Dichlorodistannoxane transesterification catalysts, pure Lewis acids

Article abstract of DOI:10.1039/b303978a

Dialkoxy- or diacyloxy-distannoxanes were transformed into unsymmetrical acyloxyalkoxydistannoxanes during a transesterification reaction where they were used as catalysts, while more active dichlorodistannoxanes were recovered unchanged.

Full text of DOI:10.1039/b303978a

Dichlorodistannoxane transesterification catalysts, pure Lewis acids
Bernard Jousseaume,* Christian Laporte, Marie-Claude Rascle and Thierry Toupance
Laboratoire de Chimie Organique et Organométallique, UMR CNRS 5802, Université Bordeaux I, 351
Cours de la Libération, 33405 Talence, France
Received (in Cambridge, UK) 10th April 2003, Accepted 25th April 2003
First published as an Advance Article on the web 16th May 2003
Dialkoxy- or diacyloxy-distannoxanes were transformed
into unsymmetrical acyloxyalkoxydistannoxanes during a
transesterification reaction where they were used as cata-
lysts, while more active dichlorodistannoxanes were re-
covered unchanged.
determined by the same method from data obtained with
identical concentrations of the reactants. It was found equal to 2
indicating a first order dependency on alcohol. Then, the
reaction was conducted in the presence of various organotin
catalysts with a stoichiometric amount of ethyl butyrate and
1-heptanol and the rate constants calculated in order to test their
activity in our reaction conditions.
Esterifications and transesterifications are very important
reactions in organic synthesis as well as in academic laborato-
ries as in industry, for instance for the preparation of polyesters
from alcohols and acids or esters. These reactions can be made
spontaneous by using Dean–Stark apparatus or high tem-
perature conditions. However, they are very often catalyzed for
a better efficiency, higher reaction rates and milder conditions.
Common catalysts include acids, Lewis acids such as titanium
or aluminium alkoxides, or bases such as alkali-metal alkoxides,
DMAP, DBL or other amines.1 Among organometallic com-
pounds, organotin derivatives as organotin oxides, alkoxides
and carboxylates are the most popular compounds used in
esterification and transesterification processes.1b,2,3 Otera et al.
recently developed this aspect of organotin chemistry by
demonstrating that diversely substituted tetraorganodistannox-
anes show very high catalytic activity allowing their use at very
low concentration when the alcohol is used in large excess.4
This work has been extended to various selective acylation
reactions and novel dimeric organotin cations.5 The mechanism
commonly admitted to explain the very high efficiency of
tetraorganodistannoxanes involves first the displacement of a
substituent on the tin by a molecule of alcohol leading to an
alkoxytetraorganodistannoxane. Then, this alkoxytetraorgano-
distannoxane would deliver the alkoxide group to a carbonyl,
after the coordination of this carbonyl to a tin atom of the
distannoxane.1b,c This scheme was logically supported by the
ability of organotin alkoxides to displace the alkoxide moiety of
esters3a and by the formation of small amounts of alkoxyte-
traorganodistannoxanes upon heating dichlorotetraorganodis-
tannoxanes and alcohols.6 We were recently interested in the
search for new organotin catalysts for silicone curing and
polyurethane preparation, especially with latent properties and
enhanced activity.7 To design more efficient catalysts for the
preparation of polyesters through transesterification reactions, it
appeared interesting to determine the nature of the active
organotin species in the catalytic cycle. The results we obtained
in this field are reported here.
The data presented in Table 1 indicate that almost identical
results were obtained with 1-acetoxy-3-methoxy-1,1,3,3-tetra-
butyldistannoxane and 1,3-diacetoxy-1,1,3,3-tetrabutyldistan-
noxane. 1,3-Dimethoxy-1,1,3,3-tetrabutyldistannoxane led to a
higher rate which was almost doubled with respect to other
catalysts. However, 1,3-dichloro-1,1,3,3-tetrabutyldistannox-
ane gave the best results. In this case the rate of the
transesterification reaction increased again by a factor of two.
