficiently long-lived to undergo a rapid reaction with co-
incorporated 4,4′-bipyridine to form the corresponding 4,4′-
bipyridine-carbene 2e ylide, but is too short-lived to be directly
observed on the microsecond time scale.
In LiY and NaY, laser irradiation at room temperature of
diazirine 1e produced the transient spectra shown in Figure 4,
which consist of two absorption bands, one centered at 320 nm
that showed little decay on the time scale of the laser system,
and an additional intense broad absorption band located at 370
nm that decayed slowly within these zeolites with a rate constant
in LiY of 1.1 × 104 s-1 under evacuated conditions. The static
absorption at 320 nm is likely the carbene-diazirine adduct
similar to that previously described for the reaction of the other
Unlike carbenes 2a-2c that were readily detectable in both
RbY and CsY, the 4-CF3-substituted carbene 2d was not directly
observed in CsY. In this zeolite, only the long-lived component
of the band at 310 nm and bleaching between 360 and 430 nm
due to photodecomposition of the diazirine precursor were
observed, suggesting that the carbene decays too rapidly to be
detected in CsY. This contrary behavior of 2d continued in
zeolites with the smaller alkali metal cations such that absorption
due to 2d remained strong in KY and continued to be readily
observed in both NaY and LiY. In addition, as shown in Table
1, the rate constants for the first-order decay of 2d decreased
(23) In hexanes, reaction of chloro(4-methoxyphenyl)carbene and 4,4′-bipyridine
produces the corresponding carbene ylide at 490 nm.
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A R T I C L E S
Moya-Barrios and Cozens
The possibility that the transient observed at 420 nm is the
chloro(4-nitrophenyl)carbene 2f can be discounted because this
carbene presents a very different absorption spectrum in
hydrocarbon solution with an absorption maximum at 320 nm.24
In addition, the lifetime of 2f in solution is only a few hundreds
nanoseconds, while the transient observed at 420 nm is persistent
on all time scales of our laser system in CsY, RbY, and KY,
and decays only slowly in NaY and LiY.
Carbene 2f has previously been shown to react with acetone
to produce a ylide with a strong absorption band at 590 nm.25
As shown in Figure 5b, vapor inclusion of acetone in RbY
rapidly quenched the intensity of the signal at 420 nm and
produced a new absorption band centered at approximately 600
nm, consistent with the formation of the acetone-2f ylide.25
This band was quenched by the inclusion of oxygen, which is
in agreement with the known reactivity of the carbene-acetone
ylide in solution.25 The formation of this ylide leads to the
conclusion that carbene 2f is generated in the zeolites and can
be trapped by acetone, but decays with a rate constant that is
too rapid for the carbene to be observed.
Figure 4. Transient diffuse reflectance spectra obtained upon 355 nm laser
excitation of 3-chloro-3-(4-methoxyphenyl)diazirine 1e ( s ) 0.5) in LiY
under dry vacuum conditions. The spectral windows were taken at 1.56 µs
(b), 3.20 µs (O), 5.48 µs (9), and 15.4 µs (0) after laser irradiation. The
inset shows the relationship between the rate constant for the decay of the
370 nm absorption and increasing methanol concentration within LiY.
carbenes with their diazirine precursors, eq 2 (Ar ) 4-CH3OPh).
The 370 nm absorption, which decays slowly on the time scale
of the experiment, is located at a longer wavelength and is much
broader in shape as compared to the known absorption properties
of the chloro(4-methoxyphenyl)carbene 2e, which in solution
has a sharp absorption band with a maximum at 340 nm.16 These
results therefore indicate that the carbene 2e is not observable
within LiY and NaY and that the 370 nm absorption is due to
a different transient species formed upon the photolysis of
diazirine 1e within these two Lewis acidic zeolites.
At -28 °C, the transient diffuse spectrum in LiY was identical
to that obtained at room temperature and was again characterized
by strong absorption bands at 320 and 370 nm, with no
absorption at 350 nm due to carbene 2e. However, laser
irradiation at room temperature of 1e in LiY or NaY with co-
incorporated 4,4-dipyridine at room temperature did result in
the formation of the 4,4′-bipyridine-2e ylide at 540 nm. Thus,
carbene 2e was indeed generated within these zeolites, but even
at a reduced temperature of -28 °C decayed too rapidly to be
observed.
