The preparation and use of polyHIPE-grafted reactants to reduce alkyl halides under free-radical conditions

Article abstract of DOI:10.1039/b009318l

PolyHIPE-supported tin hydride and thiol are synthesized to reduce alkyl bromides under free-radical conditions, taking advantage of the fully interconnected polyHIPE structure. Radical reduction of 1-bromoadamantane, 6-bromohex-1-ene and 1-allyloxy-2-bromobenzene are performed using two methodologies: one using supported tin hydride as reducing agent, and another using supported thiol as polarity-reversal catalyst in the presence of triethylsilane as reducing agent. The polyHIPE-supported tin hydride is used either in catalytic or stoichiometric amounts depending on the bromo compound to be reduced. In the case of unsaturated bromides, both methodologies lead to reductive cyclization, thus enabling radical rearrangement before hydrogen transfer. PolyHIPE-supported organotin hydride is a good alternative to tributyltin hydride to prevent tin contamination and facilitate product separation. PolyHIPE-supported thiol, in the presence of an excess of triethylsilane, shows good activity and selectivity toward reductive cyclization products. Results obtained by the two methodologies are compared and discussed.

Full text of DOI:10.1039/b009318l

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The preparation and use of polyHIPE-grafted reactants to reduce
alkyl halides under free-radical conditions
Alexandre Chemin,a Anthony Mercier,a,b Hervé Deleuze,a* Bernard Maillarda and
Olivier Mondain-Monvalb
1
a Laboratoire de Chimie Organique et Organométallique associé au CNRS,
UMR 5802 Université Bordeaux I, 33405 Talence. France.
E-mail h.deleuze@lcoo.u-bordeaux.fr
b Centre de Recherche Paul Pascal UPR 8641-CNRS, 33600 Pessac, France
Received (in Cambridge, UK) 20th November 2000, Accepted 10th January 2001
First published as an Advance Article on the web 1st February 2001
PolyHIPE-supported tin hydride and thiol are synthesized to reduce alkyl bromides under free-radical conditions,
taking advantage of the fully interconnected polyHIPE structure. Radical reduction of 1-bromoadamantane,
6-bromohex-1-ene and 1-allyloxy-2-bromobenzene are performed using two methodologies: one using supported
tin hydride as reducing agent, and another using supported thiol as polarity-reversal catalyst in the presence of
triethylsilane as reducing agent. The polyHIPE-supported tin hydride is used either in catalytic or stoichiometric
amounts depending on the bromo compound to be reduced. In the case of unsaturated bromides, both
methodologies lead to reductive cyclization, thus enabling radical rearrangement before hydrogen transfer.
PolyHIPE-supported organotin hydride is a good alternative to tributyltin hydride to prevent tin contamination
and facilitate product separation. PolyHIPE-supported thiol, in the presence of an excess of triethylsilane, shows
good activity and selectivity toward reductive cyclization products. Results obtained by the two methodologies are
compared and discussed.
solid-phase organic synthesis7 has been reported. The use of
Introduction
macroporous beads with a permanent porosity can be an alter-
native but is not completely satisfactory because of the poor
permeability of these materials8 and this reduces their useful-
ness for organic synthesis. An alternative approach consists in
the use of porous polymer foams having a low resistance to flow
which makes them usable in low-pressure, continuous-flow
methods. These materials are well known and have been pro-
duced by a wide variety of techniques ranging from leaching
soluble fillers through gas-blowing to phase separation,
although the structure of these materials is often irregular and
difficult to control.9 A novel method for producing porous
materials with a more regular structure has been developed
based on high-internal-phase emulsion (HIPE). These foams
are called emulsion-derived foams and were initially developed
by Unilever10 and called polyHIPE. The characteristics
and syntheses of such polyHIPE materials have already been
reviewed.11
Free radical reactions such as dehalogenation of alkyl, vinyl or
aryl halides, often followed by intra- or inter-molecular C–C
coupling are in increasing use in organic synthesis1 since a large
variety of functional groups is tolerated thereby avoiding the
necessity of laborious protection and deprotection sequences.
These reactions are generally performed at the laboratory stage
with tributyltin hydride. However, the toxicity of this reactant is
now well established2 and this drawback strongly limits the
development of its use in the synthesis of pharmaceutical
derivatives. A less toxic alternative consists in the combination
of triethylsilane, the reducing agent, with small amounts of a
thiol, as proposed by Roberts and co-workers.3 Reduction pro-
ceeds by a radical-chain mechanism and the thiol acts as a
polarity-reversal catalyst which mediates hydrogen-atom trans-
fer from the Si–H group of the silane to the alkyl radical
(Scheme 1). However this methodology is limited by the stench
of thiols.
