Threefold cross-linked polystyrene-triphenylphosphane hybrids: Mono-P-ligating behavior and catalytic applications for aryl chloride cross-coupling and C(sp3)-H borylation

Article abstract of DOI:10.1002/anie.201306769

Covalently bound polystyrene-phosphane hybrids were prepared by a method based on radical emulsion polymerization of styrenes in the presence of a tris(p-vinylphenyl)phosphane cross-linker. These hybrids favor mono-P-ligation to transition-metal complexes and are useful for challenging catalysis, such as Pd-catalyzed C-C/C-N couplings with unactivated chloroarenes and Ir- or Rh-catalyzed C(sp³)-H borylations. Copyright

Full text of DOI:10.1002/anie.201306769

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Angewandte
Communications
DOI: 10.1002/anie.201306769
Heterogeneous Catalysis
Hot Paper
Threefold Cross-Linked Polystyrene–Triphenylphosphane Hybrids:
Mono-P-Ligating Behavior and Catalytic Applications for Aryl Chlo-
3
À
ride Cross-Coupling and C(sp ) H Borylation**
Tomohiro Iwai, Tomoya Harada, Kenji Hara, and Masaya Sawamura*
Polystyrene (PS)-supported phosphanes are widely used for
heterogeneous transition-metal catalysis, taking advantage of
their practical merits as insoluble materials.[1,2] Owing to the
flexible nature of the PS backbone, the phosphane moieties
have generally significant mobility even in cross-linked PS
resins, and thus exhibit ligand properties generally similar to
those of their homogeneous counterparts, except for their
insoluble nature. Thus, such supported phosphanes are able to
function not only as monodentate ligands (Figure 1, A) but
also as bidentate or multidentate ligands (Figure 1, B) toward
emulsion polymerization of styrenes in the presence of tris(p-
vinylphenyl)phosphane as a threefold cross-linker.[3–5] Our
scenario is as follows: The threefold cross-linking increases
the density of the polymer chain around the Ph3P core, and
thus limits multidentae P-coordination to the metal, but the
steric demand in proximity to the P atom is only moderate
(Ph3P-like) owing to the spacer effect of the three aromatic
rings on the P atom, which projects toward trigonal pyramid
directions; consequently, the resulting mono-P-ligated metal
system allows effective access of substrates for catalysis. In
fact, this simple technique yielded novel immobilized mono-
dentate tertiary phosphane ligands that specifically formed
mono-ligated transition-metal complexes (Figure 2). The
Figure 1. A conceptual view of various metal coordination modes of
a PS-supported phosphane. The PS chains, which may cause steric
effects toward the catalytic environment, are shown as gray lines.
transition metals. In many cases, however, the flexible
polymer chain causes unfavorable steric effects in the
catalytic environment, resulting in reduced catalyst efficiency
(Figure 1, C). Thus, using the PS backbone for designing
phosphane ligands to favor a specific structure desirable for
an increase in catalytic activity is difficult.
Figure 2. A conceptual view of metal mono-P-ligation of a networked
PS resin prepared from a phosphane-centered threefold cross-linker.
PS chains are shown as gray lines.
Herein we report a new type of polystyrene–phosphane
covalently bound hybrid, which was prepared through radical
À
successful Pd-catalyzed C Cl transformation reactions such
as Suzuki–Miyaura coupling and Buchwald–Hartwig amina-
3
À
tion as well as the Ir- or Rh-catalyzed C(sp ) H borylation
[*] Dr. T. Iwai, T. Harada, Prof. Dr. M. Sawamura
Department of Chemistry, Faculty of Science, Hokkaido University
Sapporo 060-0810 (Japan)
reaction demonstrated the utility of this strategy, suggesting
that steric hindrance of the polymer chain unfavorable for
catalytic performance was successfully reduced in this system.