The decay of the transient with absorption at 370 nm in LiY
and NaY was not affected by the addition of oxygen to the
sample. The rate constant for the decay did, however, increase
in the presence of methanol. The decay rate constant of the 370
nm band increased in a linear manner with respect to methanol
concentration, and linear least-squares analysis gave a bi-
molecular rate constant for quenching of the transient at 370
nm with methanol of kq ) 5.1 × 105 M-1 s-1, Figure 4, inset.
Chloro(4-nitrophenyl)carbene 2f. Laser experiments with
3-chloro-3-(4-nitrophenyl)diazirine 1f incorporated in all of the
alkali-metal cation-exchanged Y zeolites under dry vacuum
conditions yielded transient diffuse reflectance spectra domi-
nated by an absorption maximum centered at approximately 420
nm, Figure 5a. In CsY, RbY, and KY, this absorption band was
very strong and did not decay even on the longest time scale of
our laser system (k < 1 × 103 s-1). In NaY and LiY, the
absorption at 420 nm was less intense, and the transient decay
was faster with first-order observed rate constants of 2.3 × 104
and 3.4 × 104 s-1, respectively. The introduction of oxygen
had no effect on the absorption or the rate constant of the
transient observed at 420 nm in any of the alkali metal cation-
exchanged Y zeolites.
Product Studies. Thermolysis at 80 °C for 48 h of the
unsubstituted 3-chloro-3-(phenyl)diazirine 1a incorporated with
a loading level s ) 0.5 in NaY under dry nitrogen conditions
led to the formation of benzaldehyde (95% of isolated material
after extraction) and 2,5-diphenyl-1,3,4-oxadiazole (5%) as the
major products. As mentioned above, the oxadiazole is likely
generated by addition of carbene 2a to its diazirine precursor
to form the R,R′-dichlorobenzaldazine,19-21 followed by hy-
drolysis and cyclization.22 Benzaldehyde is the expected product
from direct addition of water to the carbene; however, as
described below, benzaldehyde may also be generated by
hydrolysis of a carbene-framework complex.
Thermolysis of NaY samples containing 3-chloro-3-(aryl)-
diazirines 1b-1e yielded only the corresponding arylaldehydes
with no detectable trace of 2,5-diaryl-1,3,4-oxadiazoles upon
GC analysis of the extracted material. Given that the laser
irradiation of 1b-1e led to formation of the absorption band at
300-310 nm assigned, at least in part, to the corresponding
substituted R,R′-dichlorobenzaldazines, the absence of the
oxadiazoles was surprising. It is, however, possible that the
oxadiazoles were produced, but due to the additional substituents
on the aryl ring were too large to be effectively extracted from
the zeolite samples.
Thermolysis of 3-chloro-3-(4-nitrophenyl)diazirine 1f at 80
°C for 48 h under argon followed by continuous extraction with
dichloromethane as described above for the diazirines 1a-1e
yielded no GC-detectable products. However, addition of 4 mL
of methanol to the zeolite sample after the thermolysis at 80
°C was complete, followed by extraction of the zeolite-
methanol slurry with dichloromethane, led to the detection of
4-nitrobenzaldehyde (89%) and 4-nitrobenzaldehyde dimethyl
acetal (11%). Thermolysis (80 °C, 48 h) of a NaY sample
containing the product 4-nitrobenzaldehyde under argon, fol-
lowed by the addition of 4 mL of methanol and extraction with
dichloromethane in the same manner as above, afforded
exclusively unreacted p-nitrobenzaldehyde, and no 4-nitrobenz-
aldehyde dimethyl acetal was obtained under these conditions.
(24) Bonneau, R.; Liu, M. T. H. J. Chem. Soc., Chem. Commun. 1989, 510-
512.
(25) Bonneau, R.; Liu, M. T. H. J. Am. Chem. Soc. 1990, 112, 744-747.
9
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Reactivity of Chloro(aryl)carbenes within Zeolites
A R T I C L E S
Figure 5. Transient diffuse reflectance spectra obtained upon 355 nm laser excitation of 3-chloro-3-(4-nitrophenyl)diazirine 1f ( s ) 0.2) (a) in RbY under
dry vacuum conditions and (b) in RbY upon vapor inclusion of acetone. Inset shows the decay trace monitored at 600 nm. The spectral windows were taken
at (a) 2.04 µs (b), 3.20 µs (O), 6.84 µs (9), and 14.9 µs (0), and (b) 240 ns (b), 500 ns (O), 3.42 µs (9), and 7.46 µs (0) after laser irradiation.
