A Polystyrene-Cross-Linking Bisphosphine: Controlled Metal Monochelation and Ligand-Enabled First-Row Transition Metal Catalysis

Article abstract of DOI:10.1021/acscatal.6b02988

A polystyrene-cross-linking bisphosphine PS-DPPBz was synthesized through radical emulsion copolymerization between 4-t-butylstyrene as a monomer and tetravinylated 1,2-bis(diphenylphosphino)benzene (DPPBz) as a 4-fold cross-linker. The location of the DPPBz bisphosphine moiety at the branching points of the cross-linked network organic polymer allowed controlled bisphosphine monochelation to transition metals under conditions where homogeneous ligands may form bischelated single metal complexes or multinuclear complexes. PS-DPPBz showed high ligand performance in first-row transition metal catalysis, enabling challenging molecular transformations that were not possible through the screening of existing homogeneous ligands. In the Ni-catalyzed amination of aryl chlorides with N-alkyl-substituted primary amines, the substrate scope has been expanded to include 2,6-disubstituted aryl chlorides and N-tertiary-alkyl-substituted primary amines. PS-DPPBz was effective for the Ni-catalyzed C-H/C-O coupling between 1,3-azoles and monocyclic aryl pivalates, which showed poor reactivity in the reported homogeneous catalytic system based on 1,2-bis(dicyclohexylphosphino)ethane (DCYPE). The usefulness of the polymer-cross-linking strategy was also demonstrated in alkene hydroboration reactions catalyzed by Cu or Co.

Full text of DOI:10.1021/acscatal.6b02988

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Article
A Polystyrene-Cross-Linking Bisphosphine: Controlled Metal
Monochelation and Ligand-Enabled First-Row Transition Metal Catalysis
Tomohiro Iwai, Tomoya Harada, Hajime Shimada, Kiichi Asano, and Masaya Sawamura
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02988 • Publication Date (Web): 17 Jan 2017
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ACS Catalysis
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A Polystyrene-Cross-Linking Bisphosphine: Controlled Metal Mono-
chelation and Ligand-Enabled First-Row Transition Metal Catalysis
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Tomohiro Iwai,* Tomoya Harada, Hajime Shimada, Kiichi Asano and Masaya Sawamura*
Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060ꢀ0810, Japan
ABSTRACT: A polystyreneꢀcrossꢀlinking bisphosphine PSꢀDPPBz was synthesized through radical emulsion copolymerization between
4ꢀtꢀbutylstyrene as a monomer and tetravinylated 1,2ꢀbis(diphenylphosphino)benzene (DPPBz) as a fourfold crossꢀlinker. The location of
the DPPBz bisphosphine moiety at the branching points of the crossꢀlinked network organic polymer allowed controlled bisphosphine
monochelation to transition metals under conditions where homogeneous ligands may form bischelated single metal complexes or multiꢀ
nuclear complexes. PSꢀDPPBz showed high ligand performance in firstꢀrow transition metal catalysis, enabling challenging molecular
transformations that were not possible through the screening of existing homogeneous ligands. In the Niꢀcatalyzed amination of aryl chloꢀ
rides with Nꢀalkylꢀsubstituted primary amines, the substrate scope has been expanded to include 2,6ꢀdisubstituted aryl chlorides and Nꢀ
tertiaryꢀalkylꢀsubstituted primary amines. PSꢀDPPBz was effective for the Niꢀcatalyzed C–H/C–O coupling between 1,3ꢀazoles and monoꢀ
cyclic aryl pivalates, which showed poor reactivity in the reported homogeneous catalytic system based on 1,2ꢀ
bis(dicyclohexylphosphino)ethane (DCYPE). The usefulness of the polymerꢀcrossꢀlinking strategy was also demonstrated in alkene hyꢀ
droboration reactions catalyzed by Cu or Co.
KEYWORDS: immobilized phosphines, polystyrene, bisphosphine, first-row transition metals, C–N coupling, C–H/C–O coupling,
hydroboration
metal catalysis. Owing to their chelating nature, bisphosphines
are effective in the construction of precise catalytic environments
for various transition metal catalyses. However, monochelation to
the metal center (I), bischelation to the metal center (II), or diꢀ
merization of monochelate metal species (III) may potentially
occur. The latter two species may be inactive and thus cause a
decrease in total efficiency of the catalysis (Chart 3a).
We envisaged that the formation of II and III (Chart 3a)
could be avoided by spatially isolating the bisphosphine units
through immobilization on a solid support (Chart 3b). To achieve
this, we designed a polystyreneꢀcrossꢀlinking bisphosphine PSꢀ
DPPBz (Chart 1). The rigid 1,2ꢀbis(diphenylphosphino)benzene
(DPPBz) core was adopted as the bisphosphine unit to achieve
reliable metal chelation ability in a polymer matrix.8–12 The steric
demand in proximity to the P atoms should be only moderate
(DPPBzꢀlike) due to the spacer effect of the four aromatic rings
on the P atoms, which form a pyramidalꢀtype space; and conseꢀ
quently, the resulting PꢀPꢀchelated metal system should provide
effective access of substrates for catalysis.
INTRODUCTION
A polymerꢀsupported phosphine is a useful material for producing
heterogenized transition metal catalysts, which can provide pracꢀ
tical benefits in synthetic processes (e.g., catalyst separation and
reusability).1 Polystyrene is the most widely used support owing
to its facile preparation, and physical, thermal and chemical staꢀ
bility.2,3 However, the use of the polystyrene backbone for inꢀ
creasing the catalytic performance has been overlooked in the
design of highly active heterogeneous catalysts.4 In this context,
our group previously developed a new type of polystyreneꢀcrossꢀ
linking monophosphine PSꢀTPP (Chart 1).5 Owing to spatial isoꢀ
lation of the P center in the polymer matrix by the threefold crossꢀ
linking,6,7 PSꢀTPP achieved controlled monoꢀPꢀligation toward
metals (P/M 1:1) to enable some challenging transition metal
catalyses such as Pdꢀcatalyzed crossꢀcoupling of unactivated aryl
chlorides and Ir(Rh)ꢀcatalyzed heteroatomꢀdirected C(sp3)–H
borylation (Chart 2a).
In the present work, the concept established by the threefold
crossꢀlinking monophosphine PSꢀTPP has been expanded to a
polystyreneꢀcrossꢀlinking bisphosphine ligand for controlled
monochelation to transition metals. Thus, a fourfold polystyreneꢀ
crossꢀlinking bisphosphine ligand (PSꢀDPPBz, Chart 1) based on
1,2ꢀbis(diphenylphosphino)benzene (DPPBz) exhibited high ligꢀ
and performance in firstꢀrow transition metal catalyses such as
Niꢀcatalyzed amination of aryl chlorides with Nꢀalkylꢀsubstituted
primary amines, Niꢀcatalyzed C–H/C–O coupling between 1,3ꢀ
azoles and aryl pivalates, and alkene hydroboration reactions
catalyzed by Cu or Co (Chart 2b). Various experiments including
NMR analysis for metal coordination and comparative catalytic
studies with various ligands suggested that the enhanced catalytic
performances with PSꢀDPPBz were mainly due to controlled
monochelation of the bisphosphine unit and inhibited bischelation
to metal centers and/or dimerization of monochelated metal speꢀ
cies.