Total replacements of soluble organotin hydride by the
corresponding polymer-supported reactant are credible altern-
atives.12 Indeed, this tin functionalized polymer would prevent
tin contamination of the products in solution, tin function-
alities remaining attached to the polymer backbone after reac-
tion. Similarly, a thiol function grafted onto a polymer will not
be volatile and could be easily separated from the reaction
products, avoiding any odorous pollution. The reduction of
‘simple’ alkyl bromides producing radicals giving no rearrange-
ment would occur similarly, whatever the hydrogen-transfer
agent YH involved in solution or grafted on a polymer and
present in small or stoichiometric amounts. This could be
different when the radical, generated by abstraction of the
bromine atom, can rearrange. Thus, the relative amounts of the
products UH and RH, obtained under various conditions, may
differ because of the existing competition between hydrogen
transfer and rearrangement (Scheme 2).
Scheme 1 Radical-chain mechanism involving the reducing agent
Et3SiH and a polarity-reversal catalyst thiol.
The development of polymer-supported chemistry is one of
the main features of modern organic synthesis. The supports
generally used in these applications are based on lightly cross-
linked styrene/1,4-divinylbenzene (DVB) beads called gel
resins. These resins have no permanent porosity, and therefore
are usable only in solvents allowing the polyHIPE to swell such
as toluene or THF.4 An impressive amount of work using these
resins as supported catalysts,5 supported reagents6 and for
In this paper, we present two types of functionalized
366
J. Chem. Soc., Perkin Trans. 1, 2001, 366–370
This journal is © The Royal Society of Chemistry 2001
DOI: 10.1039/b009318l

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Scheme 2 Competition between hydrogen transfer and rearrangement
in radical mechanism.
polyHIPE for free-radical chain reduction. The post-
functionalization of a polyHIPE-supported unsaturation led
to thiol functionalities after free-radical addition of thio-
acetic acid and deprotection. Another approach consists in
copolymerization of an organotin monomer, styrene and DVB.
Using small blocks of both resins, radical reductions of
1-bromoadamantane 1, 6-bromohex-1-ene 2 and 1-allyloxy-2-
bromobenzene 3 (Chart 1) have been investigated. PolyHIPE-
Fig. 1 Scanning electron microscopy of polyHIPE.
groups of the precursor PH-VB was performed. Unfortunately,
the resulting polymer was not found to possess, after reduction,
a significant amount of Sn–H functionality, since no peak at
1800 cmϪ1 (Sn–H peak assignment) appeared in FT-IR spectro-
scopic analysis.
An alternative was to prepare a polyHIPE-supported
organotin chloride (PH-SnCl) by polymerization of a highly
concentrated reverse emulsion obtained from a mixture of
4-[2-(dibutylchlorostannyl)ethyl]styrene12 as the functional
monomer, styrene and DVB as crosslinker. The organotin
hydride counterpart (PH-SnH) was produced by reduction of
PH-SnCl with NaBH4 (Scheme 4).
Chart 1 Alkyl bromides used in radical reduction.
supported organotin hydride, denoted PH-SnH, was used in
catalytic amount, with in situ regeneration by NaBH4 for 1 and
3 and, in the case of 2, in stoichiometric amounts because
of the possible direct reduction of 2 by NaBH4. PolyHIPE-
supported thiol, noted PH-SH, was used in catalytic amount
for the reduction of the three bromo compounds, triethylsilane
being the reducing agent.
Results and discussion
1 Synthesis and physical properties of functional polyHIPEs
The (vinyl)polystyrene polyHIPE was synthesized from a
commercial-grade solution of DVB [80% DVB, 20% ethylvinyl-
benzene (EVB)]. This polyHIPE possesses vinylbenzene
groups, hence the term PH-VB, due to partial polymerization
of DVB.13 This precursor allows a very general and direct route
to a wide variety of stable and effective spacer-containing
polymers, with mild conditions and good yield. Upon radical or
other anti-Markovnikov addition of a reagent H–X, each vinyl
becomes a dimethylene spacer supporting a functional group.14
This dimethylene spacer between the polystyrene aryl and func-
tional group heteroatom has been found to improve stability
and activity of polymer-bound reagents and catalysts, com-
pared with the (mono)methylene spacer which makes the
carbon–heteroatom bonds relatively fragile.15 In particular,
the addition of thioacetic acid to the pendant vinyl groups,
followed by aminolysis, leads to supported mercapto SH
functionalities (Scheme 3).