A solution of tris(p-vinylphenyl)phosphane (1), divinyl-
benzene (DVB), monofunctional styrenes (2a–d: R = H, Me,
tBu, OMe), and AIBN (azobis(isobutyronitrile); 1/DVB/2/
AIBN 1:1:60:1.2; stoichiometry not optimized) in chloroben-
zene was dispersed in an aqueous phase containing acacia
gum and NaCl.[6] The mixture was vigorously stirred at 808C
for 24 h for 70–90% consumption of the vinyl monomers. The
resultant solids were collected by filtration, washed with
water, MeOH, THF, and toluene, and dried under vacuum to
provide the Ph3P cross-linked PS (3a–d) as white beads
E-mail: sawamura@sci.hokudai.ac.jp
Homepage:
http://wwwchem.sci.hokudai.ac.jp/~orgmet/index.php?id=25
Dr. K. Hara
Catalytic Research Center, Hokkaido University
Sapporo 001-0021 (Japan)
[**] This work was supported by Grants-in-Aid for Scientific Research on
Innovative Areas “Reaction Integration” (MEXT and ACT-C, JST) to
M.S. and by Grants-in-Aid for Young Scientists (B and Start-up)
from JSPS to T.I.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201306769.
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Scheme 1. Preparation of Ph3P-cross-linked PS (3a–d).
(Scheme 1). The polymer beads did not contain any solvent
molecules or unreacted monomers that were detectable by
13C CP/MAS NMR spectroscopy. The phosphorus loading
values ([P] 0.08–0.12 mmolgÀ1) were determined based on
their abilities to capture transition-metal complexes.[7] DVB
(not shown in 3) was used as a co-cross-linker to achieve
uniform formation of bead-shape materials.
The dried beads of 3 have diameters of about 300–400 mm
(Figure 3a), and exhibited ordinary swelling properties as
polystyrene resins. Swelling was good with aprotic solvents,
such as toluene, THF, and CH2Cl2 (Figure 3b), but poor with
Scheme 2. Effect of PS-supported triarylphosphanes in the Pd-catalyzed
Suzuki–Miyaura cross-coupling reaction.
has been well-established through experiments with bulky
and electron-rich phosphane[10,11] or NHC[12] ligands. Further-
more, Tsuji and co-workers achieved mono-ligation of Ph3P-
based ligands by introducing steric demands in the periphery
of the dendrimer-type high-molecular-weight phosphane
molecules.[13–15] In fact, these “bowl-shaped” phosphanes
also induced the Suzuki–Miyaura coupling with chloroare-
nes.[13b]
The mono-P-ligated PdII complexes [PdCl2(PhCN)(3a–
d)] (1 mol% Pd) were evaluated for catalytic efficacy in the
coupling reaction between p-chlorotoluene (6a, 0.5 mmol)
and phenylboronic acid (7a, 0.75 mmol) in the presence of
K3PO4 (1.5 mmol). To our delight, the reaction proceeded
under mild conditions (THF, 408C, 2 h, Scheme 2). The yield
of biaryl 8a was apparently dependent on the substituents (R)
on the benzene ring of the PS backbone of 3. Relatively high
yields were obtained with the substituted ligands (3b, R = Me,
82%; 3c, R = tBu, 95%; 3d, R = OMe, 84%) compared with
the parent ligand (3a, R = H, 71%). The origin of these
substituent effects are not clear; it is not explained by the
difference in the swelling properties.[8]
The threefold cross-linking is essential. The analogous
twofold cross-linked PS-Ph3P hybrid 4 (R = tBu) gave 8a in
only 52% yield under otherwise identical conditions, and the
conventional single-point linked PS-Ph3P hybrid 5 was even
less efficient (6% yield; Scheme 2). A homogeneous catalyst
system based on Ph3P did not induce the reaction at all (data
not shown).
