Role of Copper Species in the Oxidative Dimerization of Arylboronic Acids: Synthesis of Symmetrical Biaryls

Article abstract of DOI:10.1021/jo034680a

Certain Cu(I) and Cu(II) salts are able to mediate the dimerization of arylboronic acids in DMF. They provide the corresponding symmetrical biaryls in moderate to very good yields. It is possible to run the reaction catalytically under an oxygen atmosphere without a significant loss of yields.

Full text of DOI:10.1021/jo034680a

Role of Cop p er Sp ecies in th e Oxid a tive Dim er iza tion of
Ar ylbor on ic Acid s: Syn th esis of Sym m etr ica l Bia r yls
Ayhan S. Demir,* O¨ mer Reis, and Mustafa Emrullahoglu
Department of Chemistry, Middle East Technical University, 06531 Ankara, Turkey
asdemir@metu.edu.tr
Received May 21, 2003
Certain Cu(I) and Cu(II) salts are able to mediate the dimerization of arylboronic acids in DMF.
They provide the corresponding symmetrical biaryls in moderate to very good yields. It is possible
to run the reaction catalytically under an oxygen atmosphere without a significant loss of yields.
In tr od u ction
instead of air, but the effect is not uniform, providing
much better yields for certain arylboronic acids and lower
yields for others. In another study, Pd(OAc) in a DMF
2
There is a growing interest in the synthesis of valuable
natural and unnatural biaryls. Catalytic cross-coupling
reactions are the cornerstone of reactions developed for
this purpose. Ullmann-type homocoupling is another
possible approach; many useful and superior variants of
this reaction have been developed for the synthesis of
symmetrical biaryls. For the symmetrical biaryls con-
cerned, oxidative dimerization of organometallic species
emerges as another method of choice. All of the ap-
solution was shown to provide the corresponding biaryls
in a moderate yield, while the addition of phosphine
6
ligands enhanced the reaction yields. Interestingly, Koza
2
et al. reported that Pd(OAc) in DMF does not promote
1
the self-coupling of phenylboronic acid, and Pd(PPh )
3 4
7
yields only trace amounts of biphenyl. Moreover, con-
trary to the report by J ackson et al., Pd(OAc) in ethanol
2
furnished only trace amounts of the product, the only
difference being the base. Their reaction conditions and
reported yields are very similar to that of J ackson et al.
except for Cu(NO ) being the oxidant instead of Cu-
3 2
2
(OAc) . Recently, the base-free homocoupling of arylbo-
proaches mentioned have been comprehensively reviewed
recently.2
Arylboronic acids are organometallic species in the
well-known Suzuki coupling, and the homocoupling of
3
arylboronic acids occurs in Suzuki coupling when the
desired cross-coupling is slow.3 Because of their greater
stability and lower toxicity over organostannanes, it is
not surprising that there have been many studies that
have reported recently on the dimerization of arylboronic
acids as a method of symmetrical biaryl synthesis.
Almost all of the methods rely on palladium catalysis
and, as well, improvements made on the basis of the use
of different ligands, bases, and solvents. Moreno-Ma nˇ as
et al. reported on a study that aimed at understanding
the mechanism of self-coupling in arylboronic acids. They
proposed a mechanism based on Pd(0) as the active
,4
ronic esters mediated by palladium-1,3-bis(diphenylphos-
phino)propane (DPPP) complex in DMSO under an
oxygen atmosphere has been shown to provide high yields
8
of biaryls. Another improvement in the oxidative dimer-
ization of arylboronic acids is the use of Pd(OAc) in H O,
2
2
9
with oxygen as the oxidant and NaOAc as the base.
