Sonochemical reactions of lead tetracarboxylates with styrene

Full text of DOI:10.1021/jo981168u

J . Org. Chem. 1998, 63, 9561-9564
9561
Son och em ica l Rea ction s of Lea d
Sch em e 1
Tetr a ca r boxyla tes w ith Styr en e
Takashi Ando,*,† Takahide Kimura,†
†,‡,§
,‡
J ean-Marc Lev eˆ que,
J . Philip Lorimer,*
J ean-Louis Luche,*,§ and Timothy J . Mason‡
Department of Chemistry, Shiga University of Medical
Science, Seta, Otsu, Shiga 520-2192, J apan, School of
Natural and Environmental Sciences, Priory Street,
Coventry University, Coventry, CV1 5FB, UK,
and Laboratoire de Chimie Mol e´ culaire et
Sch em e 2
Environnement, Universit e´ de Savoie,
7
3376 Le Bourget du Lac Cedex, France
Received J une 17, 1998
In tr od u ction
Because of their mechanistic diversity, the reactions
of lead tetraacetate (LTA)1 with various substrates
constitute an ideal objective for sonochemical studies.2
An example of this is the reaction of LTA with styrene 1
in acetic acid solution or suspension which is able to
data also agree with a chain mechanism, easily inhibited
by oxygen or tert-butylcatechol.2
Notwithstanding this observation, many aspects of
these reactions, both conventional and sonochemical,
remain unclear. Thus, in the literature there are several
a,7-9
follow several mechanisms, each of which provides a
different product (Scheme 1).3 Reaction i implies an ionic
dissociation of the LTA oxidant, followed by an electro-
philic attack of the olefinic bond. Reaction ii involves a
preliminary one-electron oxidation of the olefin to a
references to this type of reaction, some run simply by
heating the styrene-LTA mixture,2
a,7
whereas others
4
report experiments performed in the presence of a base,
radical cation, although in this case it is also possible
2
b-d,8
for the reaction to follow a polar pathway.3b Whatever
the role of which is not clearly explained.
The
present study was undertaken to provide more mecha-
nistic elements, particularly with respect to the sonochem-
ical effect. A specific aim was to determine the experi-
mental conditions, sonochemical or conventional, that
might confer useful selectivity to the reaction. In
sonochemical reactions it is not firmly established where
the key step occurs, i.e., in the continuous liquid phase
or next to the cavitation bubble boundary, because
activation is not expected to occur inside the cavity
itself.2 It has been shown that the sonolysis of LTA is
extremely slow at temperatures up to 50 °C, leading to
the conclusion that the homolytic process can occur only
the mechanism, compound 3 is generally a minor con-
stituent of the mixture. The key step of reaction iii
consists of the addition to the double bond of a methyl
radical resulting formally from a homolysis of LTA. Most
of the published work shows that the pathway i is
predominant, although not exclusive.
Because one of the important features of sonication is
5
the enhancement of radical mechanisms, the oxidation
of styrene by LTA under ultrasound irradiation would
be expected to proceed with an increased selectivity in
c
favor of compound 4 (and possibly 3). This was indeed
observed,2a and this result constitutes an example of what
2
c
6
when LTA and styrene are present simultaneously, an
observation not explained by the currently accepted
mechanism.
has become known as sonochemical switching. All the
†
Shiga University of Medical Science.
School of Natural and Environmental Sciences.
Universit e´ de Savoie.
‡
§
Resu lts a n d Discu ssion
(1) For general accounts on LTA reactions, see: (a) Criegee, R. In
Oxidation in Organic Chemistry; Wiberg, K. B., Ed., Academic Press:
New York, 1965; Part A. (b) Sheldon, R. A.; Kochi, J . K. Org. React.