Chart 1. Polystyrene-Phosphine Hybrids Used in this Paper
RESULTS AND DISCUSSION
Design
of
polystyrene-cross-linking
bisphosphine.
Bisphosphines are one of the most common ligands in transitionꢀ
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(CDCl3); δ –13.3 (ppm)].15 Based on the results of combustion
Chart 2. Conceptual View of Metal Catalysts with Polysty-
rene-cross-linking (a) Monophosphine5 and (b) Bisphosphine
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elemental analysis for P, the loading of the DPPBz moiety to
polystyrene resins was estimated to be 0.12 mmol/g. This value
was comparable to a value (0.098 ≈ 0.1 mmol/g) calculated on the
basis of the ratio of the used monomers. This calculated value
(0.1 mmol/g) was used in metal coordination studies and catalytic
applications (vide infra).
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Scheme 1. Preparation of PS-DPPBz
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Figure 1. TGA profiles of PSꢀDPPBz (heating rate: 10 °C/min, in
air).
Chart 3. Metal Chelation Modes with Bisphosphines
Figure 2. 31P CP/MAS NMR spectrum of PSꢀDPPBz (spinning
rate 8 kHz).
Coordination of a Rh(I) species. To confirm the monoꢀ
chelating property of PSꢀDPPBz, a cationic Rh(I) complex was
employed as a metal source because it was prone to undergo bisꢀ
chelation by two bisphosphine ligands.16 In fact, the reaction of
the soluble ligand DPPBz with [Rh(cod)2]BF4 (DPPBz/Rh 1:1) at
Preparation of polystyrene-cross-linking bisphosphine.
PSꢀDPPBz was synthesized through radical emulsion polymerizaꢀ
tion with tetravinylated DPPBz (1)9a and 4ꢀtꢀbutylstyrene (1/4ꢀtꢀ
butylstyrene 1:60), as shown in Scheme 1. The obtained beads of
PSꢀDPPBz had ordinary swelling properties as a polystyrene resin
toward common organic solvents.13 The thermogravimetric analyꢀ
sis (TGA, 10 °C/min, in air) of the dried PSꢀDPPBz showed that
this material was stable below 250 °C (Figure 1).14 The 31P
CP/MAS NMR spectrum exhibited a broad signal at δ –17 (ppm)
(Figure 2), indicating that its electronic and steric properties are
virtually the same as that of a DPPBz molecule [31P NMR
rt afforded
a mixture of monoꢀDPPBzꢀchelated complex
[Rh(cod)(DPPBz)]BF4 and bisꢀDPPBzꢀchelated complex
[Rh(DPPBz)2]BF4 with NMR resonances at δ 58.0 and δ 62.9
(ppm), respectively (Figure 3a). When the DPPBz/Rh ratio was
2:1, the DPPBz bisphosphine was completely consumed for exꢀ
clusive formation of the bischelated complex [Rh(DPPBz)2]BF4
(Figure 3b). The corresponding experiments with the polymer
ligand PSꢀDPPBz showed contrasting behavior. The reaction of
PSꢀDPPBz with [Rh(cod)2]BF4 with a PSꢀDPPBz/Rh ratio of 1:1
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was conducted in CH2Cl2 at rt for 2 h. Filtration to remove a colꢀ
(a)
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orless solution, washing, and removal of volatiles gave orange
beads. The solidꢀstate 31P CP/MAS NMR spectrum of the beads
indicated the formation of a new Rhꢀbound DPPBz species,
which gave a signal at δ 53 (ppm) accompanied by significant
spinning side bands (Figure 4a). Although the highꢀfield signal of
the spinning side bands overlapped the region of the free bisphosꢀ
phine signal [δ –17 (ppm)], most of the bisphosphine moieties
including those deep inside the beads were accessible for the
metal complexation. Notably, even with excess bisphosphine
toward the Rh(I) complex (PSꢀDPPBz/Rh 2:1), [Rh(cod)(PSꢀ
DPPBz)]BF4 existed as the virtually sole Rhꢀbound P species,
along with the free bisphosphine (Figure 4b). Thus, the polystyꢀ
reneꢀcrossꢀlinking strategy appeared to be effective to realize
controlled bisphosphine monochelation to metals owing to the
spatial isolation of the coordination center in the polymer matrix.
Effect of the bridge between the two P atoms: Coordina-
tion of a Pd(II) species. The following NMR experiments eluciꢀ
dated that the bridge between the two P atoms must be sufficientꢀ
ly rigid for maintaining the chelating ability of the bisphosphine
as a polymer crossꢀlinker. The 31P CP/MAS NMR spectrum obꢀ
tained from PSꢀDPPBz and excess PdCl2(cod) in THF at rt (PSꢀ
DPPBz/Pd 1:2) showed only one signal of a bisphosphineꢀ
monochelate Pd complex PdCl2(PSꢀDPPBz) at δ 61 (ppm) (Figꢀ
ure 5a).17 In contrast, the use of a fourfold crossꢀlinked polystyꢀ
rene PSꢀDPPE (Chart 1) based on a more flexible bisphosphine
1,2ꢀbis(diphenylphosphino)ethane (DPPE) instead of PSꢀDPPBz
gave two wellꢀseparated signals at δ 63 and δ 33 (ppm) (Figure
5b), which correspond to a bisphosphineꢀmonochelate Pd comꢀ
plex PdCl2(PSꢀDPPE) and a monoꢀPꢀligated (nonꢀchelate) Pd
complex Pd2Cl4(L)2(PSꢀDPPE) (L = donor ligands), respectively
(see Supporting Information for 31P NMR studies with EtPh2P as
a model ligand, Figure S8). The P–C–C–P tetrad of the DPPE
core in PSꢀDPPE must adopt a thermally favored sꢀtrans conforꢀ
mation as prepared, and the structural change to the sꢀcis conforꢀ
mation, which is required for metal chelation, seems to be inhibitꢀ
ed due to the rigidity of the crossꢀlinked polymer matrix.
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(b)
Figure 4. 31P CP/MAS NMR spectra obtained from PSꢀDPPBz
and [Rh(cod)2]BF4 [PSꢀDPPBz/Rh (a) 1:1 and (b) 2:1] (spinning
rate 8 kHz).
(a)
(a)
(b)
(b)
Figure 3. 31P NMR spectra in CDCl3 obtained from DPPBz and
[Rh(cod)2]BF4 [(a) DPPBz/Rh 1:1 and (b) 2:1].
Figure 5. 31P CP/MAS NMR spectra obtained (a) from PSꢀ
DPPBz and PdCl2(cod) (PSꢀDPPBz/Pd 1:2), and (b) from PSꢀ
DPPE and PdCl2(cod) (PSꢀDPPE/Pd 1:2) (spinning rate 8 kHz).
