Mutual activation: Suzuki-Miyaura coupling through direct cleavage of the sp2 C-O bond of naphtholate

Article abstract of DOI:10.1002/anie.201101461

Working together: A new approach of mutual activation between naphtholates and aryl boronic acid derivatives by the formation of borates to facilitate the Suzuki-Miyaura coupling through direct cleavage of the sp² C-O bond by nickel catalysis is described (see scheme; R′: annulated ring system). Various naphtholates and aryl boronic acid derivatives could be directly coupled in good yields. Copyright

Full text of DOI:10.1002/anie.201101461

DOI: 10.1002/anie.201101461
Synthetic Methods
Mutual Activation: Suzuki–Miyaura Coupling through Direct Cleavage
2
À
of the sp C O Bond of Naphtholate**
Da-Gang Yu and Zhang-Jie Shi*
Since the 1970s, the development of various cross-coupling
reactions that start from organohalides has provided efficient
methods to build up biaryl compounds, which are important
structures in bioactive compounds, natural products, and
synthetic materials.[1] Cross-coupling reactions were recog-
nized last year with the Nobel Prize, and owing to the mild
reaction conditions, high functional-group tolerance, and the
wide commercial availability of nontoxic and stable boronic
acid derivatives, the Suzuki–Miyaura reaction is particularly
attractive among the various cross-coupling reactions.[2] The
Suzuki–Miyaura coupling is now successful both under
homogeneous and heterogeneous catalysis, and has been
widely applied to the synthesis of important drugs, polymers,
and materials in both academic and industrial laboratories.[3]
Investigations offer excellent chances to use simple arenes in
many cross-coupling reactions.[4] However, the lack of accu-
racy obtained using a directing strategy, indicates that aryl
halides are currently not replaceable.[5] Owing to the abun-
dance and availability of phenol derivatives, many methods to
activate phenols so that they could take the place of halides in
these reactions have been tested.[6] Herein, we report the new
in terms of meeting the requirements of sustainable chemis-
try: not only improving step economy and atom economy and
starting from naturally available chemicals, but also produc-
ing no carbon-containing by-products. Although the recent
success in the Kumada–Tamao–Corriu coupling using pheno-
lates with active Grignard reagents provided a potential
solution,[8] the challenge to cross-couple phenols and their
salts with much less-reactive species, for example, aryl
boronic acids, still presents difficulties. Our recent efforts
have turned to the development of an efficient method for the
application of phenolates in the Suzuki–Miyaura coupling
reaction.
In a traditional Suzuki–Miyaura coupling, the hydroxy
group of the phenol species is derivatized to form active
compounds.[6,7] Meanwhile, the formation of the borate
facilitates the transmetalation in the catalytic cycle (Sche-
me 1A and B).[9] In our novel design, the phenolate could
react with the three-coordinate aryl boronic reagents to
generate aryl borates. In these complexes, the borate might
À
activate the aryl C O bond and the borate anion might play
the same role as a derivatized hydroxy group, such as a
sulfonate and phosphate. Meanwhile, the formation of the
À
concept of mutual activation of the C O bond in phenols and
À
À
the C B bond in aryl boronic acid derivatives to realize the
borate activates the C B bond and promotes the trans-
Suzuki–Miyaura coupling of phenols.
metalation later in the reaction cycle (Scheme 1C). The
advantage is that both the coupling partners come from the
Recent advances have made less-reactive aryl ethers,
carboxylic esters, and carbamates applicable in Suzuki–
Miyaura coupling reactions.[7] Undoubtedly, the use of
phenols in the Suzuki–Miyaura coupling would be optimal
À
same in situ generated aryl borate, and the aryl C O bond
À
and the aryl C B bond are mutually activated. Notably the
two partners activate each other and so avoid the need for
preactivation of the phenol species and the addition of extra
base (Scheme 1B).
