Mechanistic elucidation of monoalkyltin(iv)-catalyzed esterification

Article abstract of DOI:10.1039/d1cy00184a

Monoalkyltin(iv) complexes are well-known catalysts for esterification reactions and polyester formation, yet the mode of operation of these Lewis acidic complexes is still unknown. Here, we report on mechanistic studies ofn-butylstannoic acid in stoichiometric and catalytic reactions, analyzed by NMR, IR and MS techniques. While the chemistry ofn-butyltin(iv) carboxylates is dominated by formation of multinuclear tin assemblies, we found that under catalytically relevant conditions only monomericn-BuSn(OAc)₃and dimeric (n-BuSnOAc₂OEt)₂are present. Density functional theory (DFT) calculations provide support for a mononuclear mechanism, wheren-BuSn(OAc)₃and dimeric (n-BuSnOAc₂OEt)₂are regarded as off-cycle species, and suggest that carbon-oxygen bond breaking is the rate-determining step.

Full text of DOI:10.1039/d1cy00184a

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Mechanistic elucidation of monoalkyltin(IV)-
catalyzed esterification†
Cite this: DOI: 10.1039/d1cy00184a
c
Lukas A. Wolzak,ab Joen J. Hermans,
*
Folkert de Vries,d Keimpe J. van den Berg,e
ad
a
Joost N. H. Reek,
b Moniek Tromp
and Ties J. Korstanje
*
*
Monoalkyltin(IV) complexes are well-known catalysts for esterification reactions and polyester formation,
yet the mode of operation of these Lewis acidic complexes is still unknown. Here, we report on
mechanistic studies of n-butylstannoic acid in stoichiometric and catalytic reactions, analyzed by NMR, IR
and MS techniques. While the chemistry of n-butyltin(IV) carboxylates is dominated by formation of
multinuclear tin assemblies, we found that under catalytically relevant conditions only monomeric
n-BuSn(OAc)3 and dimeric (n-BuSnOAc2OEt)2 are present. Density functional theory (DFT) calculations
provide support for a mononuclear mechanism, where n-BuSn(OAc)3 and dimeric (n-BuSnOAc2OEt)2 are
regarded as off-cycle species, and suggest that carbon–oxygen bond breaking is the rate-determining step.
Received 1st February 2021,
Accepted 17th February 2021
DOI: 10.1039/d1cy00184a
rsc.li/catalysis
such as SnCl4 are not considered good esterification catalysts.
The dialkyltin(IV) compounds form distinct ladder-like
Introduction
The synthesis of esters from alcohols and carboxylic acids is an
important reaction in organic synthesis of small molecules and
polymers.1,2 Although strong Brønsted acids are efficient
esterification catalysts, often the milder Lewis acids are
preferred since they provide fewer unwanted side products.3
Therefore over the past decades, a variety of Lewis acidic
organotin derivatives have been developed and these catalysts
are frequently used in industry.4,5 The inherent Lewis acidity of
tin(IV) complexes in combination with facile ligand exchange are
considered the origin of their catalytic performance in the
esterification reaction.6 Especially organometallic mono- and
dialkyltin(IV) complexes display good catalytic performance and
stability in many transesterification, esterification and
polyesterification reactions.7 The presence of the alkyl tails,
generally n-butyl or n-octyl, appears to be pivotal for the catalytic
behavior of these tin-based catalysts, since strong Lewis acids
structures, distannoxanes, which can incorporate an alcohol
and activate the carbonyl function of the carboxylic acid.8–11
Although still under debate, the catalytic activity of these
distannoxanes is often attributed to the unique cooperative
behavior of the tin atoms in the distannoxanes.12–14 For
