First direct observation of reactive carbenes in the cavities of cation-exchanged Y zeolites

Article abstract of DOI:10.1021/ol0363669

(Equation presented) Herein we report the first direct observation of reactive carbenes within the cavities of cation-exchanged Y zeolites. Chloro(phenyl)- and bromo(phenyl)carbenes were generated upon laser photolysis of 3-halo-3-phenyldiazirines incorporated within dry zeolites and the absolute reactivity of the carbenes was investigated as a function of counterbalancing cation and coincorporated quenchers in order to elucidate the behavior of these intermediates within zeolites. Product analysis performed upon thermolysis of the diazirine in Y zeolites yielded products that were identified as those derived from the carbene.

Full text of DOI:10.1021/ol0363669

ORGANIC
LETTERS
2004
Vol. 6, No. 6
881-884
First Direct Observation of Reactive
Carbenes in the Cavities of
Cation-Exchanged Y Zeolites
Reinaldo Moya-Barrios and Frances L. Cozens*
Department of Chemistry, Dalhousie UniVersity, NoVa Scotia, Canada B3H 4J3
fcozens@dal.ca
Received December 4, 2003
ABSTRACT
Herein we report the first direct observation of reactive carbenes within the cavities of cation-exchanged Y zeolites. Chloro(phenyl)- and
bromo(phenyl)carbenes were generated upon laser photolysis of 3-halo-3-phenyldiazirines incorporated within dry zeolites and the absolute
reactivity of the carbenes was investigated as a function of counterbalancing cation and coincorporated quenchers in order to elucidate the
behavior of these intermediates within zeolites. Product analysis performed upon thermolysis of the diazirine in Y zeolites yielded products
that were identified as those derived from the carbene.
Zeolites are microporous crystalline aluminosilicate materials
made up of corner-linked [SiO4]-4 and [AlO4]-5 tetrahedra,
where every aluminum present in the framework carries a
net negative charge that must be balanced with a charge-
balancing cation.1,2 The arrangement of the tetrahedra units
results in an open framework structure consisting of cavities
and pores where organic guest molecules can be readily in-
corporated. The negative network of well-defined void spaces
and associated counterbalancing cations generates a polar
matrix with localized electrostatic fields and active acidic
and basic sites. As a result, there has been considerable inter-
est in the ability of these supramolecular materials to modify
and/or promote a variety of fundamental reactions in organic
chemistry.3-11 From these studies the reactivity and decay
pathways of several different types of reactive species have
been investigated within these heterogeneous solid supports.
However, little has been reported regarding the behavior
of reactive carbenes within the motion-restricted media of
zeolites.12,13 Carbenes are neutral compounds that present a
divalent carbon, which is linked to two adjacent groups by
covalent bonds, and two nonbonding orbitals containing,
between them, two electrons. To date, no time-resolved
studies of reactive carbenes within zeolites have been
reported. The direct detection and study of reactive carbenes
within the cavities of zeolites will give important information
(7) Cozens, F. L.; Ortiz, W.; Schepp, N. P. J. Am. Chem. Soc. 1998,
120, 13543.
(8) Sankararaman, S.; Yoon, K. B.; Yabe, T.; Kochi, J. K. J. Am. Chem.
Soc. 1991, 113, 1419.
(1) Breck, D. W. Zeolite Molecular SieVes: Structure, Chemistry and
Use; John Wiley & Sons: New York, 1974.
(9) Rhodes, C. J.; Standing, M. J. Chem. Soc., Perkin Trans. 2 1992,
1455.
(10) Rhodes, C. J. Colloids Surf., A 1993, 72, 111.
(11) Kiricsi, I.; Horst, F.; Tasi, G.; Nagy, J. B. Chem. ReV. 1999, 99,
2085.
(12) Kupfer, R.; Brinker, U. H. Liebigs Ann. 1995, 1721.
(13) Kupfer, R.; Poliks, M. D.; Brinker, U. H. J. Am. Chem. Soc. 1994,
116, 7393.
(2) Rabo, J. A. Zeolite Chemistry and Catalysis; ACS Monograph;
American Chemical Society: Washington, DC, 1976.
(3) Yoon, K. B. Chem. ReV. 1993, 93, 1.
(4) Corma, A. Chem. ReV. 1995, 95, 559.
(5) Casal, H. L.; Scaiano, J. C. Can. J. Chem. 1985, 63, 1308.
(6) Turro, N. J. Acc. Chem. Res. 2000, 33, 637.
10.1021/ol0363669 CCC: $27.50 © 2004 American Chemical Society
Published on Web 02/21/2004

