Scope and limitations of the use of grafted undecyltin trichloride as a catalyst for transesterifications: Effect of tin loading on catalytic activity, recyclability, and leaching

Article abstract of DOI:10.1021/om900922k

The effect of the tin loading (functionalization degree t) on the catalytic activity and recyclability is investigated for a polystyrene-grafted undecyltin trichloride catalyst, P-C11-SnCl₃, in transesterification reactions involving either a primary or a secondary alcohol. For the latter, the achieved conversion degree in the first run is about 20% lower than with the primary alcohol. In subsequent runs, the reaction rates are strongly influenced by the tin loading of the catalyst. Unlike low-loaded P-C11-SnCl₃ catalysts (t ≈ 0.10), high-loaded catalysts (t ≈ 0.20) display a simultaneous T _g increase (from 53 °C to 107 °C after 5 runs) and conversion decrease (from 52% to 15%) upon increasing number of runs, ascribed to reduced mobility of the organotin moieties resulting from undesired cross-linking at the reaction interface. Confirmation for the fact that, in this case, the catalytic performance is dominated by conformational mobility issues is found in the comparison between transesteriflcations involving either primary or secondary alcohols. Whereas a T_g increase is not associated with a reduced conversion degree for primary alcohols, a clear decrease in conversion is observed for secondary alcohols, illustrating that steric issues are especially pronounced in a low-mobility (high-loaded) system and are of no importance in high-mobility (low-loaded) systems. This also affects the leaching resistance of the compounds, the high-loaded catalysts displaying substantially higher tin leaching (311 ± 278 ppm) than the low-loaded ones (10 ± 8 ppm).

Full text of DOI:10.1021/om900922k

Organometallics 2010, 29, 193–198 193
DOI: 10.1021/om900922k
Scope and Limitations of the Use of Grafted Undecyltin Trichloride
As a Catalyst for Transesterifications: Effect of Tin Loading
on Catalytic Activity, Recyclability, and Leaching
Vanja Pinoie, Monique Biesemans,* and Rudolph Willem
High Resolution NMR Centre (HNMR), Department of Materials and Chemistry (MACH),
Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium
Received October 21, 2009
The effect of the tin loading (functionalization degree t) on the catalytic activity and recyclability is
investigated for a polystyrene-grafted undecyltin trichloride catalyst, P-C11-SnCl3, in transesterifi-
cation reactions involving either a primary or a secondary alcohol. For the latter, the achieved
conversion degree in the first run is about 20% lower than with the primary alcohol. In subsequent
runs, the reaction rates are strongly influenced by the tin loading of the catalyst. Unlike low-loaded
P-C11-SnCl3 catalysts (t ≈ 0.10), high-loaded catalysts (t ≈ 0.20) display a simultaneous Tg increase
(from 53 °C to 107 °C after 5 runs) and conversion decrease (from 52% to 15%) upon increasing
number of runs, ascribed to reduced mobility of the organotin moieties resulting from undesired
cross-linking at the reaction interface. Confirmation for the fact that, in this case, the catalytic
performance is dominated by conformational mobility issues is found in the comparison between
transesterifications involving either primary or secondary alcohols. Whereas a Tg increase is not
associated with a reduced conversion degree for primary alcohols, a clear decrease in conversion
is observed for secondary alcohols, illustrating that steric issues are especially pronounced in a
low-mobility (high-loaded) system and are of no importance in high-mobility (low-loaded) systems.
This also affects the leaching resistance of the compounds, the high-loaded catalysts displaying
substantially higher tin leaching (311 ( 278 ppm) than the low-loaded ones (10 ( 8 ppm).
Introduction
insoluble support. As such, so-called “clean” organotin re-
agents, which can be easily removed from the final reaction
product by simple filtration, are being developed.
