Diol-tin ketal as effective catalyst in the tin mediated benzoylation of polyols

Article abstract of DOI:10.1016/j.molcata.2005.08.042

The monobenzoylation of 1-phenyl-1,2-ethanediol in the presence of different tin reagent has been studied. The stannylene ketal of 1-phenyl-1,2-ethanediol has been prepared and used as a catalyst or generated in situ by exchange with other mixed tin ketals. In either case, the acylation occurs rapidly with 2 mol% of the catalyst with the known selectivity. These experiments clearly establish the catalytic nature of the acylation reaction.

Full text of DOI:10.1016/j.molcata.2005.08.042

Journal of Molecular Catalysis A: Chemical 244 (2006) 41–45
Diol-tin ketal as effective catalyst in the tin
mediated benzoylation of polyols
∗
Ezio Fasoli, Antonio Caligiuri, Stefano Servi , Davide Tessaro
Dipartimento di Chimica, Materiali e Ingegneria Chimica G. Natta, Politecnico di Milano, Via Mancinelli 7, 20131 Milano, Italy
Received 21 March 2005; received in revised form 1 June 2005; accepted 30 August 2005
Available online 3 October 2005
Abstract
The monobenzoylation of 1-phenyl-1,2-ethanediol in the presence of different tin reagent has been studied. The stannylene ketal of 1-phenyl-1,2-
ethanediol has been prepared and used as a catalyst or generated in situ by exchange with other mixed tin ketals. In either case, the acylation occurs
rapidly with 2 mol% of the catalyst with the known selectivity. These experiments clearly establish the catalytic nature of the acylation reaction.
©
2005 Elsevier B.V. All rights reserved.
Keywords: Tin ketals; Diols; Monoacylation catalysis
1
. Introduction
than stoichiometric amount, but when the reaction is run in a
microwave apparatus a catalytic amount of Bu2SnO has been
used [3]. This clearly indicates that a stoichiometric amount of
reagent is not necessary. Also, a polymer supported tin reagent
has been used in the selective functionalisation of carbohydrates
using a catalytic [4] amount of tin containing resins. Recent
works describe the use of dimethyltin dichloride (Me2SnCl2)
as an effective catalyst in the monofunctionalisation described
in Scheme 1 [5]. Macrolactonisation and esterification proce-
dure with water removal are also catalysed by Bu2SnO [7].
Recently, a thorough study of the monotosylation reaction of
diols with catalytic Bu2SnO has been published [8]. The possi-
bility of performing reactions with non-stoichiometric amounts
of reagents is highly desirable particularly in the case of highly
toxic compounds difficult to remove from the reaction mixture
like lipophilic alkyl tinreagents [6]. Bu2SnOhas been used in the
monofunctionalisation of carbohydrates for a long time [1]. Due
to solubility requirements reactions were run in methanol were
Bu2SnO is transformed into Bu2Sn(MeO)2 [1], a much more
reactive specie [9]. This compound has been also employed in a
stoichiometric amount for the monofunctionalisation of a series
of diols [9].
Tin mediation in the monofunctionalisation of polyols allows
to reach two main objectives: high regioselectivity and rate
enhancement.
In compounds containing three or more hydroxyl groups, the
regioselectivity is the prevalent aspect of the reaction and it has
been applied in the selective functionalisation of carbohydrates
[1].
In compounds containing two hydroxyl groups functional-
isation at the primary hydroxyl group prevails, but opposite
selectivity can be eventually obtained with a particular reaction
protocol [2].
In all cases, tin mediated reactions are much faster than
reactions in normal conditions. In fact, oxygen atoms in the
intermediate stannylene ketals are activated toward the nucle-
ophilic attack on the acylating agent. When the acylating agent
is an acid chloride, after the first functionalisation the tin ligand
becomes chlorinated and loses its activation preventing further
reaction.
The literature concerning tin mediated acylations and alky-
lations describes a variety of experimental conditions without
specifying when a catalytic cycle can be active. Most reactions
with dibutyl tin oxide (Bu2SnO) employs reagents in a more
2. Results and discussion
∗
We studied the monobenzoylation of compound 1 with differ-
Corresponding author. Tel.: +39 02 23993047; fax: +39 02 23993080.