[*] D.-G. Yu, Prof. Z.-J. Shi
Beijing National Laboratory of Molecular Sciences (BNLMS) and
Key Laboratory of Bioorganic Chemistry and Molecular Engineering
of Ministry of Education
College of Chemistry and Green Chemistry Center
Peking University, Beijing 100871 (China)
Fax: (+86)10-6276-0890
Prof. Z.-J. Shi
State Key Laboratory of Organometallic Chemistry
Chinese Academy of Sciences, Shanghai 200032 (China)
E-mail: zshi@pku.edu.cn
Homepage: http://pku.edu.cn/zshi/
[**] We thank Prof. Wen-Xiong Zhang, Dr. Neng-Dong Wang, Prof. Xiang
Hao, and Shao-Guang Zhang for the X-ray crystallographic analysis,
and Prof. Zhi-Xiang Yu and Qian Li for the use of their glove box. D.-
G. Yu thanks Bi-Jie Li, Bing-Tao Guan, Yang Li, and Chang-Liang Sun
for constructive discussion, and Xiao-Bo Zhang and Xin Wang for
their help in preparation of some of the compounds. Support of this
work by NSFC (No. 20925207, 21072010, 20821062) and the “973”
Project from the MOST of China (2009CB825300) is gratefully
acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201101461.
Scheme 1. Rational design of the new concept of mutual activation by
the formation of borate from aryloxylate and boronic acid derivatives.
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Table 1: Suzuki–Miyaura coupling of 2-naphthol (1a) with different aryl
boroxines 2.[a]
2-Naphtholates 1 with different metal counterions were
treated with phenylboroxine (2a) in the presence of nickel
catalysts to prove our concept. Although the magnesium
naphtholate gave no desired product, we found that the
lithium, potassium, and zinc salts afforded a trace amount of
product 3aa, and the sodium salt showed the highest
reactivity (see Table S1 in the Supporting Information).
Notably, the mixed nonpolar solvent system of THF and o-
xylene is a key requirement to facilitate this transformation.
Although the observation of the desired product 3aa was
exciting to us, initially further investigations gave no progress.
After numerous trials in which the different parameters were
varied, no improvement was seen in the reaction efficiency.
Pleasingly, the addition of the mild Lewis acid BEt3
(triethylborane) gave a surprising result. Both the conversion
and the yield of the isolated product significantly increased in
the presence of the appropriate amount of BEt3, despite being
accompanied by a trace amount of reduced and/or ethylated
by-products. Trimethoxyborane also promoted the reaction,
albeit with a slightly lower efficiency (see Table S1 in the
Supporting Information). The combination of [Ni(cod)2]
Entry
Ar (2)
3
Yield
[%]
1
2
3
82
90
80
4
80
(cod = cycloocta-1,5-diene)
and
tricyclohexylphosphine
(PCy3) showed the best efficiency, while the prepared [NiCl2-
(PCy3)2] plus additional PCy3 gave a lower catalytic activity.
Other catalysts completely failed in this transformation (see
Table S3 in the Supporting Information). Different phenyl-
boronic reagents were tested and phenylboroxine (2a) turned
out to be the best (see Table S4 in the Supporting Informa-
tion).
Further investigations demonstrated the broad substrate
scope of this transformation. Various aryl boroxines could be
applied in the reaction with good efficiency (Table 1).
Electron-donating groups, such as OMe and NMe2, promoted
the reaction (Table 1, entries 4 and 5). In contrast, aryl
boroxines that contained electron-withdrawing groups mainly
underwent protodeborylation and gave relatively poor con-
version into the desired product (Table 1, entry 6). Steric bulk
on the aryl group did not hamper the reaction and 2-
tolylboroxine also gave the desired product in an excellent
5
81
43
65
6[b]
7
[a] Reaction conditions: 0.4 mmol of 1a, 1.0 equiv of aryl boroxine 2,
10 mol% of [Ni(cod)2], 40 mol% of PCy3, 1.0 equiv of NaH, 1.5 equiv of
BEt3 in the mixture of 0.7 mL of o-xylene and 0.2 mL of THF at 1108C for
48 h. [b] 0.2 mmol of 1a and 3.0 equiv of 5,5-dimethyl-2-(4-trifluorome-
thylphenyl)-1,3,2-dioxaborinane was used.