Within this scope, our research group has contributed to the
development and optimization of efficient cross-linked poly-
styrene-supported organotin catalysts and has explored their
application potential as sustainable, recyclable, and environmen-
tally benign transesterification catalysts.12-18 We recently re-
ported on the catalytic features of an undecyltin trichloride
grafted onto cross-linked polystyrene, P-C11-SnCl3.16 Use of
Among the large variety of organotin compounds enjoying
widespread use in a vast range of industrial applications, mono-
or dialkyltin compounds1-7 as well as tetraalkyldistannoxane
derivatives8-11 are generally recognized as highly efficient
catalysts for transesterification reactions under mild and neu-
tral conditions. A scientifically rewarding strategy to overcome
the toxicity-related limitations to the industrial exploitation of
such organotin catalysts involves their immobilization onto an
(12) Mercier, F. A. G.; Biesemans, M.; Altmann, R.; Willem, R.;
Pintelon, R.; Schoukens, J.; Delmond, B.; Dumartin, G. Organometal-
lics 2001, 20, 958.
(13) Martins, J. C.; Mercier, F. A. G.; Vandervelden, A.; Biesemans,
M.; Wieruszeski, J.-M.; Humpfer, E.; Willem, R.; Lippens, G. Chem.;
Eur. J. 2002, 8, 3431.
*Corresponding author. E-mail: mbiesema@vub.ac.be.
(1) Davies, A. G. Organotin Chemistry; VCH: Weinheim, 2004.
(2) Smith, P. J. Chemistry of Tin; Chapman & Hall, St Edmundsbury
Press: Suffolk, 1998.
(3) Omae, I. Applications of Organometallic Compounds; Wiley:
Chichester, 1998.
(4) Evans, C. J.; Karpel, S. Organotin Compounds in Modern Techno-
(14) Deshayes, G.; Poelmans, K.; Verbruggen, I.; Camacho-Camacho,
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C.; Degee, P.; Pinoie, V.; Martins, J. C.; Piotto, M.; Biesemans, M.; Willem,
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R.; Dubois, P. Chem.;Eur. J. 2005, 11, 4552.
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(15) Camacho-Camacho, C.; Biesemans, M.; Van Poeck, M.;
Mercier, F. A. G.; Willem, R.; Darriet-Jambert, K.; Jousseaume, B.;
Toupance, T.; Schneider, U.; Gerigk, U. Chem.;Eur. J. 2005, 11, 2455.
(16) Pinoie, V.; Poelmans, K.; Miltner, H. E.; Verbruggen, I.;
Biesemans, M.; Van Assche, G.; Van Mele, B.; Martins, J. C.; Willem,
R. Organometallics 2007, 26, 6718.
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Biesemans, M.; Willem, R.; Dubois, P. Chem.;Eur. J. 2003, 9, 4346.
(7) Otera, J.; Biesemans, M.; Pinoie, V.; Poelmans, K.; Willem, R. In
Tin Chemistry: Fundamentals, Frontiers, and Applications; Gielen, M.,
Davies, A. G., Pannell, K., Tiekink, E., Eds.; John Wiley & Sons: Chichester,
2008; Chapter 6, pp 667-680.
(17) Poelmans, K.; Pinoie, V.; Verbruggen, I.; Biesemans, M.;
ꢀ
Van Assche, G.; Deshayes, G.; Degee, P.; Dubois, P.; Willem, R. Appl.
(8) Otera, J. Chem. Rev. 1993, 93, 1449.
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2164.
(10) Orita, A.; Mitsutome, J.; Otera, J. J. Org. Chem. 1998, 63, 2420.
(11) Xiang, J.; Orita, A.; Otera, J. Adv. Synth. Catal. 2002, 344, 84.
Organomet. Chem. 2007, 21, 504.
(18) Poelmans, K.; Pinoie, V.; Verbruggen, I.; Biesemans, M.;
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Deshayes, G.; Duquesne, E.; Delcourt, C.; Degee, P.; Miltner, H. E.;
Dubois, P.; Willem, R. Organometallics 2008, 27, 1841.
r
2009 American Chemical Society
Published on Web 12/11/2009
pubs.acs.org/Organometallics

194 Organometallics, Vol. 29, No. 1, 2010
Pinoie et al.
