Enhanced Photodecarboxylation of an Aryl Ester in Polyethylene Films

Article abstract of DOI:10.1021/ol035852t

(Matrix presented) Photodecarboxylation is the exclusive photoreaction of 2,4,6-trimethylphenyl (S)-2-methylbutanoate In unstretched high-density polyethylene films. The sole product, (S)-1-(2-methylpropyl)-2,4,6- trimethylbenzene, is formed with complete

Full text of DOI:10.1021/ol035852t

ORGANIC
LETTERS
2
003
Vol. 5, No. 24
661-4664
Enhanced Photodecarboxylation of an
Aryl Ester in Polyethylene Films
4
Tadashi Mori,*,†,‡ Yoshihisa Inoue,†,§ and Richard G. Weiss*,‡
Department of Molecular Chemistry, Osaka UniVersity, 2-1 Yamada-oka,
Suita 565-0871, Japan, Entropy Control Project, ICORP, JST, 4-6-3 Kamishinden,
Toyonaka 560-0085, Japan, and Department of Chemistry, Georgetown UniVersity,
Washington, D.C. 20057-1227
tmori@chem.eng.osaka-u.ac.jp; weissr@georgetown.edu
Received September 24, 2003
ABSTRACT
Photodecarboxylation is the exclusive photoreaction of 2,4,6-trimethylphenyl (S)-2-methylbutanoate in unstretched high-density polyethylene
films. The sole product, (S)-1-(2-methylpropyl)-2,4,6-trimethylbenzene, is formed with complete retention of stereochemistry. In other polyethylene
films, organic solvents, and â-cyclodextrin cavities, cage-escaped products derived from Fries-type bond scission are obtained as well. The
results indicate the importance of the media in controlling the conformations of aryl esters and, thereby, their photoreactions.
The photochemistry of aryl esters, especially photo-Fries
rearrangements, has been studied extensively. In several
detail, and we have recently used the stereochemical course
of a chiral aryl ester to establish that decarboxylation is a
completely concerted process.
1
5
reports, we have focused on the excited singlet state
2
photodecarboxylations of these compounds, usually a minor
Supramolecular photochemistry continues to be exploited,
and a large number of interesting hosts or confining media
pathway, and have investigated ways to increase its impor-
3
4
6
tance. In that regard, Finnegan and Knutson have demon-
strated that the relative yields of decarboxylation products
can be increased markedly when the o- and p-positions of
the aromatic ring are substituted by methyl or other alkyl
groups. This methodology has allowed us to study the
mechanism of photodecarboxylation of aryl esters in great
have been described. Among them, zeolites have been
exploited extensively because their cationic sites can interact
strongly with a guest molecule and their stiff, well-defined
cavity walls allow relatively facile interpretations of results
7
from reactions of the guests. Although the diversity of sizes
and shapes of cyclodextrin cavities are rather limited, their
ease of handling, ready availability, and chirality have led
to their extensive use as hosts for photochemical reactions
†
Osaka University.
Georgetown University.
ICORP, JST.
‡
§
8
of guest molecules, including cases leading to photochemical
(
1) Miranda, M. A.; Galindo, F. In CRC Handbook of Organic
9
asymmetric induction. By contrast, the reaction cavities
Photochemistry and Photobiology, 2nd ed.; Horspool, W. M., Lenci, F.,
Eds.; CRC Press LLC: Boca Raton, FL, 2004; Chapter 42, in press.
Miranda, M. A.; Galindo, F. In Molecular and Supramolecular Photo-
chemistry; Ramamurthy, V., Schanze, K. S., Eds.; Marcel Dekker: New
York, 2003; Vol. 9, pp 43-131 and references therein.
afforded by polyethylene (PE) films are not as well-defined
(5) Mori, T.; Saito, H.; Inoue, Y. Chem. Commun. 2003, 2302-2303.
See also ref 4.
(
2) For a review on photodecarboxylation, see: Budac, D.; Wan, P. J.
Photochem. Photobiol. A 1992, 67, 135-166.
3) (a) Mori, T.; Takamoto, M.; Wada, T. Inoue, Y. Photochem.
Photobiol. Sci. 2003, in press. (b) Mori, T.; Wada, T.; Inoue, Y. Org. Lett.
(6) For examples, see: (a) Tung, C.-H.; Wu, L.-Z., Zhang, L.-P.; Chen,
B. Acc. Chem. Res. 2003, 36, 39-47. (b) Weiss, R. G., Ramamurthy, V.;
Hammond, G. S. Acc. Chem. Res. 1993, 26, 530-536.
