Sonochemical reactions of lead tetraacetate with 4-substituted styrenes

Article abstract of DOI:10.1246/cl.2000.1124

A good linear free energy relationship was observed between the radical reactivity of the sonochemical reactions of 4-substituted styrenes with lead tetraacetate and the vapor pressure of the styrenes, indicating a probable control of the process by an intra-bubble activation.

Full text of DOI:10.1246/cl.2000.1124

1124
Chemistry Letters 2000
Sonochemical Reactions of Lead Tetraacetate with 4-Substituted Styrenes
Takashi Ando,* Takahide Kimura, Jean-Marc Levêque,† Mitsue Fujita, and Jean-Louis Luche†
Department of Chemistry, Shiga University of Medical Science, Seta, Otsu, Shiga 520-2192
†Laboratoire de Chimie Moléculaire et Environnement, Université de Savoie, 73376 Le Bourget du Lac Cedex, France
(Received July 10, 2000; CL-000656)
A good linear free energy relationship was observed
A classical method used for the study of reaction mecha-
nisms of aromatic substrates being the linear free energy relation-
ship (LFER) observed by the introduction of substituents on the
ring, we investigated the behavior of a few 4-substituted styrenes
with the hope to discriminate the above two possibilities.
between the radical reactivity of the sonochemical reactions of
4-substituted styrenes with lead tetraacetate and the vapor pres-
sure of the styrenes, indicating a probable control of the process
by an intra-bubble activation.
The experiments were conducted in the dark under argon.
Stirred reactions7 were run at 45 °C in a thermostated vessel,
with the required styrene (2.5 mmol), LTA (3 mmol) and
AcOK (15 mmol) suspended in acetic acid (10 mL). The sono-
chemical experiments were performed at 45 °C in a thermostat-
ed cell, with an Astrason Sonicator XL 2020 delivering a 30-W
(calorimetric8) 20-kHz continuous wave. In both series of
experiments, the reaction was quenched after 3 h by addition of
aq NH4Cl, and the organic compounds were extracted with
ether. The ethereal layer was washed (aq NaHCO3, then water)
and the mixture analyzed by VPC with dodecane as an internal
standard. The results are shown in the Table 1.
The reaction of lead tetraacetate (LTA) with styrene can be
considered as a typical case of sonochemical switching.1 Under
conventional conditions, an ionic pathway predominates which
yields compound IP (Scheme 1).2
Ultrasonic irradiation leads to a dramatic reactivity change,
even at 45 °C, similar to or even greater than that observed in
refluxing acetic acid (116 °C), and the products resulting from a
radical process (RP and MP) become the major ones.3
Recently4 we proposed a mechanism which agrees with our
experimental results and explains the role of acetate ions
(Scheme 2).5 According to this Scheme, the sonochemical
excitation can occur either at the radical decomposition of hexa-
acetato-lead(IV) ion to the key Pb(III) species or at the excita-
tion of styrene to its radical cation. The former process should
take place at the gas–liquid interface because of the lack of the
vapor pressure of the Pb(IV) species and the latter in the gas
phase of the cavity under thermally extreme conditions.6
Because of the heterogeneous nature of the mixture, kinetic
studies proved to be technically difficult; only the final percent-
ages of the products were obtained with a satisfactory accuracy.
Nevertheless attempts to correlate the free energy differences to
the properties of the substituted styrenes were undertaken. The
free energy term was approximated by the following equation.9
When the ionic reactivity under the conventional condi-
tions is considered, good LFER was observed with the
Hammett σ+ (Figure 1, Curve 1, ρ+ = −2.81, r = 0.979), in
accord with the admitted mechanism of an initial electrophilic
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
1125
attack by the triacetato-lead cation on the double bond. As
expected, sonication does not change the aspect of the curve at
all (Figure 1, Curve 2, ρ+ = −2.81, r = 0.992).
sonication. The conclusion should be that in this reaction, and
probably in other sonochemical systems, the ability of a sub-