the latter catalyst in the model transesterification of ethyl acetate
and 1-octanol resulted in average conversion half-lives of less
than half an hour throughout at least 10 successive runs. Similar
results were obtained for the ring-opening polymerization
(ROP) of ε-caprolactone.18 In addition, the catalyst displayed
high leaching resistance, with leaching degrees averaging around
5 ppm of grafted tin per transesterification run of ethyl acetate
and 1-octanol.16
Table 1. Tin Leaching into the Reaction Mixture for 10 Subse-
quent Runs of Catalyst P-C11-SnCl3 with t = 0.10 in the
Transesterification of Ethyl Acetate with 1-Octanol
run
1
2
3
4
5
6
7
8
9
10 av SD
ppm Sn 7.5 4.5 4.3 3.2 2.6 2.1 1.9 4.7 3.5 2.7 3.7 1.7
On the other hand, combined use of HRMAS (high-
resolution magic angle spinning) NMR spectroscopy and
modulated temperature DSC measurements in the glass
transition region of the catalyst demonstrated that its cata-
lytic application results in chemical modifications that are
accompanied by conformational mobility restrictions.16,18
Upon successive transesterification runs, the tin functional-
ities at the spacer end lose their local rotational mobility due
to increased organotin cross-linking at the solid-liquid
interface, preventing the catalyst moieties from being visible
by 119Sn HRMAS NMR spectroscopy.16,18 This increase in
local rigidity was confirmed by a significant increase in the
glass transition temperature, Tg, of the catalyst.16,18
These observations prompted us to investigate whether
reducing the degree of functionalization, i.e., “diluting” the
catalytic organotin sites at the solid-liquid interface, would
prevent such cross-linking phenomenon between neighboring
tin atoms, simply because their mutual distance would be too
large to enable bridging. Earlier work on rhodium hydrogena-
tion catalysts immobilized onto inorganic supports reported
the benefits of reducing the surface coverage.19,20 We observed
an optimum degree of functionalization for a catalyst of the
type P-C6-SnBuCl2.21 The aim of the present work is to
investigate the impact of a reduced degree of functionalization
on the catalytic activity, recyclability, and leaching resistance
of the polystyrene-grafted organotin trichloride P-C11-SnCl3
in transesterification reactions involving either primary or
secondary alcohols.
Figure 1. 119Sn HRMAS NMR spectra (a) of pure, nonused
P-C11-SnCl3 catalyst with t = 0.10; (b) after the first run;
(c) after the fifth run; (d) after the 10th run in the transesterifica-
tion of ethyl acetate with 1-octanol.
After each transesterification run, the chemical state of the
organotin catalyst is monitored with 119Sn HRMAS NMR.
In contrast with the high-t catalyst,16 the 119Sn HRMAS
spectra of the low-t catalyst do not display the dramatic S/N
ratio decrease with increasing number of transesterification
runs (Figure 1). Furthermore, whereas the high-t catalyst
experienced a Tg increase of about 40 °C after 10 runs, a
much less pronounced increase in Tg is measured for the low-
t material, i.e., from 94.4 °C for freshly synthesized catalyst
up to 97.7 °C after the 10th run. Moreover, the 119Sn
HRMAS spectra show the generation of an additional minor
119Sn resonance at ca. -300 ppm (Figure 1, arrow), the
amplitude of which increases with the number of successive
transesterifications (integrating for about 17% after the 10th
recycle (Figure 1, d)).
On the basis of the latter 119Sn chemical shift, and in line with
results previously obtained for P-C11-SnBuCl2;its catalytic
use resulting in the observation of a grafted side-functionality of
the type P-C11-SnBu(OOCCH3)2 resonating at -203 ppm17;
the current side-functionality at -300 ppm likely indicates an
increasing modification of the catalyst into a functionality of the
type P-C11-Sn(OOCCH3)3 or P-C11-SnCl(OOCCH3)2. In
view of its visibility in 119Sn HRMAS NMR, this modified
fraction clearly involves rotationally mobile organotin grafts, an
observation that is in strong contrast with results obtained for
the high-t catalyst.16 In the latter case, the modified catalyst
functionalities did not provide a 119Sn HRMAS resonance and
caused the observation of only an additional broad, ill-defined
anisotropy pattern around -300 ppm in the 117Sn solid-state
NMR spectra.16 Considering the resemblance in this low-
frequency 119Sn chemical shift behavior, we suggest that both
modified fractions are of similar nature, however with the
rotationally immobilized ones existing in an aggregated state,
favored at higher organotin concentrations.
