Soluble polymer-supported hindered phosphine ligands for palladium-catalyzed aryl amination

Article abstract of DOI:10.1039/c4cy01498g

Strategies for synthesis of more effective soluble supported ligands for phosphine-ligated Pd(0) cross coupling catalysts have been explored. Reversible addition-fragmentation chain transfer (RAFT) polymerization has been used to prepare alkane-soluble poly(4-alkylstyrene)-bound phosphine ligands. 4-tert-Butylstyrene and 4-dodecylstyrene were copolymerized with ca. 7 mol% of 4-chloromethylstyrene or a 4-diphenylphosphinestyrene monomer using RAFT chemistry to afford poly(tert-butylstyrene-co-4-dodecylstyrene) copolymers. Polymers with chloromethyl groups were allowed to react with the phenolic group of a hindered dicyclohexylbiarylphosphine ligand. This hindered polymer-bound phosphine formed reactive Pd complexes useful in haloarene amine couplings. All aryl halide amination reactions had Pd leaching that was typically <0.1% of the charged Pd with one example having only 0.02% Pd leaching. These Pd complexes of poly(4-alkylstyrene)-bound phosphines were also compared to similar hindered phosphine complexes formed with a polyisobutylene (PIB), whose terminus was also converted into a dicyclohexylbiarylphosphine ligand. Palladium catalysts ligated by these hindered biarylphosphines on poly(4-alkylstyrene) and PIB-bound both were recyclable in the absence of oxygen, had similar activity, and very low Pd leaching. This journal is

Full text of DOI:10.1039/c4cy01498g

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Soluble polymer-supported hindered phosphine
ligands for palladium-catalyzed aryl amination†
Cite this: Catal. Sci. Technol., 2015,
5, 2378
Tatyana V. Khamatnurova,a Dongmei Zhang,a Jakkrit Suriboot,a Hassan S. Bazzib
a
*
and David E. Bergbreiter
Strategies for synthesis of more effective soluble supported ligands for phosphine-ligated Pd(0) cross cou-
pling catalysts have been explored. Reversible addition-fragmentation chain transfer (RAFT) polymerization
has been used to prepare alkane-soluble polyIJ4-alkylstyrene)-bound phosphine ligands. 4-tert-Butylstyrene
and 4-dodecylstyrene were copolymerized with ca.
7 mol% of 4-chloromethylstyrene or a
4-diphenylphosphinestyrene monomer using RAFT chemistry to afford polyIJtert-butylstyrene-co-4-
dodecylstyrene) copolymers. Polymers with chloromethyl groups were allowed to react with the phenolic
group of a hindered dicyclohexylbiarylphosphine ligand. This hindered polymer-bound phosphine formed
reactive Pd complexes useful in haloarene amine couplings. All aryl halide amination reactions had Pd
leaching that was typically <0.1% of the charged Pd with one example having only 0.02% Pd leaching.
These Pd complexes of polyIJ4-alkylstyrene)-bound phosphines were also compared to similar hindered
phosphine complexes formed with a polyisobutylene (PIB), whose terminus was also converted into a
dicyclohexylbiarylphosphine ligand. Palladium catalysts ligated by these hindered biarylphosphines on
polyIJ4-alkylstyrene) and PIB-bound both were recyclable in the absence of oxygen, had similar activity, and
very low Pd leaching.
Received 15th November 2014,
Accepted 30th January 2015
DOI: 10.1039/c4cy01498g
www.rsc.org/catalysis
of such catalysts. Such supports also recycle and recover
phosphine ligands.11,12 This paper focuses on recovery and
Introduction
Palladium-catalyzed cross-coupling is widely recognized as a
powerful synthetic tool.1 In addition to Pd-catalyzed Heck and
Suzuki reactions, the Buchwald–Hartwig amination has
become an important route to aromatic amines.2–5 These
amination reactions also tend to be more sensitive tests of Pd
cross-coupling catalyst activity. They typically require the pres-
ence of phosphine ligands although there are examples of
such chemistry with nanoparticle or ‘ligand-free’ Pd
catalysts.6–9 The use of hindered phosphine ligands has been
demonstrated to be especially important and reactions using
such ligands and Pd catalysts afford good product yields even
with relatively unreactive aryl chlorides.
The recovery and recycling of the Pd catalysts used in
cross-coupling chemistry is also important. Many papers have
detailed recycling/recovery strategies for Pd cross-coupling
catalysts.10 Most commonly insoluble polymeric or insoluble
inorganic supports have been used in recovery and recycling
recycling of reactive Pd catalysts that employ a typical hin-
dered biaryldicyclohexylphosphine ligand using alternative
soluble polyIJ4-alkylstryene) and polybutyrene supports.