Another variation that provides excellent yields for
certain arylboronic acids utilizes p-toluenesulfonyl chlo-
2
ride as the additive and PdCl as the metal, but ortho-
10
substitution significantly lowers the yields. Falck et al.
reported that alkyl-, alkenyl-, and arylboronic acids
4
species derived from a variety of sources. Later, it was
shown by J ackson et al. that Pd(OAc) in combination
2
with a base provided synthetically useful yields of biaryls
under an oxygen atmosphere. It was also shown that
2 2
undergo Ag O/CrCl -mediated homocoupling in good
yields and are important in that all hybridization types
on organoboronic acids are tolerated. This method is
compatible with allylic acetates, which would cause
problems with palladium-based methods. Labile organo-
silver and/or organo-chromium have been proposed as
5
2
Cu(OAc) can be used as an oxidant in addition to or
*
To whom correspondence should be addressed. Phone: + 90-312-
103242. Fax: +90-312-2101280.
1) (a) Miyano, S.; Shimizu, K.; Sato, S.; Hashimoto, H. Bull. Chem.
Soc. J pn. 1985, 58, 1346. (b) Ziegler, F. E.; Fowler, K. W. J . Org. Chem.
1
1
possible intermediates that subsequently dimerize.
2
(
1
1
976, 41, 1564. (c) Zhang, S.; Zhang, D.; Liebiskind, L. S. J . Org. Chem.
997, 62, 2312 and references therein.
(6) Wong, M. S.; Zhang, X. L. Tetrahedron Lett. 2001, 42, 4087.
(7) Koza, D. J .; Carita, E. Synthesis 2002, 2183.
(8) Yoshida, H.; Yamaryo, Y.; Ohshita, J .; Kunai, A. Tetrahedron
Lett. 2003, 44, 1541.
(
2) Hassan, J .; S e´ vignon, M.; Gozzi, C.; Schulz, E.; Lemaire, M.
Chem. Rev. 2002, 102, 1359.
(
(
3) Miyaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457.
4) Moreno-Ma nˇ as, M.; P e´ rez, M.; Pleixtas, R. J . Org. Chem. 1996,
(9) Parrish, J . P.; J ung, Y. C.; Floyd, R. J .; J ung, K. W. Tetrahedron
Lett. 2002, 43, 7899.
6
1, 2346.
5) Smith, K. A.; Campi, E. M.; J ackson, W. R.; Marcuccio, S.;
Naeslund, C. G. M.; Deacon, G. B. Synlett 1997, 131.
(10) Kabalka, G. W.; Wang, L. Tetrahedron Lett. 2002, 43, 3067.
(11) Falck, J . R.; Mohapatra, S.; Bondlela, M.; Venkataraman, S.
K. Tetrahedron Lett. 2002, 43, 8149.
(
1
0.1021/jo034680a CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/03/2003
1
0130
J . Org. Chem. 2003, 68, 10130-10134

Copper-Mediated Dimerization of Arylboronic Acids
SCHEME 1
side products. For example, a reaction in dichloromethane
under ordinary conditions resulted in the formation of
symmetrical diaryl ether 5, whereas acetoxylation prod-
uct 6 was observed under anhydrous conditions in the
presence of molecular sieves (GC-MS). DMF was the
solvent of choice; only desired product 2b was found in
1
the crude product ( H NMR) although no precautions
were taken to avoid the presence of water during the
handling of chemicals or to remove it from the reaction
mixture with molecular sieves.17
After a series of reactions we found that 0.5 equiv of
Recently, we have shown that arylboronic acids are a
very good source of aryl radicals that can be generated
Cu(OAc) respect to 1 equiv of arylboronic acid is enough
2
for the desired conversion. Under this condition, 1b was
by Mn(OAc)
solvents to obtain the corresponding biaryls. Later we
found that a similar oxidant, Cu(OAc) , is also able to
mediate the same transformation. Since Cu(OAc) is
known to form C-O and C-N bonds from arylboronic
acids and aryl-copper intermediates have been proposed
for these transformations,13 homolysis prone aryl-copper
and aryl-manganese intermediates might play a role in
the formation of aryl radicals in benzene.
Recently, many reports by Liebeskind14 and Buch-
wald15 have shown that excellent results can be obtained
with catalysts based on copper instead of or in addition
to palladium. Therefore, the feasibility of using copper
in a variety of reactions deserves attention; one such
reaction is the oxidative dimerization of arylboronic acids.