Results and Discussion
Transesterification of Ethyl Acetate with a Primary Alco-
hol. In a first stage, the effect of reducing the functionaliza-
tion degree t from 20% to 10% on the catalytic activity of
[P-H]1-t[P-C11-SnCl3]t is assessed in the model transesterifi-
cation of ethyl acetate with the primary alcohol 1-octanol.
Ten successive catalysis experiments were performed with
1 mol % of grafted tin with respect to the initial molar
amount of alcohol, using ethyl acetate in 4-fold excess. In
order to allow proper comparison, the reaction time is kept
constant at 2 h for all runs. Overall, a slightly better and more
reproducible average catalytic performance is noted using
catalyst P-C11-SnCl3 with a lower functionalization degree
of 0.10, since 84 ( 4% of conversion in 2 h is observed, which
compares favorably to the average conversion of 76 ( 10%
obtained for a higher t-value of 0.20.16 The complete series
of 10 catalytic runs with the low-t catalyst results in a slightly
lower average tin leaching degree (Table 1) of 3.7 ( 1.7 ppm,
which compares favorably to the average of 5 ( 3 ppm for
eight out of 10 runs using the high-t catalyst16 (peaking
values at 16 and 35 ppm for two runs).
€
(19) Merckle, C.; Blumel, J. Adv. Synth. Catal. 2003, 345, 584.
€
(20) Merckle, C.; Blumel, J. Top. Catal. 2005, 34, 5.
(21) Biesemans, M.; Mercier, F. A. G.; Van Poeck, M.; Martins, J. C.;
Dumartin, G.; Willem, R. Eur. J. Inorg. Chem. 2004, 2908.

Article
Organometallics, Vol. 29, No. 1, 2010 195
Figure 2. 2D HSQC HRMAS NMR spectrum expansion of the low-t catalyst after 10 catalytic runs (left), with an inset at lower
intensity level of the cross-peaks 1 and 10 (right).
Figure 3. Conversion degrees obtained in consecutive runs after a reaction time of 2 h in the transesterification of ethyl acetate and
1-octanol (1) or ethyl acetate and cyclohexanol (2), using catalyst P-C11-SnCl3 with different degrees of functionalization t (left). The
graph (right) shows the corresponding time evolution of the conversion in the course of the first run, within a reaction duration of 2 h.
In an attempt to confirm the formation of the organotin
carboxylate moiety, a 2D 1H-13C HRMAS experiment
(Figure 2) clearly reveals the existence of two R-CH2 cross-
peaks at (1H, 13C) chemical shifts of 2.31, 33.7 ppm and 2.23,
37.8 ppm, indeed confirming the presence of two types of
organotin species, respectively denoted as 1 (remaining fraction
of original catalyst P-C11-SnCl3) and 10 (modified fraction).
An expansion of the 2D spectrum (Figure 2, right) displays
their 2J(119/117Sn, 1H) and 1J(119/117Sn, 13C) coupling constants,
respectively 82 and 700 Hz for 1 and 99 and 770 Hz for 10.
In addition, Figure 2 (left) shows a key cross-peak at
2.03, 21.3 ppm, which can be assigned to an acetate moiety,
in good agreement with the above statement on the forma-
tion of an organotin carboxylate moiety.
Transesterification of Ethyl Acetate with a Secondary
Alcohol. In a second stage, the catalytic efficiency and
recycling ability of grafted P-C11-SnCl3 are assessed in the
transesterification of ethyl acetate with a more sterically
hindered alcohol, i.e., cyclohexanol. For the sake of compar-
ison with the above results, all experiments are likewise
performed with 1 mol % of grafted tin with respect to the
initial molar amount of alcohol, using ethyl acetate in 4-fold
excess. For each of three P-C11-SnCl3 catalysts, differing
only in their degrees of functionalization, five consecutive
transesterification runs with a constant reaction time of 2 h
are performed.