Extensive work by various groups has examined soluble
polymer-supported conventional phosphines both as ligands
for transition metal catalysts or as catalysts themselves.12–15
This work extends those studies to include designing polymer
supports that we expected to have heightened phase selective
solubility using as examples supports that could be easily
modified to include hindered biaryldicyclohexyl-phosphine
ligands. The results below show that soluble supported
polymer-bound versions of these hindered ligands are readily
accessible and that the Pd complexes formed using these
ligands are highly reactive and recyclable in Pd-catalyzed
haloarene-amine coupling reactions.
The general approach our lab has favored in immobilizing
catalysts which we have applied here to hindered phosphine
ligated Pd catalysts has been to use soluble polymers as sup-
ports for the phosphine ligands. This is an approach with
considerable precedent at least for conventional phosphine
ligands. Our work has emphasized using soluble polyolefin
supports where a liquid/liquid or liquid/solid phase separa-
tion is used to effect recycling of the phosphine ligand and/
or its associated catalyst from products.16 Alternative
a Department of Chemistry, Texas A&M University, P. O. Box 30012, College
Station, Texas 77843-3012, USA. E-mail: bergbreiter@tamu.edu
b Department of Chemistry, Texas A&M University at Qatar, P. O. Box 23874,
Doha, Qatar
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cy01498g
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strategies have also been used. For example, soluble supports
studied by other groups that incorporate polymer-bound
phosphine ligands for Pd catalysis include using polymers
like poly(ethylene glycol) that are separated from products by
solvent precipitation,12 poly(alkene oxide) copolymers that
are separable in thermoregulated systems17 or polyIJ4-
methylstyrene)s that are separable from products by solvent
precipitation or a thermomorphic separation.18 Recent work
also suggests that polyIJ4-alkylstyrene)-bound phosphines pre-
pared using a reversible addition-fragmentation chain trans-
fer (RAFT) polymerization are still another alternative.19,20
Our prior work with dansyl labeled copolymers showed that
copolymers prepared with the commercially available mono-
mer 4-tert-butylstyrene with modest mole fractions of dodecyl
groups have most of the desired phase selective solubility of
a 4-dodecylstyrene homopolymer. Thus, we chose to use
copolymers with a ca. 10 : 1 ratio of 4-tert-butyl/4-dodecyl
groups to minimize the need to synthesize monomers. Such
polyIJ4-alkylstyrene)s have previously been shown to afford
highly phase selective supports for organocatalysts.19
A further consideration in our design of phase selectively
soluble polyIJ4-alkylstyrene) supports for relatively lipophilic
hindered phosphine ligands related to the fact that we have
observed that the leaching of metals from soluble polymer
supported catalysts is often higher in the first few cycles of a
recycling study, especially when the mass of the catalyst or
ligand is large relative to the mass of the support.22 We have
previously ascribed this leaching in part to the presence of
a small fraction of lower molecular weight polymer chains
that have less phase selective solubility. The synthesis in
Scheme 1 takes advantage of more modern controlled radical
polymerization techniques using a RAFT polymerization to
prepare the product polymers with Mn values of ca. 20 kDa
and PDIs of ca. 1.2 – PDIs that are substantially lower than
Results and discussion
The studies described here use two sorts of linear lipophilic
polymer
supports
–
polyIJ4-alkylstyrene)
and
polyisobutylene.19–21 The first 4-alkyl-substituted polystyrene
support is an alternative to unsubstituted polystyrene. Such a
polyIJ4-alkylstyrene) can be separated using a liquid/liquid
separation avoiding the need for excess solvent needed for
solvent precipitation as a catalyst/product separation step. By
preparing polyIJ4-alkylstyrene)s with hydrophobic 4-alkyl sub-
stituents, we can prepare supports that are highly heptane
phase selectively soluble (Scheme 1). The quantitative phase
selective solubility of these polyIJ4-alkylstyrene) copolymers
had been previously studied using fluorescent labels.20 Those
studies showed that polymers prepared as in Scheme 1 are
quantitatively separable as heptane solutions from a polar
solvent phase with <0.1% of leaching of a dansyl labeled
polymer into the CH3CN phase of an always biphasic hep-
tane/CH3CN mixture or into the polar DMF phase of a
equivolume thermomorphic heptane/DMF solvent mixture.