applications of distannoxanes as catalysts in transesterification
reactions see the work of Otera and co-workers.15–17
Monoalkyltin(IV) catalysts have received far less attention
than the distannoxanes. Nevertheless the chemistry of
a Sustainable Materials Characterization, van 't Hoff Institute for Molecular
Sciences (HIMS), University of Amsterdam, Science Park 904, 1098 XH Amsterdam,
The Netherlands. E-mail: t.j.korstanje@uva.nl, moniek.tromp@rug.nl
b Bio-inspired, Homogeneous and Supramolecular Catalysis, van ‘t Hoff Institute
for Molecular Sciences (HIMS), University of Amsterdam, Science Park 904, 1098
XH Amsterdam, The Netherlands. E-mail: j.n.h.reek@uva.nl
c Molecular Photonics, van 't Hoff Institute for Molecular Sciences (HIMS),
University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
d Faculty of Science and Engineering, Materials Chemistry – Zernike Institute for
Advanced Materials, University of Groningen Nijenborgh 4, 9747 AG Groningen,
The Netherlands
e Akzo Nobel Car Refinishes BV, Rijksstraatweg 31, 2171 AJ Sassenheim, The
Netherlands
† Electronic supplementary information (ESI) available. CCDC 2049109. For ESI
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
d1cy00184a
Scheme 1 Stoichiometric reactions of monoalkyltin complexes with
carboxylic acids compared to their behavior under catalytic
esterification conditions.
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n-butyltin(IV) complexes with carboxylic acids has been well
studied by X-ray crystallography (Scheme 1).18,19 The complex
with the highest carboxylate to tin ratio, 3 : 1, is n-butyltin
tricarboxylate and is synthesized from n-butyltin trichloride
and the appropriate silver carboxylate (Scheme 2).20 This
hydrolytically unstable complex decomposes in the presence
of water to undefined polymeric material or to a ladder-
shaped complex, [(n-BuSn(O)OOCR)2-n-BuSn(OOCR)3]2, with a
tin to carboxylate stoichiometry of 3 : 5.21 Further hydrolysis
results in the formation of a drum-shaped cluster with a tin
to carboxylate stoichiometry of 1 : 1. This [n-BuSn(O)OOCR]6
cluster can also be obtained from the reaction of polymeric
n-butylstannoic acid with a carboxylic acid in the appropriate
ratio.22,23 Although all of these multinuclear tin complexes
have been synthesized and spectroscopically analyzed, their
relevance under catalytic conditions remains largely
unexplored. It is well possible that under catalytic conditions
these tin clusters disintegrate, in contrast to the
distannoxanes, and esterification happens via a mononuclear
mechanism. Here, we report on our mechanistic investigation
of mono-n-butyltin(IV)-catalyzed esterification. This class of
catalysts was studied under catalytically relevant conditions
with a variety of spectroscopic techniques complemented
with DFT calculations.
order to exclude the need of dehydrating agents or azeotropic
distillation.
There is a clear difference in catalytic activity between the
tin(IV) compounds and the n-butyltin(IV) substituted
derivatives, with the latter being more active. The polymeric
n-butylstannoic acid (1), the drum-shaped [n-BuSn(O)OAc]6
(2) and monomeric n-BuSn(OBz)3 (4) display the highest
activity (Table 1, entries 5, 6 and 8). These three compounds
have comparable activity, suggesting that catalytic activity is
not related to unique properties of the n-butylstannoic acid
polymer. Counter-intuitively, n-BuSn(OAc)3 (3) shows inferior
performance, which we ascribe to the slow displacement of
the tin-bound acetates for benzoate groups. In addition, an
order of 0.74 in catalyst (Fig. S2†) was found for 1 (vide infra
for interpretation).