In addition, copper species are generally the first choice
when a co-oxidant is required to restore the catalytically
active species, but their use can be disastrous. Herein
we report our investigation on the use and role of Cu-
3
and subsequently captured by aromatic
found to be providing the corresponding symmetrical
biaryl 2b in 95% yield. The progress of the reaction is
relatively slow at room temperature. Reaction time
decreased dramatically at elevated temperatures (100 °C)
without significantly affecting the yield (93%). According
to these results several arylboronic acids reacted with
1
2
2
2
Cu(OAc) in DMF at 100 °C, and results are summarized
2
in Table 1.
A closer inspection of the results revealed that the
reaction is sensitive to the position of substituents.
Substituents on the ortho-position lower the yields.
Whereas 2-naphthaleneboronic acid provided the corre-
sponding dimer in a 71% yield, sterically congested
1
-naphthaleneboronic acid provided 49% binaphthyl
together with 35% naphthalene. 2,6-Disubstituted aryl-
boronic acids (entries 22 and 23) only provided the
corresponding arene. This behavior was attributed either
to the instability of the formed aryl-copper intermediate
or the sterically slowed transmetalation of the second aryl
moiety onto the metal center as a result of substitution
on the ortho-position, eventually resulting in decomposi-
tion.
2
(OAc) and related copper species in the oxidative dimer-
ization of arylboronic acids.
Resu lts a n d Discu ssion
It is also possible to carry out this homocoupling
reaction in the presence of catalytic Cu(OAc)
out the reaction with 0.1 equiv of Cu(OAc)
equiv of 1b under an oxygen atmosphere at 100 °C. H
2
. We carried
Initially we investigated the reaction of 4-bromophen-
2
in respect to
ylboronic acid 1b and 2 equiv of Cu(OAc)
2
in a variety of
1
1
solvents for the conversion depicted in Scheme 1. Typical
side products of this reaction were expected to be the
corresponding arene 3, phenol 4, symmetrical diaryl ether
NMR analysis of the crude product showed phenol and
diaryl ether type side products in the crude mixture
opposed to the stoichiometric reaction, possibly related
to the sensitivity of catalytic reaction to the presence of
water. Molecular sieves were added to reaction mixture
to suppress the formation of these side products, and 2b
was obtained in an 86% isolated yield. Additional ex-
amples showed that slightly lower but still acceptable
yields of symmetrical biaryls can be obtained with
catalytic amounts of copper (entries 2, 4, 10, 12, 14, 16).
An oxygen atmosphere was necessary, and the use of an
inert atmosphere resulted in low conversions.
5
, and acetoxylation product 6 on the basis of previous
1
3a,16
reports.
Among the solvents tried (MeOH, MeCN,
cyclohexane, dichloromethane), THF and DMF were
found to be suitable for the desired conversion. Other
solvents resulted in poor conversions or the formation of
(
12) Demir, A. S.; Reis, O¨ .; Emrullahoglu, E. J . Org. Chem. 2003,
6
3
8, 578.
(13) (a) Evans, D. A.; Katz, J . L.; West, T. R. Tetrahedron Lett. 1998,
9, 2937. (b) Combs, A. P.; Saubern, S.; Rafalski, M.; Lam, P. Y. S.
Tetrahedron Lett. 1999, 40, 1623. (c) Collman, J . P.; Zhong, M. Org.
Lett. 2000, 2, 1233. (d) Lam, P. Y. S.; Vincent, G.; Clark, C. G.; Deudon,
S.; J adhav, P. K. Tetrahedron Lett. 2001, 42, 3415.
