Radical pairs with rotational fluidity in the photochemical reaction of acetophenone and cyclohexane in the zeolite NAY: A 13C CPMAS NMR and product analysis study

Article abstract of DOI:10.1039/b815266g

The photochemical reaction of acetophenone and cyclohexane in the zeolite NaY occurs by combination of the geminate radical pairs to give products that reveal a significant amount of rotational fluidity, which was also documented by intermolecular nuclear dipolar interaction measurements using cross polarization ¹³C NMR (CPMAS) experiments. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b815266g

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PAPER
www.rsc.org/obc | Organic & Biomolecular Chemistry
Radical pairs with rotational fluidity in the photochemical reaction
of acetophenone and cyclohexane in the zeolite NAY: a 13C CPMAS
NMR and product analysis study
Ammee Amboya,a Tina Nguyen,a Hien T. Huynh,a Ashley Brown,a Gretchen Ratliff,a Heather Yonutas,a
Deniz Cizmeciyan,*a Arunkumar Natarajanb and Miguel A. Garcia Garibay*b
Received 2nd September 2008, Accepted 17th February 2009
First published as an Advance Article on the web 16th April 2009
DOI: 10.1039/b815266g
The photochemical reaction of acetophenone and cyclohexane in the zeolite NaY occurs by
combination of the geminate radical pairs to give products that reveal a significant amount of
rotational fluidity, which was also documented by intermolecular nuclear dipolar interaction
measurements using cross polarization 13C NMR (CPMAS) experiments.
of freedom, in measures that depend on the sizes and shapes of
the reactants as compared to those of the zeolite cavities and
channels.4,5 The relation between molecular fitting and reaction
Introduction
During the last few years, a number of organized assemblies
control has been deduced by product analysis in the MX and MY
zeolites by changing the sizes of the cation M,6 and by adding
molecular “spectators”7 and chiral co-adsorbates.8 A few years
ago, while studying the reaction between benzophenone-d10 and
have been examined as reaction media to control the product
distribution of photochemically generated transients, such as
excited states and reactive intermediates.1 Examples include
molecular crystals, zeolites, inclusion complexes (both in the
solid state and in solution), liquid crystals, micelles, Langmuir–
Blodgett films, and others. The effect of these media varies
widely and depends primarily on their influence on the motion
of the reactants, generally restricting some reaction pathways and
sometimes opening new ones.2 Not surprisingly, the largest effects
as compared to reactions in solution are seen with reactions carried
out in solids. In one extreme, close-packed molecular crystals
restrict molecular translation, rotation, and large amplitude
conformational motions, so that chemical reactivity is limited to
least motion pathways.3 Alternatively, photochemical reactions
carried out in zeolites take advantage of most molecular degrees
1
cyclohexane in NaX we demonstrated that intermolecular { H}-
13C cross polarization in 13C CPMAS9 NMR experiments could
be used as a measure of close proximity and restricted mobility.10
A relatively static diphenylketyl–cyclohexyl radical pair (R =
Ph, Scheme 1) formed by intermolecular hydrogen abstraction
in the orientation depicted in Scheme 1A led to 1,1-diphenyl-
cyclohexylcarbinol as the only product (as indicated by the half
arrows). In this paper we describe an analogous study with
acetophenone and cyclohexane in NaY (R = Me, Scheme 1). We
report how the smaller reactant experiences greater mobility, so
that the radical pair formed by hydrogen transfer can rotate and
explore additional reaction pathways.11 In addition to the product
formed from orientation A, products were also obtained from
the bonding pathways indicated by orientations B and C. The
increased mobility of the reactants was also documented by
aMount St. Mary’s College, Los Angeles, CA 90049, USA. E-mail:
dcizmeciyan@msmc.la.edu; Tel: 01 310 954 4107
bDepartment of Chemistry and Biochemistry, University of California, Los
Angeles, CA 90095-1560, USA. E-mail: mgg@chem.ucla.edu; Tel: 01 310
825 3159
1
intermolecular { H}-13C cross polarization measurements, which
revealed a weaker dipole–dipole interaction.
Scheme 1 Alternative forms of C–C bond formation allowed by in-cage rotational motion of the ketyl–cyclohexyl radical pair in the zeolite NaY.
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Experimental
Photochemical experiments
Acetophenone (50 ml) was irradiated using a 400 W medium
pressure mercury lamp (<300 nm) in dry cyclohexane for 2 h.
