Radical cyclisation of 1-bromo-2-(prop-2-enyloxy)benzene using a polymer-supported organotin hydride

Article abstract of DOI:10.1081/SCC-100106029

The use of polymer-supported organotin hydride for the cyclisation 1-bromo-2-(prop-2-enyloxy)benzene I to give 2,3-dihydro-3-methylbenzofuran II has been studied. The supports were prepared by suspension copolymerisation of an organotin-functionalised monomer. Two approaches have been investigated either using a stoichiometric or a catalytic amount of organotin support.

Full text of DOI:10.1081/SCC-100106029

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RADICAL CYCLISATION OF 1-BROMO-2-(PROP-2-
ENYLOXY)BENZENE USING A POLYMER-SUPPORTED
ORGANOTIN HYDRIDE
a
b
a
c
D. Lapôtre , H. Deleuze , B. Maillard & K. Mulholland
a
Université Bordeaux I , Laboratoire de Chimie Organique et Organométallique, UMR
802-CNRS, 351 Cours de la Libération, Talence, 33405, France
5
b
Université Bordeaux I , Laboratoire de Chimie Organique et Organométallique, UMR
802-CNRS, 351 Cours de la Libération, Talence, 33405, France
5
c
Process Chemistry SmithKline Beecham Pharmaceuticals , New Frontiers Sciences Park,
Harlow Essex, CM19 5 AW, UK
Published online: 20 Aug 2006.
To cite this article: D. Lapôtre , H. Deleuze , B. Maillard & K. Mulholland (2001) RADICAL CYCLISATION OF 1-BROMO-2-
(PROP-2-ENYLOXY)BENZENE USING A POLYMER-SUPPORTED ORGANOTIN HYDRIDE, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 31:21, 3207-3218, DOI: 10.1081/
SCC-100106029
To link to this article: http://dx.doi.org/10.1081/SCC-100106029
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SYNTHETIC COMMUNICATIONS, 31(21), 3207–3218 (2001)
RADICAL CYCLISATION OF 1-BROMO-2-
(PROP-2-ENYLOXY)BENZENE USING A
POLYMER-SUPPORTED
ORGANOTIN HYDRIDE
1
1,
1
D. Lapo
ˆ
tre, H. Deleuze, * B. Maillard,
2
and K. Mulholland
1
Laboratoire de Chimie Organique et Organome
UMR 5802-CNRS, Universite Bordeaux I,
´
´
tallique,
´
351 Cours de la Liberation 33405 Talence, France
2
Process Chemistry SmithKline Beecham
Pharmaceuticals, New Frontiers Sciences Park,
Harlow Essex CM19 5 AW, UK
ABSTRACT
The use of polymer-supported organotin hydride for the
cyclisation 1-bromo-2-(prop-2-enyloxy)benzene I to give 2,3-
dihydro-3-methylbenzofuran II has been studied. The sup-
ports were prepared by suspension copolymerisation of an
organotin-functionalised monomer. Two approaches have
been investigated either using a stoichiometric or a catalytic
amount of organotin support.
*
Corresponding author.
3
207
Copyright & 2001 by Marcel Dekker, Inc.
www.dekker.com

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3
208
1
. INTRODUCTION
Tributyl tin hydride plays a important role as reagent for reductive
1
cyclisations. However, the problem of separation, toxicity and disposal of
2
trialkyltin derivatives are now broadly acknowledged. Two kinds of sol-
ution have been proposed to solve this important problem. The first
approach includes an assortment of workup procedures in order to
3
remove the trialkyl tin derivatives from the reaction mixture. In the
second approach, attempts have been made to modify the structure of the
4
tin hydride itself in order to facilitate separation and reduce toxicity.
5
Examples of this approach include the fluorous tin hydrides and the
6
polymer-bound tin hydrides.
In the course of our work on the synthesis and applications of
7
polymer-supported organotin reagents and catalysts, our results on the
radical cyclisation of 1-bromo-2-(prop-2-enyloxy)benzene using a polymer-
supported organotin hydride are described below.
2
. RESULTS AND DISCUSSION
2
.1. Radical Cyclisation of 1-Bromo-2-(prop-2-enyloxy)-
benzene Using SupportP Cl
1
2.1.1. Synthesis of Support P Cl
1
Support P Cl was obtained by suspension copolymerisation of dibu-
1
tyl-2-[(4-vinyl)phenylethyl]tin chloride 1 with bis-1,4-(4-vinylphenoxy)but-
2
8
E-ene 2 and styrene as previously reported (Scheme 1).
2.1.2. Reactivity of Organotin Polymer P Cl
1
Having the polymer-supported organotin chloride P Cl in hand, we
1
first decided to test the ability of different reducing agents to convert the
grafted Sn-Cl functions into their Sn-H counterpart to obtain support P H.
1
The polymer being insoluble, the reaction medium is heterogeneous, so we
had to find a compromise between the compatibility of the solvent with the
polymer and the solubility of the reducing agent. The Sn–Cl bonds were
reduced into Sn-H using an excess of reducing agent. After washing of
the beads by water, ethanol and ether, the efficiency of the reduction was
estimated by a titration of the grafted Sn–H bonds by reduction of