RbY, followed by a more gradual decrease from RbY to NaY,
and then a slight increase in LiY. Thus, carbenes 2a-2c are
more reactive in the more strongly Lewis acidic (i.e., electro-
philic) zeolites LiY and NaY and less reactive in the more
strongly Lewis basic (i.e., nucleophilic) zeolites RbY and CsY,
while carbene 2d is less reactive in the electrophilic zeolites
and more reactive in the nucleophilic zeolites.
A reasonable explanation for the two opposing trends in
Figure 6a is that carbenes 2a-2c, which do not contain strongly
electron-withdrawing aryl substituents, are displaying nucleo-
philic character, while carbene 2d, which contains a strongly
electron-withdrawing CF3 group, is behaving as an electrophile.
In other words, the mechanism of the reaction of the carbenes
within the zeolite cavities changes as a function of substituent,
with carbenes 2a-2c attacking an available electrophile within
the zeolite cavity, and carbene 2d reacting with an available
nucleophile.
The effect of substituents is more clearly illustrated in Figure
6b, which shows the relationship between the rate constant for
decay of the carbenes as a function of the Hammett substituent
parameter σ+ in CsY and KY. The plot includes data for
carbenes 2e (4-CH3O) and 2f (4-NO2), which could not be
observed and react within all of the cation-exchanged Y zeolites
with rate constants >106 s-1. As can be seen, the data for the
reaction in CsY are clearly curved, with a negative slope for
the carbenes containing electron-donating substituents, and a
positive slope for carbenes with electron-withdrawing substit-
uents. This behavior is consistent with the known ambiphilic
nature of chloro(aryl)carbenes26,27 and is typical for a change
in mechanism whereby the ambiphilic carbenes switch from
behaving as nucleophiles (negative slope) to electrophiles
(positive slope). In the more strongly electrophilic KY, the data
are also curved, but the change in slope is only evident with
the strongly electron-withdrawing 4-NO2 group.
While curved Hammett plots have been previously observed
for other ambiphilic carbenes,28 Hammett plots of reactions of
chloro(aryl)carbenes with quenchers are typically linear with
little evidence of curvature. As a result, it is reasonable to
suggest that the behavior of the chloro(aryl)carbenes in the
present work is due to the unusual properties of the zeolite
Figure 6. (a) Effect of zeolite composition on the rate constant for the
decay of the (0) chloro(phenyl)carbene 2a, (b) chloro(4-methylphenyl)-
carbene 2b, (9) chloro(4-chlorophenyl)carbene 2c, and (O) chloro(4-
trifluoromethylphenyl)carbene 2d at 24 ( 1 °C. (b) Relationship between
the rate constant for the decay of chloro(aryl)carbenes 2a-2f and σ+
substituent parameters in (9) CsY and (0) KY at 24 ( 1 °C. Curves were
drawn to highlight curvature and do not represent lines of best fit. Data
points with arrows represent lower limit estimates of the decay rate constants.
These results suggest that, in NaY, 4-nitrobenzaldehyde dimethyl
acetal is not produced by a reaction between methanol and
4-nitrobenzaldehyde within NaY.
Discussion
From the rate constant data presented in Table 1, several
trends are evident with regards to the effect of carbene structure
and zeolite composition on the rate constants for the decays of
the chloro(aryl)carbenes 2a-2f. Two of these trends are
illustrated in Figure 6a, which shows the rate constants for the
decay of carbenes 2a-2d as a function of zeolite composition.
As can be seen for carbenes 2a-2c, the rate constants increase
slightly as the zeolite composition changes from CsY to KY,
and then dramatically increase in NaY and LiY. Conversely,
the reactivity of carbene 2d shows the opposite trend, with the
rate constant decreasing significantly upon going from CsY to
(26) Soundararajan, N.; Platz, M. S.; Jackson, J. E.; Doyle, M. P.; Oon, S. M.;
Liu, M. T. H.; Anand, S. M. J. Am. Chem. Soc. 1988, 110, 7143-7152.
(27) Moss, R. A.; Fan, H.; Hadel, L. M.; Shen, S.; Wlostowska, J.; Wlostowski,
M.; Kroghjespersen, K. Tetrahedron Lett. 1987, 28, 4779-4782.