For photoreactions within NaY zeolite, 50 ml of acetophenone
and 3 ml of cyclohexane were mixed in a Pyrex test tube. To this
solution was added 200 mg of NaY activated for 3 h at 425 ◦C and
the resulting suspension stirred for 6 h under an argon atmosphere.
The supernatant cyclohexane layer was removed, tested for the
removal of acetophenone and the zeolite washed three times with
3 ml fresh cyclohexane. Samples were irradiated as cyclohexane
slurries for 1–6 hours in Pyrex test tubes (l <300 nm) with the
400 W medium pressure mercury lamp. Products were extracted
by stirring with ethyl acetate for 6 h. Product analyses were
carriedby GC andGC-MS with retention times and fragmentation
patterns calibrated with those of authentic samples. The identities
of products 2,12 3a13 and 3b14 and 415 were confirmed by comparing
spectral data with that previously reported in the literature.
Fig. 1 Structure of the Faujasite X and Y framework illustrating the sites
(I–III) of the charge-compensating cations.
Photochemical results
Our observations on the solution and zeolite photochemical
reactivity of acetophenone and cyclohexane are summarized
in Scheme 2. Photochemical excitation in solution results in
formation of pinacol 4 and several low molecular weight products,
including bicyclohexane and cyclohexene. It is well known that the
reaction occurs by intermolecular H-abstraction from the rapidly
formed triplet excited state to form a triplet radical pair that
separates to become free radicals.17 The results suggest that the
kinetics of termination for the ketyl and cyclohexenyl radicals
are very different as no coupling between them is observed in
solution.18 In contrast, the reaction in NaY is characterized by
a very efficient cage effect. While 1-cyclohexyl-1-phenylethanol 2
is formed by coupling the cyclohexyl and acetophenone-derived
radicals at the ketyl carbon, 2¢-cyclohexylacetophenone 3a and 4¢-
cyclohexylacetophenone 3b are formed by coupling at the ortho-
and para-positions, respectively. It is interesting that pinacol 4
and bicyclohexyl were completely absent (by GCMS) when the
photoreaction was conducted in the zeolite.
Solid state NMR measurements
The 13C NMR spectra of all zeolite samples were carried out at
75.47 MHz on a Bruker Avance instrument. The 13C CPMAS
experiments were run with the TOSS sequence16 in order to
suppress the spinning sidebands. Adamantane was employed as
an external chemical shift reference and the magic angle was
adjusted with KBr. The 90◦ pulse width was 4.25 ms. Adequate
cross polarization was achieved using 1.5 ms as contact time in
the case of natural abundance samples and 5 ms in the case
of acetophenone-d8. A rotation rate of 10 kHz was used for all
samples. Zeolite samples for 13C-CPMAS studies were prepared
with acetophenone labeled with 13C at the carbonyl and methyl
carbon to shorten the acquisition times.
Results and discussion
Zeolites X and Y are microcrystalline aluminosilicates made up
of [SiO4]4- and [AlO4]5- tetrahedra that form an extended three-
dimensional network with an architecture that results in relatively
large cavities known as supercages (Fig. 1). With an internal
˚
diameter of ca. 12 A, each supercage is connected to four others by
˚
openings that have ca. 8 A diameter, so that medium-size organic
molecules are able to diffuse through the three-dimensional
network. The rate of diffusion, rotation, and conformational
motions of the molecules included therein depend on their size
and shape, as well as the specific chemical interactions with zeolite
walls. Given that each aluminium center carries a negative charge,
the structure of the zeolites requires an equivalent number of
charge-compensating cations, which are largely responsible for
the specific binding interactions with the organic adsorbates. Na-
exchanged Y zeolites (NaY) such as the one used in this study
have a low Al content, and a smaller number of cations than
the analogous X-zeolites. While samples of dried NaY may be
considered relatively inert, they are able remove acetophenone
from a cyclohexane solution in as much as 25% of their own dry
weight, clearly suggesting a very favorable binding interaction.
Scheme 2 Photochemical reactivity of acetophenone and cyclohexane in
(a) dilute solution, and (b) co-adsorbed in the zeolite NaY.
