Oxovanadium(V)-catalyzed oxidative biaryl synthesis from organoborate under O2

Article abstract of DOI:10.1039/b610731a

Oxidative ligand coupling of organoborates was catalyzed by VO(OEt)Cl ₂ under oxygen atmosphere, which provides a versatile method for the selective synthesis of symmetrical or unsymmetrical biaryls. The Royal Society of Chemistry.

Full text of DOI:10.1039/b610731a

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Oxovanadium(V)-catalyzed oxidative biaryl synthesis from
organoborate under O2{
Hidenori Mizuno, Hidehiro Sakurai, Toru Amaya and Toshikazu Hirao*
Received (in Cambridge, UK) 26th July 2006, Accepted 20th September 2006
First published as an Advance Article on the web 12th October 2006
DOI: 10.1039/b610731a
Table 1 Oxidative ligand coupling of NaBPh4 with a catalytic
amount of vanadium compound
Oxidative ligand coupling of organoborates was catalyzed by
VO(OEt)Cl2 under oxygen atmosphere, which provides a
versatile method for the selective synthesis of symmetrical or
unsymmetrical biaryls.
Oxidative coupling of organometallic nucleophiles leads to the
formation of a new carbon–carbon bond, which is considered to
be a complementary method for the conventional nucleophile–
electrophile coupling. The oxidative homo-coupling of aryl-metal
reagents by transition-metal oxidants has been widely studied to
offer a useful method for the construction of symmetrical biaryl
backbones.1 However, oxidative cross-coupling is usually difficult
to perform because of the lack of selectivity.2 We have previously
reported the selective carbon–carbon bond formation of main-
group organometallics, such as organoaluminiums,3 organobo-
rons,4 organozincs,5 and their ate complexes using
oxovanadium(V) compounds as a stoichiometric oxidant. From
a synthetic viewpoint, a catalytic method for oxidative coupling
needs to be developed. Although tetraarylborates are transformed
to biaryls by photochemical,6 electrochemical7 or chemical
oxidation,8 there has been no report on the formation of biaryls
from tetraarylborates using a catalytic amount of a metallic
oxidant. In this paper, we wish to report a first example of the
catalytic ligand coupling of organoborates induced by VO(OEt)Cl2
under oxygen atmosphere. The chemoselective ligand coupling was
accomplished by the oxidation of unsymmetrical tetraarylborates.
Sodium tetraphenylborate (1a) was treated with a series of high-
valent vanadium compounds 3 (0.1 molar equivalents to 1a) in
acetonitrile under oxygen atmosphere (Table 1).9 No catalytic
activity was observed with vanadium(IV) compounds, like
VO(acac)2, VO(hfac)2 (hfac = hexafluoroacetylacetonate) and
Cp2VCl2 (entries 1–3). Among five-valent vanadiums, although
V2O5 exhibited no catalytic activity (entry 4), a catalytic reaction
occurred with VOCl3 (33% yield, entry 5). A better result was
obtained when VO(OEt)Cl2 was used (52% yield, entry 6).
Entry
Vanadium catalyst 3
Yield of 2a (%)
1
2
3
4
5
6
VO(acac)2
VO(hfac)2
Cp2VCl2
V2O5
VOCl3
VO(OEt)Cl2
trace
trace
9
5
33
52
VO(OEt)Cl2 to 1 (entry 7). In the case of sodium tetrakis(4-
chlorophenyl)borate (1b), however, the yield of 4,49-dichlorobi-
phenyl (2b) was low (entry 2). Thus, the higher p-electron density
on the aromatic ring is considered to permit more facile oxidation,
suggesting that the ligand coupling is controlled by the electronic
properties of the aryl groups.
Table 2 Catalytic oxidative ligand coupling of NaBAr4 with
VO(OEt)Cl2
Entry
1
Ar
1
Yield of 2 (%)
1a
87
2
3
4
1b
1c
1d
20
90
85
The present method was successfully extended to the catalytic
ligand coupling of various symmetrical tetraarylborates 1 with
VO(OEt)Cl2 (Table 2).{ Use of 0.2 molar equivalents of
VO(OEt)Cl2 to 1 gave the corresponding biaryls 2 in satisfactory
yields (85–90%) for almost all borates (entries 1, 3, 4, 5 and 6). The
catalytic reaction also proceeded with 0.1 molar equivalents of
5
1e
87
Department of Applied Chemistry, Graduate School of Engineering,
Osaka University, Yamada-oka, Suita, Osaka 565-0871, Japan.