The synthesis in Scheme 1 led to a copolymer of 4-tert-
butylstyrene and 4-dodecylstyrene (4) as the support and
incorporated 7 mol% of a third 4-chloromethylstyrene mono-
mer. The benzyl chloride groups in this group served as a
handle for subsequent incorporation of nucleophiles that are
spectroscopic probes for estimating phase selective solubility
or that are ligands for catalysis. While we could have pre-
pared homopolymers of 4-dodecylstyrene, using homopoly-
mers would require more substantial quantities of the mono-
mer 4-dodecylstyrene which is not commercially available.
what can be obtained in
a conventional radical poly-
merization of these same monomers. As shown below, the
polymers prepared by this RAFT chemistry are as effective as
supports as polymers prepared by a conventional radical
polymerization.20
1
The terpolymer 4 was characterized by GPC and H NMR
spectroscopy and contained 7 mol% –CH2Cl groups based on
1H NMR spectroscopy analysis of the relative peaks at 2.47 δ
for the benzylic protons of the 4-dodecylstyrene groups and
at 4.45 δ for the –CH2Cl protons of the chloromethyl group.
The molecular weight of the terpolymer 4 varied, ranging
from 9–25 kDa with PDI of ca. 1.2.
In initial experiments, we prepared an analog of 4 that
used a 4-diphenylphosphinylstyrene in place of the chloro-
methyl group to prepare
a
heptane soluble polyIJ4-
alkylpolystyrene)-bound phosphine ligand directly without a
post polymerization modification step. However, while this
heptane soluble polymeric phosphine formed a Pd catalyst
on reaction with PdIJdba)2 that was active and recyclable in
Suzuki coupling reactions,20 the Pd catalyst formed using this
relatively simple phosphine ligand was not active in halo-
arene amine cross coupling affording only traces of
N-phenylmorpholine in a reaction of morpholine and bromo-
benzene (Scheme 2).
To prepare a more active catalyst, we incorporated a more
electron-rich hindered phosphine onto the polyIJ4-tert-butyl-c-
4-dodecyl)styrene copolymer. As noted above, Buchwald has
shown that similar phosphine ligands lead to formation of
more active catalysts for Pd-catalyzed aminations.22 We
explored two approaches to anchor a Buchwald-like hindered
biaryldicyclohexylphosphine onto a heptane phase-selectively
soluble polystyrene support (Scheme 3). The first of these
approaches was modeled after the approach Buchwald's
Scheme
1 Synthesis of chloromethylated polyIJ4-alkylstyrene)
terpolymers suitable for conversion into a heptane phase-selectively
soluble supports.
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100 °C. The resulting nucleophile then formed the desired
lipophilic linear polyIJ4-alkylstyrene) polymer-bound phos-
phine 15 in a Williamson ether synthesis.
The second approach to coupling a hindered dicyclo-
hexylbiarylphosphine ligand to a soluble polyIJ4-alkylstyrene)
used a nucleophilic substitution reaction of the phenolic
phosphine ligand with vinylbenzyl chloride to form
a
phosphine-containing monomer (Scheme 3). Copolymeriza-
tion of this monomer 14 with a mixture of 4-tert-butylstyrene
and 4-dodecylstyrene following the approach we had success-
fully used earlier to prepare polyIJ4-alkylstyrene-c-4-
diphenylphosphine)-bound Pd catalysts for Suzuki catalysis
led to 15. These two approaches that were equally successful
in preparing the polymer-bound benzyloxy-substituted
dicyclohexyl phosphine ligand 15 are shown in Scheme 3.
The polystyrene-supported dicyclohexylbiaryl phosphine
15 prepared by either of the routes in Scheme 3 was an effec-
tive phase separable ligand for in situ formation of a Pd cata-
lyst for arylhalide amination (Scheme 4). While the loading
of phosphine on the soluble polystyrene differed in the two
approaches, experiments showed that Pd catalysts formed
with each polymer using the same Pd/phosphine ratio at 1
mol% loading of Pd had similar activity in reactions of
bromobenzene with morpholine. In using the phosphine 15
to prepare a Pd catalyst, the linear polystyrene-bound phos-
phine 15 was converted into a Pd catalyst by stirring a hep-
tane solution of this phosphine with PdIJdba)2 at ca. 90 °C for
30 min. During this process, the initial clear solution
changed to a clear yellowish solution. The polymer-bound
Pd(0) complex so formed was not isolated but was used and
recycled as a solution in heptane. ICP-MS analysis of a solu-
tion of the recycled catalyst in cycle 3 showed that the Pd
content of the heptane solution was equivalent to that in the
initial solution (i.e. a Pd content of ca. 520 ppm that was
unchanged from the initial Pd loading). Separate experiments
showed that PdIJdba)2 alone cannot be recycled as a heptane
Scheme
supported
2
Synthesis of chloromethylated polyIJ4-alkylstyrene)
triarylphosphine from hydrophobic 4-alkylstyrene
monomers and 4-diphenylphosphinylstyrene.