The heterogeneous catalyst system prepared from
[{PdCl(h3-cinnamyl)}2] (1 mol% Pd) and 3c (2 mol% P)
was examined for reusability in the coupling reaction between
6a (1.0 mmol) and 7a (1.5 mmol; K3PO4, 608C, 2 h).[16] After
the first run, the catalyst beads were contaminated with
inorganic salts, and the supernatant was colorless. The
insoluble materials were filtered, washed with H2O, and
then with THF, and dried under vacuum to obtain orange-
colored catalyst beads for use in the next run. The Pd-3c
catalyst maintained most of its activity even in the fourth run,
but the gradual color change from orange to brown over
repeated uses (yield of 8a: 1st 91%, 2nd 97%, 3rd 98%, 4th
94%, 5th 85%).[17,18] HRTEM observation of the reused Pd-
3c was indicative of gradual formation of Pd cluster species,
which should be less active or inactive (Figure 4).[19] Induc-
tively coupled plasma atomic emission spectroscopy (ICP-
Figure 3. a) Dried beads of 3c (R=tBu). b) Beads of 3c swollen in
toluene. c) Beads of [PdCl2(PhCN)(3c)] (with a stirring bar) as
prepared in THF (P/Pd 2:1).
aliphatic hydrocarbons and aprotic polar or protic solvents
such as DMF, MeOH, and water.[8] The degree of swelling was
slightly dependent on the substituent (R) on the benzene ring
of the PS backbone (5.0–5.8 mLgÀ1 in toluene).
The treatment of the THF-swollen polymer beads 3c with
[PdCl2(PhCN)2] at room temperature under magnetic stirring
caused gradual coloring of the beads to orange over several
minutes (Figure 3c). The 31P CP/MAS NMR analysis was
clearly indicative of the selective formation of the mono-P-
ligated PdII complex [PdCl2(PhCN)(3c)], irrespective of the
stoichiometry of the P ligand and the Pd source (P/Pd 2:1,
1:2). In contrast, the homogeneous reaction between [PdCl2-
(PhCN)2] and 2 equiv of (p-Tol)3P in THF gave bis-P-ligated
complex trans-[PdCl2{(p-Tol)3P}2] as a single product. Sim-
ilarly, the conventional PS-supported phosphane 5 (chemical
structure shown in Scheme 2), in which the phosphane moiety
was incorporated in the PS resin through the reaction of
monovinylated triphenylphosphane, gave the bis-P-ligated
complex [PdCl2(5)2] as a major species when P was in excess
(P/Pd 2:1).[9]
Controlled mono-ligation in the Pd-3 systems having been
confirmed, we explored their applicability toward Suzuki–
Miyaura coupling of unactivated chloroarenes, for which the
importance of mono-ligation of two-electron donor ligands
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Pd-catalyzed coupling reactions prompted us to test the best-
performing ligand 3c in the heteroatom-directed borylation
3
À
of unactivated C(sp ) H bonds, for which the silica-supported
cage-type trialkyl- or triarylphosphanes (silica-SMAP and
silica-TRIP, respectively) were effective ligands in our recent
studies.[23c,e,f,24] We expected that such experiments might give
us an answer to the question as to whether or not the cage in
silica-SMAP and silica-TRIP is essential.
In fact, the PS-Ph3P hybrid 3c did promote these
challenging catalyses. Specifically, the reaction of 2-pentyl-
pyridine (11) with bis(pinacolato)diboron (12) in the presence
of a catalytic amount of [{Ir(OMe)(cod)}2] (2 mol% Ir) and
3c (2 mol% P) afforded secondary alkylboronate 13 in 82%
yield based on the diboron 12 (Table 1, entry 1).[23f] Although
the twofold cross-linked PS-Ph3P ligand 4 and the single-point
linked ligand 5 also induced the borylation, their efficacies
were significantly lower than that of the threefold cross-
linked ligand 3c (entries 2 and 3). No reaction occurred under
homogeneous conditions with Ph3P (entry 4).
Figure 4. a,b) HRTEM images of part of the Pd-3c resins after 1st and
4th runs, respectively.