E-mail address: Stefano.servi@polimi.it (S. Servi).
ent tin reagents and found that a catalytic cycle can be obtained
1
381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2005.08.042

4
2
E. Fasoli et al. / Journal of Molecular Catalysis A: Chemical 244 (2006) 41–45
Scheme 1.
with a series of non-chlorinated tin derivatives forming the same
stannylene derivative 6 as an active specie. This simple reaction
allows to easily compare different reaction protocols by mea-
suring the reaction rate and the product distribution.
Scheme 2.
Table 1
Time of complete conversion of 1 → 2 + 3 in different conditions and 2/3 ratio
When compound 1 was treated at reflux in chloroform for
3
0 min with a catalytic amount of Bu2Sn(MeO)2 (0.02 eq)
and the resulting solution was treated with 1.1 eq of benzoyl
chloride at room temperature in the presence of triethylamine
Run
Catalyst (eq)
Solvent
t (min)
2/3
Base (eq)
1
2
3
4
5
6
7
8
6 (0.02)
6 (0.02)
CHCl3
Toluene
CHCl3
Toluene
2-PrOH
CHCl3
CHCl3
Toluene
10
14
<10
14
12
16
14
TEA (2)
TEA (2)
TEA (2)
TEA (2)
K2CO3 (2)
TEA (2)
TEA (2)
TEA (2)
(TEA) (2 eq) as a base, the benzoylation reaction was complete
10
14
11
12
11
in 10 min [11] with a product distribution similar to the one
observed with the same substrates with other reaction protocols
Bu Sn(MeO) (0.02)
2
2
Bu2Sn(MeO)2 (0.02)
Bu2Sn(2-PrO)2 (0.02)
Bu2Sn(2-PrO)2 (0.02)
Bu4(MeO)2Sn2O (0.02)
Bu4(MeO)2Sn2O (0.02)
[3,5]. Dibutyl tin diisopropoxide (Bu2Sn(2-PrO)2), prepared by
refluxing Bu2SnO in 2-propanol for 1 h and removing the sol-
vent under high vacuum, was as effective as a catalyst for the
reaction in Scheme 1. Finally, tetra butyl dimethoxide distan-
noxane (Bu4Me2Sn2O) obtained by reaction of Bu2SnO and
dimethyl carbonate and further reaction with methanol [10] was
prepared and the same reaction protocol was followed: 1 was
refluxed for 20 min in chloroform in the presence of 0.02 eq of
Bu4(MeO)2Sn2O. The mixture was then treated at room tem-
perature with 1.1 mol of benzoyl chloride and TEA. Analysis of
samples showed that the reaction was complete in about 10 min
time. These experiments strongly suggest that a common inter-
mediate might be responsible for the catalytic cycle. It is well
known that exchange reactions occur between tin alkoxides and
diols thus generating the corresponding tin cyclic ketals [1,8,9].
<10
12
9.5
10
Table 1 reports the data of reaction in Scheme 1 with four
different tin compounds used in a catalytic amount in different
solvents. t(min)indicatesthetimeatwhichthedioliscompletely
transformed after benzoyl chloride addition. 2/3 indicates the
ratio between the two isomeric monobenzoates as determined
1
by H NMR studies.
From the preparative point of view, the use of 6 as catalyst
(0.02 eq) in CHCl3 proved to be the most practical protocol.
3. Conclusions
We have established for the first time the catalytic nature of
the tin acetal 6 in the benzoylation of 1. The present method has
2
.1. Benzoylation catalysed by the stannylene ketal 6
the following peculiarities:
Compound 6 obtained by refluxing 1 with 1 eq of Bu2SnO
in methanol was isolated, characterised and proved to be
identical with the compounds obtained from 1-phenyl-1,2-
ethanediol with a stoichiometric amount of the tin reagents 7–10
(
i) A catalytic amount of tin compound is required, not involv-
ing toxic chloro tin derivatives or microwave irradiation.
(
ii) The stannylene ketal can be generated from commercially
available tin species in the appropriate solvent and then
used as catalyst in the benzoylation reaction in a different
solvent if required.
[12]. 1-Phenyl-1,2-ethanediol 1 (500 mg, 3.6 mmol) in chloro-
form (20 ml) was then treated at room temperature with com-
pound 6 (26 mg, 0.07 mmol, 0.02 eq), benzoyl chloride (509 mg,
3
.6 mmol) and TEA (730 mg, 7.2 mmol). Reaction samples anal-
(
iii) A wide range of solvents from non-polar to isopropanol
can be used allowing to work in homogeneous solution
even with less soluble substrates [13].
iv) The catalytic cycle is active with any of the tin species in
Scheme 3 and the selective acylation goes to completion
in 5–15 min at room temperature (>60 min without the tin
catalyst).