In order to study the behavior of n-butyltin(IV) complexes
under catalytically relevant conditions we investigated
solutions of 1, 2 and 3 by NMR spectroscopy. With three
NMR-active nuclei with spin ½, 119Sn being the most sensitive,
Sn NMR provides
a valuable tool for examining the
coordination environment of tin complexes in solution.24
Various organotin complexes retain their geometry in
anhydrous organic solvents, as demonstrated by 119Sn NMR
experiments.18,20 In anhydrous CDCl3 a singlet in the 119Sn
NMR spectrum at −480 ppm is observed for cluster 2 and
from 1H NMR a 1 : 1 ratio of the acetate groups and the
Results and discussion
n-butyl tails can be deduced.22,25 For monomer
3 in
anhydrous CDCl3 a signal at −532 ppm is present, which is in
We started our investigation by establishing the catalytic
activity of various tin(IV) compounds. Also n-butylstannoic
acid and acetic and benzoic acid derivatives thereof were
examined in a model esterification reaction between benzoic
acid and heptanol (in a 1 : 10 ratio) in the presence of 1
mol% catalyst (Table 1). An excess of alcohol was used in
1
the expected range of a seven-coordinate tin complex. The H
NMR spectrum displays the expected 3 : 1 ratio of the acetate
groups and the n-butyl tail. For polymeric 1 no 1H NMR
spectra could be obtained due to its insoluble nature in
common organic solvents.
After establishing the NMR shifts in CDCl3 we switched to
acetic acid-d4 as solvent and measured the tin precursors at
various temperatures (Table 2). At 298 K complex 3 shows a
singlet at −528 ppm. This minor upfield shift compared to 3
in CDCl3 indicates that the seven-coordinate geometry of
n-BuSn(OAc)3 is retained in acetic acid (AcOH). Upon
increasing the temperature to 363 K a downfield shift to −518
ppm, Δδ 10.5 ppm, is observed which is caused by a
decreased temperature-induced shielding (Table 2, column
2).26 Upon dissolving 2 in AcOD-d4 at 298 K a singlet at −528
ppm, corresponding to monomer 3, and a peak at −553 ppm
is observed (Table 2, column 3). Upon heating to 363 K we
again only observe a singlet at −517 ppm indicative for
complex 3. In addition, this process turned out to be
reversible upon lowering the temperature to 298 K. For 1
three peaks at −529, −554 and −592 ppm are present at 298 K
(Table 2, column 4). Likewise, heating to 363 K resulted in a
single signal at −517 ppm. In addition, in 1H and 13C NMR
we observed a similar trend with identical spectra for
complexes 1,
2
and
3
(Fig. S41–S47†) at elevated
temperatures.
These results indicate that these precursors in acetic acid
Scheme 2 Reaction of monoalkyltin complexes with carboxylic acids.
at catalytically relevant temperatures (≥363 K) rapidly form a
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Table 1 Catalyst screening in model esterification reaction
Entrya
Complex
Conv. [%] benzoic acid
Yield [%] heptyl benzoate
1
2
3
4
5
6
7
8
No cat.
Sn(OAc)4
SnCl4·5H2O
n-BuSnCl3
(n-BuSnOOH)n (1)
[n-BuSn(O)OAc]6 (2)
n-BuSn(OAc)3 (3)
n-BuSn(OBz)3 (4)
10
23
31
52
82
87
63
89
6
23
31
50
82
87
63
87
a
All reactions were performed with benzoic acid (5 mmol), heptanol (50 mmol), and catalyst (1 mol%, 0.05 mmol), at 150 °C for 6 h. Yield and
conversion were determined by GC analysis with pentadecane as internal standard.
mononuclear tin tricarboxylate in situ (Fig. 1). To obtain more
insight into the origin of the two high-field signals around
−554 and −591 ppm, which appear upon solvation of 1 and 2
in AcOD-d4 at 298 K, we treated complex 3 with various
equivalents of D2O (Fig. 2). The addition of D2O resulted in
identical 119Sn chemical shifts, which reveals that the
formation of these species is dependent on the concentration
of water, rather than Sn atoms.
Upon dissolution in AcOD-d4, the oxo- and hydroxo-
moieties in 2 and 1 become protonated and subsequently
water is formed in situ. Therefore, we propose that these
two shifts emerge from 3 with either one (−554–−556 ppm)
or two (−591–−592 ppm) D2O molecules coordinated.