To detect possible ligand exchange reactions occurring
during the reaction (see below), lead tetrapropanoate
1
972, 19, 279-421. (c) Mihailovic, M. L.; Cekovic, Z.; Lorenc, L. In
Organic Synthesis by Oxidation with Metal Compounds; Mijs, W. G.;
De J onge, C. R. H. I., Eds.; Plenum Press: New York, 1986; pp 741-
8
(
2
LTP) was chosen as the reagent in place of LTA (Scheme
).
There is a distinct lack of accurate data concerning the
16.
(
2) For previous papers on this topic, see: (a) Ando, T.; Bauchat, P.;
Foucaud, A.; Fujita, M.; Kimura, T.; Sohmiya, H. Tetrahedron Lett.
991, 32, 6379-6382. (b) Ando, T.; Fujita, M.; Bauchat, P.; Foucaud,
10
reaction of LTP with styrene, and so a series of
experiments were performed in the first instance to
rectify this and the results are summarized in Table 1.
The optimal stirring rate (600 rpm) was determined in a
1
A.; Sohmiya, H.; Kimura, T. Ultrasonics Sonochemistry 1994, 1, 33-
3
5
5. (c) Kimura, T.; Fujita, M.; Sohmiya, H.; Ando, T. Chem. Lett. 1995,
5-56. (d) Ando, T.; Fujita, M.; Kimura, T.; Lev eˆ que, J . M.; Luche, J .
L.; Sohmiya, H. Ultrasonics Sonochemistry 1996, 3, 223-227.
(3) (a) Criegee, R. Angew. Chem. 1958, 70, 173-181. (b) House, H.
O. Modern Organic Reactions; Benjamin/Cummings Publishing Com-
pany: Menlo Park, CA, 1972; pp 379-380.
(7) Norman, R. O. C.; Thomas, C. B. J . Chem. Soc. B 1967, 771-
779.
(8) Heiba, E. I.; Dessau, R. M.; Koehl, W. J . J . Am. Chem. Soc. 1968,
90, 2706-2707.
(9) (a) Kochi, J . K. J . Am. Chem. Soc. 1965, 87, 3609-3619. (b)
Kochi, J . K. J . Org. Chem. 1965, 30, 3265-3271.
(10) Yukawa, Y.; Sakai, M. Bull. Chem. Soc. J pn. 1963, 36, 761-
762.
(4) Norman, J . A.; Thomas, C. B.; Burrow, M. J . J . Chem. Soc.,
Perkin Trans. 1 1985, 1087-1093.
5) Luche, J . L.; Einhorn, C.; Einhorn, J .; Sinisterra-Gago, J . V.
Tetrahedron Lett. 1990, 31, 4125-4129.
6) Ando, T.; Sumi, S.; Kawate, T.; Ichihara, J .; Hanafusa, T. J .
Chem. Soc., Chem. Commun. 1984, 439-440.
(
(
1
0.1021/jo981168u CCC: $15.00 © 1998 American Chemical Society
Published on Web 11/14/1998

9
562 J . Org. Chem., Vol. 63, No. 25, 1998
Notes
Ta ble 1. Effect of P ota ssiu m P r op a n oa te on th e
Rea ction Ad va n cem en t
tion shown in Scheme 3 should then represent the (or
one of the) key step(s) of the radical formation and
addition to styrene.
EtCO2K
run (mmol) conditionsa
remaining
1 (%)
product
5 (%) 6 (%) balance
The higher conversion obtained under sonication in a
heterogeneous medium can be assigned, in part, to the
mechanical effects of sonication (crystal breakage of the
solid reagents, mass transport, etc.), even if an additional
chemical role cannot be excluded (see below). These
conditions lead to an excellent selectivity with the yield
of monoester 5 reaching almost 80%. Therefore the
optimal conditions for the addition of an alkyl group to
the styrene double bond would be the use of a mixture of
alkali and lead(IV) carboxylates suspended in the corre-
sponding acid. These requirements limit the generality
of the reaction, however, particularly when expensive
acids are used. A series of other reaction conditions were
investigated (Table 2).