Scheme 4 Preparation of polyHIPE-supported organotin hydride.
A scanning electron micrograph of polyHIPE-supported
organotin chloride PH-SnCl is shown in Fig. 1.
Note the 5–25 µm cells formed by the temporary pore former
(water) and the great number of 1–2 µm ‘window’ between
adjacent cells. The characteristic dimension of the cell diameter
(about 10 µm) is to be compared with the characteristic window
diameter (about 1 µm).
The general range of measurements/properties of the
emulsion-derived foam prepared may be summarized as
follows:
– macroscopic density as low as 0.05 g cmϪ3
– internal void volume ≈95%, i.e. 95% pore volume/5% polymer
– fully interconnected uniform structure, with cells being con-
nected to all their neighbours, allowing liquids to circulate
through the porous structure
– the large channels and the full interconnection allow a
worthwhile flow rate under low pressure.
Scheme 3 Preparation of PH-SH.
In order to synthesize PH-SnH in the same convenient way, a
first trial of the addition of Bu2SnHCl to the pendant vinyl
J. Chem. Soc., Perkin Trans. 1, 2001, 366–370
367

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2 Loading of the useful polyHIPE-supported functionalities
diffusion of only the small molecules and radicals towards
the backbone of the supported reagents.
Table 1 reports the functionalization level of PH-SnCl, PH-
SnH and PH-SH. We note that about 33% of the Sn–Cl bonds
present on PH-SnCl were transformed into Sn–H bonds by
NaBH4. The remaining chlorine atoms were probably too
buried inside the material to be easily accessible to the reducing
agent.
Once the efficiency of the supported hydrogen transfer was
confirmed, it appeared of interest to study the reduction of
unsaturated bromide since the reductive cyclizations of
unsaturated halides are useful tools in organic synthesis.1 Then,
the reductions of 6-bromohex-1-ene 2 and of 1-allyloxy-2-
bromobenzene
3 were investigated using the polyHIPE
reducing agents. These free-radical reactions would produce
two major compounds: the products of reductive cyclisation
(4 from 2, 6 from 3) and those arising from the direct reduction
(respectively 5 and 7) (Schemes 5 and 6). The results obtained
for both substrates are reported in Table 3.
The homolytic reduction of 2 by PH-SnH (sodium boro-
hydride reduces 2 directly to generate 5) and 3 by PH-SnCl/
NaBH4 allowed us to perform the reductive cyclization of both
bromides with good selectivity (entries 6 and 8). The treatment
of 2 and 3 by triethylsilane in the presence of PH-SH, under the
same conditions as the reduction of 1, did not lead to any of
the expected hydrocarbon 4 or 5 from 2 (100% of 2 recovered
unchanged); meanwhile the cyclic ether 6 was formed in a
yield of 10% from 3 (90% unchanged) after 6 h (Table 3). As
pointed out by Roberts,3 the possible addition of the thiyl
3 Radical reduction of bromides 1, 2 and 3
The reduction of 1-bromoadamantane 1 by PH-SnH and
PH-SH, respectively regenerated in situ by reduction of the
intermediates by sodium borohydride and triethylsilane, was
performed under conditions similar to the reduction of alkyl
bromides by supported organotin hydride12 and dodecane-
thiol.3 Table 2 summarizes the relative and overall yields of
formed products resulting from the reduction of 1, using various
reducing agents and supported catalysts. The results described
in entries 1,3 and 4,6 (Table 2) show that the reduction of 1 is
due to the functions grafted onto the polyHIPE via the
expected free-radical process. The observation of a relatively
high conversion in 1 h confirms the efficient regeneration of the
hydrogen-transfer agent, thiol or tin hydride, in the reaction of
the intermediate, thiyl radical or tin bromide, with the reducing
agent respectively employed, triethylsilane and sodium boro-
hydride. The first-hour conversions obtained using polyHIPE-
supported chemistry when compared with solution chemistry
are lower (entries 2,3 and 5,6). This could be due to slower
elementary reactions rates when one of the reactive species
is attached to the polyHIPE; this is certainly caused by the
Table 1 Loading of the desired polyHIPE-supported functionalities
Scheme 5 Radical-reduction mechanism of 6-bromohex-1-ene 2.
