Organotin catalysts rafted onto cross-linked polystyrene supports through polar spacers

Article abstract of DOI:10.1002/aoc.1578

The present study investigates the suitability of a HypoGel support bearing oligomeric poly(ethylene glycol) (PEG) chains to act as an insoluble carrier for grafted organotin catalysts. Through the introduction of polar spacers, an improved swelling and site accessibility in the polar media typically involved in transesterification reactions are targeted. Advanced structural investigation shows that quantitative conversion into the targeted HypoGel-supported organotin trichloride is hampered by the existence of intra-and/or intermolecular donor-acceptor O → Sn interactions caused by the presence of donor moieties in the PEG-linker. Support is provided to the proposal that the latter interactions are at the origin of the moderate catalytic performance displayed by these HypoGel-supported catalysts, achieving only 41% conversion after 2 hours in the transesterification of ethyl acetate and n-octanol. In contrast with similar organotin catalysts supported by an alkyl spacer, the HypoGel-supported materials appear to be poorly recyclable and display poor leaching resistance. Copyright

Full text of DOI:10.1002/aoc.1578

Full Paper
Received: 8 September 2009
Revised: 30 September 2009
Accepted: 30 September 2009
Published online in Wiley Interscience: 30 November 2009
(www.interscience.com) DOI 10.1002/aoc.1578
Organotin catalysts grafted onto cross-linked
polystyrene supports through polar spacers
Vanja Pinoie∗, Monique Biesemans and Rudolph Willem
The present study investigates the suitability of a HypoGel support bearing oligomeric poly(ethylene glycol) (PEG) chains to act
as an insoluble carrier for grafted organotin catalysts. Through the introduction of polar spacers, an improved swelling and site
accessibilityinthepolarmediatypicallyinvolvedintransesterificationreactionsaretargeted.Advancedstructuralinvestigation
shows that quantitative conversion into the targeted HypoGel-supported organotin trichloride is hampered by the existence
of intra- and/or intermolecular donor-acceptor O→Sn interactions caused by the presence of donor moieties in the PEG-linker.
Support is provided to the proposal that the latter interactions are at the origin of the moderate catalytic performance displayed
by these HypoGel-supported catalysts, achieving only 41% conversion after 2 hours in the transesterification of ethyl acetate
and n-octanol. In contrast with similar organotin catalysts supported by an alkyl spacer, the HypoGel-supported materials
c
appear to be poorly recyclable and display poor leaching resistance. Copyright ꢀ 2009 John Wiley & Sons, Ltd.
Keywords: tin; HypoGel resin; supported catalyst; transesterification; HRMAS NMR spectroscopy
Introduction
functionality.[12,18] Non-polar solvents were shown to be more ap-
propriate media for a supported dialkyldiphenyltin functionality
but less efficient for a dialkyltin dichloride, whereas the opposite
is true for polar solvents. These observations have demonstrated
that the swellability of the material is very sensitive to local polar-
ity effects in the vicinity of the grafted organotin functionalities.
Consequently, their reactivity and catalytic activity is a priori antic-
ipated to improve with increasing local polarity as a result of the
introduction of polar spacers, an assumption strengthened by the
fact that organotin-catalyzed reactions often take place in polar
solventsandthatdonor–acceptorinteractions, responsibleforthe
catalytic effect at the level of the tin sites, imply polar coordinative
bonds.
It is well established that organotin compounds are efficient
catalysts in a large variety of organic reactions, providing them
tremendous potential for widespread industrial application. In
particular, tin-based Lewis acids such as mono- or dialkyltin
compounds[1–6] and tetraalkyldistannoxane derivatives[7–10] dis-
play high catalytic efficiency in transesterification reactions under
mild and neutral conditions. Nevertheless, their routine use in
an industrial context is hampered by their intrinsic toxicity and
by the generally tedious quantitative removal of tin-containing
remnants or by-products from the final reaction product. A scien-
tifically rewarding strategy to overcome the latter concerns – and
nowadays a general trend in catalysis – involves grafting of the
organotin reagent onto an insoluble support.