In the first run, the achieved conversion degree toward the
ester is about 20% lower as compared to the reactions
performed previously with the primary alcohol 1-octanol
(Figure 3), for a given reaction time of 2 h. While, for this
primary alcohol, no pronounced effect of the degree of
functionalization on the catalytic activity and recyclability
could be observed (see above and Figure 3), the performance
of the P-C11-SnCl3 catalyst in transesterification involving a
secondary alcohol is shown to be highly dependent on the tin
loading. In a first run, the use of a low t-value of 0.10 leads to
about 15% higher conversion after 2 h as compared to the
reaction performed with catalyst having a higher t-value of
0.21 (Figure 3, graph).
Also the recycling ability of each of the catalysts considerably
depends on their tin loading (Figure 3, left). While, overall, the
low-tcatalyst maintains its initial catalytic efficiency throughout
five successive runs, an increasing drop in conversion is observed
upon increasing tin loadings. This observation can be readily
assigned to the occurrence of additional cross-linking of orga-
notin functionalities at the interface, since this phenomenon is
more likely for a high-t than for a low-t catalyst. This is
confirmed by the 119Sn HRMAS spectra of the catalysts after
the different runs (Figure 4), displaying similar trends to those of
the transesterification with 1-octanol (see above). Thus, appli-
cation of the high-t catalyst in the transesterification with
cyclohexanol likewise results in a severe loss of the 119Sn

196 Organometallics, Vol. 29, No. 1, 2010
Pinoie et al.
Figure 4. 119Sn HRMAS NMR spectra (a) of pure, nonused P-C11-SnCl3 catalyst with t = 0.20 (left) and t = 0.10 (right); (b) after the
first run; (c) after the third run; (d) after the fifth run in the transesterification of ethyl acetate with cyclohexanol.
Figure 5. Conversion degrees (left) and glass transition temperatures Tg of P-C11-SnCl3 (right) obtained after successive transester-
ification runs of ethyl acetate and cyclohexanol with a reaction time of 2 h.
Figure 6. 117Sn solid-state NMR spectra of P-C11-SnCl3 catalyst with t = 0.20 (left) and t = 0.10 (right) after the fifth run in the
transesterification of ethyl acetate with cyclohexanol.
HRMAS resonance with increasing number of recycling
experiments (Figure 4, left). On the contrary, for the low-t
catalyst, no dramatic S/N decrease is observed, while an
additional tin functionality around -300 ppm is generated
(Figure 4, right).
The evolution in glass transition temperature, from
52.8 °C for freshly synthesized high-t catalyst to a very broad
Tg with a maximum around 107 °C after five catalytic runs,
confirms the significant rigidity increase at the interface of
this catalyst (Figure 5, right). As explained above, this
cancels out all local rotational mobility of the C11-grafted
organotin functionalities, consequently leading to the loss of
the characteristic 119Sn HRMAS resonance around 0 ppm.
From Figure 5 (right) it is clear that this effect significantly
diminishes for the low-t catalyst, displaying a Tg increase of
only ca. 3 °C after five catalytic runs. This indicates that this
low-t catalyst does not undergo any organotin cross-linking
at the interface as a result of its reduced concentration of
organotin sites.
The latter interpretations are supported by the solid-state
117Sn NMR spectra of the high- and low-t catalyst after five
recycling experiments (Figure 6, left and right, respectively).
For the low-t catalyst the spectrum displays, next to the
anisotropy pattern of the original SnCl3 catalyst centered
around 0 ppm, an additional though noisy resonance around
-300 ppm, whereas for the high-t one only a very broad and
noisy anisotropy pattern is observed at low frequency.
As suggested above, the observed chemical modifica-
tion of the high-t catalyst likely involves an acetate moiety,
which is consistent with the bridging capacity of carboxylate

Article
Table 2. Tin Leaching into the Reaction Mixture for Five
Consecutive Runs of 2 h in the Transesterification Reaction of
Ethyl Acetate with Cyclohexanol, Using Catalyst P-C11-SnCl3
with Different Functionalization degrees t
Organometallics, Vol. 29, No. 1, 2010 197
tin can cause changes in reaction selectivity, favoring aliphatic
carbon-tin bond cleavage.24 This assumption is supported by
the stronger leaching resistance of the low-loaded catalysts,
where this cross-linking by tin coordination expansion upon
catalytic use does not take place, as shown above.
leaching (ppm Sn)
run
t = 0.10
t = 0.17
t = 0.21
Conclusion
1
2
3
4
5
av
SD
24
7
6
31
10
18
19
57
27
18
22
39
The present study addressed the influence of the functio-
nalization degree of polystyrene-supported undecyltin
trichloride on its catalytic activity and recyclability in trans-
esterification reactions.
