Palladium-catalyzed borylation of sterically demanding aryl halides with a silica-supported compact phosphane ligand

Article abstract of DOI:10.1002/anie.201103224

Immobile but active: A silica-supported "compact" phosphane, Silica-SMAP, can be used in the Pd-catalyzed borylation of aryl chlorides or bromides with bis(pinacolato)diboron (see scheme). The Silica-SMAP/Pd system significantly expands the substrate scope of the borylation toward sterically and electronically challenging aryl halides.

Full text of DOI:10.1002/anie.201103224

DOI: 10.1002/anie.201103224
Borylation
Palladium-Catalyzed Borylation of Sterically Demanding Aryl Halides
with a Silica-Supported Compact Phosphane Ligand**
Soichiro Kawamorita, Hirohisa Ohmiya, Tomohiro Iwai, and Masaya Sawamura*
Arylboronic acid derivatives are versatile intermediates in
organic synthesis because of their broad availability, air
stability, and ease of handling.[1] Among the routes to
arylboronic acid derivatives, the conventional methods that
use aryllithium or Grignard reagents have a problem with
functional-group compatibility. More recently, functional-
group-tolerating approaches such as the transition-metal-
compactness.[7a,b] We demonstrated that the surface-bound 1:1
metal/P complexes afford highly active catalysts for the
hydrosilylation and the hydrogenation of ketones (with
Rh),[7a–c] and the directed ortho borylation of functionalized
arenes (with Ir).[7d–f] In particular, these reactions showed
remarkable tolerance toward the reactions of sterically
demanding substrates. Accordingly, we envisioned that,
despite its compactness, the supported phosphane would be
useful in creating a highly active catalytic environment for the
palladium-catalyzed borylation of sterically or electronically
challenging aryl halides.[5i,8,9] Furthermore, this utility means
that the steric demand of a ligand would not be essential for
the high catalytic activity in the palladium-catalyzed boryla-
tion of aryl chlorides, as has been proved by the experiments
described below.
The reaction of 4-chlorotoluene (1a, 0.5 mmol) with
bis(pinacolato)diboron (2, 0.5 mmol) in the presence of
Pd(OAc)2 (0.5 mol%), Silica-SMAP (0.5 mol%), and
KOAc (3 equiv) in benzene at 608C for 10 h gave the desired
arylboronate 3a in 84% yield, but the reaction also formed a
significant amount of biaryl compound 3a’ (8% yield), which
likely resulted from a Suzuki–Miyaura coupling between the
arylboronate product 3a and aryl chloride 1a (Scheme 1,
catalyst precursor A).[10] The undesired biaryl formation was
almost completely inhibited by using the Silica-SMAP/Pd
system that was prepared in advance (from Silica-SMAP and
[PdCl2(cod)], P/Pd = 1:1, catalyst precursor B), instead of the
in situ generated complex (catalyst precursor A); 3a was thus
produced in an excellent yield (Scheme 1). Furthermore,
another preformed complex Silica-SMAP/[PdCl2(pyridine)2]
afforded exclusively the borylation product 3a, albeit in a
lower conversion (Scheme 1, catalyst precursor C). It should
catalyzed borylation of aryl halides[2,3] and the direct C H
À
borylation[4] of arenes have been introduced, and both are
complementarily applicable to the preparation of a wide
range of arylboronates. Specifically for the borylation of aryl
halides, aryl chlorides are the most desirable substrates
because of their low cost and broad availability; however,
they are less reactive than the corresponding bromides,
iodides, and triflates. Accordingly, only a few catalyst systems
are effective for reactions of unactivated aryl chlorides.[2f,h–m]
A common feature of effective systems is the use of electron-
rich and sterically demanding phosphane ligands in combi-
nation with a palladium source. The most efficient catalyst
systems reported to date are those based on (dicyclohex-
ylphosphino)biphenyl-type ligands such as SPhos and XPhos,
which were originally described by Buchwald and co-work-
ers.[2i,j] The use of these catalysts allowed for the borylation of
sterically or electronically challenging aryl chlorides such as
1-chloro-2,6-dimethylbenzene (2 mol% Pd/SPhos, RT, 86%)
and 4-chloroanisole (0.1 mol% Pd/XPhos, 1108C, 94%).[2i]
Buchwald and co-workers described that the efficacy of
biphenyl-type ligands for the borylation is attributed at least
in part to their sterically demanding nature, which promotes
the formation of a highly reactive 1:1 Pd0/P complex over less
reactive 1:2 Pd0/P species.[2i,5] On the other hand, we have
developed Silica-SMAP, which is a silica-supported “com-
pact” phosphane ligand.[6,7] Because of its immobilized
nature, this ligand forms 1:1 metal/P complexes exclusively
with a range of transition-metal species, despite its extreme
[*] S. Kawamorita, Prof. Dr. H. Ohmiya, Dr. T. Iwai,
Prof. Dr. M. Sawamura
Department of Chemistry, Faculty of Science
Hokkaido University
Sapporo, 060-0810 (Japan)
E-mail: sawamura@sci.hokudai.ac.jp
Homepage: http://barato.sci.hokudai.ac.jp/~orgmet/index-
Eng1.html
[**] This work was supported by Grant-in-Aid for Scientific Research on
Innovative Areas “Organic Synthesis Based on Reaction Integra-
tion”, MEXT. S.K. thanks the JSPS for a scholarship. This work was
supported by the Joint Studies Program (2011) of the Institute for
Molecular Science.
Scheme 1. Borylation of 4-chlorotoluene (1a) catalyzed by Silica-
SMAP/Pd systems. 1.0 equiv of 2 were used with catalyst precursors A
and C, and 1.1 equiv of 2 was used with catalyst precursor B.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201103224.
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8363