Similar chemical shifts are observed for the addition of
D2O to 3 dissolved in a 1 : 1 mixture of AcOH and EtOH
(Fig. S49†).
More structural information on the formed tin complexes
was accessed via 119Sn NMR DFT calculations on a B3LYP/
TZVPP all electron level of theory.27,28 Evaluation of the 119Sn
NMR chemical shift of various conformers of 3 with one (Fig.
S5†) and two water (Fig. S6†) molecules coordinated to the
central tin atom resulted in two structures with a good match
between experimental and calculated 119Sn NMR chemical
shift (Fig. 3).
groups has changed coordination mode from bidentate to
monodentate in order to accommodate a hydrogen bond
between the carbonyl oxygen and the proton of water. For
structure 3B a second water molecule is bound to the central
tin core which results in two of the acetate groups adopting a
monodentate coordination mode. The coordination of water
to structure 3A and 3B proved to be reversible upon
increasing the temperature, resulting in the sole formation of
3 (Fig. S50 and S51†).
The effect of water on 3 in AcOH was further studied by
ATR-FTIR spectroscopy via the addition of various equivalents
of H2O to 3 in AcOH. Vibrations at 1588 (νCOO), 660 (νSn–O
)
and 568 cm−1 (νSn–O) appeared to be correlated to the H2O
concentration (Fig. S52 and S53†). Similar experiments using
H218O resulted in a small shift in one of the Sn–O vibrational
modes from 568 to 554 cm−1 (Fig. 4). This isotope effect
further underlines the interaction between 3 and the oxygen
atom from the water molecule(s). Furthermore, experiments
with D2O instead of H2O resulted only in a minor shift in the
COO vibrational mode from 1588 to 1575 cm−1 (Fig. S54†).
Knowing its behavior in AcOH in both the presence and
absence of water, we turned our attention to the 119Sn NMR
spectrum of 3 in anhydrous EtOH at 298 K. Under these
conditions two signals at −532 and −541 ppm were observed,
revealing the partial conversion of 3 into another seven-
coordinate tin complex (Fig. S55†). The structure of this
For the monohydrate complex 3A, one water molecule is
bound trans to the n-butyl group and one of the acetate
Table
2
119Sn NMR chemical shifts in acetic acid-d4 at various
temperaturesa
Temp. (K)
298
3 n-BuSn(OAc)3
−528
2 [n-BuSn(O)OAc]6
1 (n-BuSnOOH)n
−528
−553
−529
−554
−592
−524
−554
−517
333
363
−523
−518
−523
−553
−517
Fig.
1 Formation of n-butyltin triacetate upon dissolution of
a 119Sn chemical shifts are given in ppm relative to Sn(CH3)4. All
multimetallic n-butyltin complexes in carboxylic acid at catalytically
samples are 0.4 M in Sn and measured in AcOD-d4.
relevant temperatures.
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Fig. 4 ATR-FTIR spectra of 3 in AcOH with 2, 5 and 10 equivalents of
H216O (dashed lines) and H218O (solid lines).
Fig. 2 119Sn{H} NMR spectra (186 MHz, 298 K) of the reaction of 3
(0.4 M) with various equivalents of D2O in AcOD-d4 at 298 K. 119Sn
chemical shifts are given in ppm relative to Sn(CH3)4.
119Sn NMR chemical shift together with the calculated free
energy differences (Fig. 5).
These data together point to the formation of the isomer
with both n-butyl groups occupying the trans-position with
respect to the bridging ethoxy groups and transoid with
respect to the other n-butyl group (complex 5A). Solvation of
5A in AcOH resulted in a 119Sn NMR chemical shift at −529
ppm, which indicates complete conversion to 3 (Fig. S62†).
We thus propose the partial formation of the dimer 5A when
3 is dissolved in EtOH, while addition of AcOH results in the
backwards reaction to 3.
