Chemoselective hydroperoxidation of alkenylarenes within thionin-supported zeolite Na-Y

Article abstract of DOI:10.1016/S0040-4039(01)00739-0

Photooxidation of several 1-aryl-2-methylpropenes within the thionin-supported zeolite Na-Y affords the ene type allylic hydroperoxides in a highly chemoselective manner. This is in contrast to their photooxygenation reaction in solution, where the ene pathway is less favorable compared to the [4+2] or [2+2] pathways. Cation-π interactions are likely to be responsible for this selectivity.

Full text of DOI:10.1016/S0040-4039(01)00739-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 4545–4547
Chemoselective hydroperoxidation of alkenylarenes within
thionin-supported zeolite Na-Y
Manolis Stratakis* and Constantinos Rabalakos
Department of Chemistry, University of Crete, 71409 Iraklion, Greece
Received 8 March 2001; accepted 2 May 2001
Abstract—Photooxidation of several 1-aryl-2-methylpropenes within the thionin-supported zeolite Na-Y affords the ene type
allylic hydroperoxides in a highly chemoselective manner. This is in contrast to their photooxygenation reaction in solution, where
the ene pathway is less favorable compared to the [4+2] or [2+2] pathways. Cation-p interactions are likely to be responsible for
this selectivity. © 2001 Published by Elsevier Science Ltd.
The photooxygenation of organic compounds adsorbed
within the dye-supported zeolite Na-Y1 and other con-
strained media2 has recently attracted considerable
attention primarily due to the significant enhancement
of product selectivity. So far it is recognized that in the
intrazeolite (Na-Y) photooxygenation of geminal
dimethyl trisubstituted alkenes, formation of the new
double bond in the ene adduct occurs preferentially at
the methyl groups3 (Markovnikov selectivity). Specific
deuterium labeling of the anti methyl group in a series
of such trisubstituted alkenes, independently by us4 and
Clennan,5 showed that the well known ‘cis effect’6
selectivity in solution does not operate within the zeo-
lite. It was proposed that electrostatic interaction
between the intermediate perepoxide and an alkali
metal cation7 plays a significant role in dictating the
distribution of the ene products. Similarly, in contrast
to the reaction in solution, an anti ‘cis effect’ selectivity
was found in the intrazeolite photooxygenation of 1-
methylcycloalkenes.8
example, b,b-dimethylstyrene (1) affords10 the ene
adduct 1a in approximately 20% yield, benzaldehyde
(1b), and mainly the two diastereomeric di-
endoperoxides11 1d and 1e in a 68/32 ratio; di-endoper-
oxides arise from initial [4+2] addition of 1O2 to 1,
followed by a second addition of 1O2 to the newly
formed diene endoperoxide 1c (Scheme 1). Also, for
1-(2-methylpropenyl)naphthalene (2), originally it was
reported that only the 1,4-endoperoxide 2b is formed.12
However, the same group reported later13 that the ene
adduct 2a is also formed in various yields (24–40%)
depending on solvent polarity (Scheme 1). We repeated
this reaction (CH2Cl2/methylene blue) and found that
the ene product is formed in low yield (around 15%).
Intrazeolite photooxygenation of alkenylarenes 1–6
within the thionin-supported zeolite Na-Y afforded sur-
prisingly the ene adducts 1a–6a14 as the major or only
products (Table 1). This is in contrast to their reactions
in solution where the allylic hydroperoxides are formed
as secondary adducts (5–20% yield).15 In a typical
experiment, 10 mg of the alkenylarene dissolved in 10
mL of hexane were added into a tube containing 1 g of
dry thionin-supported Na-Y.16 The tube was immedi-
ately irradiated for 5 min under constant flow of oxy-
gen. Short irradiation time and low loading levels of the
alkene into zeolite are the optimum conditions for
performing intrazeolite photooxygenations.3b,5 After
extractive work up as described earlier,4 the solvent was
removed under a vacuum and the products were ana-
As part of our continuing interest in the regioselectivity
and stereoselectivity of singlet oxygen reactions in
homogeneous and heterogeneous media, we report in
this communication our preliminary results on the
intrazeolite photooxygenation of 1-aryl-2-methylprop-
enes. It is well known that reaction of alkenylarenes
with 1O2 in solution affords products arising mainly
from [4+2] or [2+2] addition to the double bond.9 The
ene pathway is less favorable or even absent. For
1
lyzed by H NMR.
Apart from the ene allylic hydroperoxides, minor
amounts (3–6%) of aldehydes resulting from the cleav-
age of the alkene double bond are also produced.
Keywords: zeolite Na-Y; ene reactions; singlet oxygen; zwitterions.
* Corresponding author. Fax: +30-81-393601; e-mail: stratakis@
chemistry.uoc.gr
0040-4039/01/$ - see front matter © 2001 Published by Elsevier Science Ltd.
PII: S0040-4039(01)00739-0