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It has been previously established that radical combination
is the dominant reaction pathway for ketyl and alkyl geminate
radical pairs generated by intermolecular hydrogen abstraction
with the zeolite supercage.19,10 Some examples include the reactions
of benzaldehyde, p-methylbenzaldehyde, acetophenone, and ben-
zophenone with toluene,19 and benzophenone with cyclohexane.10
All these reactant pairs favor the coupling of the corresponding
alkyl and ketyl radicals to give a tertiary alcohol. Based on this
precedent, the absence of pinacol 4 and the formation of the
tertiary alcohol 2 from acetophenone and cyclohexane within
NaY come as no surprise. The results suggest: (1) that having two
reacting acetophenone molecules in the same supercage is very
unlikely, (2) that reacting ketones share the supercages with cyclo-
hexane molecules and, (3) that the resulting radical pairs are unable
to separate and become free radicals. However, the formation of
2¢-cyclohexylacetophenone 3a, and 4¢-cyclohexylacetophenone 3b
suggest that radical pairs are able to explore several orientations
for the ortho- and para-coupling reactions to occur (Scheme 1).
It should be noted that formation of 3a and 3b requires
ketonization and oxidation of the initially formed coupling
products. The initial coupling at the ortho- and para-positions
followed by ketonization of resulting enols leads to 2,4- and
1,4-cyclohexadienone intermediates 3a-H2 and 3b-H2 (Scheme 2).
While no evidence could be obtained for these intermediates by
solid state 13C CPMAS NMR (vide infra), control experiments with
samples of commercial 1,4-cyclohexadiene in NaY showed that
oxidation and aromatization takes place spontaneously within the
time scale of the photochemical reaction and analysis. The results
from a series of runs under various experimental conditions are
shown in Table 1. Notably, photochemical experiments carried out
for 6 h with areated, O2-purged, and Ar-puged samples showed
relatively modest differences, suggesting that chemisorbed oxygen
may be the oxidant.20 Given that the spin density at the ortho-
and para-positions are relatively similar,21 the yields of 3a and 3b
suggest that ortho-coupling is sterically hindered as compared to
para-coupling. One may also speculate that ortho-coupling should
be kinetically disfavored with respect to coupling with the nearby
ketyl position. Although the uncertainty of our measurement is
relatively high at low conversion values (17%), the relative yields of
3 (a and b) as compared to 2 are relatively constant for conversion
values of 56% and 95%. Combination at the ketyl radical center
to form 2 accounts for ca. 42% and combination at the ortho-
and para-carbons to form 3a and 3b accounts for ca. 48% of the
products formed.
Solid state 13C-CPMAS NMR studies
Reaction progress was documented in situ by solid state NMR
measurements by comparing the spectra before and after several
irradiation periods. Samples were prepared with acetophenone
labeled with 13C at the carbonyl and methyl carbons in order to
have sufficient signal intensity and chemical resolution to detect
the starting material, potential intermediates, and final products.
The 13C-CPMAS spectra of acetophenone and cyclohexane coad-
sorbed in NaY acquired before photolysis, and after 2.0 and
5.5 hours of UV irradiation are illustrated in Fig. 2. The spectrum
of acetophenone and cyclohexane before photolysis exhibits 13C
peaks at 27 ppm and 208 ppm. Independent measurements with
samples of the two components (Fig. 2) confirmed that high field
peak belongs to the overlapping signals of cyclohexane and the
methyl group of acetophenone (Fig. 1a). The signal at 208 ppm
corresponds to the carbonyl carbon and the natural abundance
aromatic signals are not visible in this experiment. Evidence of
reaction is clearly visible after 2.0 h of irradiation (Fig. 2b)
with a signal belonging to the tertiary alcohol of 2-cyclohexyl-
2-phenylethanol 2 clearly visible at 78 ppm along with some
broadening of the methyl and carbonyl signals. Further irradiation
increased the intensity of the carbinol carbon in 2 and led to
changes in the carbonyl region.
Table 1 Photoproduct distribution from acetophenone and cyclohexane
in solution and in the zeolite NaY
Photoproducts (%)a
Reaction
medium
Irradiation
time/h
% Conversion
2
3a, 3b
4
Cyclohexane
NaY, aerated
NaY, aerated
NaY, aerated
2
3
6
9
10
17
56
95
—
4
22
41
—
10
—
—
—
1, 12
5, 29
7, 45
Fig. 2 13C CPMAS NMR spectra (75.47 MHz) of 13C CO- and
Me-labelled acetophenone and cyclohexane co-adsorbed in zeolite NaY
(a) before photolysis, (b) after 2 hours of photolysis (17% conversion),
(c) after 5.5 hours of photolysis (40% conversion).
a % Photoproducts are based on the relative ratio obtained from the GC
data with an estimated error 3. The photoproduct 2 was characterized
based on comparison of retention time values with those of a sample
independently synthesized. The photoproducts 3a, 3b and 4 were char-
acterized by comparing the GC-MS data with the literature (ref. 13–15).