With the knowledge of the behavior of the components
under various conditions in hand, we turned our attention to
following the catalyst under catalytically relevant conditions.
The catalytic esterification of acetic acid and ethanol at 363
K, in the presence of molecular sieves to mimic the
azeotropic distillation conditions commonly applied, was
monitored over time (18.7% yield ethyl acetate after 1 h, Fig.
complex was further elucidated with ATR-FTIR which gave a
strong vibration at 1020 cm−1, indicative of a C–O stretching
vibration (Fig. S58†).29 In addition, a vibration at 1711 cm−1
revealed the presence of uncoordinated AcOH. Only minor
isotope effects were observed upon changing EtOH to EtOD-
d6 (Fig. S59†). These findings indicate the exchange of one of
the acetate groups for an ethoxy group. Subsequently, an
aliquot of this mixture was studied with LIFDI-MS which
resulted in the detection of the fragments [SnOAc3]+ and [n-
BuSnOAc2OEt–n-BuSnOAcOEt]+ (Fig. S60 and S61†). The in
situ formation of the dimer (n-BuSnOAc2OEt)2 (5) is in
agreement with a seven-coordinate environment around the
tin core as indicated by 119Sn NMR. In addition, the dimer 5
was independently synthesized via the reaction of
n-BuSn(OAc)3 and n-BuSnOEt3, and has
a
119Sn NMR
chemical shift of −544 ppm in CDCl3.30 The bridging mode
of the ethoxy groups in 5 becomes apparent from the distinct
quaternary carbon signal at 182.61 ppm observed for the
acetate groups in the 13C NMR spectra at 233 K, indicating
that all acetate groups are in a terminal position (Fig. S33†).
Discrimination between the different conformers is possible
upon comparison of the experimental and DFT-calculated
Fig. 3 Formation of adducts 3A and 3B from n-BuSn(OAc)3 with H2O
in AcOH. Experimental (green) and calculated (red) 119Sn chemical
shifts (in ppm).
Fig. 5 Conformers of 5 with their 119Sn NMR chemical shift (green, in
ppm) and calculated free energies (blue, in kcal mol−1 relative to
n-BuSn(OAc)3).
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Fig. 6 Time dependent 119Sn{H} NMR spectra (186 MHz, 298 K) of
complex 3 in a 1 : 1 mixture of AcOH and EtOH. All samples are 0.4 M
in Sn and measured unlocked.
S66†). During catalysis, aliquots of the catalytic reaction
mixture were examined by 119Sn NMR spectroscopy. At 363 K
a single 119Sn chemical shift is observed which moves upfield
from −525 to −539 ppm over a time period of 60 minutes
(Fig. S64†), which we attribute to a shift in equilibrium
between monomer
3
and dimer 5A. 119Sn NMR
measurements of the same samples at 298 K revealed two
broad signals with a chemical shift of −535 and −555 ppm
after 5 minutes reaction time (Fig. 6). These two signals can
be assigned to 3 and monohydrated complex 3A respectively.
After 30 minutes reaction time three broad signals at −545,
−553 and −590 ppm were observed. The two upfield signals
originate from n-BuSn(OAc)3 with either one or two water
molecules coordinated (structures 3A and 3B), while the
signal at −545 ppm is consistent with the dimer 5A. After 60
minutes reaction time two new unidentified signals at −550
and −553 are present but the main contributions are still
from structures 3A, 3B and 5A.