4546
M. Stratakis, C. Rabalakos / Tetrahedron Letters 42 (2001) 4545–4547
Formation of the aldehydes probably arises via a non-
photochemically induced electron-transfer pathway,
which competes with the energy-transfer pathway that
produces singlet oxygen.17 It is interesting that the
substrates had been completely consumed within 5 min-
utes of irradiation, whereas the same amounts of
alkenylarenes (ꢀ10 mg) in solution require reaction for
1–3 h to go to completion. The mass balances, as
Scheme 2.
estimated by GC using several calibration compounds
(2-phenylethanol, 2,6-di-tert-butyl-p-cresol or trans-
decahydronaphthalene) depending on the substrate,
were >85% for substrates 1 and 4–6, whereas for 2–3
they were ꢀ75%. Equimolar amounts of the alkenyl-
arene and the appropriate calibration compound were
subjected to reaction. After the workup, the reaction
1
mixtures were analyzed by H NMR or alternatively
triphenylphosphine was added to reduce the hydroper-
oxides to alcohols and the relative amounts of the
alcohols versus the calibration compounds were found
by GC. To establish further our results, the four prod-
1
ucts from the reaction of O2 with 1 in solution (ene,
benzaldehyde and the two di-endoperoxides) were
added to thionin-supported Na-Y and then irradiated
under a constant flow of oxygen for 5 minutes. After
1
extractive workup and analysis by H NMR, the ratio
of the oxygenated adducts before and after irradiation
was almost unchanged.
Scheme 1.
As a reasonable mechanistic rationalization for the
observed chemoselectivity, we consider that the [4+2]
and [2+2] products in the reactions of arylalkenes with
singlet oxygen are formed in a step-wise manner, via a
common open 1,4-zwitterionic intermediate.18 On the
other hand, the ene products arise from a perepoxide
intermediate or an exciplex with the structural require-
ments of a perepoxide.1d,6 It is reasonable that Na+
binding to the electron rich aryl ring19 within the cages
of Na-Y results in destabilization of the open zwitteri-
onic intermediate (ZI), because the aryl ring upon
complexation is losing electron density and is less able
to stabilize the positive charge at the benzylic position
of ZI, as shown in Scheme 2 for substrate 1. On the
other hand, in the perepoxide intermediate the benzylic
carbon has only a partial positive charge;10 thus, com-
plexation of the aryl ring to the cation is expected to
cause significantly less destabilization compared to the
zwitterion.
Table 1. Photooxygenation of alkenes 1–6 within the
thionin-supported zeolite Na-Y
a
Di-endoperoxides 1d and 1e are also formed in 8% combined yield.
An alternative explanation of the product chemoselec-
tivity could be that the simultaneous complexation of
the Na+ to the aryl ring and to the alkene double bond
within the cages, places the aryl group at an inappropri-
ate conformation for a concerted [4+2] reaction to
occur; the alkene double bond and the aryl ring cannot
b For compound 5 the reaction was performed on a 50 mg scale using
2.5 g of the supported zeolite and the pure ene hydroperoxide was
isolated in 88% yield.
c A minor product, not found in the reaction with 1O2 in solution, is
also formed in 12% yield. This product was not isolated or charac-
terized.

M. Stratakis, C. Rabalakos / Tetrahedron Letters 42 (2001) 4545–4547
4547
be coplanar. This rationalization, however, is less likely,
since it requires a synchronous mechanism for 1O2
addition to the diene moiety of the alkenylarene form-
ing the endoperoxide.
Press: Boca Raton, FL, 1985; pp. 120–133.
10. Stratakis, M.; Orfanopoulos, M.; Foote, C. S. J. Org.
Chem. 1998, 63, 1315–1318.
11. Matsumoto, M.; Dobashi, S.; Kuroda, K. Tetrahedron
Lett. 1977, 3361–3364.
Similar lack of formation of [4+2] products, in contrast
to their reaction in solution, was reported in the photo-
oxygenation of stilbene and 4,4%-dimethoxystilbene.20
The 1,2-dioxetane was formed instead and the selectiv-
ity was attributed to an electron transfer mechanism.
12. Matsumoto, M.; Kondo, K. Tetrahedron Lett. 1975,
3935–3938.
13. Matsumoto, M.; Kuroda, K. Synth. Commun. 1981, 11,
987–992.
14. 1H NMR data of the allylic hydroperoxides; compound
1a: 7.95 (s, 1H, OOH), 7.30–7.38 (m, 5H), 5.38 (s, 1H),
5.13 (br. s, 1H), 5.08 (br. s, 1H), 1.69 (s, 3H). Compound
2a: 8.21 (s, 1H, OOH), 8.14 (d, J=8.3 Hz, 1H), 7.89 (d,
J=7.8 Hz, 1H), 7.86 (d, J=8.3 Hz, 1H), 7.58 (d, J=7.1
Hz, 1H), 7.48–7.55 (m, 3H), 6.15 (s, 1H), 5.21 (br. s, 2H),
1.79 (s, 3H). Compound 3a: 8.09 (s, 1H, OOH), 7.84–7.87
(m, 4H), 7.46–7.51 (m, 3H), 5.55 (s, 1H), 5.20 (br. s, 1H),
5.13 (br. s, 1H), 1.74 (s, 3H). Compound 4a: 8.03 (s, 1H,
OOH), 7.59–7.62 (m, 4H), 7.43–7.49 (m, 4H), 7.36 (t,
J=7.3 Hz, 1H), 5.44 (s, 1H), 5.18 (br. s, 1H), 5.12 (br. s,
1H), 1.75 (s, 3H). Compound 5a: 8.09 (s, 1H, OOH), 7.64
(d, J=8.1 Hz, 2H), 7.49 (d, J=8.1 Hz, 2H), 5.44 (s, 1H),
5.12 (br. s, 1H), 5.11 (br. s, 1H), 1.70 (s, 3H). Compound
6a: 7.99 (s, 1H, OOH), 7.29 (d, J=8.5 Hz, 2H), 6.90 (d,
J=8.5 Hz, 2H), 5.33 (s, 1H), 5.14 (br. s, 1H), 5.08 (br. s,
1H), 3.81 (s, 3H), 1.70 (s, 3H).
Further work to determine the relative reactivity of the
syn and anti methyl groups of a series of geminal
dimethyl alkenylarenes are under way in order to shed
more light on the mechanistic rationalization of this
highly chemoselective ene reaction.
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