Another interesting aspect of the reaction is the source
of copper. CuCl
resulted in slow protodeboronation. Addition of sodium
acetate did not promote the reaction. Cu(NO provided
2b from 1b albeit in a lower yield than with Cu(OAc)
The conversion rates were so low with Cu(OTf) that only
2
was found to be ineffective, and heating
(14) (a) Farina, V.; Kapadia, S.; Krishnan, B.; Wang, C.; Liebeskind,
L. S. J . Org. Chem. 1994, 59, 5905. (b) Allred, G. D.; Liebeskind, L. S.
J . Am. Chem. Soc. 1996, 118, 2748. (c) Zhang, S.; Zhang, D.; Liebes-
kind, L. S. J . Org. Chem. 1997, 62, 2312. (d) Savarin, C.; Liebeskind,
L. S. Org. Lett. 2001, 3, 2149. (e) Liebeskind, L. S.; Srogl, J . Org. Lett.
3 2
)
2
.
2
2
002, 4, 979.
15) (a) Kwong, F. Y.; Klapars, A.; Buchwald, S. L. Org. Lett. 2002,
trace amounts of the product was observed after 24 h at
room temperature. Addition of sodium acetate to a DMF
(
4
3
, 581. (b) Wolter, M.; Klapars, A.; Buchwald, S. L. Org. Lett. 2001, 3,
803. (c) Klapars, A.; Antilla, J . S.; Huang, X.; Buchwald, S. L. J . Am.
Chem. Soc. 2001, 123, 7727. (d) Marcoux, J .-F.; Doye, S.; Buchwald,
S. L. J . Am. Chem. Soc. 1997, 119, 10539.
(17) It has been shown that the presence of water results in
formation of side product 5; molecular sieves trap the water released
by arylboronic acids via anhydride formation. For an example, see ref
13a.
(
16) (a) van Koten, G.; J astrzebski, J . T. B. H.; Noltes, J . G.
Tetrahedron Lett. 1976, 17, 223. (b) Cohen, T.; Wood, J .; Dietz, A., J r.
G. Tetrahedron Lett. 1974, 15, 3555.
J . Org. Chem, Vol. 68, No. 26, 2003 10131

Demir et al.
TABLE 1. Yield s of Sym m etr ica l Bia r yls
a
Isolated yields. All compounds are known except for 2h and 2u , and all analytical data are in agreement with the previously reported
data. Compounds 2h and 2u gave satisfactory analytical data (Experimental Section). When the yields were low, corresponding arenes
were isolated together with biaryls summing up to >90% of the starting material. In the case of volatile arenes, product content was
detected with GC-MS. Although all of the reactions were carried out on a 1 mmol scale, reaction with 5 mmol of 1b worked equally well.
b
Run with catalytic Cu(OAc)2.
solution of Cu(OTf)
acid provided a result similar to that of Cu(OAc)
2
prior to the addition of arylboronic
. More-
to react under the same set of conditions. Because
conditions were not optimized for each Cu(I) compound,
yields are not an indication of the potential of that
particular Cu(I) salt, but they are well useful for com-
parison with Cu(II) sources. Contrary to Cu(II) salts, all
of the examined Cu(I) salts afforded the corresponding
dimer as shown in Table 2. CuI afforded vast amount of
the corresponding iodoarene in addition to dimer (GC-
MS). CuOAc provided the highest conversion, and the
2
over, the addition of the sodium salt of N-benzoylphen-
ylglycine instead of sodium acetate furnished 2b in 51%
yield in an unoptimized reaction at 100 °C, whereas sod-
ium trifluoroacetate was ineffective. It could be envi-
sioned that acetate ion in the solution activates the aryl-
boronic acid as boronate that has a propensity to trans-
metalate onto the active metal. However, our results
seem to indicate that acetate ligand bound to copper
activates the arylboronic acid. Similar behavior was
reported by Liebeskind: copper(I) thiophene-2-carboxyl-
yield is just comparable to that of Cu(OAc)
2
. Another
similarity between CuOAc and Cu(OAc) was observed
2
for 2v not only in terms of yield of dimer but also that of
a protodeboronation product. CuTC gave mainly protode-
boronation together with small amounts of dimer under
the same conditions.18
ate (CuTC) activates arylboronic acids toward trans-
metalation onto palladium under base-free conditions.14d,e
However, the picture could be much more complicated
keeping in mind that truly active species in the reaction
medium is not known. So we investigated the reactivity
of various Cu(I) salts. Cu(I) salts (CuOAc, CuOTf.1/2PhH,
(
18) It was previously shown that 2-naphthaleneboronic 1y gave
14d
CuBr‚Me
2
S, CuCl, CuI, CuTC) and Cu(OAc)
2
were made
complete protodeboronation with CuTC in THF.