In previous work,[11–17] our research group has contributed
to the development and optimization of efficient cross-linked
polystyrene-supported organotin catalysts, and has explored
their application potential as sustainable, recyclable and envi-
ronmentally benign transesterification catalysts. Among these
investigations, the polystyrene-grafted undecyltin trichloride (P-
C11-SnCl3) displayed short half-life times of hardly half an hour
to reach transesterification equilibrium, good recyclability up to
at least 10 runs, and high leaching resistance with tin leaching
degrees averaging around 5 ppm in the end product.[15] Taking
these promising results as a benchmark, the present paper aims at
expanding the potential of polystyrene-grafted organotin deriva-
tives as catalysts in transesterification reactions by investigating
the impact of local polarity changes around the tin atom on the
catalytic activity through the introduction of polar spacers of the
polyethylene glycol type. In this way, it is targeted to improve
the swellability of the support in polar solvents, aiming at an
increased molecular mobility of the anchored functional groups
at the solid-liquid interface. In this respect, previous HRMAS (high-
resolution magic angle spinning) NMR studies have shown that
the adequacy of the solvent used for this technique is highly de-
pendent on its polarity and on the nature of the grafted organotin
Experimental Section
General Aspects
All reactions and manipulations were conducted under a strictly
dry N2 atmosphere using standard Schlenk-tube techniques. The
HypoGel resins were purchased from Rapp Polymere. Thionyl
chloride (99 + %), mercury(II) chloride (anhydrous), butyllithium
(2.5 M in hexane) and triphenyltin hydride were purchased from
Aldrich. Triphenyltin hydride was purified prior to use by column
chromatography using petroleumether–benzene (4 : 1) as an
eluent. Tetrahydrofurane (Aldrich, 99 + %) and tetrahydropyrane
(Fluka, 98%) were dried over sodium and distilled prior to
use. Diisopropylamine (Aldrich, 99%) was dried over calcium
hydride and distilled prior to reaction. Ethyl acetate (Aldrich,
∗
Correspondence to: Vanja Pinoie, High Resolution NMR Centre, Department of
Materials and Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels,
Belgium. E-mail: vpinoie@vub.ac.be
High Resolution NMR Centre (HNMR), Department of Materials and Chemistry
(MACH), Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium
c
Appl. Organometal. Chem. 2010, 24, 135–141
Copyright ꢀ 2009 John Wiley & Sons, Ltd.

V. Pinoie, M. Biesemans and R. Willem
99.5 + %) and n-octanol (Aldrich, 99 + %) were distilled prior to all
mass fractions for t = 0.235 as obtained for HG-Bz-Cl: H 6.86, C
74.43, Sn 10.34, O 8.37. HRMAS NMR (CDCl3, chemical shifts in
ppm, coupling constants in Hz): 1H: 2.95 [2J(119/117Sn,1H) 62] (Bz-
CH2), 6.73 (Bz-CHo), 6.98 (Bz-CHm), 4.08 (1-CH2), 3.85 (2-CH2), 7.45
(Ph-CHo), 7.36 (Ph-CHm), 7.36 (Ph-CHp); 13C: 18.8 [1J(119/117Sn,13C)
325] (Bz-CH2), 114.8 (Bz-Co), 128.7 (Bz-Cm), 132.6 (Bz-Cp), 155.8
(Bz-Ci), 137.0 (Ph-Co), 128.4 (Ph-Cm), 128.9 (Ph-Cp), 138.4 (Ph-Ci),
67.6 (1-CH2), 69.8 (2-CH2); 119Sn: −118.
transesterification experiments.
Synthesis Procedures
Synthesis of [P-H](1−t)[P-(CH2)2(OCH2CH2)5Cl]t (HG-Cl)
Under dry N2 atmosphere, 2 g (5.43 mmol) of [P-
H](1−t)[P-(CH2)2(OCH2CH2)5OH]t, HG-OH (elemental analysis: H
8.37, C 80.82, O 10.63, resulting in t = 0.173), was im-
mersed in freshly distilled tetrahydropyrane (THP) and 0.6 ml
of thionyl chloride (SOCl2) was added. The mixture was stirred
under reflux for 24 h. After reaction, the target compound [P-
H](1−t)[P-(CH2)2(OCH2CH2)5Cl]t, HG-Cl, was successively washed
with tetrahydrofurane (THF, 8 × 30 ml) and ethanol (2 × 30 ml).
The beads were dried under vacuum at 35 ◦C.
Experimental mass fractions (wt%): H 7.89, C 77.41, O 8.91, Cl
5.04, resulting in t = 0.204; calculated mass fractions for t = 0.173
as obtained for HG-OH: H 7.89, C 79.06, O 9.04, Cl 4.01. HRMAS
NMR (CDCl3, chemical shifts in ppm): 1H: 3.65 (α-CH2), 3.77 (β-CH2);
13C: 42.9 (α-CH2), 71.4 (β-CH2).
Synthesis of [P-H](1−t)[P-(CH2)2(OCH2CH2)5SnPhnCl3−n]t (n = 1, 2)
(HG-SnPhnCl3−n
)
At room temperature and under an inert N2 atmosphere, HgCl2
(0.66 g, 2.43 mmol) was dissolved in dry THF and transferred via a
capillary tube into a Schlenk flask containing 0.4 g of HG-SnPh3
immersed in dry THF. The reaction was monitored for reaction
times of 1 h, 2.5 h, 4 h and 7 h. After reaction, the beads were
thoroughly washed with THF (8 × 15 ml) and CH2Cl2 (2 × 15 ml),
and dried under reduced pressure at 35 ◦C.