Tin
monomer
(mol %)
Loading of
φa
PolyHIPE (%) (mol %)
Styrene
DVB b
(mol %)
functionality
(mmol gϪ1
)
PH-Ac
95
95
90
90
0
0
70
70
100
100
20
0
0
10
10
1.50 (SAcc)
0.30 (SHd )
0.34 (SnCle)
0.10 (SnHf )
PH-SH
PH-SnCl
PH-SnH
20
a Volumic fraction of pore former (aqueous phase). b 80% commercial
solution of DVB. c Estimated by S elemental analysis. d Estimated by Cl
elemental analysis after reaction with trichloroacetyl chloride. e Esti-
mated by Sn elemental analysis. f Estimated by the amount of decane
formed after stoichiometric reaction with 1-iododecane and AIBN.12
Scheme 6 Radical-reduction mechanism of 1-allyloxy-2-bromo-
benzene 3.
Table 2 Conversion and reaction conditions of radical reduction of 1 (1-bromoadamantane)
Reducing
agent (equiv.)
Reaction
time/(t/h)
Adamantane
yield (%)
Entry
R-Br
T/ЊC
Catalyst
Cat./RBr
1
2
3
4
5
6
1
1
1
1
1
1
80
80
80
80
80
80
none
NaBH4 (2)
NaBH4 (2)
NaBH4 (2)
Et3SiH (2)
Et3SiH (2)
Et3SiH (2)
0
1
1
1
1
1
1
5
100
77
0
100
70
Bu3SnCl
PH-SnCl
none
C12H25SH
PH-SH
0.1
0.1
0
0.02
0.02
Table 3 Conversion and reaction conditions of radical reduction of 2 and 3 by polyHIPEs grafted reagents
Reducing
agent
(equiv.)
Reaction
time/(t/h)
Conversion
(%)
Entry
6
R-Br
T/ЊC
Catalyst
Cat./RBr
1
2
70
PH-SnH
none
10
90
4 and 5 (84/16)
7
8
2
3
70
70
PH-SH
PH-SnCl
Et3SiH (2)
NaBH4 (2)
0.05
0.1
6
10
0
50
6 and 7 (90/10)
10
9
3
70
PH-SH
Et3SiH (2)
0.05
6
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J. Chem. Soc., Perkin Trans. 1, 2001, 366–370

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Table 4 Conversion and reaction conditions of radical reduction of 2
by PH-SH/Et3SiH (2/PH-SH = 0.05; T = 70 ЊC; 6 h)
DVB (80:20 DVB:EVB) as cross-linker, eventually a functional
co-monomer {4-[2-(chlorodibutylstannyl)ethyl]styrene} and
sorbitan monooleate (Span80) as emulsifying agent (20% w/w
of the organic phase) was placed in a reactor. The mixture was
stirred with a rod fitted with a D-shaped paddle, connected to
an overhead stirrer motor, at approximately 300 rpm. A volume
Vaq of aqueous phase was prepared separately by dissolving the
initiator, potassium persulfate K2S2O8, and sodium chloride
NaCl (1.5% w/w of the aqueous phase) in distilled water. This
solution was added dropwise, under constant mechanical stir-
ring, to the organic solution in order to obtain a thick white
homogeneous emulsion without apparent free water. Once all
the aqueous phase had been added, stirring was continued for a
further 15 min to produce an emulsion as uniform as possible.
The high-internal-phase emulsion obtained was then placed in
a polyethylene bottle. The polymerization occurred by immers-
ing the plastic bottle in a water-bath, heated to 60 ЊC for 10 h.
The container was then cut away to collect the resulting poly-
meric monolith. This was extracted with acetone in a Soxhlet
apparatus for 48 h, then was dried under vacuum at 60 ЊC for
48 h. The resulting monolith was cut into cubes (approx. 5 mm
per side). The polyHIPE thus synthesized is characterized
by the volumic fraction of pore precursor (water) φ = Vaq/
(Vaq ϩ Vorg).