Scheme 3 Proposed catalytic cycle for the esterification of ethanol
and acetic acid catalyzed by 3. Hydrogen bonds are indicated with
black dashed lines and free energy values are given in kcal mol−1
relative to 3.
nucleophilic attack (ΔG‡
intermediate F, where the hydrogen atom of the ethanol is
accepted by the Lewis basic ethoxy group. A rotation (ΔG‡
=
18.0 kcal mol−1) results in
=
17.3 kcal mol−1), which requires the breakage of a hydrogen
bond, pre-organizes intermediate G for the carbon–oxygen
bond breaking step. The actual carbon–oxygen bond breaking
has the highest barrier with ΔG‡ = 20.0 kcal mol−1 and can be
regarded as the rate determining step. The collapse of the
The adducts of 3 with H2O (structure 3A and 3B) are only
observed at 298 K, at elevated temperatures water is expected
to be expelled from these complexes and monomeric 3 is
formed, vide supra.
tetrahedral intermediate
G as turnover-limiting step is
To complement our experimental findings under
catalytically relevant conditions, we have further examined
the reaction mechanism for the n-BuSn(OAc)3-catalyzed
esterification by DFT calculations at the BP86-D3/def2-TZVP//
M06-2X-D3/def2-TZVP/def2-QZVPP level of theory (Scheme 3).31
The observed off-cycle resting states, monomer 3 and dimer
5A, are equal in free energy, and both can lead to the
formation of intermediate D (ΔG = 4.5 kcal mol−1) which is
the starting points of the catalytic cycle. The tin-bound ethoxy
group can subsequently accommodate another ethanol
consistent with previously reported metal-catalyzed
esterification and amidation reactions.32,33 From structure H
water is expelled, which was bound via two hydrogen bonds,
and finally the ester is replaced by a new acetic acid
molecule. Overall the reaction is slightly exergonic with ΔΔG
= −1.9 kcal mol−1.
The experimentally observed off-cycle resting states 3 and
5A can both form the amphoteric mixed alkoxide/carboxylate
(structure D) which is the starting point of the monomeric
catalytic cycle. Furthermore, a monomeric catalytic cycle,
with two off-cycle resting states, one monomeric and the
molecule via
a
hydrogen bond (structure E). Next,
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other dimeric, being in equilibrium with each other and
equal in energy, is in agreement with the experimentally
determined catalyst order of 0.74, vide supra.
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since it can act as a Lewis acid to activate the carboxylic acid,
and as a Brønsted base to deprotonate the alcohol during
nucleophilic attack. The importance of having an amphoteric
catalyst in esterification reactions is not only restricted to tin-
based catalysts as we have recently demonstrated for titanium-
based esterification catalysts.34 Furthermore, the function of
the n-butyl tail on the tin catalyst is to enforce a seven-
coordinate tin center which remains during the whole catalytic
cycle as established by DFT calculations.
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Conclusions
We have demonstrated here that n-butyl substituted tin(IV)
complexes have a unique coordination chemistry which
results in
a
monomeric tin catalyst under catalytic
conditions. Although the commonly applied esterification
catalyst n-butyl stannoic acid is a polymer in the solid state,
it transforms to monomeric 3 and dimeric 5A as off-cycle
resting states under catalytically relevant conditions, as
proven by NMR, ATR-FTIR and MS measurements. DFT
calculations lend support to a monomeric mechanism where
structures 3 and 5A are regarded as off-cycle species. In this
mechanism the carbon–oxygen bond breaking step is the rate
determining step. Furthermore, water formed during the
esterification reaction has limited effect on the active
catalyst, since the water adducts 3A and 3B where only
observed at 298 K. These findings shed new light on the role
of the n-butyl tail and the nuclearity of the class of mono-n-
butyl tin esterification catalysts.
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Conflicts of interest
The authors declare no conflict of interest.
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This work is part of the Advanced Research Center for
Chemical Building Blocks, ARC CBBC, which is co-founded
and co-financed by the Netherlands Organization for
Scientific Research (NWO, contract 736.000.000) and the
Netherlands Ministry of Economic Affairs and Climate. In
addition, the authors thank NWO for funding VENI grants
722.016.012 (to T. J. K.) and 016.Veni.192.05 (to J. J. H.). The
authors thank Dr. Andreas Ehlers and Ed Zuidinga for NMR
spectroscopy and mass spectrometry support.
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