670
445
381
311
278
9
5.5
10
8
Reaction rates can sometimes be strongly influenced by
the tin catalyst loading on the support. For highly loaded
catalysts, a simultaneous Tg increase and conversion de-
crease with increasing number of runs originate both from
reduced conformational mobility of the organotin moieties
resulting from additional cross-linking by tin coordination
expansion. Confirmation of the fact that the catalytic per-
formance is dominated by mobility issues was found in the
comparison between transesterifications involving either
primary or secondary alcohols. In the case of primary
alcohols, a Tg increase is not associated with a reduced
conversion, whereas it is in the case of secondary alcohols,
illustrating that steric issues are pronounced in a low-mobility
system, whereas they are of subordinate importance in high-
mobility systems.
When comparing the performances of the supported
alkyltin trichloride and a soluble analogue, n-butyltin
trichloride, the grafted system showed higher catalytic acti-
vity under comparable conditions, an effect even more pro-
nounced in the case of reactions with secondary alcohols and
low-t catalysts. Considering, furthermore, the satisfactory
tin leaching results and the easy recycling conditions of the
grafted catalyst as compared to its nongrafted analogue in
solution, the benefits of grafting organotin catalysts are
unambiguously confirmed.
ligands between tin atoms in functionalized polystyrenes.22,23
Such carboxylates bridging two tin atoms have shown to
display characteristic CdO and C-O stretching bands in the
1550-1560 and 1410-1425 cm-1 ranges.22,23 In this res-
pect, the clear appearance of new bands at 1542 and around
1400 cm-1 (shoulder) in the IR spectrum of the high-t catalyst
after five consecutive runs supports this interpretation. For
the low t-value, no such intense bands are visible, in line with
the above proposal that no acetate bridging is possible with
low functionalization degree. Similar bands were observed
after application of the catalyst to the ring-opening poly-
merization of ε-caprolactone as well.18
Figure 5 unambiguously correlates the performance of the
catalyst (conversion, left) and the local rotational molecular
mobility of its catalytic sites at the solid-liquid interface
(Tg, right): the lower the amount of remaining flexible C11
chains with increasing number of runs, the higher the value
of Tg and, thus, the more dramatic the reduction in conver-
sion degree from run to run. Noteworthy, this conclusion
only holds for the transesterifications performed with a
secondary alcohol; the additional cross-linking with high-t
catalysts is harmless for the catalyzed transesterification
with a primary alcohol (see above), showing both restricted
mobility of the catalytic organotin sites and the steric hin-
drance of the secondary alcohol to cause the catalyst to be
poorly recyclable.