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1
-BROMO-2-(PROP-2-ENYLOXY)BENZENE
3209
bromodecane into decane followed by GC (Scheme 2). The principal results
of this investigation are reported on Table 1.
Sodium borohydride is well known to smoothly convert Bu SnCl in
3
Bu SnH in polar solvents and complexing solvents such as ethyleneglycol
3
9
dimethyl ether (glyme). However, in our case, due to the poor compatibility
of the glyme with the support, we had to use a large excess of reagent to
obtain a satisfactory conversion level (Table 1, entries 1 and 2). Attempts to
use reducing agents that were more compatible with the support, in solvents
1
Scheme 1. Preparation of support P Cl.
1
Scheme 2. Reduction of support P Cl and titration of the SnH functions.

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3
210
Table 1. Reactivity of Support P1Cl Using Different Reducing Agents and
Reduction Conditions
Reducing
Agent (eq)
T
Reaction
ꢁ1
( C) Time (h) Solvent (mmol.g ) (mmol.g )
Sn–Cl
Sn–H
ꢀ
ꢁ1
Entry
1
2
3
4
5
6
7
NaBH (1.5)
4
70
70
2
2
3.5
4
3.5
2
Glyme
Glyme
THF
THF
THF
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.11
0.30
0.17
0.28
0.27
0.20
0.13
NaBH
CH
(C H ) CH NBH (20) 60
4
(20)
NBH
(C
8
H
17
)
3
3
4
(1.5) 50
8
17 3
3
4
TMDS/Bu
NaBH
DIBAL-H (20)
4
NF (1.5)
50
60
70
3
CN (20)
THF
Glyme
2
such as THF were made. Tricaprylmethylammonium borohydride readily
soluble in THF was synthesised, its use does not significantly increase the
yield of conversion of supported Sn-Cl into Sn-H functions (Table 1, entries
3
monium fluoride (TMDS/Bu NF) was inspired by an analoguous published
and 4). The use of the combination tertramethyldisiloxane/tetrabutylam-
4
1
system: PMHS/KF. The results obtained are similar with those observed
0
using NaBH (entry 5). Sodium cyanoborohydride, fairly soluble in THF,
4
gave lower results than NaBH (entry 6). Finally, the use of a large excess of
4
DIBAL-H gave only a low conversion probably due to the presence of
moisture into the polymer (entry 7).
Therefore, we selected NaBH as the reducing agent for the following
4
of the study. This reagent is cheap, easy to handle and gives a reasonable
Sn-H loading onto P H.
1
2.1.3. Cyclisation Using a Stoichiometric Amount of
Polymer-Supported Tin Hydride P H
1
The radical cyclisation of 1-bromo-2-(prop-2-enyloxy)benzene I is a
good model for the research of a clean and effective synthesis of 2,3 dihy-
1
drobenzofurane derivatives. The kinetics of the cyclisation of I has been
1
1
studied by Beckwith et al. They found that, whatever the concentration of
2
Bu SnH used, the cyclisation arises almost exclusively by the exo mecha-
3
nism, leading to the dihydrobenzofuran II.
The cyclisation reaction was initially carried out using a stoichiometric
amount of P H. Starting from an excess of supported tin chloride P Cl, the
first step produces the Sn-H functions using NaBH . A part of the reduced
1
1
4