(28) Moss, R. A.; Perez, L. A. Tetrahedron Lett. 1983, 24, 2719-2722.
9
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A R T I C L E S
Moya-Barrios and Cozens
cavities. One of these properties is the availability of both
nucleophilic and electrophilic sites within the zeolite framework
that allow the ambiphilic nature of the chloro(aryl)carbenes to
be fully expressed in a single environment. Previous work has
demonstrated that the oxygen atom of [Si-O-Al]- bridges is
nucleophilic, with the nucleophilicity of the oxygen increasing
as the alkali metal cation is changed from Li+ to Cs+.5,6 This
framework nucleophile can react with electron-deficient species
such as carbocations to generate framework bound alkoxy
species.11,29,30 A similar reaction can occur between the frame-
work [Si-O-Al]- active sites and the electrophilic carbenes
2d and 2f, which in this case would lead to the formation of
framework bound ylide-like intermediates, as shown in eq 3.
reaction with the carbenes. In CsY and RbY, the incorporated
water is only loosely bound to the framework, as illustrated in
Scheme 1a. As a result, water in CsY and RbY will be
substantially more nucleophilic than that in LiY or NaY, and
more electrophilic carbenes will become increasingly longer-
lived as the alkali-metal cation is changed from Cs+ to Li+. On
the other hand, in LiY and NaY, the oxygen of water interacts
strongly with the smaller metal cations. This weakens the
nucleophilicity of the incorporated water, but can increase its
acidity, thus providing an electrophilic proton to react with the
more nucleophilic carbenes, Scheme 1b.
In addition, it is well known that the alkali metal cations,
especially Li+ and Na+, interact with lone-pair electrons of guest
molecules such as pyridine and pyrrole and that these cations
have strong Lewis acid properties in Y zeolites and can act as
effective electrophiles.6,31,32 Furthermore, results from studies
in solution have shown that carbenes are reactive toward metal
cations. In particular, fluoro(phenyl)carbene and chloro(fluoro)-
carbene react rapidly (k ≈ 107-108 M-1 s-1) with Li+ in
acetonitrile to generate R-halo lithium carbenoids.33,34 In addi-
tion, stable carbene-alkali metal complexes have been isolated
where the carbenic center is coordinated directly to either Li+,
Na+, or K+.35,36 Thus, a reasonable mechanism for the reaction
of the more electron-rich carbenes within the Y zeolites is rate-
determining addition of the carbene to the alkali metal cation
to generate an R-chloro-metal cation complex, eq 4. This
complex is likely to be unstable, especially for weakly coordi-
nating cations such as Rb+ and Cs+, and would presumably
react further with the zeolite framework to generate a framework
bound carbenoid-like structure, eq 4.
An alternative possibility that cannot be entirely discounted
at the present time is that, instead of the zeolites providing
nucleophilic and electrophilic sites for the reaction with the
carbenes, they modulate the properties of an additional quencher
that may be present within the zeolite. In particular, despite all
attempts to ensure that water is excluded from the zeolites during
sample preparation, we can only be confident that the zeolite
samples prepared in the present work contain less than 1% water,
which may still be sufficient to act as the primary reagent for
Scheme 1
Because both possible sets of mechanisms described above
highlight the ability of the zeolite framework to provide both a
nucleophilic and an electrophilic environment, it is difficult to
determine which one is more prevalent, or whether both are
contributing to the observed trends. Furthermore, results from
the product studies do not provide clear evidence for one
particular mechanism. The results, which showed that the
arylaldehydes are the major products, are consistent with water
adding directly to the carbenic center; however, both of the
carbene-framework complexes in eqs 3 and 4 are likely to be
prone to hydrolysis, presumably by slow reaction with trace
amounts of intrazeolite water, which would ultimately give the
arylaldehydes as the final products.
(29) Mota, C. J.; Esteves, P. M.; de Amorim, M. B. J. Phys. Chem. 1996, 100,
12418-12423.
(30) Kramer, G. J.; van Santen, R. A. J. Am. Chem. Soc. 1993, 115, 2887-
2897.
Nucleophilic addition of the framework to carbene 2f to
generate a carbene-framework bound complex rather than
addition of water is suggested by the formation of 4-nitrobenz-
aldehyde dimethyl acetal upon irradiation of 1f in NaY. This
product was isolated only when methanol was added to the
zeolite sample after thermolysis was complete. Because the
chloro(4-nitrophenyl)carbene decays rapidly within 100 ns, and
(31) Yashima, T.; Hara, N. J. Catal. 1972, 27, 329-341.