1
2
3
4
5
6
0
0
10
10
20
20
c, 45 °C
)))), 32 °C
c, 45 °C
)))), 32 °C
c, 45 °C
)))), 32 °C
48
28
44
19
25
7
0b
49
3
4
3
5
4
6
51
81
83
90
89
91
36
66
60
78
c
a
c stands for “stirring”, and )))) for “ultrasonic irradiation”.
b
Under the same conditions, LTA in AcOH yields 4 (11%) and 2
6%). Under the same conditions, LTA and AcOK in AcOH yield
(80%).
c
(
4
Sch em e 3
The gem- and vic-diacetates (Scheme 2) 2 and 3 are
obtained in small amounts and are mentioned in Table
2
only when their percentage becomes significant. Only
in one case (run 5) does the percentage of 2 reach a
relatively high level (15%). However even then it is
significantly diminished by sonication in accordance with
expectations. This result shows that ultrasonic irradia-
tion not only plays a mechanical role (which should be
limited to a rate increase), but also produces the sonochem-
ical switching. In the reactions of LTA in propanoic acid
in the presence of potassium propanoate, two other esters
can be formed, acetates 4 and 10. 1-Phenylpropanol
propanoate (11) was never observed. The di-propanoates
preliminary study and kept constant throughout this
work.1
1,12
Along with the normal product 1-phenylbutyl
propanoate 5, the lactones 6 were also found in small
amounts. Products 7 and 8, analogues to 2 and 3,
represent less than 2% of the mixture under any reaction
conditions. The balance of the material consists of
several minor unidentified constituents.
These results establish that the carboxylate anions
directly influence the reaction in that the conversion of
styrene occurs more rapidly than in their absence. The
selectivity is also higher, as evidenced by an improved
product balance, and the radical process is favored.
These effects are further increased by sonication, even
at lower temperatures, confirming that under these
specified conditions sonochemical switching does occur.
The diastereomeric mixture of lactones 6 is also the result
of a free-radical process.
7
and 8 were not evident in the GC-MS spectra of the
crude mixtures.
From the comparison of runs 5-6 in Table 1 and runs
-2 in Table 2, it appears that the selectivity in favor of
1
ester 5 is practically unaffected by the replacement of
LTP by LTA in the presence of propanoate anion and
propanoic acid. With the system LTP-EtCOOK in AcOH
(runs 3 and 4 in Table 2), although the addition of an
ethyl group remains the preferred process, a significant
amount of methylation occurs. Because the homolysis
of the propanoate moiety occurs only in the lead carboxyl-
ate complex, we can conclude that a fast exchange must
occur between LTA and the solvent and/or potassium
The enhancement of the radical reactivity of LTA
under the influence of bases has been reported previously
for several reactions, e.g., the oxidation of primary
1
3
alcohols, the oxidative decarboxylation of carboxylic
acids,1 and the thermal decomposition itself.
b,9
14
For
9
propanoate (a shift to the right in Scheme 4).
reactions run in the presence of carboxylate anions,
Benson et al.1 proposed an explanation based on the
formation of a hexaliganded complex of Pb(IV) (Scheme
According to Kochi,9b the selectivity of the cleavage is
governed by the stability of the incipient alkyl radical.
However the formation of 1-phenylpropanol acetate 4 in
runs 3-8 shows that this preference is not respected in
all cases. In the same experiments, the predominant
pathway leads to compound 10 via the addition of an
ethyl group and formation of the acetate ester. A second
4b
3
), even if only a few articles have described such high-
1
b,15
coordination lead species.
Intermediates of this type
are implicitly supposed to undergo homolysis pre-
ferentially.1b
Higher propanoate concentrations favor the formation
of 9 (R ) Et), which reacts with styrene preferentially
via a radical process. In parallel, the electrophilic attack
of the double bond is significantly decreased. The reac-
factor must have a determining role in this case.