1
ysed by TLC and H NMR showed that compound 1 was com-
pletely transformed in about 10 min [11] giving a 10:1 mixture
of 2 and 3. We believe that these results show that in all the above
experiments there is an exchange reaction between the initially
formed tin alkoxide and the diol to give the ketal 6, which is
the active reaction catalyst. The proposed mechanism for the
catalytic cycle, is reported in Scheme 2.
(
We have not explored the effect of the nature and amount of
base on the selectivity and the rate of the reaction. A few solvents
have been tested but from previous data from the literature [8]
it is reasonable to believe that the choice is much larger.
4
. Experimental
Bu2SnO and the other dibutyl tin derivatives 7 and 8 were
purchased from Sigma–Aldrich. Bu2Sn(2-PrO)2 was prepared

E. Fasoli et al. / Journal of Molecular Catalysis A: Chemical 244 (2006) 41–45
43
Scheme 3.
from Bu2SnO in 2-PrOH as described above. Bu4(MeO)2Sn2O
4.1.5. From Bu4(MeO)2Sn2O (10)
was prepared from dibutyl tin oxide and dimethyl carbonate as
described in the literature [10].
Compound 1 (0.5 g, 3.6 mmol) was dissolved in 25 ml of
CHCl3. Bu4(MeO)2Sn2O (1.25 g, 3.6 mmol) was added and the
solution was refluxed for 2 h. Compound 6 was obtained after
solvent removal and used as such for the acylation reaction.
4
.1. Preparation of the stannylene ketal 6
4
.1.6. (± )4-Phenyl-2,2-di-n-butyl-1,3,2-dioxa stannolan
Compound 6 was prepared from each of the following stan-
(
6)
nylene derivatives: the compound prepared from Bu2SnO 5
was isolated purified and characterised. In all other cases, the
compound was not purified and used as such in the benzoyla-
tion reaction. The presence of compound 6 in the mixture was
1H NMR (250 MHz,CDCl3): δ = 0.92 (m, 6H); 1.1–2.0 (m,
2H); 3.19 (br, dd, 1H, J = 9.5 and 9.0); 3.79 (Br, dd, 1H, J = 9.0
and 4.0); 4.50 (br, dd, 1H, J = 9.5 and 4.0); 7.1–7.6 (m, 5H).
1
13
1
13
3
C NMR (250 MHz,CDCl ): δ = 13.61 (q); 22.36 (t); 26.87
assessed by inspection of the H and C NMR spectra of the
crude material.
(
(
t); 27.36 (t); 69.28 (t); 75.74 (d); 126.76 (d); 127.64 (d); 128.21
d); 143.02 (s).
4
.1.1. From Bu2SnO (5)
Compound 1 (5.0 g, 36 mmol) was dissolved in 50 ml of
4
.2. Benzoylation of compound 1 with benzoyl chloride in
different solvents and conditions
toluene. Dibutyltin oxide (8.9 g, 35.7 mmol) is added. The sus-
pension was refluxed for 24 h in a Dean Stark apparatus. Com-
pound 6 was isolated after solvent removal and stored in a dry
box for further use. H and C NMR were identical with the
one reported in the literature.
4.2.1. With 6 as catalyst in CHCl3
1
13
Compound 1 1-phenyl-1,2-ethane diol (0.5 g, 3.6 mmol) was
dissolved in 20 ml of CHCl3. Compound 6 (0.026 g, 0.07 mmol,
.02 eq) in 5 ml of CHCl3 was added to the solution together
with TEA (1.0 ml, 7.2 mmol) and benzoylchloride (0.42 ml,
.6 mmol). The reaction was stirred at room temperature until
0
4
.1.2. From Bu2SnCl2 (8)
3
Compound 1 (5.0 g, 36 mmol) was dissolved in 50 ml
the starting material had disappeared (10 min from TLC hex-
ethyl ac. 1:1, rf PED 0.18, primary monoac. 0.34). The reaction
was quenched with water (10 ml) and the two phases separated.