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First-row transition metal catalysis. Firstꢀrow transition
Chart 5. Comparison between PS-DPPBz/Ni(cod)2 and
(BINAP)Ni(η2-NC-Ph)
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metal catalysis has attracted great attention as a strategy for enviꢀ
ronmentally benign organic synthesis because of their high earth
abundance and low toxicity.18 However, homogeneous catalytic
systems with firstꢀrow transition metals often suffer from facile
ligand dissociation, aggregation, and/or disproportionation of
metal complexes. We expected that the polystyreneꢀcrossꢀlinking
bisphosphine PSꢀDPPBz should be useful for eliminating probꢀ
lems associated with aggregation and disproportionation reactions
that demand proximity of different catalytic centers, thus realizing
highly efficient firstꢀrow transition metal catalysis.
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Nickel-catalyzed amination of aryl chlorides with N-alkyl-
substituted primary amines. As the initial firstꢀrow transition
metal catalysis to test the feasibility of our idea described above,
we adopted Niꢀcatalyzed amination of unactivated aryl chlorides
with Nꢀalkylꢀsubstituted primary amines,19 for which
(BINAP)Ni(η2ꢀNCꢀPh) was identified to be an exceptionally efꢀ
fective catalyst precursor by Hartwig and coꢀworkers (Chart 4).20
Mechanistic studies by Hartwig’s group indicated the importance
of controlled Ni monochelation by the BINAP bisphosphine ligꢀ
and (A) to form a catalytically active species that reacts with aryl
chlorides. In addition, they identified four catalytically inactive
species, (BINAP)2Ni(0) (B), [(BINAP)NiI(ꢁ2ꢀCl)]2 (C),
(amine)2NiAr(Cl) (D), and a nickelacycle complex (E), the forꢀ
mation of which limited the catalytic activity and scope of the
reaction. Specifically, the reaction of 4ꢀchloroanisole (2a), which
was deactivated by an electronꢀdonating pꢀMeO group, with nꢀ
octylamine (3a) required relatively high catalyst loading (4
mol%) to obtain an acceptable yield of coupling product 4a.20–22
We envisioned that the use of PSꢀDPPBz could suppress the
formation of the inactive species B and C (Chart 4), both of
which have two bisphosphine ligands. Moreover, the rigid DPPBz
core should be favorable for suppressing the ligand degradation
due to P–C bond cleavage as in the formation of E.
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Next, the effect of PSꢀDPPBz was evaluated by comparison
with those of various homogeneous and heterogeneous ligands.
PhosphineꢀNi catalysts were prepared in situ by mixing Ni(cod)2
(1 mol%) and the ligands, and were subjected to the coupling
between 2a and 3a at 80 °C for 20 h. The results are summarized
in Table 1. BINAP was much less efficient as a ligand for in situ
preparation of a Ni catalyst than PSꢀDPPBz, irrespective of ligand
loading (1 or 1.5 mol% BINAP; entries 1–3). Importantly, the use
of DPPBz (1.5 mol%) ligand as a homogeneous reference against
PSꢀDPPBz induced little activity (entry 4). The DPPBz derivative
SciOPP23 (Chart 6), which has increased steric demands at the Pꢀ
phenyl groups, gave a better yield than the parent DPPBz, but it
was much less efficient than PSꢀDPPBz (entry 5). Thus, the polꢀ
ymer effect in the ligand performance of PSꢀDPPBz is crucial.
During this study, it was revealed that the P,P,P’,P’ꢀ
tetracyclohexylꢀsubstituted Josiphosꢀtype ligand 5 (Chart 6) was a
particularly effective homogeneous ligand for promoting the reacꢀ
tion, giving 71% yield of 4a (Table 1, entry 6).24 This is in acꢀ
cordance with independent reports by Hartwig’s25a and Straꢀ
diotto’s25b groups on the efficacy of this type of ligand in the
amination of aryl chlorides with ammonia. The performance of 5
shown here indeed approached that of PSꢀDPPBz, but the adꢀ
vantage of PSꢀDPPBz over the homogeneous systems, including
5, appeared to be overwhelming in the reaction with lower cataꢀ
lyst loading at even higher temperature [0.1 mol%, 120 °C, ligꢀ
and/Ni 2:1; 93% yield (isolated) with PSꢀDPPBz vs. 9% yield (1H
NMR) with 5, eq 1] or in the transformations of more challenging
substrates, which required harsher reaction conditions (vide inꢀ
fra).
Addition of the homogeneous DPPBz ligand (1.5 mol%) to
the reaction system with (PSꢀDPPBz)ꢀNi (DPPBz/PSꢀDPPBz 1:1,
at 80 °C) inhibited the amination completely (Table 1, entry 7).
During this reaction, the solution phase was reddish orange in
color. This observation was indicative of ligand exchange beꢀ
tween the homogeneous and polymerꢀbound DPPBz ligands.26,27
On the other hand, the increase in the loading of PSꢀDPPBz from
1.5 mol% (entry 1) to 3 mol% (PSꢀDPPBz/Ni 3:1) had almost no
effect on the reaction efficiency, indicating that the excess
bisphosphine moiety in PSꢀDPPBz did not participate in the reacꢀ
tion (entry 8).28,29 The loading of PSꢀDPPBz could be reduced to
1 mol% (PSꢀDPPBz/Ni 1:1) without significant reduction in the
product yield (entry 9).
Chart 4. Ni-catalyzed Amination of Aryl Chlorides with N-
Alkyl-substituted Primary Amines reported by Hartwig’s
Group20
Initially, the reaction between 2a and 3a was conducted at 50
°C with PSꢀDPPBz/Ni(cod)2 or (BINAP)Ni(η2ꢀNCꢀPh) systems
with 1 mol% Ni loading (Chart 5). Both of the catalytic systems
induced the amination, but with comparable low yields of 4a
(<12%). To our delight, however, the (PSꢀDPPBz)ꢀNi system
achieved substrate conversion as high as 93% at 80 °C, while
(BINAP)Ni(η2ꢀNCꢀPh) gave only a slight improvement (16%).
These results suggest that the (PSꢀDPPBz)ꢀNi system has higher
thermal stability than the BINAPꢀNi system.
In contrast to the crucial polymer effect obtained by the fourꢀ
fold bisphosphineꢀcrossꢀlinking (Table 1, entry 1), a twofold
crossꢀlinking in HemiꢀPSꢀDPPBz (Chart 1), which was prepared
together with a divinylbenzene coꢀcrossꢀlinker to achieve a deꢀ
gree of crossꢀlinking comparable to that of PSꢀDPPBz, did not
exhibit a useful polymer effect as indicated by the low yield of 4a
in the reaction with this heterogeneous ligand (entry 10). In this
case, the polystyrene chains should be more accessible to the
catalytic center, causing unfavorable steric effects. This result
also suggests that potential polymer effects such as swelling and
polarity are not significant compared with the monochelation
effect.