An almost identical ratio was found when the reaction was
conducted at 90 °C. These results are in agreement with the
reports where higher yields were obtained with 1,3-dichloro-
1,1,3,3-tetrabutyldistannoxane, using a ten-fold excess of
alcohol.4d
Then the same type of reaction was conducted with ethyl
acetate and ethanol, for an easier recovery of the catalyst after
the reaction, under reflux at 100 °C for 6 h in the presence of a
5% amount of tetrabutyldistannoxane. After evaporation of
ethyl acetate and ethanol, the catalysts were analyzed by 119Sn,
1H and 13C NMR spectroscopy. The spectra were compared
either to authentic samples or to relevant data from the
literature. The amount of recovered catalyst was higher than
95%. When 1,3-diacetoxy-1,1,3,3-tetrabutyldistannoxane was
used as catalyst, NMR analysis revealed its transformation into
1-acetoxy-3-ethoxy-1,1,3,3-tetrabutyldistannoxane. Under the
same conditions, 1-acetoxy-3-methoxy-1,1,3,3-tetrabutyldis-
tannoxane gave 1-acetoxy-3-ethoxy-1,1,3,3-tetrabutyldistan-
noxane.9 Analogous distannoxanes were already formed when
dibutyltin oxide was used as catalyst in a transesterification
reaction at high temperature.10 Then, the more active 1,3-di-
chloro-1,1,3,3-tetrabutyldistannoxane was used. It was re-
covered unchanged after the reaction (Table 2).11 When a ten-
fold excess of alcohol was used, 2% of the complex
ClBu2SnOSnBu2Cl; ClBu2SnOSnBu2OAc12 were detected in
the recovered catalyst. When the amount of catalyst was
reduced to 0.5% and the reaction heated for 24 h, the same by-
product was accompanied by 5% of the complex ClBu2SnOSn-
Bu2Cl; ClBu2SnOSnBu2OR.13,14
The kinetic parameters of the transesterification of ethyl
butyrate with 1-heptanol were first determined, in order to
compare the activity of the tetraorganodistannoxanes used in
this study.
Table 1 Rate constants of organotin catalysed transesterifications8
The reaction order in methyl butyrate was determined by
heating a solution of methyl butyrate, a 10-fold excess of
1-heptanol, dodecane as internal standard and 1,3-diacetoxy-
1,1,3,3-tetrabutyldistannoxane in toluene at 100 °C. A 5%
amount of catalyst was used to facilitate its analysis after the
reaction as will be described later. A short path column allowed
ethanol to escape. Aliquots of the reaction mixture were
analyzed by GC to monitor the conversion of ethyl butyrate in
heptyl butyrate. An order of 1 was obtained by comparing the
variation of concentration in ester formed with calculated
curves for different values of order. The global order was
Catalyst
Rate constant/L mol21 min21
(AcO)Bu2SnOSnBu2(OAc)
(AcO)Bu2SnOSnBu2(OMe)
(MeO)Bu2SnOSnBu2(OMe)
(MeO)Bu2SnOSnBu2(OMe)
(MeO)Bu2SnOSnBu2Cl
ClBu2SnOSnBu2Cl
7.8 3 1023 a
6.4 3 1023 a
13 3 1023 a
3 3 1023 b
20 3 1023 a
27 3 1023 a
7.5 3 1023 b
ClBu2SnOSnBu2Cl
a Reaction conducted at 100 °C. b Reaction conducted at 90 °C.
1428
CHEM. COMMUN., 2003, 1428–1429
This journal is © The Royal Society of Chemistry 2003

Table 2 Nature of the catalyst after transesterification
3 (a) M. Pereyre, G. Colin and J-P. Delvigne, Bull. Soc. Chim. Fr., 1969,
262; (b) R. C. Poller and S. P. Retout, J. Organomet. Chem., 1979, 173,
C7.