A
structural effect in the intermediate complex can be
envisaged as an hypothesis. The NMR spectra of 9 (R )
1
5b
3
CH ) exhibit two signals for the acetate groups, which
is explained by the presence of two types of ligands,
unidentate and symmetrically chelating bidentate. The
preferential reaction of the less tightly bound ligand could
then be invoked to explain the presence of 1-phenylpro-
panol acetate 4. In the present case, this ligand could
be the acetate or the propanoate, according to the rate
of exchange (Scheme 4) and the order of addition of the
ligands to the central atom (i.e., addition of propanoate
ions to LTA or of acetate ions to LTP). Further modifica-
tion of the system by replacement of potassium pro-
panoate by acetate suppresses the formation of ester 5,
(
11) The conversion yield increases with the stirring rates (300, 600,
2
400 rpm), but the relative amounts of the radical to ionic reaction
products (see Table 2) is higher at 600 rpm. The parallel between
ultrasonic and mechanochemical activations is not straightforward in
this case.
(12) For a discussion of mechanochemical activation, see: Suslick,
K. S. Proc. Int. Conf. Mechanochem. 1993, 1, 43-48.
(
(
84. (b) Benson, D.; Sutcliffe, L. H.; Walkley, J . J . Am. Chem. Soc.
959, 81, 4488-4492.
(
Soc., Dalton Trans. 1976, 2238-2242. (b) Alcock, N. W.; Tracy, V. M.;
Waddington, T. C. J . Chem. Soc., Dalton Trans. 1976, 2243-2246.
13) Partch, R.; Montony, J . Tetrahedron Lett. 1967, 4427-4431.
14) (a) Norman, R. O. C.; Poustie, M. J . Chem. Soc. B 1968, 781-
7
1
15) (a) Alcock, N. W.; Tracy, V. M.; Waddington, T. C. J . Chem.

Notes
run
J . Org. Chem., Vol. 63, No. 25, 1998 9563
Ta ble 2. Rea ction s of LTP a n d LTA u n d er Va r iou s Con d ition s
reagent, additive,a
residual 1
polar reaction
products (%)
radical reaction
products (%)
material
balance
b
solvent, conditions
(%)
1
2
3
4
5
6
7
8
9
LTA, EtCO2K, EtCO2H, stirring
LTA, EtCO2K, EtCO2H,))))
LTP, EtCO2K, AcOH, stirring
LTP, EtCO2K, AcOH,))))
LTP, AcOK, AcOH, stirring
LTP, AcOK, AcOH,))))
LTA, EtCO2K, AcOH, stirring
LTA, EtCO2K, AcOH,))))
LTP, AcOK, EtCO2H, stirring
LTP, AcOK, EtCO2H,))))
7
2
21
3
3 (3)
3 (4)
2 (3)
2 (1)
2 (15) + 3 (1)
2 (1) + 3 (3)
2 (9) + 3 (2)
2 (2) + 3 (5)
not found
not found
3 (2)
5 (57) + 6 (4) + 10 (9)
5 (73) + 6 (6) + 10 (9)c
4 (14) + 5 (9) + 10 (44)c
4 (10) + 5 (13) + 10 (61)c
4 (22) + 10 (35)
80
94
93
90
80
91
83
82
93
93
79
96
8
traces
25
5
traces
traces
24
4 (35) + 10 (55)c
4 (13) + 10 (36)
4 (19) + 10 (56)c
5 (80) + 6 (5) + 10 (8)
5 (79) + 6 (3) + 10 (11)
5 (40) + 6 (3) + 10 (10)
5 (71) + 6 (7) + 10 (11)
1
1
1
0
1
2
LTA, AcOK, EtCO2H, stirring
LTA, AcOK, EtCO2H,))))
traces
3 (7)
a
0 equiv in all cases. b )))) stands for “ultrasonic irradiation”. c Lactone 6 and 4-phenylbutyrolactone were detected in small amounts
2
(<1% of each) by GC-MS of the crude mixture.
Sch em e 4
Sch em e 6
Sch em e 5
Sch em e 7
but 1-phenylbutanol acetate 10 remains the major prod-
uct. This result confirms that the presence of a highly
coordinated species of lead is necessary to the reaction,
and the homolysis takes place essentially in this complex.