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1
-BROMO-2-(PROP-2-ENYLOXY)BENZENE
3211
support obtained P H was used for the titration of the Sn-H functions
1
according to the method previously described. The Sn-H loading obtained
was 0.3 mmol/g. The second step consists in the radical cyclisation of I. This
reaction was performed using benzene in the presence of AIBN which is a
good solvent for the support and for radical reactions (Scheme 3 and Table
2).
The results obtained showed that the five-membered ring product
II can be obtained with the same selectivity and yield with the polymer-
1
supported tin hydride in benzene as with Bu SnH. For this cyclisation,
2
3
ꢁ
1
we used 12 g of polymer supported organotin reagent (0.3 mmol.g of
tin hydride) to cyclise 0.589 g (2.8 mmol) of substrate. Tin residue analysis
by ICP/AESof the product gave 120 ppm, which is a promising result
Scheme 3. Radical cyclisation of 1-bromo-2-(prop-2-enyloxy) benzene I by
1
support P H.
Table 2. Radical Cyclisation of I Using a Stoichiometric Amount of SnH
1
Functions Supported on P H
Reaction
Time (h)
Conversion
of I (%)
Ratio
II : III
ꢀ
Solvent
T ( C)
Benzene
80
7
99
96 : 4

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3
212
considering that no special precautions have been taken during the synthesis
of II i.e. the glassware used was not washed in a particular manner in order
1
3
to remove tin residues. It was thought that decreasing the temperature of
the reaction would probably reduce the level of residual tin as the polymer
backbone would be less damaged and therefore tin leaching would be
reduced. In that case, we would have to find another initiator, as AIBN is
ꢀ
poorly efficient below a temperatures of 60 C.
The recycling of the polymer P H failed. The yield of recovered
1
compound II was only 20% on the second use. Treatment of P H with I
1
2
was attempted in order to break the Sn–Sn bond which could have been
formed during the cyclisation. We recovered some of the reactivity (42%
of conversion of I) but not completely. This behaviour may be attributed
to NaBH remaining from the first step and obstructing the pores of the
4
polymer.
2.1.4. Cyclisation Using a Catalytic Amounts of
Polymer-Supported Tin Chloride P Cl
1
In order to reduce the large quantities of support needed by the stoi-
chiometric reaction, we decided to explore the possibility of synthesising II
using a catalytic amount of support. We also thought that, by using less
support, the level of tin residue would be reduced. This approach has
1
4
already been described using Bu SnCl/NaBH .
4
3
Several reactions using different reducing agents and a catalytic
amount (10% mol) of supported tin chloride were carried out. The results
are reported in Table 3.
The nature of the solvent seems crucial for the reactivity of the
support. NaBH is significantly soluble only in polar and complexant sol-
4
vents such as glyme. The reaction solvent must solubilise the product and
at the same time, it must be compatible with the polymer. Therefore, the
Table 3. Radical Cyclisation of I Using a Catalytic Amount (10%) of SnCl
Functions Supported on P Cl
1
Reducing
Agent (eq.)
Reaction
Time (h)
Conversion
of I (%)
Ratio
II : III
ꢀ
Solvent
Glyme
Glyme/Benzene
Glyme
T ( C)
NaBH (2)
4
80
80
80
16
16
16
37
10
89
93 : 7
91 : 9
93 : 7
NaBH
NaBH
4
4
(2)
(20)

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-BROMO-2-(PROP-2-ENYLOXY)BENZENE
3213
reaction was performed in glyme using 2 equivalents of Sodium
Borohydride and 10% molar (with respect to the substrate) of polymer-
supported tin chloride P Cl (Table 3, entry 1). The conversion was rather
1
low whereas the selectivity was good. This results is in agreement with the
non recycling capacity of support P Cl. The use of a 1 : 1 mixture of gly-
1
me : benzene did not improve the conversion (entry 2). However, the use of
a large excess of reducing agent (20 equivalents) gave an acceptable conver-
sion level (entry 3). The tin residue analysis by ICP/AESof the product gave
15 ppm which is an acceptable level. For further work, it would be desirable
to reduce this amount. One approach would be to increase the compatibility
of the polymer with a polar solvent such as glyme, the solvent of choice
for NaBH4.
2
.2. Radical Cyclisation of 1-Bromo-2-(prop-2-enyoxy)-
benzene Using SupportP Cl
2
In order to make a polymer more compatible with glyme, we copoly-
merised the organotin monomer 1 with di(ethyleneglycol)bis-(4-vinylbenzyl)
ether 3 and styrene to give support P Cl (Scheme 4). Compound 3 was
2
already successfully used as crosslinking agent in the preparation of poly-
1
mers containing a chiral Schiff base-Zinc complex. The support obtained
5
ꢁ
1
had a loading in tin of 1 mmol.g
.
2
Scheme 4. Preparation of support P Cl.
The results of two successive cyclisation reactions using the same cata-
lytic amount (10% mol) of supported tin chloride P Cl are reported in Table
2
4
. Using P Cl as the support, product II was obtained in a very good yield in
2
ꢀ
3
support P Cl was increased compared with P Cl. Recycling of this polymer
h using 10 equivalents of NaBH in glyme at 80 C. So the reactivity of the
4
2
1