Experimental mass fractions (wt%): (1 h) H 6.99, C 68.09, Sn 7.91,
O 10.35, Cl 4.88; (2.5 h) H 7.01, C 67.61, Sn 8.03, O 10.37, Cl 4.65;
(4 h) H 7.04, C 67.67, Sn 7.91, O 10.06, Cl 4.44; (7 h) H 7.14, C 67.75,
Sn 7.03, O 10.44, Cl 5.10.
HG-Bz-Cl [P-(CH2)2(OCH2CH2)5O-Bz-Cl] was synthesized from
HG-Bz-OH (elemental analysis: H 8.18, C 79.61, O 10.83, resulting
in t = 0.189), using exactly the same procedure as applied for the
synthesis of HG-Cl.
Experimental mass fractions (wt%): H 7.58, C 76.31, O 9.73, Cl
5.03, resulting in t = 0.235; calculated mass fractions for t = 0.189
as obtained for HG-Bz-OH: H 7.65, C 78.35, O 10.22, Cl 3.78.
HRMAS NMR (CDCl3, chemical shifts in ppm): 1H: 4.56 (Bz-CH2),
6.92 (Bz-CHo), 7.31 (Bz-CHm), 4.15 (1-CH2), 3.88 (2-CH2); 13C: 46.4
(Bz-CH2), 114.8 (Bz-Co), 130.0 (Bz-Cm), 130.0 (Bz-Cp), 158.9 (Bz-Ci),
67.6 (1-CH2), 69.8 (2-CH2).
Catalysis Experiments
Ethyl acetate and n-octanol were used in a 4 : 1 molar ratio.
A mixture of ethyl acetate (3.9 g, 44 mmol), n-octanol (1.4 g,
11 mmol) and supported catalyst (0.11 mmol Sn; 1 mol% Sn)
was gently stirred at reflux temperature while the generated
ethanol was distilled off to drive the reaction to completion. For
monitoring the time evolution of the conversion in the course
of the transesterification, negligibly small aliquots of 50 µl of
reaction mixture were regularly tapped and analyzed by 1H NMR
spectroscopy after evaporation of ethyl acetate and ethanol. The
ratio of initial octanol to obtained octyl acetate was determined
by integration ( 1%) of the respective CH2O 1H NMR resonances.
After each transesterification run, the catalyst was filtered off and
gently washed with ethyl acetate.
Synthesis of [P-H](1−t)[P-(CH2)2(OCH2CH2)5SnPh3]t (HG-SnPh3)
HG-SnPh3 was synthesized from HG-Cl using the same procedure
asdescribedpreviouslyforthecross-linkedpolystyrene-supported
P-C11-SnPh3.[15] Diisopropylamine (DIA, 0.11 g, 1.13 mmol) and
butyllithium(n-BuLi,2.5M inhexane,1.13 mmol)wereaddedtodry
THF (2 ml) at 0 ◦C under a dry nitrogen atmosphere. After 10 min,
Ph3SnH (0.41 g, 1.18 mmol) was added and the mixture was stirred
for 20 min. Using a capillary tube and under N2 atmosphere, the
obtained solution of Ph3SnLi was transferred into another Schlenk
tubecontaining0.5 gofHG-ClindryTHFatroomtemperature.The
mixture was stirred for 4 h at room temperature. Subsequently,
the beads were washed twice with 30 ml of dry THF and the
lithiostannylation reaction was repeated with the same amounts
of reagents overnight. The obtained target organotin graft [P-
H](1−t)[P-(CH2)2(OCH2CH2)5SnPh3]t, HG-SnPh3, was washed with
THF–H2O (2 : 1; 30 ml), THF (6 ×30 ml) and CH2Cl2 (2 ×30 ml). The
beads were dried under vacuum at 35 ◦C.
Results and Discussion
Synthesis
In the present work, it is aimed to develop novel supported
organotin catalysts starting from readily available materials. The
followed strategy consists of anchoring organotin functionalities
onto commercially available solid supports called HypoGel
resins.[19] In these systems, linear poly(ethylene glycol) (PEG)
chains are grafted onto a weakly cross-linked (1% divinyl
benzene comonomer – DVB) polystyrene matrix, carriers of which
have already been successfully used in solid-phase peptide
synthesis.[20–24] In the present study, two types of HypoGels
are used as suitable precursors for the desired organotin grafts,
differing only by the nature of the terminal functional group X of
the PEG spacer (Fig 1).