The organic solvent was removed under vacuum to obtain a
of toluene. Dibutyltin dichloride (11 g, 36 mmol) was added
together with triethylamine (800 mg, 2 eq). The solution was
refluxed for 2 h. The reaction mixture containing compound 6
was used as such for the acylation reaction.
1
pale yellow solid directly analysed by H NMR without further
purification. The outcome of the reaction was determined from
the integration of the benzylic proton whose chemical shifts are
highlighted below. The ratio of the primary monobenzoylation
to the secondary one was 14/1.
4
.1.3. From Bu2Sn(MeO)2 (7)
Compound 1 (5.0 g, 36 mmol) was dissolved in 50 ml of
methanol. Bu2Sn(MeO)2 (10.6 g, 36 mmol) was added and the
solution was refluxed for 2 h. Compound 6 was obtained after
solvent removal and used as such for the acylation reaction.
1H NMR 1-phenyl-1,2-ethane diol (CDCl3): δ = 3.61–3.79 (m,
2
H); 4.78–4.85 (dd, 1H); 7.29–7.39 (m, 5H).
1
4
.1.4. From Bu2Sn(2-PrO)2 (9)
H NMR 1-phenyl-1-hydroxy-2-benzoyloxy-ethane (CDCl3):
δ = 3.20 (br, s, 1H); 4.38–4.57 (m, 2H); 5.15 (dd, 1H);
7.20–7.60 (m, 8H); 7.90–8.30 (m, 2H).
Compound 1 (0.5 g, 3.6 mmol) was dissolved in 10 ml of
-propanol. Bu2Sn(2-PrO)2 (1.25 g, 3.6 mmol) was added and
2
1
the solution was refluxed for 2 h. Compound 6 was obtained
after solvent removal and used as such for the acylation
reaction.
H NMR 1-phenyl-1-benzoyloxy-2-hydroxy-ethane (CDCl3):
δ = 2.20 (br s, 1H); 3.90–4.15 (m, 2H); 6.05 (q, dd, 1H);
7.20–7.60 (m 8H); 7.90–8.30 (m, 2H).

4
4
E. Fasoli et al. / Journal of Molecular Catalysis A: Chemical 244 (2006) 41–45
1H NMR 1-phenyl-1,2-dibenzoyloxy-ethane (CDCl3):
δ = 4.55–4.80 (m, 2H); 6.35 (t, 1H); 7.15–8.15 (m, 15H).
and stirred until the starting material had disappeared (TLC,
4 min). Thereactionwasquencedwithwaterandthetwophases
separated. The ratio of the primary monobenzoylation to the
secondary one was 11.
1
4
.2.2. With 6 as catalyst in toluene
Compound 1 1-phenyl-1,2-ethane diol (0.5 g, 3.6 mmol) was
dissolved in 20 ml of toluene. Compound 6 (0.026 g, 0.07 mmol,
.02 eq) in 5 ml of toluene was added to the solution together
with TEA (1.0 ml, 7.2 mmol) and benzoylchloride (0.42 ml,
.6 mmol). The reaction was stirred at room temperature until
4.2.7. With Bu2Sn(2-PrO)2 9 as catalyst in CHCl3
0
Compound 1 1-phenyl-1,2-ethane diol (0.5 g, 3.6 mmol) was
dissolved in 20 ml of CHCl3. Compound 9 (0.024 g, 0.07 mmol,
0.02 eq) in 5 ml of CHCl3 was added and the solution refluxed
for 30 min. TEA (1 ml, 7.2 mmol) and benzoylchloride (0.42 ml,
3.6 mmol) in 2 ml of CHCl3 were added at room temperature
and stirred until the starting material had disappeared (TLC,
10 min). Thereactionwasquencedwithwaterandthetwophases
separated. The ratio of the primary monobenzoylation to the
secondary one was 12.
3
the starting material had disappeared (TLC, 14 min). The reac-
tion was quenced with water and the two phases separated.
The organic solvent was removed under vacuum to obtain a
1
pale yellow solid directly analysed by HNMR without further
purification. The ratio of the primary monobenzoylation to the
secondary one was 10.