1
0132 J . Org. Chem., Vol. 68, No. 26, 2003

Copper-Mediated Dimerization of Arylboronic Acids
TABLE 2. Com p a r ison of Cu (I) Sa lts w ith Cu (OAc)2a
SCHEME 2. P ossible Mech a n ism s of
Hom ocou p lin g of Ar ylbor on ic Acid s
entry
copper salt
arylboronic acid
yield (%)
1
2
3
Cu(OAc)2
CuOAc
CuOTf‚1/2PhH
Cu(O2CCF3)
CuTC
CuBr.Me2S
CuCl
CuOAc
2b
2b
2b
2b
2b
2b
2b
2v
2v
77
72
44
31
48
34
65
51
16
b
4
5
6
7
8
9
c
d
CuTC
a
All reactions were carried out under the same set of condi-
tions: 0.5 mmolof 2a and 0.25 mmol of Cu(I) or Cu(II) in 1 mL of
DMF at room temperature for 24 h. Isolated yields; no side
SCHEME 3
b
products were observed. Sodium trifluoroacetate was mixed with
c
CuOTf‚1/2PhH in DMF prior to the reaction. 48 h for complete
d
conversion; 36% naphthalene was isolated. 74% naphthalene was
isolated
nism was also proposed by Piers for the oxidative
dimerization of organostannes by stoichiometric CuCl.
Although the mechanism of dimerization of arylboronic
acids by copper salts is not obvious at this stage, we want
to emphasize the following points: (1) investigation with
21
If the route through 8 is operational in catalytic reaction
with either Cu(I) or Cu(II) as the active species, regen-
eration of active copper salt from Cu(0) with oxygen is
necessary, which was shown to be feasible (see above).
However, a critical role of oxygen might also be an
indication of a Cu(III) oxidation state that is advocated
by some researchers while others criticized it being
Cu(I) salts showed that although Cu(OTf)
and CuCl were not suitable for this transformation,
corresponding Cu(I) salts were useful. Moreover, CuOAc
and Cu(OAc) gave very similar results in terms of
product yield and distribution (Table 1, entry 27; Table
, entry 8). These seem to be pointing out the necessity
2 2 3 2
, Cu(O CCF ) ,
2
2
2
22
energetically unfavorable. This invokes a possible Cu(I)/
of the formation of corresponding Cu(I) salts that are
either chiefly active species or act together with Cu(II)
salts.19 This could be understandable if one considers that
highly electronegative ligands (triflate and trifluoroac-
etate) may retard the reduction of the presumably
inactive Cu(II) salts to the corresponding Cu(I) salts
Cu(III) catalytic cycle. Although it is difficult to figure
out the exact timing of the transmetalation, one possible
route is as follows: a second aryl moiety transmetalates
onto 7 to form diarylcopper(I) 9, which is oxidized by
oxygen to diarylcopper(III) 10, subsequently releasing
Cu(I) upon reductive elimination. Considering that there
is no information currently available on the active
species, it is hard to justify a more lengthy discussion
based on speculation
Although investigation of both stoichiometric and
catalytic formation of arylcopper intermediates from
arylboronic acids is still in progress in order to develop
new synthetic applications, preliminary results have
revealed the following potentially important results:
First, homocoupling reaction does not interfere with the
Suzuki-type cross-coupling reactions under the specified
conditions; 4-iodoanisole stayed intact during the homo-
(which were shown to be active), even though chloride
does not perfectly fit into this picture. A deeper under-
standing of these possibilities requires the detection of
oxidation states of copper species present in reaction
mixture and prior knowledge of the redox behavior of
various copper species under the specified reaction condi-
tions, for which studies are currently underway. (2) In
most cases, we observed the precipitation of metallic
copper. Moreover, we also observed the complete or
partial disappearance of the precipitated copper. Sepa-
rate control reactions were carried out with metallic
copper and sodium acetate both under oxygen and
nitrogen atmosphere, even though these conditions are
not a perfect imitation of the real process. Reaction under
inert gas showed no sign of change, and only starting
material was obtained. However, the formation of green
color was observed under oxygen, providing a low yield
of biaryl together with side products. Moreover homo-
coupling reaction under inert atmosphere gave inferior
2
coupling reaction of 1b in the presence of Cu(OAc) as
Cu(II) source and CuOAc (or CuTC) as Cu(I) source both
at room temperature and 100 °C. Second, it is possible
to cross-couple phenylacetylene 11 and 1b. Addition of
1
2
b and Cu(OAc) to a solution of 11 and NaH in dry THF
(MS 4A°) at room temperature afforded a mixture of 12
and 2b (GC-MS) in an unoptimized reaction as shown
in Scheme 3.
