Oxidative ligand coupling of tetraarylborates promoted by chlorosilane and molecular oxygen

Article abstract of DOI:10.1246/cl.2001.1084

Oxidative ligand coupling of organoborates is promoted by chlorosilane and molecular oxygen. When tetraarylborate compounds are treated with Ph₂SiCl₂ or PhSiCl₃ in acetonitrile under an oxygen atmosphere, the corresponding

Full text of DOI:10.1246/cl.2001.1084

1084
Chemistry Letters 2001
Oxidative Ligand Coupling of Tetraarylborates Promoted
by Chlorosilane and Molecular Oxygen
Hidehiro Sakurai, Chihiro Morimoto, and Toshikazu Hirao*
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamada-oka, Suita, Osaka 565-0871
(Received August 6, 2001; CL-010748)
Oxidative ligand coupling of organoborates is promoted by
On the other hand, no ligand cross-coupling between boron and
silicon compounds occurred. PhSiCl3 was more effective to
promote the coupling reaction than MeSiCl3 (entries 9 and 10).
chlorosilane and molecular oxygen. When tetraarylborate com-
pounds are treated with Ph2SiCl2 or PhSiCl3 in acetonitrile
under an oxygen atmosphere, the corresponding biaryls are
obtained in good yields. Use of unsymmetrical borates results
in the selective formation of unsymmetrical biaryls.
Alkenyltrialkylborates undergo formal ligand coupling
reactions induced by I2 or BrCN, giving the corresponding
alkylated alkenes, and the reactions have been applied to stereo-
selective alkene synthesis.1 Recently, we reported that oxo-
vanadium(V) compounds can induce the ligand coupling of
alkenyltrialkylborates.2,3 As for the oxidative transformation of
tetraarylborates, biaryl formation has been reported to occur by
electrochemical,4 photochemical,5 and chemical oxidation, for
example with Ir(IV).6 In this paper we demonstrate a novel
oxidative ligand coupling of tetraarylborates promoted by
chlorosilane and molecular oxygen.
When NaBPh4 (1a) was treated with Me3SiOTf in acetoni-
trile, the color of the solution changed from colorless to pale
yellow immediately. Under an argon atmosphere, no further
reaction was detected, but formation of biphenyl (2a) was
observed under an oxygen atmosphere in 30% yield. In the
absence of Me3SiOTf, 2a was produced only in 5% yield even
under an oxygen atmosphere (eq 1).
Table 2 demonstrates the results starting from various
kinds of organoborate compounds.8 As shown in entry 4, lig-
and coupling of tetramethylammonium salt (1a') also proceeded
in 83% yield. Me4NB(3-MeC6H4)4 (1b) was similarly convert-
ed to 3,3'-dimethylbiphenyl (2b) without formation of any
regioisomer, indicating that the coupling occurs at the ipso
position of the borate. Only a trace amount of B–Si cross-cou-
pling product 3b was detected by GC (entries 5 and 6). From
Me4NB(4-MeC6H4)4 (1c), 2c was obtained exclusively (entries
7 and 8). In the reaction of unsymmetrical borates, the more
nucleophilic aryl group was susceptible to ligand coupling, giv-
ing an unsymmetrical biaryl compound as a major product
(entries 9–12). For example, the reaction of Me4NBPh3(4-
MeC6H4) (1d) with Ph2SiCl2 afforded 2d in 65% yield, while
2a was produced only in 9% yield (entry 9).
Although the precise reaction mechanism has not been
established, one possible reaction pathway may be explained by
a combination of each reported step as illustrated in Scheme 1.
When borate and chlorosilane are mixed, a silylium ion equiva-
lent (R3Si+) is considered to be generated as reported.9 The
silylium ion is known to react with an aromatic compound to
form a silylated cyclohexadienyl cation (Wheland intermediate
or σ-complex).10 Since electrophilic attack of the silylium ion
to an aromatic ring is reversible, no further reaction would pro-
ceed under an argon atmosphere. The reaction of alkynyltri-
The influence of halosilanes was monitored by employing
various silicon compounds in the biphenyl formation from
NaBPh4. The representative results are shown in Table 1.
When Me3SiCl was used, 2a was obtained only in 6% yield
(entry 1), but the reaction with Ph3SiCl afforded 2a in 42%
yield (entry 3). Next, the substituent effect of R2SiX2 was
examined to reveal that the more phenyl groups introduced on
silicon increase the yield of 2a (entries 4–6). However, intro-
duction of a diethylamino group did not promote the coupling
reaction as shown in entry 7. In order to confirm whether lig-