Communications
be noted that the corresponding homogeneous catalyst
completed within 2 h (monitored by GC) to afford boronates
3d and 3e in quantitative yields (Table 1, entries 5 and 6).
Notably, in both cases, no biaryl was formed even with the
in situ generated Silica-SMAP/Pd(OAc)2 system (catalyst
precursor A). This result seems to be due to lower reactivities
(nucleophilicities) of the arylboronate products 3d and 3e
toward the subsequent Suzuki–Miyaura coupling.
Sterically challenging ortho-substituted aryl chlorides
such as 2-chlorotoluene (1 f) and 2-chloro-1,1’-biphenyl (1g)
were also suitable substrates, and their reactions produced the
corresponding arylboronates quantitatively without the for-
mation of homocoupling products, even under conditions A
(Table 1, entries 7 and 8). The steric hindrance of the ortho
substituents seems to prevent the overreaction of the
arylboronates 3 f and 3g.
systems based on Ph-SMAP[6] and Pd(OAc)2 (0.5 mol% Pd,
Pd/P = 1:1 or 1:2) resulted in no reaction under conditions
that were otherwise the same, thus indicating that the
immobilization of the phosphane ligand is crucial for the
borylation activity.[11]
In heterogeneous systems, the catalysts were easily
separated from the products by filtration through Celite.
Inductively coupled plasma atomic emission spectroscopy
(ICP-AES) analysis indicated that the Pd leaching was below
the detection limit (0.02% of the loaded Pd). Unfortunately,
however, our attempts to reuse the immobilized catalysts
were unsuccessful.
The effect of the substituent on aryl chlorides is shown in
Table 1. The Silica-SMAP/Pd systems were also effective for
the reaction of 4-chloroanisole (1b), which is an electronically
deactivated substrate. While the in situ generated Silica-
SMAP/Pd(OAc)2 system (catalyst precursor A) afforded
arylboronate 3b and biaryl 3b’ in 80% and 10% yield,
respectively, the reaction with the preformed Silica-SMAP/Pd
system (catalyst precursor B) resulted in an excellent yield of
the arylboronate 3b (96%) with the formation of only a trace
of 3b’ (2%; Table 1, entries 1 and 2). Similarly, the reaction of
4-(trifluoromethyl)chlorobenzene 1c, which is a weakly
activated substrate, afforded a mixture of 3c (86%) and 3c’
(7%) with catalyst precursor A, while boronate 3c was
obtained quantitatively with catalyst precursor B (Table 1,
entries 3 and 4).
Table 2 shows the excellent applicability of the Silica-
SMAP/Pd catalyst systems for the borylation of even more
challenging 2,6-disubstituted aryl halides, which furnished
highly hindered arylboronates 3h–p. These molecular trans-
formations have not been described to date, except for the
conversion of 1h to 3h (Table 2, entry 1).[2i,12] Specifically, the
reactions of 2,6-dimethyl- (1h), 2-methyl-6-phenyl- (1i), and
2,6-diphenylchlorobenzenes (1j) with the use of catalyst
precursor A proceeded in the temperature range 60–908C to