Scheme
3 Syntheses of polyIJ4-alkylstyrene) copolymer bound
dicyclohexylbiarylphosphine.
group used to attach similar phosphines to insoluble
divinylbenzene-crosslinked polystyrene supports11 and
involved a post-polymerization modification of the benzyl
chloride groups of 4. This approach began by using the reac-
tion of an ortho-methoxyphenylmagnesium bromide with
benzyne that was formed in situ. The 2′-methoxybiaryl-2-mag-
nesium bromide formed in situ by this chemistry was then
allowed to react with dicyclohexylchlorophosphine to form
a
2′-methoxy-2-dicyclohexylphosphinylbiphenyl phosphine.
Removal of the methoxy group followed by dimethylation
formed a phenolic group that could then be converted into a
nucleophilic phenolate group by reaction with CsCO3 in a
monophasic equivolume mixture of heptane and DMF at
Scheme
prepared using
dicyclohexylbiarylphosphine.
4
Synthesis and use of
a Pd(0) aryl amination catalyst
a
phase-separable polyIJ4-alkylstyrene)-bound
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solution and that PdIJdba)2 is not competent as a catalyst in
aryl aminations.
82–95% (Table 1). While the yellow catalyst phase did not
discolour and while the product phases were colorless, induc-
tively coupled plasma mass spectroscopy (ICP-MS) analyses
were used in cycle 3 to provide a quantitative analysis of
leaching. These ICP-MS analyses of the product isolated
from the polar methanol phase showed 0.08, 0.04, or 0.04%
Pd leaching in cycle 3 for experiments that used 1 mol% of
the Pd catalyst 16 in reactions of bromobenzene with
morpholine, bromobenzene with N-methylaniline, or chloro-
benzene with morpholine, respectively. These leaching levels
correspond to 1–10 ppm Pd in the product arylamine when
catalyst loadings of ca. 1–2 mol% are used.
Polyisobutylene (PIB) is an alternative heptane-soluble
support that our group has studied extensively.21 We there-
fore also examined syntheses of PIB-bound hindered phos-
phines. While our initial efforts to prepare PIB-bound
dicyclohexyl or di-tert-butylphosphine were unsuccessful or
formed phosphines that were easily contaminated by phos-
phine oxide, the use of hydroxyl-substituted dicyclo-
hexylbiaryl phosphine as a nucleophile was more successful
and led to the relatively stable PIB-bound dicyclo-
hexylbiarylphosphine 21 (Scheme 5). This phosphine is
formed as a mixture of diastereomers because of the pres-
ence of two elements of chirality in this soluble polymer
bound phosphine ligand – a chiral tetrahedral carbon on the
PIB group due to the carbon containing a single methyl
group and an axially chiral biarylphosphine ligand. No
attempt was made to separate these diastereomers.
To effect catalysis, the heptane solution of the Pd catalyst
16 formed in situ was mixed with bromobenzene or chloro-
benzene along with a secondary aliphatic or benzylic amine
and KO-tert-Bu was added. These reactions were carried out
on a 1 mmol scale in 3 mL of heptane in a sealed 10 mL vial.