À
À
yield (Table 1, entry 3). Notably the C OMe and C F bonds
(Table 1, entries 5 and 7), which can be easily cleaved by
nickel catalysis, were tolerated well, and therefore could
provide the opportunity for orthogonal cross-coupling reac-
tions with different functionalities.[10,11]
that the nickel species played a vital role in the activation and
À
cleavage of the C O bond of the phenolates. Importantly the
significant effect of the BEt3 is due to its Lewis acidity. Several
different hypotheses were proposed, as illustrated in
Scheme 2. As a Lewis acid, BEt3 might directly react with
À
From the aspect of the phenols (Table 2), 1-naphthol gave
the desired product (3bb), although in a relatively lower
yield, under the slightly modified reaction conditions
(Table 2, entry 2). Phenanthren-9-ol (1c) also showed high
reactivity (Table 2, entry 3). Notably the selective cleavage of
the phenolate to form a borate and thus activate the C O
bond (Scheme 2A). Another possibility is a second Lewis
acid/Lewis base interaction between the in situ generated
borate and BEt3, which can be defined as a Lewis acid assisted
Lewis acid effect (LA/LA) to promote the reactivities of both
[12]
À
À
À
À
the C OH bond over the C OR bond was observed; for
example, the tert-butoxy group on the naphthyl ring (1e) was
tolerated (Table 2, entry 5). Notably, an ester group (1 d) is
also stable under the reaction conditions (Table 2, entry 4).
Most importantly, the selective cleavage of simple phenol
derivatives could also take place, albeit in a low yield (Table 2,
entry 6), thus indicating the potential applicability of this
method to the functionalization of simple phenol derivatives.
Mechanistically, the absence of the desired product when
the reaction was carried out without a nickel catalyst implied
C O and C B bonds (Scheme 2B).
Experimentally, both BEt3 and the phenylboronic reagent
could indeed react with sodium naphtholate to form borate, as
shown by 11B NMR spectroscopy (Scheme 3, spectra (1)–(4)).
However, the 11B NMR signal of BEt3 in the presence of the
phenyl boronate ester and sodium naphtholate moved only
slightly upfield, thus implying that the formation of the borate
from BEt3 and naphtholate (Scheme 2A) did not occur in the
presence of the phenyl boronate ester (Scheme 3, spectrum
(5)). The observed reactivity in the absence of BEt3 also
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Angew. Chem. Int. Ed. 2011, 50, 7097 –7100

Table 2: Suzuki–Miyaura coupling of 2b with different phenols 1.[a]
Entry
1
3 (Ar=4-nBuC6H4)
Yield
[%]
1
2
88
68
Scheme 3. 11B NMR spectra: (1) 2-phenylbenzo[d]1,3,2-dioxaborole,
d=32.1 ppm; (2) NapONa + 2-phenylbenzo[d]1,3,2-dioxaborole,
d=32.1 and 10.9 ppm; (3) BEt3, d=80.8 ppm; (4) BEt3 + NapONa,
d=5.5 ppm; (5) NapONa + 2-phenylbenzo[d]1,3,2-dioxaborole +
BEt3, d=76.4, 32.0 and 11.4 ppm. Nap=naphthyl.
3
4
82
62
intermediate. Pleasingly, the isolation of the key intermediate
was achieved and its structure, which was determined by X-
ray crystallographic analysis, gave support to our hypothesis
(Scheme 4). This compound was isolated from the reaction
performed under similar reaction conditions as shown in
Scheme 4 in the absence of the catalysts, thus THF played the
role of a ligand in the isolated dimer. From the X-ray
5
63
18
À
crystallographic data we found that both the C O bond
6[b]
À
(1.370 ꢀ) and C B bond (1.606 ꢀ) were slightly longer than
the corresponding bonds in 1a (1.336 ꢀ) and phenyl boronic
ester (1.537 ꢀ).[8a,14] This information gave strong support for
our concept of mutual activation. Notably, the bridged sodium
ions might play a role similar to that of the additional Et3B to
further facilitate the double activation. Direct application of
this borate to the optimized reaction conditions also afforded
the desired product in 60% yield (see the Supporting
Information).