Et3SiH/2
2
10
20
40
100
4 Yield (%)a
0
0
100
5
0
95
55
0
45
90
0
10
100
0
0
6 Yield (%)a
Unchanged 2 (%)a
a Determined by GLC.
radical to the double bonds present in the medium could be
responsible for such an inefficient reduction. The increase in
the concentration of the silane would allow direct reaction of
the thiyl radical towards the regeneration of the thiol by
hydrogen abstraction from the silane and, then, allow the
free-radical chain reduction of the unsaturated bromide to
occur. The results summarized in Table 4 indicate that the
reductive cyclization of 2 could be performed to completion
in 6 h with a one hundred fold excess of silane relative to the
bromide with PH-SH.
Comparison of the reductive cyclisation of 2 using PH-SH/
Et3SiH rather than PH-SnH shows a higher selectivity with the
first reagent, generating only the cyclic product 4. The possible
use of a catalytic system – even with primary alkyl bromide
– involving an organic reducer totally soluble in the organic
medium (triethylsilane) and this selectivity are good arguments
to select the thiol/triethylsilane system instead of the tin one.
Nevertheless, it is important to underline the necessary use of a
high ratio of silane to thiol to perform the reduction of bromide
compounds possessing a terminal vinyl group. This could be a
drawback when the required reaction product has a boiling
point close to that of triethylsilane (107–108 ЊC). In this case,
the commercial availability of numerous other silanes would
allow one to circumvent this problem.
4. Preparation of PH-SnH
1 mole equivalent (hereafter abbreviated to equiv.) of SnCl
from PH-SnCl (100 mg, 0.034 mmol SnCl) was treated with 20
equiv. of NaBH4 (31 mg, 0.68 mmol) in diethylene glycol bis-
(methyl ether) (20 mL) at 70 ЊC for 2 h. The mixture was then
filtered and the resulting polymer washed successively with
water, ethanol and diethyl ether.
5. Preparation of PH-SH
The reductive cyclisation of 3 with the thiol–silane system
(PH-SH/Et3SiH/3 = 0.05:75:1) is quantitative in 6 h at 70 ЊC,
while the use of tin catalyst (PH-SnCl/NaBH4/3 = 0.1:2:1)
produced a mixture of 6 and 7 (respectively 9:1) with a yield of
only 50%. These results confirmed the superiority of the
supported thiol reagent over the tin one.
Small cubes of PH-VB [500 mg, 3.00 mmol C᎐C gϪ1, 1.50 mmol
᎐
C᎐C] were impregnated with toluene by the freeze/thaw method
᎐
and suspended in 40 mL of toluene. 5 Equiv. of thioacetic acid
(570 mg, 7.50 mmol) and AIBN (1% mol/C᎐C) were then
᎐
added. The suspension was heated at 70 ЊC for 48 h. The poly-
mer was filtered off, extracted with acetone overnight on a
Soxhlet apparatus, and dried under vacuum at 60 ЊC, overnight.
FT-IR (KBr) νmax 1690 cmϪ1 (C᎐O). Elemental analysis: S,
Experimental
1. Materials
᎐
1.50 mmol gϪ1
.
The resulting PH-5Ac (500 mg, 0.75 mmol SAc) was then
treated with 10 equiv. of n-butylamine (550 mg, 7.5 mmol) and
NaBH4 (4 mg) in DMF (20 mL) to produce PH-SH.17
Unless otherwise noted, all materials were purchased from
Aldrich Chemical Company and used as received. 4-[2-(Chloro-
dibutylstannyl)ethyl]styrene, was prepared by hydrostannation
of p-divinylbenzene with Bu2SnHCl according to a published
procedure.12 Dibutylchlorostannane was prepared by the
reduction of Bu2SnCl2.16
6. Radical reduction of 1
1 Equiv. of 1-bromoadamantane 1 (73 mg, 0.34 mmol) was
reduced by 0.1 equiv. of SnCl (from PH-SnCl or Bu3SnCl)
(0.034 mmol SnCl) in the presence of 2 equiv. of NaBH4
(26 mg, 0.68 mmol) and AIBN. The mixture was allowed to
react at 80 ЊC in ethylene glycol bis(methyl ether) (10 mL).
1 Equiv. of 1-bromoadamantane 1 (322 mg, 1.50 mmol) was
also reduced by 0.02 equiv. of thiol (PH-SH or dodecanethiol)
(100 mg, 0.03 mmol) in the presence of 2 equiv. of Et3SiH (348
mg, 3.0 mmol) and dilauroyl peroxide (0.05 equiv.). The mix-
ture was allowed to react at 80 ЊC in cyclohexane (10 mL).