AES) analysis indicated that Pd leaching for each run was
below the detection limit (< 0.02% of the loaded Pd).[20]
À
The chemical stability of the PS resin and the C C bond
linkage between the phosphane moiety and the resin is the
advantage of the PS-Ph3P hybrids 3 over heterogenized
ligands based on oxide surfaces in the cases of using them
under rather harsh reaction conditions. For example, a catalyst
system prepared from [{PdCl(h3-allyl)}2] (1.5 mol% Pd) and
3c (3 mol% P) was successfully applied to the amination of p-
butylchlorobenzene (6b, 0.25 mmol) with aniline (9a,
0.3 mmol).[21] The reaction was conducted under basic and
high-temperature conditions employing KOtBu (0.35 mmol),
which is stronger base than K3PO4, and a toluene–tBuOH
(4:1) mixture as the base and solvent, respectively, at 1008C
over 20 h to afford the corresponding diarylamine 10a in 80%
yield (Scheme 3).[22] Notably, when the previously reported
3
[a]
À
Table 1: Ir-catalyzed C(sp ) H borylation.
Entry
Ligand
Yield [%][b]
1
2
3
4
3c
4
5
82
61
45
0
Ph3P
[a] Conditions: 11 (0.9 mmol), 12 (0.3 mmol), [{Ir(OMe)(cod)}2]
(0.006 mmol Ir), ligand (0.006 mmol P), cyclopentyl methyl ether
(CPME, 1 mL), 608C, 15 h. [b] Yield determined by 1H NMR analysis
based on 12.
The threefold cross-linked PS-Ph3P ligand 3c was also
3
À
effective for the Rh-catalyzed N-adjacent C(sp ) H boryla-
tion of urea 14 and 2-aminopyridine 16 (1 mol% Rh,
Scheme 4), for which the silica-TRIP triarylphosphane
ligand was effective but the silica-SMAP trialkylphosphane
was not in our previous study.[24] As the pinB-H also served as
Scheme 3. The C-N coupling reactions with the Pd-3c catalyst system.
silica-supported monophosphanes (silica-SMAP[23] and silica-
TRIP[24]) with similar mono-P-ligating features were applied
to this Pd catalysis instead of the PS-Ph3P hybrid, the
complete leaching of metal and P atoms owing to degradation
of the oxide surface was evident, and no substrate conversion
was observed, showing the merit of the PS resin over silica gel
as a robust solid support. It is also to be noted that use of the
twofold cross-linked phosphane 4, the single-point linked
phosphane 5, and the homogeneous ligand 1 were again less
efficient (22%, 5%, and 0% yields, respectively).[25]
À
a reagent for the C H borylation, the a-aminoalkylboronates
15 and 17 were obtained in good or high yields (15, 73%; 17,
90%) based on the boron atom used. These results clearly
show that achieving mono-P-ligation and having moderate
ligand steric effect together is important for producing
a C(sp ) H borylation catalyst (mono-P-ligating, homoge-
3
À
This procedure was also applicable for the synthesis of
a sterically congested diarylamine (10b) and a triarylamine
(10c, from diphenylamine). The amination of the chloroarene
6b with morpholine to give the N,N-dialkylaniline derivative
10d was successfully performed under even more basic
conditions using LiHMDS as a base.
The mono-ligating feature of the threefold cross-linked
PS-Ph3P hybrids (3) and their excellent performance in the
3
À
Scheme 4. Rh-catalyzed N-adjacent C(sp ) H borylation.
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neous bulky phosphanes were not useful[23c,e,f,24]), and using
a cage structure as in silica-SMAP and silica-TRIP is not
always necessary. Advantages of the PS-Ph3P hybrid system
over the cage-type silica-supported phosphanes are found in
the easiness of the synthesis and structural modification and
in the flexibility for expanding the concept.
[7] All of the phosphorus atoms participated in the metal binding
upon treatment with an excess amount of [{RhCl(cod)}2] to form
the mono-P-ligated complex [RhCl(cod)(3)]. See the Supporting
Information for details (Supporting Information, Figure S8).
[8] See the Supporting Information for the swelling properties of the
PS-Ph3P hybrids 3a–d, 4, and 5 in various solvents (Supporting
Information, Table S1).