afford the corresponding 2,6-disubstituted arylboronates 3h–j
in excellent yields (Table 2, entries 1–3). Notably, 2,4,6-
triethyl- (1k-Cl and 1k-Br),[13] 2,4,6-triisopropyl- (1l-Cl and
1l-Br), and 2,4-tert-butyl-6-methyl-substituted (1m-Br) aryl
halides also underwent an efficient borylation, despite the
extremely crowded and electronically inactivated nature of
the substrates (Table 2, entries 4–8).[14] Furthermore, the
electronically even more deactivated 2,4,6-trisubstituted aryl
chloride 1n-Cl underwent the borylation when catalyst
precursor C was used, while the use of catalyst presursor A
resulted in a lower substrate conversion (Table 2, entries 9
and 10). The corresponding bromide 1n-Br also served as a
suitable substrate (catalyst precursor A, Table 2, entry 11).
The borylation of chloronaphthalene derivative 1o with a
bulky oxygen-based functional group (-OCO2tBu) at the
ortho position proceeded more efficiently with catalyst
precursor C rather than with catalyst presursor A (Table 2,
entries 12 and 13). The anthracenyl chloride 1p underwent
efficient borylation at a relatively low temperature (608C)
with catalyst precursor A (Table 2, entry 14).
While Buchwald and co-workers described reaction con-
ditions in which sterically demanding biphenyl-based phos-
phane ligands such as XPhos and SPhos were useful for the
borylation of 2,6-dimethylchlorobenzene (1h),[2i] this trans-
formation did not occur at all when these ligands were used in
place of Silica-SMAP under the reaction conditions described
for Table 2, entry 1 (0.5 mol% Pd(OAc)2, Pd/P = 1:1 or
1:2).[11] It should be noted, however, that the use of XPhos
(Pd/P = 1:2) in place of Silica-SMAP resulted in a quantitative
conversion in the borylation of the sterically less demanding
2,4,6-trimethoxychlorobenzene (1n-Cl). On the other hand,
the use of SPhos under the same conditions resulted in only
7% conversion.
Strongly electron-withdrawing substituents such as
methoxycarbonyl (1d) or acetyl (1e) groups at the para
position caused marked rate enhancement; the reactions were
Table 1: Silica-SMAP/Pd-catalyzed borylation reaction.[a]
Entry
Aryl chloride
1
Arylboronate
3
Cat.
Yield [%][c]
prec.[b]
1[d]
A
B
80 (55)
96 (82)
2[d,e]
3[d]
4[e]
A
B
86
99 (89)
5
6
A
A
99 (75)
98 (80)
7
8
A
A
96 (78)
98 (74)
[a] Conditions: 1 (0.5 mmol), 2 (0.5 mmol), Silica-SMAP/Pd (0.5 mol%),
KOAc (3 equiv), benzene (1.0 mL) at 608C for 10 h. All reactions
proceeded with full conversion. [b] See Scheme 1. [c] Yield (based on 1)
determined by 1H NMR spectroscopy. The yield of the isolated product is
given in parentheses. [d] Biaryl 3’ was detected in the crude mixture:
10% (entry 1), 2% (entry 2), 7% (entry 3). [e] 0.55 mmol of 2 was used.
In summary, a silica-supported “compact” phosphane,
Silica-SMAP, was successfully used for the palladium-cata-
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Angew. Chem. Int. Ed. 2011, 50, 8363 –8366