The rigidity of the bridge between the two P atoms in polyꢀ
styreneꢀcrossꢀlinking bisphosphines was important for the high
catalytic activity. Thus, PSꢀDPPE, having the flexible ethylene
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bridge, induced almost no reaction (Table 1, entry 11). Note that
the inexpediency of chelation by PSꢀDPPE had been indicated by
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the 31P CP/MAS NMR studies on the Pd(II) coordination (Figure
5b). The catalytic inefficiency of the (PSꢀDPPE)ꢀNi system is in
accord with Hartwig’s report, in that chelation of Ni by the
bisphosphine is mandatory.20 The importance of chelation in the
heterogeneous system was also indicated by the inefficiency of
the polystyreneꢀcrossꢀlinking monophosphine ligand PSꢀTPP
(entry 12).5
The high performance of the (PSꢀDPPBz)ꢀNi catalyst system
was demonstrated in the amination of sterically demanding aryl
chlorides. As shown in Chart 7, diꢀorthoꢀsubstituted aryl chloꢀ
rides such as 2,6ꢀdimethylchlorobenzene (2b) and 2,6ꢀ
diisopropylchlorobenzene (2c) reacted with 3a at 100–120 °C in
the presence of 1 mol% of (PSꢀDPPBz)ꢀNi, giving 4b and 4c,
respectively, in high yields. Sterically and electronically deacꢀ
tivated aryl chloride 2,4,6ꢀtrimethoxychlorobenzene (2d) was also
a suitable substrate. In contrast, DPPBz, BINAP or 5 were not
effective to promote these challenging transformations: only 5
showed a slight conversion of 2b, and all of the homogeneous
ligands were totally ineffective for the reactions of 2c and 2d.
Chart 7 showcases more examples of usable aryl chlorides.
Electronꢀrich (2e–2g), electronꢀneutral (2h), electronꢀdeficient (2i
and 2j), and πꢀextended (2k) aryl chlorides successfully underꢀ
went the amination with 3a at 80–100 °C, giving desired products
4e–4k in good to high yields (69–96%). In the reaction of 4ꢀ
chlorobenzotrifluoride (2j), use of LiOtBu instead of NaOtBu as a
base was effective for the selective monoarylation of 3a to give
the corresponding secondary amine 4j in high yield. 3ꢀPyridyl
chloride (2l) served as a suitable substrate. Amination of monoꢀ
orthoꢀsubstituted aryl chlorides such as 2ꢀchlorotoluene (2m) and
2ꢀchloroanisole (2n) smoothly proceeded at 80 °C to give steriꢀ
cally congested anilines 4m and 4n, respectively, in high yields.
Although the separation of (PSꢀDPPBz)ꢀNi beads from the
reaction mixture was easy, the intrinsically labile nature of phosꢀ
phineꢀNi coordination hampered efficient reuse of the catalyst in
the reaction between 2a and 3a (1 mol% Ni, 100 °C, 2 h): 1st run,
90%; 2nd run, 99%; 3rd run, 93%; 4th run, 73%. In fact, solution
phases appeared to include Ni species as indicated by ICPꢀAES
analysis (1.8–3.3% of the loaded Ni for each run) (see Supporting
Information for details; Table S5, Figure S9). Thus, the formation
of (amine)2NiAr(Cl) (D), which was identified to be an inactive
species by Hartwig and coꢀworkers (Chart 4),20 or metallic Ni
may be inevitable even with the PSꢀDPPBz ligand. Meanwhile,
the heterogeneous nature of catalytically active species was conꢀ
firmed by a hot filtration test and a mercury poisoning test (see
Supporting Information for details; Figure S10).30
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48
49
50
51
52
53
54
55
56
57
58
59
60
Table 1. Ligand Effects in the Ni-catalyzed Amination of 2a
with 3aa
Chart 7. Scope of Aryl Chloridesa,b
entry
1
ligand
PSꢀDPPBz
yield (%)b
93 (91)
10
24
11
15
71
0
2
BINAP (1 mol%)
BINAP
3
4
DPPBz
5
SciOPP
6
7c
5
PSꢀDPPBz + DPPBz
PSꢀDPPBz (3 mol%)
PSꢀDPPBz (1 mol%)
HemiꢀPSꢀDPPBz
PSꢀDPPE
8
89
84
11
5
9
10
11
12
PSꢀTPP (3 mol%)
0
a
Conditions: 2a (0.25 mmol), 3a (0.375 mmol), Ni(cod)2 (1 mol%), ligꢀ
and (1.5 mol%, structures shown in Chart 6), NaOtBu (0.375 mmol),
toluene (1 mL), 80 °C, 20 h. b Yields were determined by 1H NMR analyꢀ
c
sis of the crude product. Isolated yields are shown in parentheses. PSꢀ
DPPBz (1.5 mol%), DPPBz (1.5 mol%).
Chart 6. Homogeneous Bisphosphine Ligands Used for the
Ni-catalyzed Amination (Table 1)
a
Conditions: 2b–2n (0.25 mmol), 3a (0.375 mmol), Ni(cod)2 (1 mol%),
PSꢀDPPBz (1.5 mol%), NaOtBu (0.375 mmol), toluene (1 mL), 80–
120 °C, 20 h. b Isolated yields. c 1H NMR yields. d For 24 h. e LiOtBu was
used instead of NaOtBu.
With the (PSꢀDPPBz)ꢀNi system, the scope of primary
amines was also expanded (Chart 8).31 Isoamyl (3b), 2ꢀ
phenylethyl (3c), and 4ꢀmethoxybenzyl (3d) amines smoothly
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Page 6 of 12
reacted under the standard conditions (1 mol% Ni, 80 °C, 20 h) to
fact, the reaction of benzoxazole (6a, 0.4 mmol) with phenyl
1
2
3
4
5
6
7
8
give the corresponding secondary amines 4o–4q in high yields.