(32) Ward, J. W. J. Colloid Interface Sci. 1968, 28, 269-280.
(33) Moss, R. A.; Fan, H.; Gurumurthy, R.; Ho, G.-J. J. Am. Chem. Soc. 1991,
113, 1435-1437.
(34) Tippmann, E.; Platz, M. S. J. Phys. Chem. A 2003, 107, 8547-8551.
(35) Alder, R. W.; Blake, M. E.; Bortolotti, C.; Bufali, S.; Butts, C. P.; Linehan,
E.; Oliva, J. M.; Orpen, A. G.; Quayle, M. J. Chem. Commun. 1999, 241-
242.
(36) Arduengo, A. J., III; Tamm, M.; Calabrese, J. C.; Davidson, F.; Marshall,
W. J. Chem. Lett. 1999, 1021-1022.
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Reactivity of Chloro(aryl)carbenes within Zeolites
A R T I C L E S
methanol was only added after several hours of thermolysis,
the dimethyl acetal cannot be a product from the direct reaction
of the carbene with methanol. On the other hand, a framework
bound complex may be sufficiently long-lived to still be
available for reaction after thermolysis is complete and would
then upon addition of methanol be readily converted to the
observed product as shown in eq 5.
the 4-CH3O carbene 2e, is not limited to the highly electrophilic
LiY and NaY zeolites, but appears to continue in the less
electrophilic and more nucleophilic CsY and RbY zeolites.
Similarly, the electrophilic 4-NO2 carbene 2f is too reactive to
be observed both in the more nucleophilic CsY and RbY zeolites
as well as in the less nucleophilic NaY and LiY zeolites. Thus,
while changing the substituents from electron-donating to
electron-withdrawing causes a change in the carbene’s selectivity
toward the different kinds of reactive sites in the zeolites, the
reaction of each of the individual carbenes 2a-2c, 2e, and 2f
remains unchanged upon going from the more electrophilic LiY
to the more nucleophilic CsY.
The situation appears to be somewhat different for the CF3-
containing carbene 2d whose reactivity decreases upon going
from CsY to NaY, but then increases slightly in LiY. This may
indicate that 2d reacts as an electrophile in zeolites that possess
the stronger nucleophiles, while in a weakly nucleophilic, but
strongly electrophilic zeolite like LiY, the carbene adds as a
nucleophile to the zeolites framework.
Conclusions
While the precise mechanism of the reaction of the carbenes
within the Y zeolites cannot be unambiguously determined, the
results described above clearly indicate that the Y zeolite
framework can provide both a nucleophilic and an electrophilic
environment for reactions of potentially ambiphilic materials
such as the chloro(aryl)carbenes studied in the present work.
One consequence of this amphoteric behavior is that these Y
zeolites are not likely to be suitable media for capturing and
stabilizing carbenes, at least not simple singlet carbenes. For
example, our results suggest that adding strongly electron-
donating substituents to a normally electrophilic carbene in an
attempt to capture the carbene within a zeolite cavity is not
likely to be a successful strategy: the zeolite will instead reveal
its electrophilic character and rapidly react with the electron-
rich carbene. Whether or not this is valid even for strongly
stabilized carbenes, such as diamino carbenes that are stable in
solution, has not yet been determined.
Also of interest is the observation that, while the chloro(4-
nitrophenyl)carbene was not directly observed, a transient at
420 nm was generated after irradiation of 1f in all of the Y
zeolites, Figure 5a. Species with absorption maxima in this
region have previously been observed upon the addition of
nucleophiles to this carbene. In particular, the reaction of carbene
2f with ethyl acetate37 and tetrahydrofuran38 generates ylides
with absorption maxima at 490 nm, while the reaction with
oxygen produces the carbonyl oxide at 400 nm.39 In addition,
the 4-nitrophenyldichloromethide carbanion was recently gener-
ated by addition of chloride to 2f, and it has a maximum at 420
nm.40 Thus, while a definitive assignment of the 420 nm band
is not possible, its spectral properties are consistent with the
formation of the framework-2f ylide shown in eq 3 (X ) NO2).