The resulting mixture was stirred for ca. 20 h at 90 °C. After
cooling to room temperature, 2 mL of degassed methanol
solution saturated with heptane was added to the reaction
vial by forced siphon via a cannula. After brief stirring, this
biphasic mixture was centrifuged to separate the heptane
containing catalyst phase from the product containing meth-
anol rich phase. After the colorless methanol phase was phys-
ically separated from the yellowish heptane phase containing
the Pd catalyst, the catalyst containing heptane phase was
transferred to another sealed test tube containing fresh sub-
strates and base. In this way, the catalyst was recycled five
times without any observed loss of catalytic activity. The con-
versions in each cycle were >98% by 1H NMR spectroscopic
analysis. In cycles 1 and 2, the reaction mixture was analyzed
1
by H NMR spectroscopy every 6–8 h. These analyses showed
that the reactions were complete (i.e. >98% conversion) after
20 h. Cycles 3–5 were then analyzed at 20 h and in each case
the conversion of starting aryl bromide was complete. These
reaction times are similar to the 15–20 h reaction times
reported by Parrish and Buchwald for a similar Pd(0) catalyst
formed in situ by a DVB-crosslinked polystyrene supported
ligand analogous to 15. The five methanol product phases
were combined and the product arylamine from these five
cycles was isolated after column chromatography with an
average yield of product for five cycles that varied from
The catalytic activity of Pd catalyst formed in situ from the
PIB-supported phosphine ligand 21 with PdIJdba)2 was equiva-
lent to that of the Pd complexes prepared from a polyIJ4-
alkylstyrene) supported ligand. The conversions in each cycle
Table 1 Aryl halide amination using soluble Pd catalysts ligated by hep-
tane soluble polymer-bound biaryldicyclohexylphosphine ligandsa
Aryl halide
Amine
Product
Cycles
5
Average yield (%)
82(82)b
5
4
85(84)b
95(94)b
a
Reactions carried out on a 1 mmol scale with 1 mol% of 16 (or
b
1.5 mol% of 22) at ca. 90 °C for ca. 20 h. Yields in parentheses refer
to average yields for reactions using catalyst 22 for 8, 8, or 6 cycles in
reactions with morpholine, N-methylaniline, or morpholine
respectively isolating the product using either MeOH extraction of
PS–SO3H sequestration.
Scheme 5 Synthesis of a PIB-bound hindered phosphine 21 and its
reaction to form a Pd catalyst 22.
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were >98% by 1H NMR spectroscopic analysis. These reaction
times are similar to the 15–20 h reaction times reported by
Parrish and Buchwald for a similar Pd(0) catalyst formed
in situ by a similar DVB-crosslinked polystyrene supported
ligand and by the soluble polystyrene ligand described above.
Isolated yields of products from are listed in Table 1. These
isolated yields like those reported above are an average yield
for 5 cycles since the five 1 mmol scale reactions were com-
bined prior to workup.
Recycling of the PIB catalyst 21 was effected with product
separation in methanol and catalyst recycling in heptane.
This separation was effective as was the case with polyIJ4-
alkylstyrene) supported Pd ligated by the phosphine 15 with
Pd leaching of 0.1% Pd leaching or 0.02% in reactions that
used 1.5 mol% of the Pd catalyst 22 in coupling of bromo-
benzene with N-methylaniline or morpholine, respectively.
The product phases were colorless.
synthesis. Either of these polymers form Pd catalysts that
show excellent catalytic activity in Buchwald–Hartwig
amination reactions affording good isolated yields of
arylamines from secondary alkyl or benzylic amines and
bromobenzene or chlorobenzene. Successful catalyst
recycling was effective and using biphasic liquid/liquid sepa-
ration. Pd leaching was analyzed by ICP-MS analysis and was
uniformly very low with typical values for Pd leaching consis-
tently <0.1% for reactions that used 1.5 or 1 mol% of the Pd
catalyst.
Acknowledgements
Initial support of our work on these phase selectively soluble
polymers by the R. A. Welch Foundation (grant A-0639) and of
our work on polyIJ4-alkylstyrene) and polyisobutylene
supported Pd catalysts by the Qatar National Research Foun-
We did not examine the polar phase that contained
ca. 0.1% of leached Pd for catalytic activity in any of these
reactions. The concentration of Pd in that phase was ca.
1000-fold lower that the concentration of Pd in the heptane
phase and at these Pd concentrations, catalyst activity would
not be significant.
Both the polyIJ4-alkylstyrene) terpolymer support above
and the commercial PIB used below have low PDI values (the
PIB reportedly has a PDI of ca. 1.3).23 While the narrow PDI
may affect leaching of catalyst in the first or second cycle, the
high heptane phase solubility of these polymers is likely the
most important feature in achieving low Pd leaching. For
example, a simple diphenylphosphine-ligated Pd catalysts
attached to a similar polyIJ4-alkylstyrene) terpolymer prepared
by conventional AIBN polymerization had Pd leaching that is
only slightly higher than that for Pd ligated by 15.20
We also explored an alternative product isolation scheme
with the PIB-bound catalyst 22. This scheme used Amberlyst
15 as an in situ sequestrant following a protocol we had previ-
ously described.24
dation
acknowledged.
(project
number
4-081-1-016)
is
gratefully
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In summary, we have prepared two versions of a soluble poly-
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RAFT chemistry afforded highly phase selectively soluble
polyIJ4-alkylstyrene)supports that incorporate such ligands by
post-polymerization phosphinylation or directly using
a
phosphine-containing monomer. Polyisobutylene with termi-
nal dicyclohexylbiarylphosphine groups can be prepared from
a bromo-terminated polyisobutylene by a Williamson ether
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