Cyclohexylamine (3e) was also a suitable substrate. Although the
reactions with more sterically hindered primary amines containꢀ
ing an acyclic secondary Nꢀalkyl group [benzhydrylamine (3f)]
and tertꢀalkyl groups [1ꢀadamantyl (3g), tꢀoctyl (3h), and cumyl
(3i) amines] were less efficient under the present optimal condiꢀ
tions (data not shown), the use of LiOtBu instead of NaOtBu as a
base allowed the reactions of these sterically demanding amines
to occur with reasonable catalyst loading (1–2 mol% Ni) at 100–
120 °C, giving the corresponding secondary amines 4s–4y in
good to high yields. This is the first use of Nꢀtertꢀalkylꢀsubstituted
primary amines for the C–N coupling with Ni catalysts.32
pivalate (7b, 1.5 equiv) catalyzed by the DCYPEꢀNi system (10
mol% Ni) gave much lower product yield (8b, 11%) for C–H/C–
O coupling than that with 2ꢀnaphthyl pivalate (7a) (8a, 95%,
Chart 9).33
Chart 9. Ni-catalyzed C–H/C–O Coupling between 1,3-Azoles
and Aryl Pivalates Reported by Itami and Co-workers33
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
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40
41
42
43
44
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47
48
49
50
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52
53
54
55
56
57
58
59
60
Chart 8. Scope of N-Alkyl-substituted Primary Aminesa,b
Thus, the (PSꢀDPPBz)ꢀNi system was prepared in situ from
PSꢀDPPBz (6 mol%) and Ni(cod)2 (5 mol%), and applied to the
C–H/C–O coupling between 6a (0.1 mmol) and 7b (1.5 equiv)
with Cs2CO3 as base in mꢀxylene at 150 °C. To our delight, this
reaction afforded the desired product 8b in 85% isolated yield
(Table 2, entry 1).37,38 In contrast, the use of DCYPE resulted in
only 3% yield of 8b in accord with Itami’s report (entry 2).33 No
reaction occurred with the soluble DPPBz ligand (entry 3). Thus,
the polymerꢀcrossꢀlinking was again crucial. The sterically deꢀ
manding substituents introduced in the Pꢀphenyl groups of the
homogeneous DPPBz ligand (SciOPP) induced catalytic activity
(entry 4), but this effect was much smaller than the polymer efꢀ
fects with PSꢀDPPBz. The twofold crossꢀlinking bisphosphine
HemiꢀPSꢀDPPBz was far less effective and PSꢀDPPE induced no
catalytic activity as in the case of the Niꢀcatalyzed amination
(entries 5 and 6). These results suggested that the fourfold polyꢀ
merꢀcrossꢀlinking was important for the sterically and electroniꢀ
cally moderate DPPBzꢀtype bisphosphine to be effective for this
transformation, which involves the C(aryl)–O bond cleavage of
phenolꢀbased pivalates.39
a
Conditions: 2a or 2h (0.25 mmol), 3b–3i (0.375 mmol), Ni(cod)2 (1
mol%), PSꢀDPPBz (1.5 mol%), NaOtBu (0.375 mmol), toluene (1 mL),
80–120 °C, 20 h. b Isolated yields. c LiOtBu was used instead of NaOtBu.
d Ni(cod)2 (2 mol%), PSꢀDPPBz (3 mol%), toluene (0.5 mL).
Table 2. Ligand Effects in Ni-catalyzed C–H/C–O Coupling
between 6a with 7ba
Nickel-catalyzed C–H/C–O coupling between 1,3-azoles
and aryl pivalates. Next, we sought to use PSꢀDPPBz to solve a
problem in C–H functionalization. Itami and coꢀworkers reported
the Niꢀcatalyzed C–H/C–O coupling between 1,3ꢀazoles and aryl
pivalates.33 The use of aryl pivalates, which are available from
phenol derivatives and cheap acylating reagent pivaloyl chloride,
as electrophilic reagents is an attractive feature of this reaction.34
In this catalysis, sterically demanding and electronꢀrich bisphosꢀ
phine 1,2ꢀbis(dicyclohexylphosphino)ethane (DCYPE) is specifiꢀ
cally effective (Chart 9).35 Mechanistic studies by Itami’s group
suggested two important roles of the DCYPE ligand.36 Firstly, the
strongly electronꢀdonating ability based on the peralkylated
bisphosphine renders the metal center of the Ni(0) complex more
electronꢀrich, enabling oxidative addition of the C(aryl)–O bond
of aryl pivalates possible. Secondly, the sterically demanding
chelating structure by four cyclohexyl substituents on the two P
atoms could stabilize the oxidative addition species Ar–Ni(II)–
OC(O)tBu.
entry
ligand
PSꢀDPPBz
DCYPE
yield (%)b
1
2
3
4
5
6
83 (85)
3
0
DPPBz
SciOPP
42
8
HemiꢀPSꢀDPPBz
PSꢀDPPE
0
a
Conditions: 6a (0.1 mmol), 7b (0.15 mmol), Ni(cod)2 (5 mol%), ligand
(6 mol%), Cs2CO3 (0.15 mmol), mꢀxylene (1 mL), 150 °C, 24 h. b Yields
were determined by 1H NMR analysis of the crude product. Isolated yield
is shown in parentheses.
Unfortunately, however, the protocol with the DCYPEꢀNi
system showed limited applicability toward the reaction of monoꢀ
cyclic phenolꢀbased pivalates, while various naphtholꢀbased (πꢀ
extended) pivalates participated favorably in the reactions to couꢀ
ple with different azole derivatives. This is in accord with a genꢀ
eral trend in oxidative addition of aryl electrophiles toward a low
With the (PSꢀDPPBz)ꢀNi system, various phenolꢀbased
pivalates (7) served as C–O electrophiles (Chart 10). Electronꢀ
rich (7c–e), electronꢀneutral (7f and 7g) and electronꢀdeficient
valent metal that the reactivity increases with π
ꢀextension.34 In
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ACS Catalysis
(7h) aryl pivalates successfully reacted with 6a (5–10 mol% Ni,
dropped significantly in the third use (1st run, 95%; 2nd run, 94%;
3rd run, 34%).
For comparison, the effects of homogeneous ligands are also
1
2
3
4
5
6
7
8
Cs2CO3, in mꢀxylene, 150 °C, 24 h), affording the corresponding
2ꢀarylꢀbenzoxazoles 8c–8h in good to high yields (44–94%).
orthoꢀSubstituted phenyl pivalates 7i and 7j were also suitable
substrates, providing sterically congested products 8i and 8j, reꢀ
spectively, in useful yields. The reaction with 3ꢀpyridyl pivalate
(7k) proceeded at lower temperature (10 mo% Ni, at 120 °C) in
1,4ꢀdioxane to give 2ꢀ(3ꢀpyridyl)azole 8k.
provided in Table 3. DPPBz was far less effective than PSꢀ
DPPBz (entry 1 vs. entry 2), again highlighting the importance of
the polymer crossꢀlinking strategy. Although the use of sterically
demanding bisphosphines such as SciOPP and (R)ꢀDTBMꢀ
Segphos (Chart 12) may be effective to form lessꢀaggregated
copper(I) hydride species,46 their catalytic performances were
smaller than that of PSꢀDPPBz (entries 3 and 4). Largeꢀbiteꢀangle
bisphosphine Xantphos, bulky peralkylated bisphosphine
DCYPE, and bulky Nꢀheterocyclic carbene IPr (Chart 12) induced
no activity (entries 5–7). A significant advantage of PSꢀDPPBz
over homogeneous bisphosphine ligands such as DPPBz, SciOPP
and (R)ꢀDTBMꢀSegphos was also observed in the reaction of 1,1ꢀ
disubstituted alkene 9b (1 mol% Cu, 80 °C, 4 h; entries 8–11).