[a] Reaction conditions: 0.2 mmol of 1, 1.0 equiv of 2b, 10 mol% of
[Ni(cod)2], 40 mol% of PCy3, 1.0 equiv of NaH, 3.0 equiv of BEt3 in the
mixture of 0.35 mL of o-xylene and 0.1 mL of THF at 1108C for 48 h.
[b] 1.5 equiv of BEt3 was added.
In summary, starting from in situ generated sodium
phenolates, the first direct cross-coupling with aryl boroxines
to produce biaryl compounds was carried out using nickel
catalysis. The key point is the formation of the borate, which
offered the mutual activation of the two coupling partners.
Additional preactivation of phenols and the addition of
Scheme 2. The possible role of BEt3 in the catalytic cycle. A direct
activation of phenolate; B double activation through a Lewis acid
assisted Lewis acid (LA/LA) pathway.
supports the pathway involving the Lewis acid
assisted Lewis acid effect (Scheme 2B). Also,
the slight upfield shift of the 11B NMR signal of
BEt3 in the presence of the phenyl boronate
ester and sodium naphtholate (Scheme 3, spec-
tra (3) and (5)) indicated a weak interaction
between the formed borate and the additional
Et3B by the above-mentioned LA/LA interac-
tion (Scheme 2B). These studies provide
strong evidence in support of the hypothesis
of double activation. Notably, the possibility
that the BEt3 plays the role of a radial initiator
at the initial stage of the oxidative addition, as
previously reported, could not be ruled out.[13]
Mass spectrmeric analysis of the reaction
mixture also provided strong evidence to sup-
port the formation of borate complexes (see
the Supporting Information).
Scheme 4. X-ray crystal structure of borate to support the mutual activation concept.
For clarity, the hydrogen atoms are omitted. Thermal ellipsoids are shown at 30%
probability.[15]
To further prove our concept, many
attempts have been made to isolate the key
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Communications
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strong bases was not required and thus made such a cross-
coupling both step and carbon atom economical. The promo-
tion of efficiency of the reaction by the addition of BEt3
À
implies a new pathway to activate the “inert” C O bond by
the double activation through the LA/LA interaction. Not
only is this an efficient method to produce biaryl compounds
from easily available phenols, but it also demonstrates the
novel concept of mutual activation, which could be used for
the design of further sustainable transformations.
Experimental Section
An oven-dried Schlenk tube containing a stir bar was charged with 2-
naphthol (57.6 mg, 0.4 mmol), phenylboroxine (124.8 mg, 0.4 mmol),
and NaH (12.0 mg, 0.4 mmol) in a glovebox. After being taken out of
the glove box, the tube was degassed and refilled with N2 3 times.
Then, THF (1.0 mL) was injected in by using a syringe under N2 and
the reaction mixture was stirred at room temperature for 5 min. The
solvent was removed under reduced pressure at room temperature
(308C). Then THF (0.2 mL) and BEt3 (1m in hexane; 0.6 mL) were
injected in and the solution was stirred at room temperature for
5 min. After the solution of [Ni(cod)2] (11 mg, 0.04 mmol) and PCy3
(44.8 mg, 0.16 mmol) in o-xylene (0.7 mL) was injected into the tube
under N2, the reaction mixture was stirred at 1108C in the sealed tube
for 48 h. The reaction mixture was then cooled to room temperature,
quenched with EtOAc, and filtered through a short pad of silica gel.
The solvent was removed and the product was purified by flash
column chromatography on silica gel (eluent: petroleum ether).
Received: February 28, 2011
Published online: June 24, 2011
À
Keywords: C O activation · cross-coupling ·
homogeneous catalysis · Lewis acids · nickel
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