E-mail: hirao@chem.eng.osaka-u.ac.jp; Fax: +81-6-6879-7415;
Tel: +81-6-6879-7413
6
7
1f
1f
90
70a
{ Electronic supplementary information (ESI) available: Compound
characterization data (1H, 13C, 11B NMR and ESR) for selected
compounds. See DOI: 10.1039/b610731a
a
0.1 molar equivalents of VO(OEt)Cl2 to 1 were used.
5042 | Chem. Commun., 2006, 5042–5044
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To obtain further insight into the ligand coupling, a crossover
experiment was conducted. Two different borates, sodium
tetraphenylborate (1a) and sodium tetrakis(4-methoxyphenyl)bo-
rate (1f), were treated with two molar equivalents of VO(OEt)Cl2
(a molar equivalent for each substrate) in one pot (Scheme 1). The
products were found to be biphenyl (2a) and 4,49-bismethoxybi-
phenyl (2f), and the formation of 4-methoxybiphenyl (6f) was not
detected at all. These findings indicate that the ligand coupling
proceeds only intramolecularly.
Table 3 Ligand coupling of unsymmetrical borate 5
The different reactivities based on the electronic properties of
the aryl groups and the intramolecular coupling mode permitted a
selective carbon–carbon bond formation in the reaction of the
unsymmetrical borates 5. A variety of 5 were prepared from the
corresponding aryl bromide 4, BuLi and BPh3 in situ. With use of
the electron-rich aryl bromide as a starting substrate, the
unsymmetrical biaryl was obtained as a major product by the
treatment of 0.2 molar equivalents of VO(OEt)Cl2 to 4 (Table 3,
entries 2–8).§ The coupling of methyl-substituted aryl bromides
4c–e was revealed to depend on the substituent position. Use of
4-methylphenyl bromide (4c) resulted in 4-methylbiphenyl (6c) in
60% yield (entry 2). However, starting from 2-methylphenyl
bromide (4e), both yield and selectivity were lowered (entry 4).
Since a similar low reactivity was observed with the naphthyl
derivative (entry 7), a steric effect appears to operate in the
coupling in addition to the electronic effect. In the case of
4-chlorophenyl bromide (4b), the cross-coupling product, 4-chloro-
biphenyl (6b) was obtained only in 10% yield, while biphenyl (2a)
was predominantly produced in 45% yield (entry 1). This method
was applied to the coupling of the heterocyclic group to give the
unsymmetrical biaryl via selective carbon–carbon bond formation
(entry 8).
Yield (%)
Selectivity
Entry
1
Ar
4
6
2a
6 : 2a
4b
10
45
18 : 82
87 : 13
84 : 16
2
3
4c
4d
60
47
9
9
4
4e
14
8
64 : 36
A similar mechanism seems to operate as proposed in the
stoichiometric reaction.4 During the ligand coupling reaction, the
vanadium(V) species is reduced to a vanadium(IV) one, which was
detected in the ESR spectrum of the reaction mixture. Thus-
obtained V(IV) species might be reoxidized to a vanadium(V) one
with molecular oxygen, forming a catalytic redox cycle.10
To develop an atom economical process, potassium diphenyldi-
fluoroborate (7a)11 was employed in the ligand coupling. Although
5
6
7
4f
4g
4h
70
58
30
9
7
8
89 : 11
89 : 11
79 : 21
8
4i
62
8
89 : 11
the reactivity was lower, the ligand coupling proceeded at 70 uC as
shown in Scheme 2.
In conclusion, the oxovanadium-catalyzed ligand coupling of
the organoborates was demonstrated to provide a versatile method
for the selective oxidative coupling between two aryl nucleophiles
on boron. It should be noted that the ligand coupling to
unsymmetrical biaryls was achieved with in situ generated
unsymmetrical borates.
Thanks are due to the Analytical Center, Graduate School
of Engineering, Osaka University, for the measurement of
the 11B NMR and ESR spectra. This work was supported by a
Scheme 1 Crossover experiment.