The long-lived species with absorption at 370 nm generated
upon laser irradiation of 3-chloro-3-(4-methoxyphenyl)diazirine
1e in NaY and LiY, Figure 4, is also noteworthy, because it
has properties that are typically observed for benzylic carbo-
cations, especially 4-methoxybenzylic cations. In particular, the
4-methoxyphenethyl cation has an absorption maximum near
340 nm,41 which is in the same region as the 370 nm absorption
maximum for the transient from 1e. In addition, the 370 nm
band was reactive toward methanol, as expected for an elec-
trophilic species such as the carbene-metal cation complex.
Furthermore, the 370 nm band is only observed in LiY and NaY,
which is consistent with the expectation that only Li+ and Na+
would form a strong enough bond with the carbene to generate
a transient carbocation with a lifetime sufficiently long to be
observed.
Experimental Section
Materials. The 3-chloro-3-(aryl)diazirines 1a-1f were prepared
following the Graham reaction.42 All diazirines were characterized by
1H and 13C NMR spectroscopy (Bruker 500 MHz) using CDCl3 as the
solvent. Spectroscopic grade acetone and 4,4′-bipyridine (Aldrich) were
used as received. Spectroscopic grade solvents (<0.02% water) used
in zeolite sample preparation were commercially available (OmniSolv,
BDH) and were used without additional purification. Methanol was
doubly distilled prior to use.
NaY (Si/Al ) 2.4) was purchased from Aldrich and used as received.
The alkali metal cation Y zeolites were prepared by stirring ∼5 g of
NaY with 3 M aqueous solutions of the corresponding chloride salts
(LiCl, KCl, RbCl, CsCl) at 80 °C for 1 h in Nanopure water. The
zeolites were then filtrated and washed with Nanopure water until no
more chlorides appeared in the filtrate. The presence of chlorides was
tested using a saturated silver nitrate solution. This procedure was
repeated three times, and the zeolite was calcinated between exchanges.
NaY has three types of exchangeable cations. The number of cations
per unit cell of zeolite Y are 16 site I cations, 32 site II cations, and 8
site III cations. Only site II and site III cations are accessible to guest
An additional feature of the trends shown in Figure 6a is
that the nucleophilic behavior of carbenes 2a-2c, as well as
(37) Chateauneuf, J. E.; Liu, M. T. H. J. Am. Chem. Soc. 1991, 113, 6585-
6588.
(38) Celebi, S.; Tsao, M.-L.; Platz, M. S. J. Phys. Chem. A 2001, 105, 1158-
1162.
(39) Liu, M. T. H.; Bonneau, R.; Jefford, C. W. J. Chem. Soc., Chem. Commun.
1990, 1482-1483.
(40) Moss, R. A.; Tian, J. Org. Lett. 2006, 8, 1245-1247.
(41) Cozens, F. L.; Kanagasabapathy, V. M.; McClelland, R. A.; Steenken, S.
Can. J. Chem. 1999, 77, 2069-2082.
(42) Graham, W. H. J. Am. Chem. Soc. 1965, 87, 4396-4397.
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A R T I C L E S
Moya-Barrios and Cozens
molecules.43 The degree of cation exchange in Y zeolites is typically
47% for LiY, 97% for KY, 44% for RbY, and 47% for CsY. For larger
cations such as Rb+ and Cs+, only the Na+ that occupy type II and
type III sites can be readily exchanged.44 Hydrated Li+ does not readily
exchange the type I cations, which results in a lower percent exchange.
Preparation of Diazirine-Zeolite Samples. Diazirines used in this
work were incorporated within the zeolite samples by the following
general procedure. The hydrated zeolite was heated for at least 24 h in
an oven at 450 °C to remove the adsorbed water. The zeolite was
removed from the oven and poured into a flamed-dry test tube. Rapidly,
to prevent the absorption of water from the atmosphere, 20 mL of
hexanes (HPLC grade) was added, and the test tube was covered with
a septum. A certain volume (typically < 100 µL) of a concentrated
(typically 0.1-1 M) hexanes or dichloromethane stock solution
containing diazirine was added to the zeolite, suspended in hexanes.
The zeolite-hexanes slurry was stirred for 3 h at room temperature.
After this time, the test tube was centrifuged, the supernatant layer
was decanted, and another 20 mL of hexanes was added to the zeolite
to remove any compound on the zeolite surface. The slurry was stirred
for at least 10 min, centrifuged, and the supernatant liquid separated
from the zeolite. The supernatant layer was kept to determine the actual
loading level of the zeolite sample.