strate to vaporize into the gas phase of the cavitation bubble is a
critical step. In addition, our mechanism proposed for the reac-
tion of LTA with styrene is further confirmed; the sonochemi-
cal effect is attributed to the excitation of styrene to its radical
cation and not the sonication enhanced radical decomposition
of hexaacetato-lead(IV) ion.
The authors gratefully acknowledge the valuable com-
ments concerning the LFER treatment by Profs. Y. Kondo
(Osaka Women’s University), K. Takeuchi (Kyoto University),
and M. Sawada (Osaka University). J. M. L. thanks the
Ministry of Education, Science, Sports and Culture, Japan, for a
grant. The present work was supported by a Grant-in-Aid for
Scientific Research No. 11640531 from the Ministry of
Education, Science, Sports and Culture.
Practically no RP is formed under stirring. In the sonicat-
ed reactions, all the attempted LFER between the radical reac-
tivity and various substituent constants including those repre-
senting the radical reactivity such as σJJ fail.10 We have estab-
lished previously that the decomposition of LTA in the absence
of styrene is not greatly influenced by sonication,3d but this
decomposition can generate a lead(III) species,11 which can ini-
tiate a chain process. By considering the characteristics of
sonochemistry and the mechanism proposed, it can be deduced
that (i) cavitation is most important to induce the radical reac-
tivity and (ii) the only compound able to penetrate into the cavi-
tation bubble is the styrene partner. Then, some relationship
between the radical reactivity and the volatility of styrenes
should exist. The vapor pressures of substituted styrenes at 45
°C were estimated by the use of the Clapeyron–Clausius equa-
tion and listed in Table 1. Enough data are available for H-, Cl-
and Me-substituted styrenes to establish the slopes and inter-
cepts of the equation.12 For F-, Br- and t-Bu-styrenes, for
which only scattered data of the boiling points at reduced pres-
sures are available, estimation was carried out by taking advan-
tage of the similarity to the above styrenes.
References and Notes
1
2
3
T. Ando, S. Sumi, T. Kawate, J. Ichihara, and T. Hanafusa,
J. Chem. Soc., Chem. Commun., 1984, 439.
H. O. House, “Modern Organic Reactions,” Benjamin/
Cummings, Menlo Park, CA (1972), p. 379.
a) T. Ando, P. Bauchat, A. Foucaud, M. Fujita, T. Kimura,
and H. Sohmiya, Tetrahedron Lett., 32, 6379 (1991). b) T.
Ando, M. Fujita, P. Bauchat, A. Foucaud, H. Sohmiya, and
T. Kimura, Ultrasonics Sonochemistry, 1, S33 (1994). c) T.
Kimura, M. Fujita, H. Sohmiya, and T. Ando, Chem. Lett.,
1995, 55. d) T. Ando, M. Fujita, T. Kimura, J.-M. Levêque,
J.-L. Luche, and H. Sohmiya, Ultrasonics Sonochemistry,
3, S223 (1996). e) Although an alternative polar pathway is
possible for MP, the variation of the yield of MP under
various conditions was mostly parallel to that of RP.
T. Ando, T. Kimura, J.-M. Levêque, J. P. Lorimer, J.-L.
Luche, and T. J. Mason, J. Org. Chem., 63, 9561 (1998).
E. I. Heiba, R. M. Dessau, and W. J. Koehl, J. Am. Chem.
Soc., 90, 2706 (1968).
4
5
6
7
K. S. Suslick, D. A. Hammerton, and R. E. Cline, Jr., J.
Am. Chem. Soc., 106, 5641 (1984).
Stirring at 45 °C at 300 and 600 rpm (magnetically) and at
2400 rpm (with an ultra-turrax device) slightly changes the
selectivity in favor of the RP product for the latter, but this
effect remains much smaller than that of sonication. J.-M.
Levêque, Ph. D. Thesis, Université de Savoie, Chambéry,
France, 1998.
8
9
T. J. Mason, J. P. Lorimer, D. M. Bates, and Y. Zhao,
Ultrasonics Sonochemistry, 1, S91 (1994); T. Kimura, T.
Sakamoto, J.-M. Levêque, H. Sohmiya, M. Fujita, S. Ikeda,
and T. Ando, Ultrasonics Sonochemistry, 3, S157 (1996).
Equation 1 was used for a pseudo-first order reaction with
high conversions.
10 X.-K. Jiang and G.-Z. Ji, J. Org. Chem., 57, 6051 (1992).
11 R. O. C. Norman and M. Poustie, J. Chem. Soc. B, 1968,
781.
The good LFER between the vapor pressure term and the
radical reactivity term estimated from RP yields and ΣRP
yields is apparent in Figure 2, Curves 1 and 2, in spite of the
rough estimation and the inaccuracy of the reactions run under
12 D. R. Stull, Ind. Eng. Chem., 39, 517 (1947).