Furthermore, even if it cannot be established that a
complete exchange occurs, a mixed acetate-propanoate
complex undergoes a preferential homolysis of the pro-
panoate ligands. This selectivity is also demonstrated
in runs 7 and 8 (Table 2).
Before a general scheme is proposed, a few important
elements should be considered. (i) The existence of a
chain mechanism was demonstrated previously by sev-
eral authors,7 using inhibition [with benzoquinone and
-9
during which the carboxymethyl radical and a lead(III)
species form in sufficient amounts. The former is re-
sponsible for the presence of lactones in detectable
amounts in most of the experiments,16 and the latter
should be the effective transfer agent in the propagation
step (step 2, Scheme 6).4
4
-tert-butyl catechol (4-BC)] and initiation experiments
(with diisopropyl peroxydicarbonate). Some of us ob-
served the complete quenching of the formation of 4 when
benzoquinone or 4-BC (20% of the stoichiometric amounts)
were added to the reaction mixture.2 (ii) In the absence
of styrene, LTA displays a surprising stability: an acetic
acid solution heated at 50 °C under nitrogen for 1 h gives
only 5% decomposition of LTA, and 7% under sonication
at the same temperature (iodometric determination).2d
On the other hand, the hexaacetatoplumbate complex
decomposes to the carboxymethyl radical, not to the
methyl radical and carbon dioxide (Scheme 5).14 The
presence of styrene is then necessary to induce the
homolytic cleavage of the R-COO bonds, probably via
the formation of a transitory species containing the lead
reagent and the olefinic partner. Indeed, authors have
observed previously that the UV-vis spectrum of a
styrene-LTA mixture differs from the sum of those of the
a
In agreement with previous interpretations, the lead-
(III) complex should then oxidize styrene to its radical
cation,4 able to react (step 3) with complex 9 to give an
unstable intermediate, in which the methyl radical
transfers to the styrenic moiety by a mechanism which
deserves further study. The lead(III) species formed
during this step can continue the chain sequence.
This mechanism offers several advantages by giving
evidence of (i) the role of the base, here the carboxylate
ions, (ii) the relationship of the selectivity to the structure
,17
(16) In addition to lactone 6, 4-phenylbutyrolactone was detected
in runs 3, 4, 7, 8 in small amounts (<1%) by GC-MS spectrometry.
(17) An addition of the Pb(III) complex to the olefinic bond seems
constituents with the presence of a low-intensity peak
incompatible with experiment (Scheme 7): (i) the radical 13, further
at 410 nm.7
oxidized by a second molecule of 9 (R ) CH ), would give a carbocation
3
precursor of compound 2; (ii) 13 can be transformed to 14 via an
From this, we can conclude that step 1 in Scheme 5
should constitute the initiation step of the reaction,
intramolecular disproportionation. The collapse of 14 would lead to
the formation of 4 and a “lead(I)” species.

9
564 J . Org. Chem., Vol. 63, No. 25, 1998
Notes
of the intermediate hexacoordinated lead complex, and
iii) the chain character, consistent with previous obser-
propanoate, LTA, and LTP were dried in vacuo before use.