4
.2.3. With 6 as catalyst in 2-propanol
Compound 1 1-phenyl-1,2-ethane diol (0.5 g, 3.6 mmol)
4.2.8. With Bu2Sn(2-PrO)2 9 as catalyst in 2-propanol
Compound 1 1-phenyl-1,2-ethane diol (0.5 g, 3.6 mmol)
was dissolved in 20 ml of 2-propanol. Compound 9 (0.024 g,
0.07 mmol, 0.02 eq) in 5 ml of 2-propanol was added and the
solution refluxed for 30 min. TEA (1.1 ml, 8 mmol) and ben-
zoylchloride (0.46 ml, 4 mmol) in 2 ml of 2-propanol were added
at room temperature and stirred until the starting material had
disappeared (TLC, 12 min). The reaction was quenced with
water and the two phases separated. The ratio of the primary
monobenzoylation to the secondary one was 12.
was dissolved in 20 ml of 2-propanol. Compound 6 (0.026 g,
.07 mmol, 0.02 eq) in 5 ml of 2-propanol was added to the solu-
tion together with TEA (1.0 ml, 7.2 mmol) and benzoylchloride
0.46 ml, 4 mmol). The reaction was stirred at room temperature
0
(
until the starting material had disappeared (TLC, 10 min). The
reaction was quenced with water and the two phases separated.
The ratio of the primary monobenzoylation to the secondary one
was 12.
4
.2.4. With Bu2SnO 5 as catalyst in MeOH/CHCl3
4.2.9. With Bu4(MeO)2Sn2O 10 as catalyst in toluene
Compound 1 1-phenyl-1,2-ethane diol (0.5 g, 3.6 mmol) was
dissolved in 20 ml of MeOH. Compound 5 (0.018 g, 0.07 mmol,
.02 eq) was added and the suspension refluxed for 2 h. The
solvent was evaporated in vacuum and the residue dissolved
in 10 ml of CHCl3 and treated with TEA (1 ml, 7.2 mmol) and
benzoylchloride (0.42 ml, 3.6 mmol) in 2 ml of CHCl3. The mix-
ture was stirred at room temperature until the starting material
had disappeared (TLC, 10 min). The reaction was quenced with
water and the two phases separated. The ratio of the primary
monobenzoylation to the secondary one was 14.
Compound 1 1-phenyl-1,2-ethane diol (0.5 g, 3.6 mmol)
was dissolved in 20 ml of toluene. Compound 10 (0.038 g,
0.07 mmol, 0.02 eq)in5 mloftoluenewasaddedandthesolution
refluxed for 30 min. TEA (1 ml, 7.2 mmol) and benzoylchlo-
ride (0.42 ml, 3.6 mmol) in 5 ml of toluene were added at room
temperature. The reaction was stirred until the starting material
had disappeared (TLC, 12 min). The reaction was quenced with
water and the two phases separated. The ratio of the primary
monobenzoylation to the secondary one was 10.
0
4.2.10. With Bu4(MeO)2Sn2O 10 as catalyst in CHCl3
4
.2.5. With Bu2Sn(MeO)2 7 as catalyst in CHCl3
Compound 1 1-phenyl-1,2-ethane diol (0.5 g, 3.6 mmol) was
Compound 1 1-phenyl-1,2-ethane diol (0.5 g, 3.6 mmol) was
dissolvedin20 mlofCHCl3. Compound10(0.038 g, 0.07 mmol,
0.02 eq) in 5 ml of CHCl3 was added and the solution refluxed
for 30 min. TE (1 ml, 7.2 mmol) and benzoylchloride (0.42 ml,
3.6 mmol) in 5 ml of CHCl3 were added at room temperature.
The reaction was stirred until the starting material had disap-
peared (TLC, 12 min). The reaction was quenced with water and
the two phases separated. The ratio of the primary monobenzoy-
lation to the secondary one was 10.
dissolved in 20 ml of CHCl3. Compound 7 (0.021 g, 0.07 mmol,
.02 eq) in 5 ml of CHCl3 was added and the solution refluxed
for 60 min. TEA (1 ml, 7.2 mmol) and benzoylchloride (0.42 ml,
.6 mmol) in 2 ml of CHCl3 were added at room temperature
and stirred until the starting material had disappeared (TLC,
0 min). Thereactionwasquencedwithwaterandthetwophases
0
3
1
separated. The ratio of the primary monobenzoylation to the
secondary one was 14.
References
4
.2.6. With Bu2Sn(MeO)2 7 as catalyst in toluene
Compound 1 1-phenyl-1,2-ethane diol (0.5 g, 3.6 mmol) was
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the monomeric one.
5
(
(
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[
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