(7) (a) Turro, N. J. Acc. Chem. Res. 2000, 33, 637-646. (b) Scaiano, J.
C.; Garcia, H. Acc. Chem. Res. 1999, 32, 783-793. (c) Ramamurthy, V.;
Garcia-Garibay, M. A. Comput. Supramol. Chem. 1996, 7, 693-719.
(
2
000, 2, 3401-3404.
4) Finnegan, R. A.; Knutson, D. J. Am. Chem. Soc. 1967, 89, 1970-
972.
(
1
1
0.1021/ol035852t CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/29/2003

product 3 was always present to an extent that varied with
the reaction conditions employed.
Scheme 1. Photoreactions of 2,4,6-Trimethylphenyl
S)-(-)-2-Methylbutanoate (1)
(
Chiral substrates have been employed to investigate the
mechanisms of a wide variety of photochemical reactions
that are thought to involve radical pair intermediates.12 For
example, we have investigated recently the photoreactions
of 1-naphthyl (R)-2-phenylpropanoate in PE films to reveal
the degree to which PE reaction cavities influence the
1
3
motions of pro-chiral radical pairs. Those radical pair
recombinations occur with significant enantiospecificity but
significantly less than complete stereospecificity. Thus, a
nonconcerted decarboxylation of 1 in PE films should not
be stereospecific; the observations that they are in PE films
supports their concerted nature in these confining media.
In our attempt to improve the yield of the photodecar-
boxylation product 2 from 1, we initially investigated the
influence of varying the solvent and temperature; however,
1
4
lysis leading to 3 could not be eliminated. For instance, in
methylcyclohexane, temperature dependence of the photo-
product yields indicated the importance of more than one
competing process: the relative yield of 2 was 60% at 90
°C and increased as temperature was lowered, reaching the
highest value, 87%, at ca. -10 °C and decreasing thereafter
(79% at -45 °C). At -80 °C, the rate of photoreaction was
too slow to be monitored. Regardless of the reason for this
effect, it indicates that modification of temperature alone is
unlikely to suppress the processes leading to 3 completely.
Thus, we investigated a different strategy, employing PE
films as the reaction medium, to improve the relative yields
in size or shape as those of zeolites or cyclodextrins, and
they exist as a distribution. The walls of the PE cavities can
exert only passive pressure on guest molecules during their
reactions. As a result, the selectivity generally expected of
reactions in PE films is less than in zeolites or cyclodex-
1
0
trins.
Here, we report that high reaction selectivity can be
achieved during irradiations of (S)-1-(2-methylpropyl)-2,4,6-
trimethylbenzene (1) in polyethylene films and that the course
of the reaction can be exclusively decarboxylation (2) when
the appropriate type of PE film, one of high density, is
employed. The results demonstrate that the reaction cavities
afforded by PE films can affect the conformations of 1 to
enhance the decarboxylation mode and to suppress products
from the lysis modes (3 and products not isolated from the
1
5
of 2. The PE films selected represent very different
morphologies that are indicated by their percent of crystal-
linity (given as a suffix to their PE acronyms).16
Evidence for the importance of the conformation of 1 on
the photochemical course of 1 is provided by DFT calcula-
tions for an isolated molecule. The energetically preferred
s-trans conformation (vide infra) cannot yield 2 unless
significant intramolecular motions occur in the excited singlet
state. The results in Table 1 indicate that the concentration
of the s-cis conformation is increased significantly within
the ground or excited singlet state of 1 when the ester is
placed in the confining reaction cavities afforded by PE
2-methylbutanoyl radical) (Scheme 1).
Some conformational control of decarboxylation has been
already achieved by irradiating 1-naphthyl esters in PE films
at varying temperatures.11 However, even under the most
favorable conditions, the major products were from photo-
Fries and related radical-rearrangement reactions. Previously,
we found that photodecarboxylation of 1 in solutions and
under a wide variety of conditions proceeds with complete
5
films. Furthermore, the importance of reaction cavity shape
(12) (a) Gao, F.; Boyles, D.; Sullivan, R.; Compton, R. N. Pagni, R. M.
J. Org. Chem. 2002, 67, 9361-9367. (b) Griesbeck, A. G.; Kramer, W.;
Lex, J. Angew. Chem., Int. Ed. 2001, 40, 577-579. (c) Bhanthumnavin,
W.; Bentrude, W. G. J. Org. Chem., 2001, 66, 980-990.
retention of configuration, indicating that extrusion of CO
2
is concerted and involves the spiro-lactonic transition state
4
(13) Xu, J.; Weiss, R. G. Org. Lett. 2003, 5, 3077-3080.
suggested previously. In these experiments, the phenolic
(14) Details will be discussed in full paper. See Supporting Information
for typical data.