2. Characterization
Gas chromatographic studies were performed with a VARIAN
3400 coupled to a SPECTRAPHYSIC CHROMJET integ-
rator. The capillary column used was DB5 type (5% Ph), 30 m
in length, 0.25 mm in inner diameter and with a film thickness
of the stationary phase of 0.25 µm; the carrier gas was nitrogen
(0.5 bar).
Scanning electron microscopy was performed with a JEOL
840 ME apparatus. Microanalyses of samples of functional
polymers were performed in the ‘Service central d’analyse
élémentaire’ C.N.R.S. (Vernaison, France).
FT-IR spectra were taken with a Perkin–Elmer Paragon 1000
spectrometer.
7. Radical reduction of 2
1 Equiv. of 6-bromohex-1-ene 2 (8 mg, 0.05 mmol) was reduced
by 1 equiv. of SnH from PH-SnH (500 mg, 0.05 mmol SnH) in
the presence of AIBN (0.05 equiv.). The mixture was allowed to
react at 70 ЊC in benzene (10 mL).
In a typical experiment, 1 equiv. of 6-bromohex-1-ene 2 (98
mg, 0.60 mmol) was also reduced by 0.05 equiv. of thiol from
PH-SH (100 mg, 0.03 mmol SH) in the presence of 2 equiv.
of Et3SiH (139 mg, 1.2 mmol) and dilauroyl peroxide (0.05
3. Preparation of PH-VB and PH-SnCl
In a typical experiment, a volume Vorg of organic phase gener-
ally constituted of styrene, a commercial-grade solution of
J. Chem. Soc., Perkin Trans. 1, 2001, 366–370
369

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equiv.). The mixture was allowed to react at 70 ЊC in n-heptane
(10 mL).
ination of odorous pollution, due to the thiol, in the reduction
system of Roberts and co-workers.3 The general possibility
of using a catalytic system with a reducer (silane), and its
reaction product (halogenosilane) highly soluble in organic
solvents, and the use of reactants with no toxicity in this last
methodology make it more attractive than the one involving
even a grafted tin reagent. However, in the case of the reduction
of unsaturated organic halides there is a limitation due to the
necessity to operate with a great excess of silane, and then
efforts have to be made to define a more reactive silane to trap
readily the thiyl radicals before they themselves react with the
unsaturated substrates.
8. Radical reduction of 3
1 Equiv. of 1-allyloxy-2-bromobenzene 3 (72.5 mg, 0.34 mmol)
was reduced by 0.1 equiv. of SnCl from PH-SnCl (100 mg,
0.034 mmol SnCl) in the presence of 2 equiv. of NaBH4 (26 mg,
0.68 mmol) and AIBN (0.05 equiv.). The mixture was allowed
to react at 70 ЊC in ethylene glycol bis(methyl ether) (10 mL).
1 Equiv. of 1-allyloxy-2-bromobenzene 3 (128 mg, 0.60
mmol) was also reduced by 0.05 equiv. of thiol from PH-SH
(100 mg, 0.03 mmol SH) in the presence of 2 equiv. of Et3SiH
(139 mg, 1.20 mmol) and dilauroyl peroxide (0.05 equiv.). The
mixture was allowed to react at 70 ЊC in n-heptane (10 mL).
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Conclusions
This work confirms that polyHIPE-supported hydrogen
transfer reagents can be:
– obtained by copolymerization of functional monomers or by
post-functionalization of
divinylbenzene
a polyHIPE generated from
– used either in catalytic or stoichiometric amounts to reduce
alkyl and aryl bromides under free-radical conditions. This
reaction could be synthetically useful according to the
possibility of realizing a free-radical rearrangement of the
generated alkyl radical, involved in the reduction, before its
trapping by the hydrogen-transfer reagent.
The supports used are of great interest since they offer a
permanent porosity, providing better accessibility to active sites
and allowing a wider range of solvents than do beads prepared
by suspension polymerization. The highly interconnected
porous structure allows the development of low-pressure
continuous-flow application. We are currently developing this
strategy in order to improve conversions and optimize
polyHIPE-supported functionality efficiency. Indeed, the next
step is to use these materials in columns in order to be able to
operate chemical transformations in continuous-flow regimes.
PolyHIPE-supported organotin hydride is a good alternative
to tributyltin hydride to prevent tin contamination and to
facilitate product separation. PolyHIPE-supported thiol used in
conjunction with triethylsilane showed good activity, with elim-
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370
J. Chem. Soc., Perkin Trans. 1, 2001, 366–370