The same transient species with an absorption maximum
at 320 nm as well as bleaching between 350 and 400 nm is
observed upon laser photolysis of PhClCN2 incorporated
within the cavities of dry CsY under vacuum (10-4 Torr)
conditions. In KY, bleaching along with a small absorption
band at 320 nm due to the carbene is also evident, but the
spectrum is now dominated by the long-lived 300 nm band.
In LiY and NaY, bleaching in the 350-400 nm region is
still seen, but no absorption due to the carbene at 320 nm
could be observed. Since the photochemical quantum yield
of the diazirine precursor is not expected to be highly
dependent on the nature of the counterbalancing cation, the
similar bleaching observed in the zeolites suggests that a
similar amount of carbene is generated in all cases. Thus,
the absence of carbene absorption in NaY and LiY suggests
that the carbene is too short-lived to be directly observed
using the nanosecond laser technique, whereas in KY only
a small fraction of the photogenerated carbenes are suf-
ficiently long-lived to be observed. For the transient diffuse
reflectance spectra of PhClCN2 in the five cation-exchanged
zeolites, see Supporting Information.
Figure 1. Transient diffuse reflectance spectrum generated (0)
1.52, (9) 2.68, (O) 4.24, and (b) 13.0 µs after 355 nm laser
irradiation of 3-chloro-3-phenyldiazirine in dry RbY under vacuum
(10-4 Torr) conditions. Insert shows the observed rate constant for
the decay of the 320 nm band as a function of increasing
2-chloroacrylonitrile concentration.
about the behavior of this reactive intermediate within the
intrazeolite environment.
In the present work, we describe results concerning the
generation and reactivity of chloro(phenyl)- and bromo-
(phenyl)carbenes within cation-exchanged Y faujasites using
nanosecond diffuse reflectance spectroscopy. These results
constitute the first direct observation of carbenes within
zeolites.
The transient diffuse reflectance spectrum generated upon
355-nm laser flash photolysis (Nd:YAG laser, e 10 mJ, <8
ns/pulse) of 3-chloro-3-phenyldiazirine14 (PhClCN2) incor-
porated into dry RbY (loading level of s ) 0.1 molecules
of PhClCN2 per zeolite supercage) under vacuum conditions
(10-4 Torr) is dominated by an intense absorption band
centered at 320 nm, Figure 1. The 320 nm transient decays
in a first-order fashion with an observed rate constant of kobs
) 3.6 × 105 s-1. At long time scales after the 320 nm band
has completely decayed the spectrum is dominated by a long-
lived absorption band centered at 300 nm.
The position and narrow shape of the absorption band at
320 nm closely resembles that of chloro(phenyl)carbene
(PhClC:) previously observed in solution.15 In addition, the
transient observed at 320 nm is not affected by the introduc-
tion of oxygen into the zeolite but is rapidly quenched in
the presence of typical carbene quenchers such as methanol
(kMeOH ) 2.1 × 105 M-1 s-1) and 2-chloroacrylonitrile,
2-CAN (k2-CAN ) 3.4 × 105 M-1 s-1), Figure 1. These
observations are similar to those typically observed for singlet
halo(aryl)carbenes in solution15 and lead to the conclusion
that the transient species at 320 nm is the chloro(phenyl)-
carbene formed within the zeolite upon laser photolysis of
PhClCN2, eq 1 (X ) Cl). In agreement with this identification
is the weak bleaching (i.e., loss of absorbance) in the 350-
400 nm region of the spectrum, which is due to the
photodecomposition of the diazirine precursor upon laser
excitation.
To provide further evidence that the carbene is generated
in NaY, experiments were carried out using 4,4′-bipyridine
to trap the carbene and generate a long-lived ylide.16 Pyridine
is more commonly used to generate such ylides, but it binds
strongly to the zeolite framework, thus reducing its ability
to trap the carbene.17-19 4,4′-Bipyridine will also bind to the
framework, but its second reactive nitrogen will remain free
for addition to the carbene. Figure 2 shows the transient
diffuse reflectance spectrum generated after 355 nm laser
irradiation of 3-chloro-3-phenyldiazirine co-incorporated in
NaY with 4,4′-bipyridine under dry vacuum conditions. The
transient spectrum is dominated by a large broad band
centered at 550 nm. The location of the absorption maximum
at 550 nm and the broad shape of the band coincide nicely
with the absorption spectrum of the PhClC:-4,4′-bipyridyl
ylide in solution20 and is formed upon a fast reaction between
the co-incorporated 4,4′-bipyridine with the photogenerated
carbene, eq 2.
Ylide formation in NaY, where the carbene is too reactive
to be directly observed, provides strong additional evidence
(16) Jackson, J. E.; Platz, M. S. In AdVances in Carbene Chemistry;
Brinker, U. H., Ed.: Stamford, 1994; Vol. 1, pp 89-160.
(17) Hughes, T. R.; White, H. M. J. Phys. Chem. 1967, 71, 2192.
(18) Dewing, J.; Monks, G. T.; Youll, B. J. Catal. 1976, 44, 226.
(19) Corma, A.; Rodelles, C.; Fornes, V. J. Catal. 1984, 88, 374.
(14) Graham, W. H. J. Am. Chem. Soc. 1965, 87, 4396.
(15) 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.
882
Org. Lett., Vol. 6, No. 6, 2004