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3
214
Table 4. Radical Cyclisation of I Using a Catalytic Amount (10%) of SnCl
2
Functions Supported on P Cl
Reducing
Agent (eq.)
Reaction
Time (h)
Conversion
of I (%)
Ratio
II : III
ꢀ
Run
Solvent
T ( C)
1
2
NaBH
NaBH
4
4
(10)
(10)
Glyme
Glyme
80
80
3
20
99
93
98 : 2
95 : 5
was also possible. After the first use, the polymer was washed successively
with ether, water, ethanol and acetone, then extracted for one day in a
soxhlet apparatus with THF. Finally the polymer was dried. In that case,
the tin residue analysis by ICP/AESof the product gave 10 ppm which is an
acceptable level. On the second run, a conversion of 93% for compound I
was obtained but a longer period of time (20 h) was necessary to reach this
value. The third run of the recycling has failed (22%). Between the second
and the third run, the polymer was only washed with ether, water and
ethanol, therefore, it seems that an exhaustive washing of the support is
necessary to recycle the polymer.
The quantities of NaBH used can probably be reduced by performing
4
the reaction at lower temperature.
3
. CONCLUSION
Two methods for the cyclisation of 1-bromo-2-(prop-2-enyloxy)ben-
zene I have been carried out. The cyclisation of I using a stoichiometric
amount of polymer-supported organotin reagent P H behaves similarly as
1
the homogeneous one with Bu SnH. However the recycling of the support
3
failed. This behaviour may be attributed to the obstruction of the pores of
the beads by tetrahydroborates by-products. A possible solution to this
problem could be to wash the beads more thoroughly.
The cyclisation of I using a catalytic amount of polymer-supported
organotin reagent P Cl behaves similarly as the homogeneous reaction with
1
Bu SnCl using at least a 20 fold excess of reducing agent with respect to the
3
substrate. Reduction of this quantity was attempted by the development of a
new polymer P Cl more compatible with glyme, a good solvent of NaBH .
2
4
The amount of reducing agent could be lowered and the stability toward
recycling of the support increased.

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-BROMO-2-(PROP-2-ENYLOXY)BENZENE
3215
4
. EXPERIMENTAL PART
4
.1. Preparation of Monomers and Reducing Agents
4.1.1. Preparation of Dibutyl-2-[(4-vinyl)phenyl]ethyl Tin Chloride 1
The synthesis of dibutyl-2-[(4-vinyl)phenyl]ethyl tin chloride have been
7
published by our group.
4.1.2. Preparation of bis-1,4-(4-vinylphenoxy)but-2E-ene 2
The synthesis of bis-1,4-(4-vinylphenoxy)but-2E-ene have also been
8
published by our group.
4
.1.3. Preparation of Di(ethylene glycol) bis(4-vinylbenzyl)ether 3
Compound 3 was synthesised according to a published procedure.
1
5
4.1.4. Preparation of Tricaprylmethylammonium Borohydride
In a 100 ml three-necked flask were introduced tricaprylmethylammo-
nium chloride (10 g–24.7 mmol) and water (5 ml). The suspension was stirred
at room temperature. An aqueous solution of 5 M NaOH (3 ml) was added
dropwise. The mixture was cooled at room temperature, then sodium bor-
ohydride (1.05 g, 27.7 mmol) in water (5 ml) was added. The solution was
stirred overnight, then extracted with dichloromethane (2 ꢂ 25 ml). The
organic phase was washed to neutrality and dried over anhydrous K CO .
2
3
CH Cl was removed in vacuo to afford a pale yellow oil (9 g, 95%).
2
2
4
.2. Preparation of the support P Cl and P Cl
1 2
The preparation of supports P Cl and P Cl was made by suspension
1
2
7
polymerisation according to the procedure already published. Both poly-
ꢁ
1
mers have a nominal crosslink of 20% and a loading from 0.6 to 1 mmol.g
of tin chloride.