Acetic and propanoic acids were distilled in the presence of their
anhydrides on chromic anhydride or potassium permanganate,
respectively. All the experiments were run in the dark under
argon. Reactions under stirring were run at 45 °C in a
thermostated vessel, with a mixture of styrene (2.5 mmol), LTP
(3 mmol), and potassium propanoate (0 to 20 mmol) suspended
in propanoic acid (10 mL). The sonochemical experiments were
performed at 32 °C in a thermostated cell, with a “Heat &
Systems” generator delivering a 20-kHz continuous wave. The
intensity was measured calorimetrically23 and kept to the
constant value of 30 W. In both series of experiments, the excess
of oxidant was quenched after 3 h by addition of an aqueous
ammonium chloride solution, and the organic compounds were
extracted with ether. The ethereal layer was washed twice
(
vations. A question is still open, the origin of the
sonochemical effect. The more simple interpretation
based only on the mechanical phase mixing is not
sufficient, because other results demonstrate a sonochem-
ical effect even in homogeneous solutions.18 The sono-
chemical activation cannot occur out of the bubble, ruling
out the activation of the unvolatile lead compound. This
necessitates the activation of the volatile styrene itself,
possibly by ionization to the radical cation (indeed this
species has a crucial role in the mechanism) in the bubble
gas phase. Styrene radical cation was evidenced spec-
troscopically in radiolysis experiments,1
9,20
(NaHCO , then water) and the mixture analyzed by vapor-phase
and analogies
3
chromatography, on a DB 624 “J &W” column with dodecane as
an internal standard, with the following temperature se-
quence: 80 °C isotherm, 4 min; temperature raise 15 °C per min;
in the chemical effects of ultrasound and ionizing radia-
tion (the radiomimetic effect of sonication) are known.21
The sonochemical reaction would then be initiated by this
ionization, by-passing step 1 (Scheme 5) and step 2
2
10 °C final isotherm, 10 min. The products were identified by
comparison with literature data and GC-MS spectrometry.
7
(Scheme 6). The higher efficiency of the sonochemical
1-Phenylpropyl acetate 4 : m/z 178, 149, 136, 117, 107.
1-Phenylbutyl propanoate 510: m/z 206, 163, 150, 132, 117, 107.
reaction, the switching in favor of the radical pathway,
2
4
and the role of volatility of the substrate18 should find a
4-Phenyl-2-methylbutyrolactone 6
1
: m/z 176, 132, 117, 105.
-Phenylbutyl acetate 1025: m/z 192, 150, 117, 107.
reasonable explanation here.
Ack n ow led gm en t. J .M.L. thanks the Ministry of
Exp er im en ta l Section
Education, Science and Culture, J apan, for a grant, and
Coventry University for financial support. The authors
thank Profs. C. Petrier and G. Reverdy (Universit e´ de
Savoie) for their interest in this work, and Prof. J . Cadet
and Dr. T. Douki (CEA Grenoble) for their help in GLC-
MS measurements.
Chemicals and solvents were purchased from Aldrich. LTA
95% pure) was recrystallized from acetic acid. LTP was
prepared from lead(II,III)oxide, propanoic acid, and anhydride,
and recrystallized in propanoic acid. Potassium acetate and
(
2
2
(
18) Ando, T.; Fujita, M. to be published.
(
19) Eastland, G. W.; Kurita, Y.; Symons, M. C. R. J . Chem. Soc.,
J O981168U
Perkin Trans II 1984, 1843-1850.
(20) Ionization potentials of styrene, acetic acid, and propanoic acid
are 8.47, 10.69, and 10.24 eV, respectively; Handbook of Chemistry
and Physics, 69th ed.; CRC Press: Boca Raton, 1989, E 85-91.
(23) Mason, T. J .; Lorimer, J . P.; Bates, D. M.; Zhao, Y. Ultrasonics
Sonochemistry 1994, 1, S91-S95.
(
21) (a) Fuchs, E.; Heusinger, H. Ultrasonics Sonochemistry 1995,
(24) Flores-Parra, A.; Gutierrez-Avela, D. M.; Guzman-Vasquez, Y.
J .; Ariza-Castelo, A.; Contreras, R. J . Org. Chem. 1992, 57, 6067-
6071.
2
, 105-109. (b) Fang, X.; Mark, G.; von Sonntag, C. Ultrasonics
Sonochemistry 1996, 3, 57-63.
22) Vinczer, P.; Kajtar-Peredi, M.; J uvancz, Z.; Novak, L.; Szantay,
C. Org. Prep. Proc. 1989, 21, 346-348.
(
(25) Katritsky, A. R.; Rachwal, S.; Rachwal, B. Synthesis 1991, 69-
73.