Organotin moieties are incorporated stepwise at the free chain
endofthesupportedPEG, initiallyfollowingthestandardsynthetic
methodology used for the polystyrene-grafted P-C11-SnCl3,[15]
and aiming likewise at synthesizing a functionally pure grafted
organotin trichloride. The general reaction scheme toward the
HypoGel-grafted organotin derivatives is presented in Scheme 1.
Experimental mass fractions (wt%): H 7.23, C 76.15, Sn 7.34, O 7.69,
resulting in t = 0.171; calculated mass fractions for t = 0.204
as obtained for HG-Cl: H 7.03, C 75.06, Sn 10.70. HRMAS NMR
(CDCl3, chemical shifts in ppm, coupling constants in Hz): 1H: 1.90
[2J(119/117Sn,1H) 58] (α-CH2), 3.85 (β-CH2), 7.62 (Ph-CHo), 7.38 (Ph-
CHm), 7.38 (Ph-CHp); 13C: 13.6 [1J(119/117Sn,13C) 380] (α-CH2), 69.0
(β-CH2), 137.2 (Ph-Co), 128.5 (Ph-Cm), 128.9 (Ph-Cp), 139.1 (Ph-Ci);
119Sn: −103.
HG-Bz-SnPh3 [P-(CH2)2(OCH2CH2)5O-Bz-SnPh3] was synthe-
sized from HG-Bz-Cl using exactly the same procedure as applied
for the synthesis of HG-SnPh3. Experimental mass fractions (wt%):
H 7.02, C 75.99, Sn 6.87, O 7.89, resulting in t = 0.180; calculated
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Appl. Organometal. Chem. 2010, 24, 135–141

Organotin catalysts grafted onto cross-linked polystyrene supports
HgCl2 both on molecular BuSnPh3 and on polystyrene-grafted
P-C11-SnPh3, yielding quantitative conversion into the targeted
alkyltin trichloride functionality after respectively 4 and 14 h at
reflux,thesamereactionprocedureisinvestigatedontheHypoGel-
supportedorganotingraftHG-SnPh3.Aseriesofexperimentswere
performed both at reflux and room temperature, monitoring the
obtainedorganotingraftaftervariousreactiontimes.Inallcases,its
119SnHRMASspectrumpointedtowardamixtureoftwosupported
organotin species, which were identified to be the respective
PEG-grafted organotin mono- and dichloride, i.e. HG-SnPh2Cl
and HG-SnPhCl2. In contrast with the C11-grafted organotins,
further substitution of the tin-bound phenyl groups toward the
targeted organotin trichloride does not occur with these PEG-
grafted derivatives. In order to elucidate why the incorporation
of a more polar spacer impedes the successful substitution of
all three phenyl groups, these particular PEG-grafts have been
the object of a detailed and advanced structural investigation
presented hereunder.
Figure 1. HypoGel (HG) resins used as precursors in the synthesis toward
grafted organotins.
In the first step, the hydroxyl group at the spacer end is substi-
tuted for a chlorine atom. In this way, the polystyrene–PEG grafts
become susceptible to subsequent lithiostannylation, allowing for
the end-functionalization with the organotin moiety. The chlo-
rination reaction of both types of hydroxy-terminated HypoGels
is carried out with thionyl chloride (SOCl2) at reflux temperature,
usingtetrahydropyrane(THP)asthesolvent(Scheme1). After24 h,
the reaction mixture is cooled to room temperature and the target
graft undergoes a thorough washing procedure, after which the
resin beads are dried under reduced pressure. Subsequently, the
stannylated HypoGel grafts are synthesized following the same
procedureaspreviouslyusedforthepolystyrene-supportedunde-
cyltriphenyltin, P-C11-SnPh3.[15] Triphenyltin lithium is prepared
by lithiation of purified, commercially available triphenyltin hy-
dride and is subsequently allowed to react with the supported
chlorine-terminated spacer, yielding the desired grafted triph-
enyltin moiety.
The last synthesis step in the reaction pathway toward
HypoGel-grafted organotin catalysts consists of substituting the
three tin-bound phenyl groups for chlorine atoms. Since a first
attempt based on a previously established procedure involving
the use of a HCl–MeOH solution[15] failed to yield the desired
organotin graft, other strategies were explored in order to
chlorinate the tin atom under phenyl group splitting in HG-
SnPh3 and HG-Bz-SnPh3. Kemmer et al. reported the synthesis
of (4,7-dioxaoctyl)phenyldichlorostannane by treatment of the
corresponding triphenyltin compound with anhydrous HCl at
−78 ◦C in methylene chloride.[25] Extending this procedure to
the HypoGel-supported triphenyltin derivatives did however
not result in the targeted organotin trichloride. For HG-SnPh3,
the reaction provided no observable 119Sn HRMAS resonance
at all, while for HG-Bz-SnPh3, a single sharp resonance at
−28 ppm indicated the generation of non-grafted Ph2SnCl2 (see
below). Therefore, as an alternative to the above mentioned
synthesis routes, a procedure in which the phenyl groups on
tin are chlorinated using mercury(II) chloride (HgCl2), has been
explored. After successfully performing model reactions with
Structural Investigation
Reaction monitoring and structural elucidation of the HypoGel
resin and its supported (organotin) functionalities is achieved
using a combination of 1D and 2D 1H, 13C and 119Sn HRMAS NMR.