The significant increase in the product yield for the
oxadiazole from 3% to 57% as the loading level of the
diazirine increases from s ) 0.5 to s ) 2 is consistent
with this product being generated by reaction of the carbene
with its precursor, Table 1.
Results from laser experiments with 3-bromo-3-phenyl-
diazirine26 (PhBrCN2) incorporated within KY and RbY
under vacuum conditions are closely similar to those obtained
for PhClCN2 in RbY and CsY. Laser irradiation of PhBrCN2
incorporated within these cation-exchanged zeolites ( s )
0.1) yields transient diffuse reflectance spectra dominated
by a strong absorption band centered at 340 nm that decays
on the microsecond time scale. Weak bleaching in the 370-
410 nm region is observed, indicating the consumption of
the diazirine precursor upon laser excitation. The 340 nm
absorption maximum and the shape of the absorption band
closely match those of the bromo(phenyl)carbene (PhBrC:)
previously reported in solution.27 In addition, the introduction
of oxygen has no influence on the decay rate constant or
absorption characteristics of the transient spectra. Further-
more, vapor inclusion of methanol or 2-CAN rapidly
quenches the intensity of the signal at 340 nm and increases
the observed rate constant for the decay of the transient. All
of these observations allow the absorption band observed at
340 nm generated upon irradiation of the PhBrCN2 in dry
KY and RbY under vacuum conditions (10-4 Torr) to be
confidently assigned to PhBrC:, eq 1 (X ) Br).
Figure 2. Transient diffuse reflectance spectrum generated (0)
1.44, (9) 3.92, (O) 8.48, and (b) 14.3 µs after 355 nm laser
irradiation of 3-chloro-3-phenyldiazirine ( s ) 0.1) and 4,4′-
bipyridine ( s ) 2) co-incorporated in NaY under dry vacuum
conditions (10-4 Torr).
for the formation of the carbene within this supramolecular
host and also indicates that the reactive carbene can be
trapped within the cavities of the zeolite.
Thermolysis at 70-80 °C for 24 h of zeolite samples of
NaY and CsY incorporated with PhClCN2 under dry nitrogen
conditions leads to the formation of benzaldehyde and 2,5-
diphenyl-1,3,4-oxadiazole as the major products, Table 1.
In CsY laser photolysis of PhBrCN2 under vacuum
conditions leads to a transient absorption spectrum that shows
a small decaying absorption band centered at 340 nm. The
reduced intensity of the band at 340 nm in CsY indicates
that in this cation-exchanged zeolite most of the photoge-
nerated carbenes are too reactive to be observed on the
nanosecond time scale. PhBrC: was not directly detected in
either LiY or NaY, presumably because the carbene is too
short-lived in these zeolites. For the transient diffuse
reflectance spectra of PhBrCN2 in the five cation-exchanged
zeolites, see Supporting Information.
Table 1. Relative Product Yields Obtained upon Thermolysis
at 70-80 °C of 3-Chloro-3-phenyldiazirine Incorporated in Dry
Y Zeolites under Nitrogen
PhClCN2 occupancy benzaldehyde 2,5-diphenyl-1,3,4-
zeolite
level ( s )
(%)
oxadiazole (%)
NaY
NaY
CsY
0.5
2
0.5
94
43
88
3
57
12
The observed first-order rate constants for the decay of
the halo(phenyl)carbenes in the Y zeolites are presented in
Table 2. In each zeolite where the carbenes could be
observed, both carbenes are quite short-lived and decay with
rate constants similar to those observed for the same carbenes
in hydrocarbon solvents.15 Thus, the zeolite cavity does not
provide an environment with the ability to substantially
increase the lifetime of the carbenes. The rate constants for
the decay of chloro(phenyl)carbene in Table 2 also reveal a
general trend whereby the rate constant for the decay of the
carbene in the different alkali-metal exchanged zeolites
decreases as the size of the cation increases. This trend is
difficult to rationalize if, as suggested by the product studies,
the main reactions causing the rapid decay of the carbenes
involve addition to water and the precursor diazirine. In both
The formation of benzaldehyde and 2,5-diphenyl-1,3,4-
oxadiazole can be reasonably explained through carbene
decay pathways. The formation of benzaldehyde may be
rationalized by a mechanism involving O-H insertion of
PhClC: into intrazeolite water, followed by loss of HCl.21
2,5-Diphenyl-1,3,4-oxadiazole can be produced from the
reaction between PhClC: and the diazirine, PhClCN2. This
reaction initially gives bis(R-chlorobenzylidine)hydrazine,22-24
which reacts further with intrazeolite water over the lengthy
reaction time to give the observed product.25
(20) In hexane solution chloro(phenyl)carbene reacts rapidly with 4,4′-
bipyridine to form the PhClC:-4,4′-bipyridyl ylide which, typical of carbene
ylides, has a broad band with an absorption maximum at 510-520 nm.
The decay of the carbene at 320 nm and the growth of the ylide at 520 nm
increase in a linear fashion upon increasing concentration of 4,4′-bipyridine
leading to a bimolecular rate constant of 2.0 × 109 M-1 s-1
.
(21) Rosenberg, M. G.; Brinker, U. H. J. Org. Chem. 2003, 68, 4819.
(22) Liu, M. T. H.; Toruyama, K. J. Phys. Chem. 1972, 76, 797.
(23) Bonneau, R.; Liu, M. T. H. J. Phys. Chem. A 2000, 4115.
(24) Bis(R-chlorobenzylidine)hydrazine has a strong absorption at 300
nm (ref 23) and is likely the long-lived species that is observed after the
carbene has decayed.
(25) Cronin, J.; Hegarty, A. F.; Cashell, P. A.; Scott, F. L. J. Chem.
Soc., Perkin Trans. 2 1973, 1708.
(26) Moss, R. A.; Terpinski, J.; Cox, D. P.; Denny, D. Z.; Krogh-
Jespersen, K. J. Am. Chem. Soc. 1985, 107, 2743.
(27) Cox, D. P.; Gould, I. R.; Hacker, N. P.; Moss, R. A.; Turro, N. J.
Tetrahedron Lett. 1983, 24, 5313.
Org. Lett., Vol. 6, No. 6, 2004
883