(
8) (a) Ueno, A.; Ikeda, H. Mol. Supramol. Photochem. 2001, 8, 461-
(15) Films employed in this study are an amorphous polymer from
DuPont-Dow Elastomers (PE0; Nordel IP 3430 containing 42.5 wt %
ethylene, 57 wt % propylene, 0.5 wt % ethylidenenorbornene), low-density
PE (PE46; Sclairfilm SL-1) from DuPont of Canada and linear low-density
PE (PE50; LL-3001.63) from Exxon Chemical Co., and high-density PE
from Polialden Petroquimica, Brazil (PE68; type ES-300) or from Exxon
Chemical Co. (PE74; Exxon HDPE 7745.10). Films were cold-stretched
by hand to ca. three (PE46 and PE50) or four (PE68 and PE70) times
their original lengths. All films were extracted with chloroform at least
three times each before use to remove antioxidants, plasticizers, and other
additives, rinsed with fresh hexanes, and dried in vacuo.
5
03. (b) Bortolus, P.; Monti, S. AdV. Photochem. 1996, 21, 1-133. (c)
Bortolus, P.; Grabner, G.; Koehler, G.; Monti, S. Coord. Chem. ReV. 1993,
1
2
25, 261-268. (d) Ramamurthy, V.; Eaton, D. F. Acc. Chem. Res. 1988,
1, 300-306.
(9) (a) Sivaguru, J.; Natarajan, A.; Kaanumalle, L. S.; Shailaja, J.; Uppili,
S.; Joy, A.; Ramamurthy, V. Acc. Chem. Res. 2003, 36, 509-521. (b) Turro,
N. J. Proc. Nat. Acad. Sci. U.S.A. 2002, 99, 4805-4809. (c) Brunet, E.
Chirality 2002, 14, 135-143. (d) Constable, E. C.; Housecroft, C. E. Chimia
1
999, 53, 187-191. (e) Inoue, Y. Chem. ReV. 1992, 92, 741-770.
(10) (a) Gu, W.; Warrier, M.; Ramamurthy, V.; Weiss, R. G. J. Am.
Chem. Soc. 1999, 121, 9467-9468. (b) Gu, W.; Warrier, M.; Schoon, B.;
(16) By X-ray diffraction with MDI Jade 5 software for X-ray diffraction
pattern processing from Materials Data, Inc., Livermore, CA. Also see: (a)
Matthews, J. L.; Peiser, H. S.; Richards, R. B. Acta Crystallogr. 1949, 2,
85-90. (b) Reference 13.
Ramamurthy, V.; Weiss, R. G. Langmuir 2000, 16, 6977-6981.
(11) Gu. W.; Abdallah, D. J.; Weiss, R. G. J. Photochem. Photobiol. A
2
001, 139, 79-87.
4662
Org. Lett., Vol. 5, No. 24, 2003

Table 1. Photodecarboxylation of 2,4,6-Trimethylphenyl (S)-(-)-2-Methylbutanoate 1 in Polyethylene Filmsa
[
1]
film thickness
T
irrad time
(min)
conv
(%)
product distrib
ee
mass balance
(%)
reaction mediumb
(mmol (kg film)-1
)
(µm)
(°C)
(2: 3)
(%)
3
P E0(u)
P E46(u)
2.0
2.7
1.2 × 10
23
2
60
90
40
28.0 ( 5.7
22.6 ( 1.0
12.9 ( 1.3
48.3 ( 9.6
26.3 ( 3.2
11.2 ( 1.8
30.5 ( 5.4
14.1 ( 1.6
28.1 ( 4.9
17.4 ( 3.0
30.8 ( 4.7
18.5 ( 2.3
19.9 ( 3.3
43
81.7:18.3
89.7:10.3
75.5:24.5
71.6:28.4
48.9:51.1
57.7:42.3
65.1:34.9
38.2:61.8
37.1:62.9
98.8:01.2
82.6:17.4
98.8:01.2
80.1:19.9
64.0:36.0
82.1:17.9
>98
>98
>98
>98
>98
>98
>98
>98
>98
>98
>95
>98
>98
92
89
93
64
83
97
78
87
74
90
88
88
90
63
99
82
2
3
6
0
6
5
20
40
P E46(s)
2.7
34
23
6
0
P E50(u)
P E50(s)
P E68(u)
P E68(s)
P E74(u)
P E74(s)
â-CD in H2O
methylcyclohexane
5.0
5.0
19
19
12
12
(0.5 mM)
(3.1 mM)
25
16
21
12
17
13
(1 cm)
(1 cm)
23
23
23
23
23
23
25
25
40
40
60
60
60
60
20c
5c
(2.0)d
>99e
15.6 ( 0.4
a
Films were immersed in dichloromethane solutions of the ester (ca. 10 mM) for 3 min to 3 h to obtain the desired concentrations, dried, sealed in
evacuated quartz cuvettes (unless noted otherwise), and irradiated (low-pressure Hg lamp; ethanol filter; λ ) 254 nm). Photoproducts and remaining 1 were
removed by exhaustive extraction and analyzed by HPLC using a DAICEL OJ-H column. See Supporting Information Figure S-3. Each datum is the average
of at least three analyses on each of three different films. “u” and “s” refer to unstretched and cold-stretched films, respectively; see text. c Quartz filter for
b
d
e
irradiations. Racemic ester as substrate. By chiral GC with a SUPELCO â-DEX 325 column (0.25 µm; 30 m).