Mass balances are typically in the 65–85% range.
A decrease in the intensity of the signal at 208 ppm was accom-
panied by the appearance of a broad peak at ca. 216 ppm, which
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is consistent with the formation of 4¢-cyclohexylacetophenone 3b
along with a small amount of the ortho-isomer 3a. Deconvolution
of the carbonyl signals in terms of two Lorentzian functions
revealed that the reactant carbonyl had broadened from 516 Hz
before photolysis (Fig. 2a) to 944 Hz after 5.5 h irradiation
(Fig. 2c). This broadening is indicative of an increased hetero-
geneity, which leads us to suggest that molecular motion within
the zeolite cage is more restricted once the products are formed.
We analyzed the peak at 27 ppm in the spectrum obtained after
5.5 h of irradiation using the program dmfit22 as a superposition
of lines in order to account for the two major products (2 and
3a) and the two reactants. The cyclohexane peak in Fig. 3b
can be fitted to a Lorentzian with a shift of d = 27.56 ppm
and a line width of 34.95 Hz. Keeping those parameters for
cyclohexane constant, and modeling the peak around 27 ppm
in Fig. 2a (corresponding to cyclohexane and acetophenone) as
a superposition of two Lorentzians, we obtained a shift of d =
25.91 and width of 197 Hz for acetophenone. We found that the
experimental spectrum could not be reproduced by these values
as well as the literature values of the chemical shift product 3b
(d = 26.55) constant. A satisfactory fit was obtained by fitting
co-added cyclohexane and acetophenone with a line width of
299 ppm, and the two products with line widths of 533 and 234 Hz,
respectively. The estimated errors in this model are relatively small
( 0.01–0.2 ppm and 8–30 Hz) and the spectrum is relatively well
simulated. While highly tentative, this analysis suggests a nearly 10-
fold increase in the line width of cyclohexane and a 50% increase
in the line width of the methyl group of acetophenone, as expected
for a highly anisotropic sample. In further agreement with this
conclusion we determine that the approximate line width of the
static spectra increases from 486 Hz before photolysis to 782 Hz
after 40% reaction.
The relative rigidity and mobility of the two reactants within
the cavity of the zeolite NaY was probed in a qualitative manner
with 13C CPMAS NMR measurements using acetophenone-d8
and natural abundance cyclohexane. The cross polarization (CP)
experiment involves the selective excitation of the 1H nuclei
followed by dipolar magnetization transfer to the less abundant
13C. A reasonable analogy may be drawn between the dipolar
interactions in the nuclear CP experiment and those that dictate
the Forster type energy transfer mechanism (FRET).9,10 The
rate and efficiency of cross polarization rely on the strength of
1
the H-13C dipolar coupling, which depends on their distances
and orientations. The most interesting difference between the
magnetic and electronic experiments is that a dismal spectral
1
overlap between H and 13C, with resonance frequencies at 300
and 75 MHz (in our instrument), is converted into a perfect
frequency match at fields that correspond to ca. 40 kHz by using
the spin-locking RF pulse established by Hartmann and Hahn.23
1
Under these conditions, isoenergetic H-13C coupled transitions
transfer the magnetization from the abundant 1H to the rare
13C over time scales that vary from 0.1 to 10 ms. While most
13C CPMAS experiments involve intramolecular magnetization
transfer, our experiment relies on the transfer of magnetization
from one molecule to the other.
As a reference point we first acquired a 13C CPMAS spectrum of
natural abundance acetophenone and cyclohexane coadsorbed in
zeolite NaY. A relatively noisy spectrum was obtained in 7200
scans with a cross polarization time (contact time) of 1.5 ms
(Fig. 3b). The peak at 27 ppm is attributed to the methyl group
of acetophenone and the equivalent carbons of cyclohexane.