Chart 10. Scope of Aryl Pivalatesa,b
9
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11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
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31
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33
34
35
36
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40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Chart 11. Cu-catalyzed Alkene Hydroboration with HBpin40
a Conditions: 6a (0.1 mmol), 7c–7k (0.15 mmol), Ni(cod)2 (5 mol%), PSꢀ
DPPBz (6 mol%), Cs2CO3 (0.15 mmol), mꢀxylene (1 mL), 150 °C, 24 h. b
Isolated yields. Ni(cod)2 (10 mol%), PSꢀDPPBz (12 mol%). Cs2CO3
(0.2 mmol). e In 1,4ꢀdioxane (1 mL), at 120 °C.
c
d
Copper-catalyzed alkene hydroboration. To explore the
utility of PSꢀDPPBz in the catalysis of Cu, which is also a highly
earthꢀabundant metal,18d we applied it to the Cuꢀcatalyzed hyꢀ
droboration of alkenes (9) with pinacolatoborane HBpin (10). Cuꢀ
catalyzed (asymmetric) alkene hydroboration was originally reꢀ
ported by Yun and coꢀworkers and recently extended to reactions
with challenging alkene substrates by Hartwig and coꢀworker
(Chart 11) (vide infra).40 We noted this reaction because the
DPPBz bisphosphine was used as a ligand and the proposed reacꢀ
tion mechanism involved the reaction between the nonꢀpolar C–C
double bond and copper(I) hydride species supported by chelation
of the DPPBz bisphosphine ligand.41 Generally, copper(I) hydride
species are prone to aggregate through hydride bridges, and the
resulting aggregates are less labile than the monomeric copper(I)
hydride species.42 This inhibitory effect of aggregation should be
more significant in a case where the Cu center demands preꢀ
coordination of spaceꢀdemanding substrates such as nonꢀpolar
alkenes. We expected that the siteꢀisolating polymer effect in PSꢀ
DPPBz would prevent the undesired aggregation of bisphosphineꢀ
monochelated copper(I) hydride species.43
Indeed, the reported alkene hydroboration catalyzed by the
homogeneous bisphosphineꢀCu system had a significant reactivity
dependence on the alkene substrate. While a variety of viꢀ
nylarenes underwent hydroboration, its scope toward aliphatic
alkenes was restricted to specially activated alkenes such as
alkenylboron derivatives, bicyclic strained alkenes, or aliphatic
internal alkenes bearing electronꢀwithdrawing functional groups
at a homoallylic position (Chart 11).40,44
The (PSꢀDPPBz)ꢀCu system, which was prepared from PSꢀ
DPPBz (1.5 mol%) and Cu(OAc)2 (1 mol% Cu) by mixing in
THF at 80 °C for 10 min followed by evaporation of the volaꢀ
tiles,45 catalyzed hydroboration of unactivated aliphatic terminal
alkene 4ꢀphenylꢀ1ꢀbutene (9a) with 10 at 80 °C in toluene over 2
h, to give primary alkyl boronate 11a in 92% yield (Table 3, entry
1). The (PSꢀDPPBz)ꢀCu catalyst recovered by decantation mainꢀ
tained most of the activity in the second run, but the yield
Table 3. Ligand Effects in Cu-catalyzed Hydroboration of 9a
entry
1
ligand
PSꢀDPPBz
DPPBz
yield (%)b
9
11
92 (79)
9a
9a
9a
9a
9a
9a
9a
9b
9b
9b
9b
11a
11a
11a
11a
11a
11a
11a
11b
11b
11b
11b
2
0
3
SciOPP
19
4
(R)ꢀDTBMꢀSegphos
Xantphos
28
5
0
6
DCYPE
0
7
IPr
0
8c
9c
10c
11c
PSꢀDPPBz
DPPBz
97 (91)
0
0
0
SciOPP
(R)ꢀDTBMꢀSegphos
a
Conditions: 9a or 9b (0.2 mmol), 10 (0.24 mmol), Cu(OAc)2 (1 mol%),
ligand (1.5 mol%, structures shown in Chart 12), toluene (1 mL), 80 °C, 2
b
1
h. Yields were determined by H NMR analysis of the crude product.
Isolated yields are shown in parentheses. c For 4 h.
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Chart 12. Homogeneous Ligands Used for the Cu-catalyzed
Hydroboration (Table 3)
Procedure for Polystyrene-cross-linking Bisphosphine PS-
1
2
3
4
5
6
7
8
DPPBz (Scheme 1). A solution of acacia gum (2.40 g) and NaCl
(3.00 g) in water (60 mL) was placed in a 200ꢀmL roundꢀbottom
flask equipped with a magnetic stirring bar, and was deoxygenatꢀ
ed by purging with Ar. Meanwhile, a solution of 4ꢀtꢀbutylstyrene
(2.75 mL, 2.41 g, >90% purity, 15 mmol), 1 (0.1375 g, 0.25
mmol) and AIBN (0.050 g, 0.30 mmol) in chlorobenzene (3 mL)
was degassed by three freezeꢀpumpꢀthaw cycles, and was injected
into the rapidly stirred aqueous solution. The mixture was stirred
at 80 °C for 18 h. After being cooled to rt, the mixture was filꢀ
tered through a glass adaptor equipped with a cotton plug, washed
successively with H2O, MeOH, toluene, THF and MeOH, and
dried in vacuo at 100 °C for 12 h to give the polymer PSꢀDPPBz
as white beads (2.30 g, 90 wt%). The obtained beads were passed
through a series of sieves, and the beads with 250–710 ꢁm size in
diameter were used for catalytic reactions.
A Typical Procedure for Ni-catalyzed Amination of Aryl
Chlorides with N-Alkyl-substituted Primary Amines (Table 1,
Entry 1). In a nitrogenꢀfilled glove box, PSꢀDPPBz (37.5 mg,
0.00375 mmol, 1.5 mol%), toluene (0.72 mL) and a solution of
Ni(cod)2 (0.69 mg, 0.0025 mmol, 1 mol%) in toluene (0.28 mL)
were placed in a 10ꢀmL glass tube containing a magnetic stirring
bar. After stirring at rt for 5 min, NaOtBu (36.0 mg, 0.375 mmol),
4ꢀchloroanisole (2a, 30.6 µL, 0.25 mmol) and nꢀoctylamine (3a,
62.1 µL, 0.375 mmol) were added successively. The tube was
sealed with a screw cap and was removed from the glove box.
The mixture was stirred at 80 °C for 20 h. After being cooled to
rt, the mixture was filtered through a Celite pad (eluting with
Et2O). The volatiles were removed under reduced pressure, and
an internal standard (1,2ꢀdiphenylethane) was added to determine
the yield of 4ꢀmethoxyꢀNꢀoctylaniline (4a, 93% yield). The crude
product was purified by silica gel column chromatography (hexꢀ
ane/EtOAc 100:0 to 95:5) to give 4a as a pale yellow oil (53.6
mg, 0.228 mmol, 91% yield).