on the reaction course of 1 (and the conformations imposed
upon it by the PE films) is demonstrated by the marked
decreases in the relative yields of the decarboxylation product
2/3 ratios of PE46(u) and PE50(u), films of nearly the same
crystallinity, perhaps the similarity between the ratios in
methylcyclohexane and PE0 is fortuitous.
2
when the films are stretched. It is known that film
As mentioned above, film stretching leads to decreased
relative yields of the photodecarboxylation product 2. Film
stretching of partially crystalline PE films redistributes
microcrystallites more evenly within the amorphous domains,
increases the area of the interfacial region, decreases mean
hole free volumes, and increases the fraction of guest
molecules in interfacial cavities. Despite these perturba-
tions, the formation of 2 remains stereospecific and, presum-
ably, concerted; a radical mechanism must not be involved,
even in the confining reaction cavities of the various
unstretched and stretched PE films.
stretching causes a net migration of guest molecules such
as 1 from reaction cavities in amorphous regions to cavities
in the interfacial regions (along the lamellar surfaces of
17
polyethylene crystallites). These results may be compared
with those obtained from irradiations in the isotropic solvent,
methylcyclohexane, and in aqueous solutions of â-cyclo-
dextrin,1 where the relative yields of 2 are less than the
optimal ones obtained in PE. However, there is no clear trend
between PE crystallinity or other single physical property
of the films (see Table S-1 of Supporting Information) and
the relative yields of 2. The natures of the reaction cavities
of each film, as well as the effect that they have on the
conformations of 1, must be determined by a combination
of the properties. However, the two high density PE films
in their unstretched state yield only trace amounts of 3; the
oVerwhelming product is 2, and its ee is >98%. The reaction
cavities of the two high-density PE films either force all of
the molecules of 1 to adopt s-cis conformations, or they
prohibit the radical pairs from the alternative reaction
pathway leading eventually to 3 to recombine and reform 1.
Whether the decarboxylation pathway is promoted or the
escape of radical pairs is attenuated, the result is the same.
In all cases, the material balance is quite high at <30%
conversion (where most of the irradiations were analyzed to
avoid secondary reactions). In completely amorphous PE0,
which lacks interfacial sites, the 2/3 ratio was quite similar
to those obtained in methylcyclohexane solution at a
comparable temperature. Given the large differences in the
1
7
8
We were unable to obtain reliable fluorescence spectra of
1 in all of the PE films except PE0 at a high doping
-
1
concentration, ∼50 mmol (kg film) (see Supporting
Information Figure S-1). The shapes of the absorption and
the excitation spectra of 1 are almost the same in methyl-
cyclohexane and PE films, although there appears to be some
self-absorption in the former. However, the dynamics of the
fluorescence decays (λex 266 nm; λem 290 nm) in methyl-
cyclohexane (single-exponential decay of 1.6 ns) and PE0
(two components of 2.9 ns (37%) and 4.8 ns (63%)) suggests
that the distributions of conformations for 1 differs in the
two media.
DFT calculations of 1 at the B3LYP/6-31(d) level19
identify four possible conformational minima (See Support-
ing Information Table S-2). Among them, the two s-cis
conformers are potential immediate precursors of the spiro-
lactonic transition state required for concerted decarboxy-
(
18) Some chiral induction (2.0% ee) was obtained in 2 when racemic
(
17) Wang, C.; Xu, J.; Weiss, R. G. J. Phys. Chem. B 2003, 107, 7015-
ester 1 was irradiated in aqueous â-cyclodextrin. Details will be presented
elsewhere.