Instead, the decrease in the rate constants parallels the
electron-accepting ability, or Lewis acidity, of the zeolites
upon going from LiY to CsY.29,30 This leads to the suggestion
that the main reaction responsible for the time-resolved
disappearance of the carbene involves an interaction of the
carbene with Lewis acid sites within the zeolites cavities.
This interaction is likely to be sufficiently weak for the
carbene to eventually be displaced from the framework by
water or the diazirine as these quenchers migrate into a cavity
possessing a carbene-framework complex. Future investiga-
tions are ongoing to provide more insight into the decay
pathway of reactive carbenes in cation-exchanged Y zeolites
in order to elucidate further mechanistic details for these
interesting reactions.
Table 2. Absolute First-Order Rate Constants (kobs) for Decay
of Halo(phenyl)carbenes in Dry Alkali Metal Cation Exchanged
Y Zeolites under Vacuum (10-4 Torr) Conditions
kobs (105 s-1
)
zeolite
chloro(phenyl)carbene
bromo(phenyl)carbene
LiY
NaY
KY
not observed
not observed
5.2 ( 0.5
not observed
not observed
4.5 ( 0.4
RbY
CsY
3.6 ( 0.4
3.1 ( 0.3
2.5 ( 0.2
4.9 ( 0.5
cases, the quenchers should be more strongly bound to the
framework in the small cation zeolites and therefore less
available for reaction. Thus, the carbene would actually be
less reactive as the size of the alkali-metal cation decreases,
a trend observed previously in the quenching of carbocations
with quenchers such as methanol.28 Furthermore, the amount
of both quenchers in the zeolites is small (<1 wt % of water)
and presumably cannot account for the rapid quenching of
the carbene in NaY and LiY. In addition, changing the
loading level of the diazirine precursor between s ) 0.5
and 0.05 had no influence on the decay rate constant of the
PhClC: in RbY. Only at very high loading levels of diazirine
( s > 1) does the observed rate constant increase slightly
with increasing loading level, which is consistent with the
coupling reaction occurring at high loading levels.
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada (NSERC) for
supporting this research.
Supporting Information Available: Time-resolved dif-
fuse reflectance spectra generated upon photolysis of 3-chloro-
3-phenyldiazirine and 3-bromo-3-phenyldiazirine in cation-
exchanged Y zeolites, methods used for sample preparation,
and a brief description of the nanosecond laser system. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL0363669
(28) O’Neill, M. A.; Cozens, F. L.; Schepp, N. P. J. Am. Chem. Soc.
2000, 122, 6017.
(29) Barthomeuf, D. J. Phys. Chem. 1984, 98, 7877.
(30) Liu, X.; Iu, K.; Thomas, J. K. J. Phys. Chem. 1994, 98, 7877.
884
Org. Lett., Vol. 6, No. 6, 2004