A Typical Procedure for Ni-catalyzed C–H/C–O Coupling
between Benzoxazole and Aryl Pivalates (Table 2, Entry 1). In
a nitrogenꢀfilled glove box, PSꢀDPPBz (60 mg, 0.0060 mmol, 6
mol%), mꢀxylene (0.5 mL) and a solution of Ni(cod)2 (1.4 mg,
0.005 mmol, 5 mol%) in mꢀxylene (0.5 mL) were placed in a 10ꢀ
mL glass tube containing a magnetic stirring bar. After stirring at
rt for 10 min, Cs2CO3 (48.9 mg, 0.15 mmol), phenyl pivalate (7b,
26.7 mg, 0.15 mmol) and benzoxazole (6a, 11.9 mg, 0.1 mmol)
were added. The tube was sealed with a screw cap, and was reꢀ
moved from the glove box. The mixture was stirred at 150 °C for
24 h. After being cooled to rt, the mixture was filtered through a
short silica pad (eluting with EtOAc). The volatiles were removed
under reduced pressure, and an internal standard (1,1,2,2ꢀ
tetrachloroethane) was added to determine the yield of 2ꢀ
phenylbenzo[d]oxazole (8b, 83% yield). The crude product was
purified by preparative thinꢀlayer chromatography (silica gel,
hexane/EtOAc 10:1) to give 8b as a white solid (16.6 mg, 0.085
mmol, 85% yield).
A Typical Procedure for Cu-catalyzed Alkene Hydrobora-
tion with Pinacolborane (Table 3, Entry 1). In a nitrogenꢀfilled
glove box, PSꢀDPPBz (30 mg, 0.0030 mmol, 1.5 mol%), THF
(0.5 mL) and a solution of Cu(OAc)2 (0.36 mg, 0.0020 mmol, 1
mol%) in THF (0.5 mL) were placed in a 10ꢀmL glass tube conꢀ
taining a magnetic stirring bar. The tube was sealed with a screw
cap and was removed from the glove box. The mixture was
stirred at 80 °C for 10 min. After being cooled to rt, the volatiles
were removed under vacuum. The mixture was moved to the
glove box again. Toluene (1 mL), 4ꢀphenylꢀ1ꢀbutene (9a, 26.4
mg, 0.20 mmol) and pinacolborane (10, 30.7 mg, 0.24 mmol)
were added to the mixture. The tube was sealed with a screw cap
and was removed from the glove box. The mixture was stirred at
80 °C for 2 h. After being cooled to rt, the mixture was filtered
through a Celite pad (eluting with Et2O). The volatiles were reꢀ
moved under reduced pressure, and an internal standard (1,1,2,2ꢀ
9
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Cobalt-catalyzed alkene hydroboration. Coꢀcatalyzed alꢀ
kene hydroboration47,48 was another example of firstꢀrow transiꢀ
tion metal catalysis in which marked polymer effects of PSꢀ
DPPBz were observed. A (PSꢀDPPBz)ꢀCo system, which was
prepared in situ from PSꢀDPPBz (1.5 mol%) and CoI2 (1 mol%)
in the presence of NaBEt3H (5 mol%) as a metal reductant cataꢀ
lyzed the hydroboration of αꢀmethylstyrene (9c) with 10 in THF
at 25 °C, affording primary alkylboronate 11c in 78% yield (eq
2).49 In contrast, the use of homogeneous DPPBz ligand resulted
in no reaction.
As in the cases of other firstꢀrow transition metal catalyses
mentioned above (Niꢀcatalyzed C–N and C–H/C–O crossꢀ
coupling, and Cuꢀcatalyzed alkene hydroboration), the polymerꢀ
crossꢀlinking strategy may have suppressed intermolecular cataꢀ
lyst deactivation pathways such as bischelation to Co, or disproꢀ
portionation reactions and dimerization of monochelated Co speꢀ
cies, thus resulting in the increase of total catalytic efficiency
compared to that of the corresponding homogeneous catalyst
systems. It should be mentioned that Lin and coꢀworkers disꢀ
cussed similar siteꢀisolation effects in the Coꢀcatalyzed alkene
hydroboration by using 2,2’ꢀbipyridineꢀbased metal–organic
frameworks (MOFs) as heterogeneous catalysts (for 11c; 1 mol%
Co, 60 °C, 36 h, 82% yield).10d
CONCLUSION
We have developed a new type of polystyreneꢀcrossꢀlinking
bisphosphine PSꢀDPPBz. Locating the DPPBz bisphosphine moiꢀ
ety at the branching points of a crossꢀlinked network organic
polymer enabled controlled bisphosphine monochelation to tranꢀ
sition metals. Owing to the polymer effect, PSꢀDPPBz showed
high ligand performance in some reactions catalyzed by firstꢀrow
transition metals such as Niꢀcatalyzed amination of aryl chlorides
with Nꢀalkylꢀsubstituted primary amines, Niꢀcatalyzed C–H/C–O
coupling between 1,3ꢀazoles and aryl pivalates, and alkene hyꢀ
droboration catalyzed by Cu or Co, enabling challenging molecuꢀ
lar transformations that were not possible through screening of
the existing homogeneous ligands. Furthermore, results of the
present work support the existence of previously proposed cataꢀ
lyst deactivation pathways in the Niꢀcatalyzed C–N coupling,
demonstrating the usefulness of immobilized bisphosphine ligꢀ
ands such as PSꢀDPPBz as tools for mechanistic studies in organꢀ
ometallic chemistry. Further studies for generalizing the concept
of ligand design based on polymerꢀcrossꢀlinking and for explorꢀ
ing efficient heterogeneous catalytic reactions are ongoing in our
laboratory.
EXPERIMENTAL SECTION
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ACS Catalysis
tetrachloroethane) was added to determine the yield of 4,4,5,5ꢀ
Y.; Riduan, S. N. Chem. Soc. Rev. 2012, 41, 2083−2094. (g)
Itsuno, S.; Hassan, M. M. RSC. Adv. 2014, 4, 52023–52043. (h)
Sun, Q.; Dai, Z.; Meng, X.; Wang, L.; Xiao, F. S. ACS Catal.
2015, 5, 4556−4567. (i) Sun, Q.; Dai, Z.; Meng, X.; Xiao, F.ꢀS.
Chem. Soc. Rev. 2015, 44, 6018–6034.
1
2
3
4
5
6
7
8
tetramethylꢀ2ꢀ(4ꢀphenylbutyl)ꢀ1,3,2ꢀdioxaborolane (11a, 92%
yield). The crude product was purified by silica gel column chroꢀ
matography (hexane/EtOAc 95:5) to give 11a as a colorless oil
(41.0 mg, 0.158 mmol, 79% yield).
A Typical Procedure for Co-catalyzed Alkene Hydrobora-
tion with Pinacolborane (Eq 2). In a nitrogenꢀfilled glove box,
PSꢀDPPBz (30 mg, 0.0030 mmol, 1.5 mol%), a solution of CoI2
(0.63 mg, 0.0020 mmol, 1 mol%) in THF (0.13 mL), and THF
(0.27 mL) were placed in a 10ꢀmL glass tube containing a magꢀ
netic stirring bar. After stirring at rt for 5 min, NaBEt3H in THF
(1 M, 10 µL, 0.01 mmol, 5 mol%) was added, and the resulting
(4) Related works on using polystyreneꢀbased resins for heterogeneꢀ
ous catalysis: (a) Uozumi, Y.; Yamada, Y. M. A. Chem. Rec.