7
025.
Org. Lett., Vol. 5, No. 24, 2003
4663

and excited states may affect the 2/3 ratio. In PE0 (and the
/
other PE films), it is reasonable to assume that k , k
2
and
Scheme 2. Possible Kinetic Steps Involved in s-cis and
s-trans Equilibration and Photodecarboxylation
1
/
k
2
, k ′. Assuming the reaction cavities of the high
-
1
density PE films do not affect the values of k-1 and k-1′,
they may promote formation of the s-cis isomer (i.e., larger
values of K) and faster reversion of the radical pair to starting
22
1 3
ester 1 (i.e., increased k /k ratios). Although both probably
contribute to the observed nearly exclusive formation of 2,
we suspect that the major influence is on the value of K.
To understand better these processes, the influence of
temperature on the irradiation of 1 in PE46(u) and in
methylcyclohexane has been compared. Within the range
2-65 °C, the changes of ln(2/3) with respect to the reciprocal
of temperature are linear in the polymer, but they are clearly
nonlinear in the isotropic paraffin (See Supporting Informa-
tion Figure S-2). In the latter medium, a linear correlation
exists over only a part of the temperature range explored,
suggesting that different steps in Scheme 2 may be rate-
determining at different temperatures. Additional studies will
be required to analyze the specific reasons for the distinctly
different temperature dependencies in PE and methylcyclo-
hexane.
lation. All of the conformers have significantly larger
molecular volumes than the mean free void volumes of the
In conclusion, photodecarboxylation of the ester 1 can be
made the nearly exclusive reaction pathway by performing
the irradiations in a high density PE film. The lack of the
competitive products from photo-Fries-type lyses can be
rationalized on the basis of ground-state conformational
control of 1 or by suppression of escape of radical pairs
(forcing their recombination and reformation of 1) by the
PE reaction cavities. Ancillary evidence suggests that con-
formational control is the more important factor, but ad-
ditional experiments will be required to determine the extent
that radical-pair recombinations contribute. The environ-
mentally benign and inexpensive nature of the PE media and
the complete retention of chirality in the photodecarboxylated
product justify further exploration of the synthetic potential
of PE-mediated asymmetric photoreactions.
3
20
PE films (113-177 Å ), and therefore it seems probable
that the reaction cavities do apply severe constraints on the
shapes of the guest molecules. Although the calculated “gas
phase” distributions predict a ca. 0.1% equilibrium population
of s-cis conformers, they are more polar than the s-trans
conformers (See calculated dipole and quadrupole moments
in Supporting Information Table S-2) and may be preferred
2
1
within the somewhat more polarizable interfacial regions.
A possible mechanism involving the interconversion of
conformers and the possible photochemical reactions from
each (assuming no change of geometries within the short
excited-state lifetimes when irradiations are performed in the
constraining environments of the PE films) is illustrated in
Scheme 2. In low viscosity isotropic solutions, the equilib-
rium between s-cis and s-trans conformers in their ground
Acknowledgment. Financial support of this work by the
COE for Integrated EcoChemistry (to T.M.) and the National
Science Foundation (to R.G.W.) are gratefully acknowledged.
We thank Mr. Jinqi Xu for his help with several aspects of
the experiments.
(19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin, K.
N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.;
Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.;
Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li,
X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.;
Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.;
Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.;
Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels,
A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.;
Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.;
Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz,
P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
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B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03,
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Supporting Information Available: Schematic presenta-
tion of the possible conformations of 1 involved, tables of
physical properties of the films employed, DFT calculations
of possible conformers, figures of fluorescence spectra of 1
in methylcyclohexane and in PE0, the temperature depen-
dence of the 2/3 ratio in methylcyclohexane and in PE46(u),
and typical chiral HPLC chromatograms. This material is
available free of charge via the Internet at http://pubs.acs.org.
(
20) Gu, W.; Hill, A. J.; Wang, X.; Cui, C.; Weiss, R. G. Macromolecules
000, 33, 7801-7811.
21) (a) Lamotte, M.; Joussot-Dubien, J.; Mantione, M. J.; Claverie, P.
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(
Chem. Phys. Lett. 1974, 27, 515-520. (b) Lamotte, M.; Joussot-Dubien, J.
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1999, 64, 5156-5161.
4664
Org. Lett., Vol. 5, No. 24, 2003