After substitution of the hydroxyl group at the spacer end in HG-
OH and HG-Bz-OH for a chlorine atom, both 1H and 13C spectra
of HG-Cl and HG-Bz-Cl display a new characteristic resonance for
a terminal chloromethyl moiety, while the one from the terminal
hydroxymethyl group of the precursor is no longer present. This
presence or absence of characteristic fingerprint resonances from
one synthesis step to the next enabled us to ascertain the degree
of completion of the reaction. The assignments of most important
1H and 13C chemical shifts are summarized in the Experimental
section.
After lithiostannylation, the functional identity of both organ-
otin grafts HG-SnPh3 and HG-Bz-SnPh3 was confirmed in their
119Sn HRMAS spectra (Fig 2, left contour plot), displaying a res-
onance at −103 and −118 ppm, respectively, characteristic for
alkyl-[26,27] and benzyltriphenyltins.[26a] However, the 119Sn spec-
tra of the stannylated target grafts HG-SnPh3 and HG-Bz-SnPh3
indicated the additional, undesired presence of a minor side-
functionality at −100 ppm (accounting for ca 6%) for the former
and −103 ppm (accounting for ca 7%) for the latter. Since these
triphenylstannylated grafts serve as precursors for subsequent
grafted target catalysts, it is important to identify the nature of
these undesired side-functionalities, an issue which is addressed
using information from 2D 1H–119Sn HSQC HRMAS NMR spectra
(Fig 2).
Scheme 1. General synthesis procedure toward the HypoGel-grafted organotins (n = 1, 2). The second class of HypoGels with benzyl anchoring group
(denoted as HG-Bz-X; X = OH, Cl, SnPh3) are synthesized according to the same procedure.
c
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V. Pinoie, M. Biesemans and R. Willem
Figure 2. 1H-119Sn HSQC HRMAS spectra of the target HG-SnPh3 (left) and HG-Bz-SnPh3 (right) grafts, with indication of the side-functionalities
(encircled).
In the 119Sn spectrum of HG-SnPh3, only a slight difference
in chemical shift between the side-functionality and the target
resonance is observed, indicative of very similar chemical
environments around the tin atom. On the other hand, the non-
existence of a 1H–119Sn correlation between the 119Sn nucleus of
theside-functionalityandethyleneoxideprotons(Fig 2,left),along
withtheobservationoftwocross-peaksat1Hchemicalshiftsof1.75
and 1.54 ppm, suggests the absence of a PEG spacer in the grafted
side-functionality. Combining the latter data with the existence
of a H–119Sn correlation peak with an aromatic H resonance,
the grafted side-product has to involve a functionality of the type
P-(CH2)2SnPh3. In the case of HG-Bz-SnPh3, the side-functionality
found at δ(119Sn) −103 ppm (Fig 2, right encircled) displays the
same 2D cross-peak pattern as the identified target graft HG-
SnPh3 (Fig 2, left), only the cross-peak of the meta phenyl protons
collapsing into the noise in the case of the side-functionality of the
HG-Bz-SnPh3 target graft.