[9] See the Supporting Information for the 31P CP/MAS NMR
spectra obtained in the experiments with 3a–d, 4, and 5 (P/Pd
2:1, 1:2; Figure S9–S14) together with the 31P NMR (CDCl3)
spectra obtained with (p-Tol)3P (P/Pd 2:1, 1:1, 1:2; Figure S15).
[10] For reviews, see: a) R. Martin, S. L. Buchwald, Acc. Chem. Res.
2008, 41, 1461; b) G. C. Fu, Acc. Chem. Res. 2008, 41, 1555;
c) G. A. Molander, B. Canturk, Angew. Chem. 2009, 121, 9404;
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Plenio, Chem. Soc. Rev. 2010, 39, 694; e) R. J. Lundgren, K. D.
Hesp, M. Stradiotto, Synlett 2011, 2443; f) S. M. Wong, C. M. So,
F. Y. Kwong, Synlett 2012, 1132; g) R. J. Lundgren, M. Stra-
diotto, Chem. Eur. J. 2012, 18, 9758.
[11] Selected examples for related studies, which used sophisticated
polymer-supported monophosphane ligands, see: a) Q.-S. Hu, Y.
Lu, Z.-Y. Tang, H.-B. Yu, J. Am. Chem. Soc. 2003, 125, 2856;
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Ed. 2009, 48, 2383; c) G. C. Fortman, S. P. Nolan, Chem. Soc.
Rev. 2011, 40, 5151.
[13] a) T. Iwasawa, T. Komano, A. Tajima, M. Tokunaga, Y. Obora, T.
Fujihara, Y. Tsuji, Organometallics 2006, 25, 4665; b) H. Ohta,
M. Tokunaga, Y. Obora, T. Iwai, T. Iwasawa, T. Fujihara, Y.
Tsuji, Org. Lett. 2007, 9, 89; c) T. Fujihara, S. Yoshida, H. Ohta,
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[15] For recent examples for employing triarylphosphanes in Pd-
catalyzed cross-coupling reactions of chloroarenes, see:
a) D. J. M. Snelders, G. van Koten, R. J. M. K. Gebbink, J. Am.
Chem. Soc. 2009, 131, 11407; b) S. Mom, M. BeaupØrin, D. Roy,
S. Royer, R. Amardeil, H. Cattey, H. Doucet, J.-C. Hierso, Inorg.
Chem. 2011, 50, 11592; c) W. K. Chow, O. Y. Yuen, C. M. So,
W. T. Wong, F. Y. Kwong, J. Org. Chem. 2012, 77, 3543.
[16] For a heterogeneous Pd catalyst system based on a polymeric
imidazole support that showed high activity for the Suzuki –
Miyaura coupling between chloroarenes and arylboronic acids
in water, see: Y. M. A. Yamada, S. M. Sarkar, Y. Uozumi, J. Am.
Chem. Soc. 2012, 134, 3190.
In summary, a new type of polystyrene(PS)–phosphane
covalently bound hybrid was synthesized through radical
emulsion copolymerization of styrenes, divinylbenzene, and
a tris(p-vinylphenyl)phosphane threefold cross-linker. These
hybrids were used for catalyst systems to favor a mono-P-
ligated metal complex with minimal synthetic effort. The
applicability of the PS-Ph3P hybrids toward Pd-catalyzed
cross-coupling reactions of unactivated chloroarenes as well
3
À
as Ir- or Rh-catalyzed borylation reactions of C(sp ) H bonds
demonstrates the utility of this simple heterogeneous strategy.
Expanding the applicability of this strategy for exploring new
catalysis and the development of new functional polymer–
phosphane hybrids will be the subject of future study in our
laboratory.
Received: August 2, 2013
Revised: August 26, 2013
Published online: October 2, 2013
À
Keywords: borylation · C H activation · cross-coupling ·
phosphanes · polystyrene-supported catalysts
.
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(Scheme S1).
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