2009, 9, 51–65. (b) Kobayashi, S.; Miyamura, H. Aldrichimica
Acta 2013, 46, 3–19.
(5) Iwai, T.; Harada, T.; Hara, K.; Sawamura, M. Angew. Chem.,
Int. Ed. 2013, 52, 12322–12326.
9
(6) Threefold crossꢀlinked polymerꢀmonophosphine hybrids reportꢀ
ed by other groups: (a) Li, B.; Guan, Z.; Wang, W.; Yang, X.;
Hu, J.; Tan, B.; Li, T. Adv. Mater. 2012, 24, 3390–3395. (b)
Fritsch, J.; Drache, F.; Nickerl, G.; Böhlmann, W.; Kaskel, S.
Microporous Mesoporous Mater. 2013, 172, 167–173. (c) Hauꢀ
soul, P. J. C.; Eggenhuisen, T. M.; Nand, D.; Baldus, M.; Weckꢀ
huysen, B. M.; Klein Gebbink, R. J. M.; Bruijnincx, P. C. A.
Catal. Sci. Technol. 2013, 3, 2571–2579. (d) Sun, Q.; Jiang, M.;
Shen, Z.; Jin, Y.; Pan, S.; Wang, L.; Meng, X.; Chen, W.; Ding,
Y.; Li, J.; Xiao, F.ꢀS. Chem. Commun. 2014, 50, 11844–11847.
(e) Zhou, Y.ꢀB.; Li, C.ꢀY.; Lin, M.; Ding, Y.ꢀJ.; Zhan, Z.ꢀP. Adv.
Synth. Catal. 2015, 357, 2503–2508. (f) Li, C.; Yan, L.; Lu, L.;
Xiong, K.; Wang, W.; Jiang, M.; Liu, J.; Song, X.; Zhan, Z.;
Jiang, Z.; Ding, Y. Green Chem. 2016, 18, 2995–3005.
(7) Preparation and use of polystyreneꢀcrossꢀlinking taddolꢀ, binolꢀ
or salenꢀbased metal complexes as Lewis acid catalysts: (a) Sellꢀ
ner, H.; Seebach, D. Angew. Chem., Int. Ed. 1999, 38, 1918–
1920. (b) Sellner, H.; Faber, C.; Rheiner, P. B.; Seebach, D.
Chem. Eur. J. 2000, 6, 3692–3705. (c) Sellner, H.; Karjalainen,
J. K.; Seebach, D. Chem. Eur. J. 2001, 7, 2873–2887. In these
works, effectiveness of the crossꢀlinking strategy for reducing
unfavorable steric effects of polymer chains was demonstrated.
(8) Early works of fourfold crossꢀlinked polymerꢀbisphosphine
hybrids: (a) Taylor, R. A.; Santora, B. P.; Gagné, M. R. Org.
Lett. 2000, 2, 1781–1783. (b) Brunkan, N. M.; Gagné, M. R. J.
Am. Chem. Soc. 2000, 122, 6217–6225. A threefold crossꢀlinked
polymerꢀbisphosphine hybrid was also reported: (c) Nozaki, K.;
Itoi, Y.; Shibahara, F.; Shirakawa, E.; Ohta, T.; Takaya, H.;
Hiyama, T. J. Am. Chem. Soc. 1998, 120, 4051–4052.
(9) Porous organic polymers containing bisphosphines: (a) Sun, Q.;
Dai, Z.; Liu, X.; Sheng, N.; Deng, F.; Meng, X.; Xiao, F.ꢀS. J.
Am. Chem. Soc. 2015, 137, 5204–5209. See also: (b) Hausoul, P.
J. C.; Broicher, C.; Vegliante, R.; Göb, C.; Palkovits, R. Angew.
Chem., Int. Ed. 2016, 55, 5597–5601.
(10) Metalꢀorganic frameworks containing bisphosphines: (a)
Bohnsack, A. M.; Ibarra, I. A.; Bakhmutov, V. I.; Lynch, V. M.;
Humphrey, S. M. J. Am. Chem. Soc. 2013, 135, 16038–16041.
(b) Falkowski, J. M.; Sawano, T.; Zhang, T.; Tsun, G.; Chen, Y.;
Lockard, J. V.; Lin, W. J. Am. Chem. Soc. 2014, 136, 5213–
5216. (c) Sawano, T.; Thacker, N. C.; Lin, Z.; McIsaac, A. R.;
Lin, W. J. Am. Chem. Soc. 2015, 137, 12241–12248. See also:
(d) Zhang, T.; Manna, K.; Lin, W. J. Am. Chem. Soc. 2016, 138,
3241–3249.
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mixture was stirred for an additional 10 min.
αꢀMethylstyrene
(9c, 23.7 mg, 0.20 mmol) and pinacolborane (10, 30.7 mg, 0.24
mmol) were added successively. The tube was sealed with a
screw cap and was removed from the glove box. The mixture was
stirred at 25 °C for 16 h. The mixture was filtered through a
Celite pad (eluting with Et2O). The volatiles were removed under
reduced pressure, and an internal standard (1,1,2,2ꢀ
tetrachloroethane) was added to determine the yield of 4,4,5,5ꢀ
tetramethylꢀ2ꢀ(2ꢀphenylpropyl)ꢀ1,3,2ꢀdioxaborolane (11c, 78%
yield). The crude product was purified by silica gel column chroꢀ
matography (hexane/EtOAc 100:0 to 95:5) to give 11c as a colorꢀ
less oil (31.0 mg, 0.126 mmol, 63% yield, contaminated with
trace amounts of impurities).
ASSOCIATED CONTENT
Supporting Information. Experimental details and characterizaꢀ
tion data for new compounds (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
* iwaiꢀt@sci.hokudai.ac.jp
* sawamura@sci.hokudai.ac.jp
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENTS
This work was supported by JSPS KAKENHI Grant Number
JP25810056 to Young Scientists (B) to T.I. and by ACTꢀC and
CREST from JST, and JSPS KAKENHI Grant Number
JP15H05801 in Precisely Designed Catalysts with Customized
Scaffolding to M.S. T.H. thanks the JSPS fellowship for young
scientists. Support from Tosoh Organic Chemical Co., Ltd. is
gratefully acknowledged. We thank Prof. Kenta Kokado (Hokꢀ
kaido University) for the TGA measurement.
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(49) The recovered (PSꢀDPPBz)ꢀCo system by decantation showed
decreased catalytic activity in the second run (~40%).
(45) After stirring the mixture of PSꢀDPPBz and Cu(OAc)2 in THF at
rt, the color of the solution phase remained blue, indicating inꢀ
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