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3
216
4
.3. Preparation of the Support P H and P H
2
1
and Titration of the SnH Functions
The reduction of supported SnCl and the titration of the obtained SnH
7
,8
functions supported on P H and P H have already been described.
2
1
4
.4. Preparation 1-Bromo-2-(prop-2-enyloxy)benzene 1
1
-bromo-2-(prop-2-enyloxy)benzene has been synthesised according to
1
1
a published procedure.
4
.5. Preparation of 2,3-Dihydro-3-methylbenzofurane II
by the Stoichiometric Route
Reduction step: A 250 ml three-necked flask was charged with polymer
ꢁ
1
supported organotin chloride P Cl (13.0 g, 0.6 mmol.g
1
of Sn-Cl) and
diglyme (100 mL). The flask was purged with argon, then sodium borohy-
dride (6.0 g, 158 mmol) was added. The mixture was gently stirred during 2 h
ꢀ
at 70 C. The polymer was filtered and washed with water, ethanol then
ether.
Cyclisation step: A 250 ml three-necked flask was charged with
-bromo-2-(prop-2-enyloxy)benzene I (0.589 g, 2.8 mmol), decane (200 mg)
1
as an internal reference and benzene (100 mL). The mixture was purged with
argon. AIBN (20 mg) was added and 12/13 of the freshly reduced polymer
was added (12 g about 4.8 mmol of Sn-H, according titration with bromo-
ꢀ
adamantane, (see below). The reaction mixture was stirred at 80 C for 16 h
to obtain a quantitative conversion for I. The suspension was filtered and
the filtrate concentrated under reduced pressure to give a colorless oil
(314 mg, 85%). Mixture of II : III ¼ 97 : 3 as determined by G.C analysis.
Tin residue analysis by ICP/AESgave 120 ppm.
NMR: dH (200 MHz; CDCl ); 7.25–6.75 (m, 4H, arom H); 4.75–4.6,
3
4
.20–4.05 (ABX, 2H, HA,B); 3.60–3.25 (ABX, 1H, H ); 1.30 (d, 3H, CH ,
C 3
J ¼ 6.8 Hz). d (75 MHz; CDCl ); 159.8 (C ); 132.1 (C ); 127.8 (C ); 123.5
C
3
7a
3a
4
(
are in accordance with the full spectroscopic data previously published.
C ); 123.5 (C ); 109.3 (C ); 78.2 (C ); 36.5 (C ); 19.1 (CH ). These results
6 5 7 2 3 3
1
1
The remaining 1/13 of the freshly reduced polymer P H was used to
1
reduce bromodecane. The Sn-H loading of the polymer was estimated to
0.3 mmol.g .
ꢁ1

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-BROMO-2-(PROP-2-ENYLOXY)BENZENE
3217
4
.6. Preparation of 2,3-Dihydro-3-methylbenzofurane II
by the Catalytic Route
A 250 ml three-necked flask was charged with polymer supported
ꢁ
1
organotin chloride P Cl (1.4 g, 1 mmol.g of Sn-Cl), I (1.5 g, 70 mmol)
2
and glyme (100 mL). The solution was purged with argon, then AIBN
500 mg–3 mmol) and sodium borohydride (5.3 g, 140 mmol) were added.
(
The suspension was gently stirred during 3 h at 80 C to obtain 99% of
ꢀ
conversion of I by GC. The mixture was filtered and washed with ether
(
3 ꢂ 50 mL). 50 mL of water were added to remove NaBH4 residue.
Organic phase was extracted with ether (2 ꢂ 50 mL) and concentrated to
afford 1.1 g of a mixture II and III as determined by G.C. analysis
(
II : III ¼ 49 : 1) (94%). Tin residue analysis by ICP/AESgave 10 ppm.
5
. REFERENCES
1
. a) Curran, D.P. in Comprehensive Organic Synthesis, Trost, B.M.;
Flemming, I. Eds.; Pergamon, Oxford, UK, 1991, Vol. 4, 715.
b) Motherwell, W.; Crich, D. Free Radical Chain Reactions in
Organic Synthesis, Academic Press, London, UK, 1992, 259.
2
. a) Davies, A.G. Organotin Chemistry, VCH, Weinheim, Germany,
1997, 327. b) Smith, P.J. (Ed.), Chemistry of Tin, 2nd Ed., Blackie,
London, UK, 1997, 578.
3. a) Crich, D.; Sun, S.X. J. Org. Chem. 1996, 61, 7200. b) Curran, D.P.;
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. a) Junggebauer, J.; Neumann, W.P. Tetrahedron 1997, 53, 1301.
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Organomet. Chem. 1993, 444, C18.
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re, B.; Donnard, O.F.X.;
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J.M.J. J. Org. Chem. 1990, 55, 304.
1
1
Received in the UK October 6, 2000

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