Experimental Section
For comparison, performing a single run with the soluble
nongrafted catalyst BuSnCl3, under comparable reaction
conditions, results in a conversion degree of only 43% after
2 h. Moreover, the high-t catalysts display a lower leaching
resistance as compared with the low-t ones (Table 2). For the
catalyst with a t-value of 0.21, reasonably low tin contents
of 22 and 39 ppm were detected in the reaction product after
the first and second runs, respectively. In the subsequent
three runs, however, extremely high residual tin leaching
values averaging at 499 ( 152 ppm were observed. Con-
versely, five consecutive runs of catalyst with lower t-values
of 0.17 and 0.10 resulted in acceptable tin leaching contents,
averaging at 27 ( 18 and 10 ( 8 ppm, respectively. The
pronounced weaker leaching resistance of the high-loaded
catalyst can be readily related to its above-mentioned che-
mical modification upon catalytic use. In this respect, it has
been reported that nucleophilic coordination expansion at
The synthesis and full characterization of P-C11-SnCl3 have
been described in detail in a previous report.16
Catalysis Experiments. Ethyl acetate (Aldrich, 99.5þ%),
1-octanol (Aldrich, 99þ%), and cyclohexanol (Aldrich, 99%)
were distilled prior to all transesterification experiments. Ethyl
acetate and 1-octanol or cyclohexanol were used in a 4:1
molar ratio. A mixture of ethyl acetate (3.5 g, 40 mmol), alcohol
(10 mmol), and supported catalyst (0.10 mmol Sn; 1 mol % Sn)
was gently stirred with a magnetic stirrer at reflux temperature
while the generated ethanol was distilled off to drive the reac-
tion to completion. The reaction time was limited to 2 h. For
monitoring the time evolution of the conversion during the
transesterification, negligibly small aliquots of 50 μL of reac-
tion mixture were regularly tapped and analyzed by solution
1H NMR spectroscopy using CDCl3 as a solvent, after evapora-
tion of ethyl acetate and ethanol. The ratio of initial 1-octanol
(cyclohexanol) to obtained 1-octyl acetate (cyclohexyl acetate)
was determined by integration ((1%) of the respective CH2O
(CHO) 1H NMR resonances. After each transesterification run,
the catalyst was filtered off and gently washed with ethyl acetate.
NMR Spectroscopy. The 1H, 13C, and 119Sn 1D and 2D
HRMAS NMR spectra were recorded on a Bruker Avance II
500 instrument operating at 500.13, 125.77, and 186.50 MHz,
respectively, or on a 700 MHz Bruker Avance II spectrometer
operating at 700.13, 176.05, and 261.08 MHz, respectively, both
with a dedicated Bruker 1H/13C/119Sn HRMAS probe equipped
(22) Angiolini, L.; Biesemans, M.; Caretti, D.; Salatelli, E.; Willem,
R. Polymer 2000, 41, 3913.
(23) Dalil, H.; Biesemans, M.; Willem, R.; Angiolini, L.; Salatelli, E.;
Caretti, D.; Chaniotakis, N. A.; Perdikaki, K. Helv. Chim. Acta 2002, 85,
852.
(24) Wardell, J. L. In Chemistry of Tin; Smith, P. J., Ed.; Chapman &
Hall, St Edmundsbury Press: Suffolk, 1998; pp 95-137.

198 Organometallics, Vol. 29, No. 1, 2010
Pinoie et al.
with gradient coils. Sample rotors were filled with ca. 15 mg of
resin beads, swollen in approximately 80 μL of CDCl3; magic
angle spinning rate was 4 kHz. 119Sn NMR measurements were
referenced to Ξ = 37.290665 MHz.25 The 119Sn spectra were
acquisition (t2) and the evolution (t1) time domains, respectively.
A total of 32 or 64 scans were used for each t1 increment.
IR Spectroscopy. IR spectra were recorded on a Bruker
Equinox 55 FT-IR spectrometer, equipped with a MIR source,
a KBr beam splitter, and a DGTS detector, from 200 mg of dry
KBr pellets with ca. 3 mg of sample.
1
recorded under broadband H decoupling during acquisition.
A diffusion LED sequence26 was used to eliminate contributions
from any mobile species from one-dimensional 1H HRMAS
spectra, using a 50 ms diffusion delay and 1.5 ms sine-shaped
gradient pulses. 2D 1H-13C HSQC HRMAS spectra were
recorded on the 700 MHz Bruker Avance II spectrometer.
A total of 2048 and 512 complex points were acquired in the
Acknowledgment. Financial support by the Fund of
Scientific Research Flanders (FWO, Belgium; Grant
G.0469.06) and the Research Council (Onderzoeksraad)
of the Vrije Universiteit Brussel (Concerted Research
Action, Grant GOA31) to R.W. and M.B. is gratefully
(25) Mason, J. Multinuclear NMR; Plenum Press: New York, 1987;
p 627.
(26) Warrass, R.; Wieruszeski, J.-M.; Lippens, G. J. Am. Chem. Soc.
1999, 121, 3787.
acknowledged. V.P. acknowledges a Ph.D. grant
(Concerted Research Action, Grant GOA31).