In addition, characteristic 2D cross-peak satellites from the
unresolved 1J(119/117Sn,13C) = 380 and 325 Hz and 2J(119/117Sn,1H)
=58and62 Hzcouplingsinthe2D1H–13CHSQCHRMASspectrum
unambiguously identify the organotin grafts HG-SnPh3 and HG-
Bz-SnPh3, respectively. These coupling constants, together with
the respective 119Sn chemical shift values (see Experimental
section), unambiguously provide evidence for four-coordination
at the tin center, as expected from the properties of the grafts and
the non-coordinating nature of the solvent, CDCl3, used for the
measurement.[26]
ThefinalsynthesisstepinthereactionpathwaytowardHypoGel-
grafted organotin catalysts originally aimed at substituting all
three tin-bound phenyl groups for chlorine atoms. 119Sn HRMAS
monitoring of the functionalized resin after various reaction times
however indicates that the targeted organotin trichloride could
not be obtained. After treatment of HG-SnPh3 with HgCl2 during
one hour at reflux temperature, the 119Sn spectrum displays two
broad resonances at −16 and −40 ppm. This observation points
toward a mixture of two supported organotin species, which are
identified as the PEG-grafted organotin mono- and dichlorides,
HG-SnPh2Cl and HG-SnPhCl2, as shown by 2D 1H–13C and
1H–119Sn HSQC HRMAS NMR experiments (Fig 3). Next to an
additional minor resonance at −101 ppm, attributed to residual
unconverted precursor HG-SnPh3, a sharp resonance at −27 ppm
can be assigned to the presence of a non-grafted organotin
compound that could not be completely removed from the resin
beads in spite of thorough washing after reaction. On the basis
of its 1H and 119Sn chemical shifts, this undesired side-product
turns out to be diphenyltin dichloride, Ph2SnCl2, as shown by 1H
and 119Sn NMR measurements performed on the obtained graft
to which aliquots of Ph2SnCl2 have been added.[26]
Unfortunately, next to the formation of organotin grafts
HG-SnPh2Cl and HG-SnPhCl2, characteristic cross-peaks in
the 1H–13C HSQC HRMAS spectrum indicate the additional
generation of an undesired alcoholic side-functionality HG-OH.
This observation clearly indicates that only partial phenyl group
cleavage from the grafted organotin functionalities can occur
and that even part of the organotin itself is cleaved from its
polar spacer, consequently leading both to functionally non-pure
grafts and undesired recovery of starting HG-OH functionality.
Treating HG-SnPh3 with HgCl2 for longer reaction times, up to
2.5 and 4 h, results in an additional loss in 119Sn signal, while the
corresponding 13C spectra reveal the increasing generation of the
alcoholic functionality HG-OH. At this point, the exact origin of this
split-off of grafted organotins is not yet fully understood since all
reactions are conducted under strictly inert N2 atmosphere using
dried and freshly distilled solvents for the reaction and washing
procedures, in the same way as for the successfully obtained
grafted undecyl-SnCl3 catalyst.[15]
1
1
1
Fig 3 (top right) illustrates the power of the 2D H–13C HSQC
HRMAS technique, as both carbon atoms adjacent to the tin
atom in the two organotin grafts can be differentiated, and their
1J(119/117Sn, 13C) coupling constants can be extracted separately.
The 1J(13C, 119Sn) coupling constants of HG-SnPh2Cl and
HG-SnPhCl2, respectively 555 and 643 Hz, are fairly higher
than those of the similar molecular compounds (nBu)SnPh2Cl
[1J(13C, 119Sn) = 429] and (nBu)SnPh2Cl [1J(13C, 119Sn) =
503 Hz], taken as references. These moderately high 1J(13C,
119Sn) coupling constant values point toward the existence
of an intra- or intermolecular coordination expansion at tin
in HG-SnPh2Cl and HG-SnPhCl2, most probably due to the
presence of oxygen atoms in the PEG spacer. Obviously, such a
nucleophilic interaction toward tin is not possible in the reference
compounds (nBu)SnPh2Cl and (nBu)SnPhCl2. Expansion of the
coordination state of tin by additional intramolecular interactions
with electronegative substituents containing an O or N atom
has indeed been reported before.[25,28] More specifically, similar
coordination expansions were observed for the compound (4,7-
dioxaoctyl)phenyldichlorostannane[25] (Fig 4).
As compared with the investigated graft HG-SnPhCl2, the
latter compound, having a tin chemical shift of −74 ppm in
solution,[25] displays only a slight low-frequency shift of about
1
30 ppm. Also, its J(119/117Sn,13C) coupling constant of 629 Hz is
not very different from the value of 643 Hz measured for HG-
SnPhCl2. These additional observations are further evidence for
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Appl. Organometal. Chem. 2010, 24, 135–141

Organotin catalysts grafted onto cross-linked polystyrene supports
Figure 3. (left) 119Sn HRMAS NMR spectra of HG-SnPh3 (a), and after reaction with HgCl2 at room temperature during (b) 1 h; (c) 7 h. (top right) Detail of
the α-(CH2) cross-peaks of HG-SnPh2Cl (I) and HG-SnPhCl2 (II) in the 2D 1H-13C HSQC spectrum, displaying the fine structure as a result of scalar coupling
interactions involving the 119/117Sn nuclei. (bottom right) 2D 1H-119Sn HSQC HRMAS NMR spectrum displaying, next to the α- and β-(CH2) cross-peaks,
correlations with the aromatic ortho and meta protons of the tin-bound phenyl groups.