Table 2: Borylation of 2,6-disubstituted aryl halides.[a]
Entry Aryl halide
Arylboronate
3
Cat.
T
Conv. Yield
Entry Aryl halide
Arylboronate
3
Cat.
T
Conv. Yield
1
prec.[b] [8C] [%]
[%][c]
1
prec.[b] [8C] [%]
[%][c]
1[d]
A
A
60 90
90 (82)
7
8
A
A
110 100
110 100
93 (89)[e]
2
80 100
85 (84)[e]
83 (88)[e]
9[g]
A
C
100 55
100 97
45[e]
3
A
90 90
80 (80)[e]
10[g]
88 (81)[e]
4
A
A
A
90 100
80 82
110 76
91 (77)[e]
76 (73)[e]
64 (64)[e]
11[g]
A
100 96
86 (80)[e]
12[g]
13[g]
A
C
100 67
100 100
66
99 (88)
5
6[f]
14[d]
A
60 95
87 (92)[e]
[a] Conditions: 1 (0.5 mmol), 2 (0.5 mmol), Silica-SMAP/Pd (0.5 mol%), KOAc (3 equiv), toluene (1.0 mL) for 16 h. [b] See Scheme 1. [c] Yield (based
on 1) determined by 1H NMR spectroscopy. The yield of the isolated product is given in parentheses. [d] Benzene was used as the solvent. [e] The
protonated arene was detected in the crude mixture: 15% (entry 2), 10% (entry 3), 9% (entry 4), 6% (entry 5), 12% (entry 6), 7% (entry 7), 17%
(entry 8), 10% (entry 9), 9% (entry 10), 10% (entry 11), 8% (entry 14). [f] 1.5 equiv of 2 were used. [g] 1,4-Dioxane was used as the solvent.
lyzed borylation of aryl chlorides or bromides with bis(pina-
colato)diboron. The Silica-SMAP/Pd systems significantly
expanded the substrate scope of the borylation toward
sterically and electronically challenging aryl halides. As for
the borylation of moderately challenging substrates such as
2,6-dimethylchlorobenzene, the efficacy of the immobilized
catalyst systems with the “compact” phosphane is comparable
or exceeds that of homogeneous palladium systems with
biphenyl-based “bulky” phosphane ligands such as XPhos
and SPhos. Furthermore, borylation reactions of several
sterically more challenging substrates represented by 2,4,6-
triethyl- and 2,4,6-triisopropylchlorobenzenes have been
demonstrated for the first time. Accordingly, it is concluded
that the steric demand of a ligand is not essential for high
catalytic activity in the palladium-catalyzed borylation of
unactivated aryl chlorides if a monodentate ligand is immo-
bilized on a solid support, so that only one ligand participates
in the coordination to the metal center.[8,9] Investigations on
the effects of the steric and electronic natures of immobilized
phosphane ligands toward aryl halide activation by palladium
and related catalysis are ongoing.
Experimental Section
Typical procedure for the borylation reaction (catalyst precursor A):
In a glove box, Silica-SMAP (0.069 mmolgÀ1, 36.0 mg, 0.0025 mmol,
0.5 mol%), 2 (127.0 mg, 0.50 mmol), KOAc (149.0 mg, 1.5 mmol),
and anhydrous, degassed benzene (0.72 mL) were placed in a 10 mL
glass tube containing a magnetic stirring bar. Then, a solution of
Pd(OAc)2 (0.56 mg, 0.0025 mmol, 0.5 mol%) in benzene (0.28 mL)
and 1a (61.1 mg, 0.48 mmol) were added. The tube was sealed with a
screw cap and removed from the glove box. The mixture was stirred at
608C for 10 h, then filtered through a Celite pad. The solvent was
removed under reduced pressure. An internal standard (1,1,2,2-
tetrachloroethane) was added to the residue. The yield of the product
(84%) was determined by 1H NMR spectroscopy. The crude material
was then purified by column chromatography on silica gel (EtOAc/
hexane = 1:9) to give arylboronate 3a (75.3 mg, 0.35 mmol) in 72%
yield.
Received: May 11, 2011
Published online: July 12, 2011
Keywords: arylboronates · borylation · heterogeneous catalysis ·
.
palladium · silica
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8365

Communications
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dendric P-substituents for the Pd-catalyzed Suzuki–Miyaura
coupling with unactivated aryl chlorides. The phosphane ligands
have moderate steric demands in the vicinity of the coordinating
atoms, but exert considerable steric effects in the periphery of
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[10] The Silica-SMAP/Pd system is also useful for the Suzuki–
Miyaura cross-coupling reaction of unactivated aryl halides.
[11] See the Supporting Information for details of ligand effects. The
lower activity of the homogeneous catalysts with soluble
phosphane ligands such as Ph-SMAP should be due to the
formation of multi-phosphane-coordinated, catalytically inac-
tive palladium complexes.
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[12] Arylboronates 3l and 3p have been synthesized through the Pd-
catalyzed borylation of the corresponding bromides, see
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yield through the nano-Fe2O3-catalyzed borylation of 1,3,5-
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[13] The increase of the reaction temperature did not improve the
yield. The reason for the lower reactivity of 1k-Br relative to 1k-
Cl is unclear.
[14] The reaction of 2,4-di-tert-butyl-6-methyl-substituted chloroben-
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