Catalyst
Recovered catalyst
4 (a) J. Otera, T. Yano, H. Kabawata and H. Nozaki, Tetrahedron Lett.,
1986, 27, 2383; (b) J. Otera, S. Ioka and H. Nozaki, J. Org. Chem., 1989,
54, 4013; (c) J. Otera, N. Dan-oh and H. Nozaki, J. Chem. Soc., Chem.
Commun., 1991, 1742; (d) J. Otera, N. Dan-oh and H. Nozaki, J. Org.
Chem., 1991, 56, 5307; (e) A. Orita, K. Sakamoto, Y. Hamada, A.
Mitsutome and J. Otera, Tetrahedron, 1999, 55, 2899.
(AcO)Bu2SnOSnBu2(OAc)
(AcO)Bu2SnOSnBu2(OMe)
(MeO)Bu2SnOSnBu2(OMe)
ClBu2SnOSnBu2Cl
(AcO)Bu2SnOSnBu2(OEt)
(AcO)Bu2SnOSnBu2(OEt)
(AcO)Bu2SnOSnBu2(OEt)
ClBu2SnOSnBu2Cl
5 K. Sakamoto, Y. Hamada, H. Akashi, A. Orita and J. Otera,
Organometallics, 1999, 18, 3555; S. S. Durand, K. Sakamoto, T.
Fukuyama, A. Orita, J. Otera, A. Duthie, D. Dakternieks, M. Schulte and
K. Jurkschat, Organometallics, 2000, 19, 3220; K. Sakamoto, H. Ikeda,
H. Akashi, T. Fukuyama, A. Orita and J. Otera, Organometallics, 2000,
19, 3242.
6 J. Otera, T. Yano and R. Okawara, Organometallics, 1986, 5, 1167.
7 B. Jousseaume, V. Gouron, B. Maillard, M. Pereyre and J. M. Frances,
Organometallics, 1990, 9, 1330; B. Jousseaume, V. Gouron, M. Pereyre
and J. M. Frances, Appl. Organomet. Chem., 1991, 5, 135; B.
Jousseaume, N. Noiret, M. Pereyre and J. M. Frances, J. Chem. Soc.,
Chem. Commun., 1992, 739; B. Jousseaume, N. Noiret, M. Pereyre, J.
M. Frances and M. Petraud, Organometallics, 1992, 11, 3910; B.
Jousseaume, N. Noiret, M. Pereyre and A. Saux, Organometallics,
1994, 13, 1034.
8 The following procedure is representative. A solution of dry ethyl
butyrate (4.31 mmol, 0.500 g), dry 1-heptanol (4.31 mmol, 0.500 g),
decane (2.12 mmol, 0.360 g) and 1,3-dichloro-1,1,3,3-tetrabutyldis-
tannoxane (0.21 mmol, 0.118 mg) in dry toluene (15 ml) was heated at
100 °C under dry nitrogen in a flask equipped with a short path column.
Aliquots were removed every 15 min and analyzed by GC. After heating
for 6 hours, heptyl butyrate was recovered by distillation. Yield 74%.
Eb12 108 °C.
9 We ensured that the exchange of methoxy for ethoxy is almost
instantaneous by mixing methoxydistannoxane with excess of etha-
nol.
10 J. Bonetti, C. Gondard, R. Petiaud, M-F. Llauro and A. Michel, J.
Organomet. Chem., 1994, 481, 7.
11 While this work was completed, it was shown that fluorous dis-
tannoxanes were partially recovered unchanged after reaction. J. Xiang,
A. Orita and J. Otera, Angew. Chem., Int. Ed., 2002, 41, 4117.
12 D. L. Hasha, J. Organomet. Chem., 2001, 620, 296.
13 1,3-Dichloro- and 1,3-dibromo-1,1,3,3-tetrabutyldistannoxanes can
give the corresponding dialkoxide under heating in the presence of an
alcohol. However, the reaction is very slow: a 5 and 30% conversion
were respectively measured after 2 hours at reflux with a 750 fold excess
of alcohol. J. Otera, T. Yano and R. Okawara, Organometallics, 1986,
5, 1167.