Figure 4. Dynamic equilibrium for (4,7-dioxaoctyl)phenyldichlorostannane as proposed by Kemmer et al.[25]
.
the existence of such O → Sn interactions in the latter organotin
graft. It is reasonable to assume that they are also responsible
for the partial cleavage of the organotin functionality observed in
the course of the final reaction step. It has indeed been reported
that fairly remarkable changes in reactivity can occur due to
intramolecular nucleophilic assistance at tin,[29] with examples in
which the cleavage of alkyltin bonds is preferred over aryltin bond
cleavage.
A second set of experiments in which the reaction temperature
is reduced to room temperature seems to avoid the dramatic
loss in 119Sn resonance with increasing reaction times (Fig 3,
left), even though the formation of some undesired HG-OH
is still observed in the 13C spectra of all reaction products.
Furthermore, independently of the reaction duration, the 119Sn
spectra continuously display a mixture of two different organotin
grafts, most likely involving HG-SnPh2Cl and HG-SnPhCl2 as
suggested above. The further substitution of the tin-bound phenyl
groups appears to be hampered by the intra- and/or interspacer
donor–acceptor O → Sn interactions due to the presence of
oxygen atoms in the linker. This assumption is strengthened by a
2D 1H–119Sn HSQC correlation experiment (Fig 3, right) in which
both 119Sn resonances are found to be correlated to aromatic
1H resonances, confirming the incomplete splitting off of all three
phenylgroupsontin.Additionally,forbothgraftsHG-SnPh2Cland
HG-SnPhCl2, the spectrum exhibits clear 1H–119Sn correlations
between the tin resonance and the protons of methylene groups
α- and β-(CH2).
HypoGel-supported organotin grafts HG-Bz-SnPh3. Overall, this
approach does not lead to successful results. After a reaction
duration of 2.5 h, the 119Sn HRMAS spectrum displays a sharp
resonance at −28 ppm (similar to the one previously observed
for the other HypoGels) on top of a weakly intense and broad
signal, rising from an organotin graft we failed to identify. The low
signal-to-noise ratio of this broad 119Sn resonance presumably
indicates loss of grafted tin with respect to the original input
quantity of grafted organotin. 119Sn HRMAS monitoring of the
reaction product at a shorter reaction time of half an hour again
demonstrates the generation of Ph2SnCl2 resonating at −28 ppm
next to residual unconverted HG-Bz-SnPh3. In view of the latter
data and considering the moderate results previously obtained
for the first type of HypoGels, along with the hypothesized intra-
and/or interspacer O → Sn associations, it was decided not to
proceed with further refinement of the synthesis pathway for the
benzyl-containing HypoGels.
Catalysis
In order to investigate the effect of increased spacer polarity on
the catalytic activity, the performance of the HypoGel-supported
organotin catalyst, i.e. a mixture of grafted HG-SnPh2Cl and
HG-SnPhCl2, is explored in the same model transesterification
of ethyl acetate and n-octanol (4 : 1) used earlier for the cross-
linked polystyrene-supported catalysts.[15] As such, unambiguous
comparison can be made between catalytic activities obtained by
both type of grafted organotin catalysts.
Finally, the same reaction procedure using HgCl2 in THF at
room temperature is also investigated on the second type of
Fig 5 shows that in a first transesterification run with the
HypoGel-supported catalyst (ꢀ), only 41% conversion is achieved
c
Appl. Organometal. Chem. 2010, 24, 135–141
Copyright ꢀ 2009 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/aoc

V. Pinoie, M. Biesemans and R. Willem
erate catalytic efficiency observed for these HypoGel-supported
organotins. In addition, as nucleophilic assistance at tin has been
reported to aid the cleavage of the aliphatic carbon–tin bond,[29]
the poor leaching resistance of the catalyst is readily explained.
Conclusion
The object of the present study was to increase the local
polarity around the tin atom in polystyrene-supported organotin
derivatives. In this context, an increased Lewis acidity of the tin
site was aimed for through the introduction of polar spacers; the
suitability of a HypoGel support bearing oligomeric poly(ethylene
glycol) chains to act as an insoluble carrier for organotin catalysts
was therefore investigated. Yet the targeted HypoGel-supported
organotin trichloride could not be obtained; the rather simple
and straightforward synthesis procedure set up for the C11-
grafted polystyrenes proved to be inappropriate when applied
to this type of support. Only by reaction of a grafted triphenyltin
moiety with the highly toxic mercury derivative HgCl2 did we
succeed in obtaining a mixture of a PEG-grafted diphenyltin
chloride and phenyltin dichloride. The further substitution of the
tin-bound phenyl groups appeared to be hampered by intra-
and/or interspacer donor-acceptor O → Sn interactions due to
the presence of oxygen atoms in the polar linker. It is also
assumed that these O → Sn interactions are at the origin of
the moderate catalytic efficiency observed for these HypoGel-
supported organotins. For this reason, and in view of their low
leaching resistance and their inability to be recycled, the HypoGel-
supported catalysts show to be less suitable for transesterification
than the C11-grafted polystyrenes. As a conclusion, it can be
stated that tuning the spacer toward higher polarities by the
incorporation of electronegative atoms such as oxygen does not
offer a new track toward improved catalytic performances of
grafted organotins. Rather is it even recommended from the
present study to avoid any additional source of coordination
expansion of the tin atom, except for the one caused by the
transesterification reaction components themselves.