2
yields either with CuOAc or Cu(OAc) .
In conclusion, we showed that certain copper species
are able to mediate the homocoupling of arylboronic acids
in DMF with moderate to very good yields. This method
allows the use of a cheap Cu(I) or Cu(II) species for this
popular transformation and offers very good results for
certain substrates. Reaction is sensitive to steric crowding
around the reacting center, and disubstitution completely
hinders the dimerization. Additionally, a variety of
Possible simplistic mechanisms for both Cu(I) and Cu-
II) compounds are depicted in Scheme 2. Formation of
(
metallic copper suggests the intermediacy of diarylcop-
per(II) 8, which reductively eliminates to afford 2; 8 could
2
be generated by Cu(OAc) via double transmetalation of
1
. Alternatively, arylcopper(I) intermediates 7 formed by
CuOAc can disproportionate to form 8 according to a
mechanism proposed by van Koten.20 A similar mecha-
(
20) van Koten, G.; J astrzebski, J . T. B. H.; Noltes, J . G. J . Org.
(19) Oxidation of DMF by Cu(II) was reported: Dhar, S. K.;
Chem. 1977, 42, 2047.
Abdelaziz, J .; Cozzi, P.; J asien, P.; Mason, C.; Zalenas, R. J . Chem.
Soc., Chem. Commun. 1984, 8.
(21) Piers, E.; Yee, J . G. K.; Gladstone, P. L. Org. Lett. 2000, 2, 481.
(22) Lindley, J . Tetrahedron 1984, 40, 1433 and references therein.
J . Org. Chem, Vol. 68, No. 26, 2003 10133

Demir et al.
acetate ligands can be bound on the metal surface for a
specific reason, such as salts of amino acids as exempli-
fied above. Moreover, it is possible to run the reaction
catalytically under an oxygen atmosphere without a
significant loss of yields. A variety of Cu(I) compounds
have activity and can be potentially useful in other
reactions based on copper and arylboronic acids; excellent
examples have already been reported.14,15 Development
of new synthetic applications through the generation of
arylcopper intermediates in this context are under in-
vestigation.23
mmol) in 1 mL DMF was added 4-bromophenylboronic acid
(100 mg, 0.5 mmol), and the resulting mixture was stirred at
1
00 °C for 1 h. After completion of the reaction (formation of
the products was monitored by TLC and GC-MS), the mixture
was filtered through a pad of silica using hexane or petroleum
ether as eluent. Concentration under reduced pressure fur-
nished 4,4′-dibromobiphenyl (2b) (73 mg, 93%) confirmed by
NMR and GC-MS.
Typ ica l P r oced u r e for Sym m etr ica l Bia r yls by Ca ta -
lytic Cu (OAc)
molecular sieves and heated under vacuum for 2-3 h. After
the flask cooled to room temperature under argon, Cu(OAc)
2
. A flask was charged with powdered 4Α°
2
(
9 mg, 0.05 mmol) and 1 mL DMF were added followed by
4
-bromophenylboronic acid (100 mg, 0.5 mmol). The resulting
Exp er im en ta l Section
mixture was stirred at 100 °C under an oxygen atmosphere
for 3 h. After that period, the mixture was worked up as for
the stoichiometric reaction to obtain 4,4′-dibromobiphenyl 2b
(67 mg, 86%).