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purification of the crude product with preparative TLC (hexane–
dichloromethane = 3 : 2) gave the corresponding unsymmetrical biaryl 6
(Table 3).
1 For a comprehensive review, see: J. Hassan, M. Se´vignon, C. Gozzi,
E. Schulz and M. Lemaire, Chem. Rev., 2002, 102, 1359. For recent
reports, see: (a) T. Nagano and T. Hayashi, Org. Lett., 2005, 7, 491; (b)
G. Cahiez, C. Chaboche, F. Mahuteau-Betzer and M. Ahr, Org. Lett.,
2005, 7, 1943; (c) D. S. Surry, X. Su, D. J. Fox, V. Franckevicius,
S. J. F. Macdonald and D. R. Spring, Angew. Chem., Int. Ed., 2005, 44,
1870; (d) S. Carrettin, J. Guzman and A. Corma, Angew. Chem., Int.
Ed., 2005, 44, 2242; (e) Y. Miyake, M. Wu, M. J. Rahman and
M. Iyoda, Chem. Commun., 2005, 411; (f) C. Gonza´lez-Arellano,
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D. S. Surry, D. J. Fox, S. J. F. Macdonald and D. R. Spring, Chem.
Commun., 2005, 2589.
Scheme 2 Ligand coupling of diphenyldifluoroborate (7a).
2 Transition-metal catalyzed cross-coupling reactions are well-known, but
examples for the oxidative cross-coupling reactions are relatively rare.
Manganese(III) acetate induced oxidative cross-coupling between
arylboronic acid and aryl solvent has been reported: A. S. Demir,
Grant-in-Aid for Scientific Research on Priority Areas ‘‘Advanced
Molecular Transformations of Carbon Resources’’ from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan.
¨
O. Reis and M. Emrullahoglu, J. Org. Chem., 2003, 68, 578.
3 T. Ishikawa, A. Ogawa and T. Hirao, J. Am. Chem. Soc., 1998, 120,
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Notes and references
4 T. Ishikawa, S. Nonaka, A. Ogawa and T. Hirao, Chem. Commun.,
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{ General procedure for symmetrical biaryls 2. In a 2-necked 20 mL round-
bottom flask, a stirring bar and sodium tetraarylborate (1, 0.20 mmol) were
placed and dried. Under an oxygen atmosphere, acetonitrile (2 mL) and
dichloromethane (0.95 mL) were added into the flask, followed by the
addition of VO(OEt)Cl2 (0.80 M solution in dichloromethane, 0.05 mL,
0.040 mmol). The mixture was stirred for 20 h at room temperature, and
then quenched by the addition of 3 mL of pH 7 buffer (KH2PO4–
Na2HPO4). After extraction with Et2O (3 6 10 mL), the combined organic
layer was washed with brine, dried over MgSO4, and evaporated. The
resulting crude product was purified with preparative TLC (hexane) to
afford the corresponding biaryl 2 (Table 2).
§ General procedure for unsymmetrical biaryls 6. To a 2-necked 20 mL
round-bottom flask equipped with a reflux condenser and a stirring bar,
aryl bromide (0.20 mmol) in THF (1 mL) was added. The mixture was
cooled to 275 uC (ethanol–dry ice). BuLi (1.35 M solution in hexane,
0.148 mL, 0.20 mmol) was added, and the mixture was stirred for 1 h at the
same temperature. A THF (1 mL) solution of BPh3 (48.4 mg, 0.20 mmol)
was added at 275 uC. After stirring for 1 h, the reaction flask was allowed
to reach room temperature and the solvent was exchanged from THF to
acetonitrile (2 mL)–dichloromethane (0.95 mL). Under an oxygen atmo-
sphere, VO(OEt)Cl2 (0.80 M solution in dichloromethane, 0.05 mL,
0.040 mmol) was added, and the reaction mixture was stirred for 12 h
under reflux. The reaction was quenched by the addition of pH 7 buffer
(3 mL). After extraction with Et2O (10 mL 6 3), the combined organic
layer was washed with brine, dried over MgSO4, and evaporated. The
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5044 | Chem. Commun., 2006, 5042–5044
This journal is ß The Royal Society of Chemistry 2006