Figure 5. Time evolution towards equilibrium in a first transesterification
runofethylacetateandn-octanol,usingthepolystyrene-supportedP-C11-
SnCl3 (ꢁ) and the HypoGel-supported HG-SnPhnCl3−n (ꢀ) as a catalyst.
The dashed vertical line corresponds to a reaction time of 2 hours.
after two hours. Consequently, the latter proves to be much
less efficient than the polystyrene-supported alkyltin trichloride
P-C11-SnCl3, (Fig 5, ꢁ) leading to chemical equilibrium at 80%
conversion in a reaction time of ca 2 h.
Inviewofthepossiblepresenceofinter-and/orintraspacerO→
SninteractionsintheHypoGel-supportedcatalyst,itislegitimateto
assume that the reaction rate – even if dealing with a hypothetic
functionally pure HypoGel-supported organotin trichloride – is
considerably influenced by these interactions, hampering the
accessibility of the reactants to the catalytic sites, and therefore
negatively affecting the overall conversion rate. Intramolecular
interactions are believed to lower the Lewis acidity of the tin atom
aswell, andaccordingly, itscatalyticactivity. Moreover, monitoring
of the catalyst with 119Sn HRMAS NMR after the first catalytic run
(Fig 6, bottom left) revealed the complete collapse of the two
119Sn resonances characteristic for HG-SnPh2Cl and HG-SnPhCl2,
which were present in the non-used catalyst (Fig 6, top left). No
119Sn HRMAS resonance was observed either at lower frequencies,
whereas the solid-state 117Sn MAS NMR spectrum of the used
catalyst displayed an anisotropy pattern around −600 ppm. In
line with previous results obtained for the polystyrene-supported
catalyst P-C11-SnCl3, these parallel observations can be ascribed
to organotin cross-linking at the interface.[15]
Furthermore, the HypoGel-supported catalyst shows to be
poorly recyclable. Reusing the catalyst in a second transesterifica-
tion run, results in a severe decrease in catalytic activity, reaching
a conversion degree of only 9% after 2 h. An explanation for this
observation can be found in the high leaching degree displayed
by the HypoGel catalyst. After a single run with fresh catalyst, a tin
content as high as 5860 ppm was measured in the final reaction
product. The latter corresponds to a dramatic loss of 70% with
respect to the initial amount of grafted tin present in non-used
catalyst. After the second run, another 420 ppm of leached Sn
was detected in the reaction mixture. Again, it is reasonable to
assume that the O → Sn interactions are at the origin of the mod-
In terms of characterization of any grafted species, 1D and
2D 1H, 13C and 119Sn HRMAS NMR spectroscopy confirmed its
high potential for monitoring solid-phase synthetic procedures,
conveniently allowing the assessment of the functional purity of
the synthesized grafts. Clearly identifiable tin coupling patterns in
1
the 2D H–13C HSQC spectra provided crucial information with
regard to the tin coordination state and the identification of the
target organotin graft.
Acknowledgments
Financial support by the Fund of Scientific Research Flanders
(FWO, Belgium; grant G.0469.06) and the Research Council
Figure 6. (left) 119Sn HRMAS NMR spectrum of non-used HypoGel-supported catalyst (top) and after the first transesterification run (bottom). (right) 117Sn
CP-MAS spectrum after the first run.
c
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Copyright ꢀ 2009 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2010, 24, 135–141

Organotin catalysts grafted onto cross-linked polystyrene supports
(Onderzoeksraad) of the Vrije Universiteit Brussel (Concerted
Research Action, grant GOA31) to R.W. and M.B. is gratefully
acknowledged. V.P. acknowledges a PhD grant (Concerted
Research Action, grant GOA31).
[18] F. A. G. Mercier, Synthesis, characterization and catalytic activity
of dialkyltin compounds grafted onto insoluble cross-linked
polystyrene, Ph.D. Thesis, VUB, 2002.
[19] Rapp Polymere GmbH, http://www.rapp-polymere.com.
[20] H. S. Hiemstra,
W. E. Benckhuijsen,
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Copyright ꢀ 2009 John Wiley & Sons, Ltd.
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