and scrambling between boron and silicon occurs, the reaction
with (4-MeC6H4)2SiCl2 was carried out. NMR analysis showed
that 2a and 4,4'-dimethylbiphenyl (2c) were produced in 91%
and 11% yields, respectively, and no 4-methylbiphenyl (2d)
was detected (entry 8). The result indicates that the ligand cou-
pling reaction not only occurs in the organoborate compound
but also in the organosilicon compound as a minor process.7
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
1085
This work was supported by a Grant-in-Aid for Scientific
Research (No. 11450341) from the Ministry of Education,
Culture, Sports, Science, and Technology, Japan. H. S. thanks
The Kurata Foundation for financial support.
Dedicated to Professor Hideki Sakurai on the occasion of his
70th birthday.
References and Notes
1
2
3
A. Pelter, K. Smith, and H. C. Brown, "Borane Reagents,"
Academic Press, London (1988).
T. Ishikawa, S. Nonaka, A. Ogawa, and T. Hirao, Chem.
Commun., 1998, 1209.
Related works: ligand coupling of organoaluminums, T.
Ishikawa, A. Ogawa, and T. Hirao, J. Am. Chem. Soc., 120,
5124 (1998); Organozincs, T. Hirao, T. Takada, and A.
Ogawa, J. Org. Chem., 65, 1511 (2000); T. Hirao, T. Takada,
and H. Sakurai, Org. Lett., 2, 3659 (2000); T. Takada, H.
Sakurai, and T. Hirao, J. Org. Chem., 66, 300 (2001).
D. H. Geske, J. Phys. Chem., 63, 1062 (1959); D. H. Geske,
J. Phys. Chem., 66, 1743 (1962).
A recent review; A. Pelter, R. T. Pardasani, and P. Pardasani,
Tetrahedron, 56, 7339 (2000).
P. Abley and J. Halpern, J. Chem. Soc., Chem. Commun.,
1971, 1238.
The result also indicates that the yields of 2a in entries 3 and
6 (Table 1) may include those derived from the silicon com-
pounds. The ligand coupling reaction of organosilicon com-
pounds will be discussed elsewhere.
4
5
6
7
8
General procedure: To an acetonitrile (5 mL) solution of the
borate 1 (0.25 mmol) was added Ph2SiCl2 or PhSiCl3 (0.25
mmol) via a syringe at room temperature under an oxygen
atmosphere and the mixture was stirred for 5–10 h. The
reaction was quenched with 1 M HCl solution, and the mix-
ture was extracted with ether. The combined organic layer
was washed with brine, and dried over MgSO4. The organic
solvent was evaporated and the resulting crude product was
purified by PTLC to give 2 as listed in Table 2.
alkylborate with chlorosilane has been reported to afford the β-
silylated alkene accompanied with 1,2-migration.11 Molecular
oxygen might promote the 1,2-migration of a nucleophilic lig-
and on boron to the ipso position on the aromatic ring.12
Finally, deboration13/desilylation14 via a radical process is pro-
moted by molecular oxygen, giving the corresponding ligand
coupling product. Alternative mechanisms such as an electron
transfer process via a peroxo species derived from a silylium
ion and molecular oxygen cannot be ruled out.
9
C. A. Reed, Acc. Chem. Res., 31, 325 (1998). For structural
information of R3Si(CH3CN)+, see M. Kira, T. Hino, and H.
Sakurai, Chem. Lett., 1993, 153; Z. Xie, D. J. Liston, T.
Jelínek, V. Mitro, R. Bau, and C. A. Reed, J. Chem. Soc.,
Chem. Commun., 1993, 384.
10 G. A. Olah, T. Bach, and G. K. S. Prakash, J. Org. Chem.,
54, 3770 (1989); G. A. Olah, G. Rasul, H. A. Buchholz, X-Y.
Li, and G. K. S. Prakash, Bull. Soc. Chim. Fr., 132, 569
(1995), and references cited therein.
11 P. Binger and R. Köster, Synthesis, 1973, 309; E. J. Corey
and W. L. Seibel, Tetrahedron Lett., 27, 905 (1986). See
also, B. Wrackmeyer, E. V. Klimkina, and Y. N. Bubnov, J.
Organomet. Chem., 620, 51 (2001).
12 Strong electrophiles are known to induce such 1,2-migration
to the aromatic ring. See, E. Negishi and R. E. Merrill, J.
Chem. Soc., Chem. Commun., 1974, 860; K. Utimoto, K.
Okada, and H. Nozaki, Tetrahedron Lett., 1975, 4239.
13 K. Nozaki, K. Wakamatsu, T. Nonaka, W. Tückmantel, K.
Oshima, and K. Utimoto, Tetrahedron Lett., 27, 2007 (1986);
K. Nozaki, K. Oshima, and K.Utimoto, J. Am. Chem. Soc.,
109, 2547 (1987).
As described above, the present reaction provides a novel
oxidative ligand coupling of organoborates promoted by a com-
bination of chlorosilane and molecular oxygen. It is noteworthy
that unsymmetrical biaryls can be selectively obtained from the
corresponding unsymmetrical borate compounds. Further appli-
cation and mechanistic studies are now under investigation.
14 A. Studer and S. Amrein, Angew. Chem. Int. Ed., 39, 3080
(2000).