Gen er a l Meth od s. All arylboronic acids 1a -y and copper
salts (CuTC was prepared as reported previously)1 were
commercially available and used as obtained. Compounds
4b
2
4a
24b
24c
24d
24e
24f
24g
24h
2
a ,b,f,g,i,k ,s, 2c, 2d , 2e,j, 2l,n ), 2m , 2o, 2t,
4,4′-Bis(t r iflu or om et h oxy)b ip h en yl (2h ). Yield 93%,
7
1
and 2v,y are known. Although no special precautions were
taken during weighing and handling of chemicals, DMF was
distilled under vacuum and stored over 4Α° molecular sieves.
Because most Cu(I) salts are relatively air-sensitive, they were
stored under a blanket of argon and handled quickly.
colorless oil; H NMR (400 MHz, CDCl ) δ 7.21 (4H, d, J ) 8.4
3
Hz), 7.46 (4H, d, J ) 8.4 Hz); 13C NMR (100 MHz, CDCl ) δ
3
1
20.8 (q, J ) 256 Hz), 121.6, 128.7, 138.9, 149.3. Anal. Calcd
for C14
8 6 2
H F O : C, 52.19; H, 2.50. Found: C, 52.31; H, 2.76.
3
,3′-Difor m yl-2,2′-bith ien yl (2u ). Yield 35%, bright yellow
Typ ica l P r oced u r e for Sym m etr ica l Bia r yls by Stoich -
1
solid; mp 131-131.5 °C; H NMR (400 MHz, CDCl
3
) δ 7.17
iom etr ic Cu (OAc)
2
. To a mixture of Cu(OAc)
2
(45 mg, 0.25
13
(
2H, d, J ) 4.8 Hz), 7.75 (2H, J ) 4.8 Hz), 9.74 (2H, s);
C
NMR (100 MHz, CDCl
3
) δ 127.4, 132.3, 141.2, 141.9, 183.0.
6 2 2
(23) It is worth mentioning that Mn(OAc)
3
mediates the homocoup-
Anal. Calcd for C10
H, 2.91.
H O S : C, 54.03; H, 2.72. Found: C, 54.15;
ling of arylboronic acids albeit in lower yields; this is thought to be an
indication of formation of arylmanganese intermediates in a manner
similar that of to arylcopper compounds.
(
24) (a) These compounds are commercially available. (b) Dougherty,
Ack n ow led gm en t. Financial support from Middle
East Technical University (BAP-2002), the Scientific
and Technical Research Council of Turkey (TUBITAK),
Turkish Academy of Sciences (T U¨ BA), and Turkish
State Planning Organization (DPT) is gratefully acknow-
ledged.
T. K.; Lau, K. S. Y.; Hedberg, F. L. J . Org. Chem. 1983, 48, 5273. (c)
Tarpley, A. R.; Goldstein, J . H. J . Phys. Chem. 1971, 75, 421. (d)
Courtois, V.; Barhdadi, R.; Troupel, M.; P e´ richon, J . Tetrahedron 1997,
5
3, 11569. (e) Lourak, M.; Vanderesse, R.; Fort, Y.; Caub e´ re, P. J . Org.
Chem. 1989, 54, 4840. (f) Ligtenbarg, A. G. J .; van den Beuken, E. K.;
Meetsma, A.; Veldman, N.; Smeets, W. J . J .; Spek, A. L.; Feringa, B.
L. J . Chem. Soc., Dalton Trans. 1998, 263. (g) Helms, A.; Heiler, D.;
McLendon, G. J . Am. Chem. Soc. 1992, 114, 6227. (h) Mitsumori, T.;
Inoue, K.; Koga, N.; Iwamura, H. J . Am. Chem. Soc. 1995, 117, 2467.
J O034680A
1
0134 J . Org. Chem., Vol. 68, No. 26, 2003