These results showed that tetraorganodistannoxanes substi-
tuted with two acetoxy or alkoxy groups were transformed
during the reaction into unsymmetrically substituted 1-acyloxy-
3-alkoxy-1,1,3,3-tetrabutyldistannoxanes which thus could be
in this case the real catalysts of the transesterification reaction.
Their presence was in favour of a transfer of alkoxy group from
the tin to the carbonyl of the ester.15 However, the results
obtained with 1,3-dichloro-1,1,3,3-tetrabutyldistannoxane,
which was recovered unchanged, gave the higher rate constants
and where the transfer of an alkoxy group cannot take place,
favours the occurence of an alternate more efficient catalytic
process where no transfer of alkoxy group linked to the metal
occurs. Thus, the catalytic efficiency of 1,3-dihalodistannox-
anes is better described in terms of Lewis acidity, the chlorine
atoms inducing higher accepting properties than alkoxy or
acyloxy groups.16 The presence of two electropositive tin atoms
could favour either the formation of a six-membered transition
state involving catalyst, ester and alcohol, or the double
activation of the ester through the coordination of each oxygen
atom to a tin of the distannoxane.17 This interpretation is fully
consistent with the observation of very strong solvent effects in
the distannoxane-catalysed transesterification reaction where
polar solvents such as acetonitrile or diglyme decrease the
reaction rate in a spectacular way.4c This strong inactivation of
the catalyst would come from the coordination of vacant sites by
the solvent acting as a ligand.
In summary, it is shown that 1,1,3,3-tetrabutyldichlor-
odistannoxane was recovered unchanged in a transesterification
reaction where it was used as catalyst, while dialkoxy- and
diacyloxy-distannoxanes were transformed into unsymmetrical
acyloxyalkoxydistannoxane. Dichlorodistannoxanes showed a
higher activity than other distannoxanes, and being recovered
unchanged after reaction, their efficiency in the transesterifica-
tion reaction comes more from their unique bidentate coordina-
tion properties as Lewis acids than from their ability to transfer
alkoxide groups. This finding is in agreement with the discovery
of very high activity for novel organotin cations5 and could be
helpful for the design of new catalysts.18
14 D. C. Gross, Inorg. Chem., 1989, 28, 2355.
15 A. J. Bloodworth and A. G. Davies, in Organotin Compounds, ed. A.
Sawyer, Marcel Dekker, New York, 1971, p. 153.
16 The higher rate observed with 1,3-dimethoxy-1,1,3,3-tetrabutyldis-
tannoxane compared to 1-acetoxy-3-methoxy-1,1,3,3-tetrabutyldistan-
noxane could be explained by a higher activity of the dialkox-
ydistannoxane at the beginning of the reaction due to a lower steric
congestion than that of the acetoxy derivative which is linked to a
bidentate acyloxy substituent.
We are indebted to Dr J.-M. Bernard for fruitful discus-
sions.
17 Coordination of esters by an organotin trihalide through an alkoxy group
of ester has already been reported. M. Biesemans, R. Willem, S.
Damoun, P. Geerlings, M. Lahcini, P. Jaumier and B. Jousseaume,
Organometallics, 1996, 15, 2237.
18 J.-M. Bernard, B. Jousseaume, C. Laporte and T. Toupance, PCT Int.
Appl., 2002, WO 0230565; J.-M Bernard, B. Jousseaume, C. Laporte
and T. Toupance, PCT Int. Appl., 2002, WO 0231014.
Notes and references
1 (a) E. Haslam, Tetrahedron, 1980, 36, 2409; (b) J. Otera, Chem. Rev.,
1993, 93, 1449; (c) J. Otera, in Advances in Detailed Reaction
Mechanisms, vol. 3, ed. J. M. Coxon, JAI Press, London, 1994, p.
167.
2 O. A. Mascaretti and R. L. E. Furlan, Aldrichim. Acta,, 1997, 30, 55.
CHEM. COMMUN., 2003, 1428–1429
1429