Although the two signals overlap, they have significantly different
line widths. Two broad signals at 127 ppm and 145 ppm correspond
to the aromatic carbons but the signal of the carbonyl carbon is
not visible with the unlabeled samples. The spectrum in Fig. 3b
was obtained with a sample prepared with acetophenone-d8 and
natural abundance cyclohexane using a cross polarization time of
5 ms and 31 900 scans. Since no signals from acetophenone were
observed despite the longer contact time and the greater number of
transients, we conclude that there is no significant dipolar coupling
between acetophenone and cyclohexane, and that intramolecular
dipolar coupling in the cyclocarbon is very weak. Given that the
two molecules share the same supercages for the reaction to take
place, we conclude that the local environment must be highly fluid
so that the lifetime of a given acetophenone–cyclohexane pair must
be sufficiently long for hydrogen transfer in the excited state (ca.
>10-9 s), but shorter than the timescale needed for magnetization
transfer, which is ca. 10-3 to 10-2 seconds. A great deal of rotational
freedom is also suggested by the sharp line width of cyclohexane in
Fig. 3 and by the low intensity of its signal, even after 31 900 scans,
as expected for a very inefficient intramolecular cross polarization.
When the photochemical and CPMAS results obtained with
acetophenone are compared to those previously determined for
benzophenone, one may conclude that a relatively small difference
in the volume of the two ketones translates in substantial fluidity
changes in the zeolite. To support this, one can estimate the average
number of reactant molecules that can fit within a supercage by
considering their van der Waals volume with respect the volume
of the supercage. The molecular volumes (Vm) of benzophenone,
acetophenone and cyclohexane can be estimated with a van der
Waals increment approach first suggested by Bondi24 with the
Fig. 3 13C CPMAS spectra (75.47 MHz) of (a) acetophenone and
cyclohexane co-adsorbed in zeolite NaY obtained with a contact time of
1.5 ms and 7200 scans, (b) acetophenone-d8 and cyclohexane co-adsorbed
in zeolite NaY obtained with a contact time of 5 ms and 31 900 scans.
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values later refined by Gavezzotti.25 The method assigns van
der Waals volume to different molecular components (methyls,
methylenes, aromatic methines, carbonyls, hydroxyls, halides, etc.),
which are added together to approximate the total volume of the
Notes and references
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3
3
molecule.24,25 Values (VM) of 177 A and 120 A are obtained
˚
˚
for benzophenone and acetophenone, respectively, and a value of
3
˚
101 A for cyclohexane. The average free volume of the supercages
3
of NaY is reported26 to be VSC = 839 A . To estimate the number
˚
of molecules that may be accommodated within a supercage,
we assume the known packing coefficient of benzene in NaY
as an upper limit. The packing coefficient, CK, was defined by
Kitaigorodskii27 as the ratio between the filled volume, based
on the van der Waals molecular volume, and the total available
volume. In the case of benzene, it has been determined that, on
average, there are 5.4 benzene molecules per supercage.22 With
3
˚
a van der Waals volume of 85.4 A per benzene molecule, the
packing coefficient of benzene becomes CK = (5 ¥ 85.4)/839 =
0.55. Assuming the same packing coefficient for the reactants in
our study, one may expect that supercage occupancies will occur
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with a number of molecules that have a total added volume VTOT
ª
3
˚
VSC * CK = 462 A . While this volume is surpassed slightly with
one benzophenone and three cyclohexane molecules for a VTOT
=
3
˚
478 A , an analogous occupancy in the case of acetophenone
3
˚
would occur with a VTOT = 421.4 A , which could help explain the
significantly higher molecular and radical pair rotational fluidities.
Conclusions
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where the time scales for molecular and intermolecular degrees
of freedom are strongly reorganized as compared to solution
and most other reaction media. While slow diffusion allows
for reactants and product to enter and exit the zeolite interior,
bimolecular reactions take advantage of relatively long-lived
cage effects. While reactions derived from random encounters
of free radicals are efficiently prevented, the options available to
the geminate radical pair depend on how the latter fits within
the rigid environment of the zeolite supercages. Intermolecular
cross polarization experiments indicate that acetophenone and
cyclohexane have a significantly enhanced rotational freedom
as compared with that of benzophenone and cyclohexane. The
greater rotational fluidity allows the radical pair derived from the
former reactant couple to explore reaction pathways that are not
available to the latter.
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Acknowledgements
This research was supported by NIH grant MARC U*STAR
2T34GM008415-16, Mount St. Mary’s College Professional De-
velopment Grant #1311-11, and NSF grant CHE0551938. We
thank Drs R. Taylor and J. Strouse from UCLA for help and
advice.
27 A. I. Kitaigorodskii, Molecular Crystals and Molecules, Academic
Press, New York, 1973.
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