The hexanes-saturated zeolite was placed in a desiccator under
vacuum overnight to eliminate most of the hexanes presented in the
sample. The zeolite-adsorbate composite was transferred from the
desiccator to a 3 × 7 mm2 quartz cell for laser experiments. This part
of the procedure was conducted in a glovebag under dry nitrogen to
avoid the absorption of water vapor from the air. After the zeolite-
adsorbate composite was transferred to the laser cell, it was connected
to a vacuum line equipped with a diffusion pump (10-4 mmHg) and
left overnight to ensure the complete removal of any remaining hexanes
and oxygen from the zeolite sample.
accurately determined. The typical zeolite mass used for the laser
experiments was approximately 300-400 mg. To add the coadsorbed
carbene quencher, the laser cells were sealed with a rubber septum.
Next, a small amount of the quencher (typically 1-2 µL) was injected
into the top of the cell using a syringe. Care was taken to prevent the
contact of the zeolite powder with the liquid. The sample cell was then
gently heated with hot air to vaporize the quencher molecule, during
which time the cell was rotated to ensure even distribution. Special
care was taken to prevent the heating of the part of the laser cell where
the diazirine-zeolite composite was deposited. The sample was then
allowed to cool and was thoroughly shaken before the laser experiments
were carried out.
Product Studies. 3-Chloro-3-(aryl)diazirines were thermally de-
composed within the Y zeolites, and the products obtained were
extracted from the zeolite and analyzed. The procedure followed is
herein described. A sample of the appropriate zeolite of approximately
2 g containing the diazirine to be studied was placed in a test-tube
sealed with a rubber septum and under UHP nitrogen or argon. The
sample was stirred and heated at 80 °C (using an oil bath and a
temperature controller) for 48 h. After thermolysis, 20 mL of di-
chloromethane was added to the zeolite, and the resulting slurry was
stirred for 24 h. The slurry was centrifuged and the supernatant liquid
collected. The zeolite was washed with an additional 20 mL of
dichloromethane and centrifuged. The liquid was decanted and trans-
ferred to the previous extract. Product distribution and identification
were accomplished by GC/FID and GC/MS by comparison with
authentic reference samples.
Nanosecond Laser Flash Photolysis. The computer-controlled
nanosecond laser flash photolysis system was employed in this study
and has been previously described.11 The excitation source was the
third harmonic of a Spectra Physics Nd:YAG laser (355 nm, e20 mJ/
pulse, <8 ns/pulse). Diffuse reflectance experiments involve measuring
the fraction of reflected light absorbed by the transient, denoted as the
reflectance change, ∆J/Jo, where Jo is the reflectance intensity before
laser excitation and ∆J is the change in reflectance intensity after
excitation due to absorption by photogenerated transients. The diffuse
reflected light is focused through a grating monochromator into a
photomultiplier system before being captured with a Tektronix 620A
digitizing oscilloscope and transferred via a GPIB interface to a Power
Macintosh 7100 computer, which controls the laser system using a
program written with LabView 3.0 software.
Analysis of the hexanes washing decants by UV-visible spectros-
copy was used to determine the loading level, s (defined as molecules
of diazirine per zeolite supercage), of organic guest molecules included
within the zeolite sample. The absorbance of decants was compared to
calibration curves of the different substrates, and the occupancy level
of the sample was determined.
Preparation of zeolite samples containing diazirine and 4,4′-
bipyridine was carried out in a manner similar to the one described
above, by incorporating the two molecules separately. The diazirine
was incorporated first, followed by 4,4′-bipyridine.
Acknowledgment. This work was supported by the Natural
Sciences and Engineering Research Council of Canada (NSERC).
We thank NSERC for the continuous support of our research
program. We thank Prof. Norman P. Schepp of Dalhousie
University for his helpful comments on the preparation of this
manuscript.
Addition of methanol, acetone, and alkenes for the quenching plots
was conducted after diazirine incorporation and evacuation. The laser
cells containing the diazirine-zeolite composites were preweighed. In
this manner, the mass of the zeolite-diazirine samples could be
(43) Ramamurthy, V.; Turro, N. J. J. Inclusion Phenom. Mol. Recognit. Chem.
1995, 21, 239-282.
(44) Van Bekkum, H.; Flanigen, E. M.; Janson, J. C. Introduction to Zeolite
Science and Practice; Elsevier Science Ltd.: Amsterdam, 1991.
JA064779+
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