Palladium-catalyzed synthesis of biaryls from arylzinc compounds using N-chlorosuccinimide or oxygen as an oxidant

Article abstract of DOI:10.1246/bcsj.74.2415

In the presence of a catalytic amount of Pd²⁺ or Pd⁰, oxidative homo-coupling of arylzinc compounds was achieved by the use of N-chlorosuccinimide (NCS) or O₂ as an oxidant. Different reaction pathways were involved in the catalytic reactions depending on the oxidants; NCS or O₂ probably oxidized I^- or Pd⁰ to I⁺ or Pd²⁺, respectively. This reaction disclosed a new and facile synthetic method of biaryls from aryl halides or arenes via arylzinc intermediates.

Full text of DOI:10.1246/bcsj.74.2415

Bull. Chem.
Soc. Jpn.
74
12
© 2001 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 74, 2415–2420 (2001) 2415
The Chemical
Society of Ja-
pan
The Chemical
Society of Ja-
Palladium-Catalyzed Synthesis of Biaryls from Arylzinc Compounds
Using N-Chlorosuccinimide or Oxygen as an Oxidant
pan
OB
12
Catalytic Synthesis of Biaryls from Arylzincs
K. M. Hossain et al.
15
Kabir M. Hossain, Toru Kameyama, Takanori Shibata, and Kentaro Takagi*
2001
2415
2420
BCSJA8
0009-2673
01190
6
8
2001
8
Department of Chemistry, Faculty of Science, Okayama University, Tsushima, Okayama 700-8530
(Received June 8, 2001)
In the presence of a catalytic amount of Pd2+ or Pd0, oxidative homo-coupling of arylzinc compounds was achieved
by the use of N-chlorosuccinimide (NCS) or O2 as an oxidant. Different reaction pathways were involved in the catalytic
reactions depending on the oxidants; NCS or O2 probably oxidized I− or Pd0 to I+ or Pd2+, respectively. This reaction
disclosed a new and facile synthetic method of biaryls from aryl halides or arenes via arylzinc intermediates.
15
2001
A wide variety of transition metals (Mn+) achieve the oxida-
Table 1. Effect of Reaction Conditions on Pd-Catalyzed
tive cleavage of organometallic compounds (R–m) to yield
4.10.2
Homo-Coupling of 1a)
homo-coupling products (R–R) efficiently; such cleavage pro-
Run Oxidant
Pd catalyst
2 (Yield/%)
vides a unique synthetic method of symmetrical organic com-
pounds (Eq. 1).1 In the reaction, transition metals act as an ox-
idant for R− fragments and, accordingly, are required to be
used in stoichiometric amounts.2,3 Recently, novel catalytic re-
actions have been developed for such organometallic com-
pounds as Sn, B, Si, or Bi by the use of auxiliary oxidants (Ox)
like CuCl2, organic dihalide, or O2 (air), which make transition
metals retain their reactive oxidation states (Mn+) through the
reaction (Eq. 2).4 This methodology has greatly increased the
utility of the homo-coupling reaction as a
1
2
3
4
5
6
O2
NCS
CuCl2
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
86
87
29
27
79
85
37
< 5
PhCHBrCHBrCO2Et [PdCl2(PPh3)2]
O2
[Pd(PPh3)4]
[Pd(PPh3)4]
[Pd(PPh3)4]
Pd(OAc)2
[PdCl2(C6H5CN)2] < 5
[PdCl2(PPh3)2] < 5
NCS
7b) NCS
8
9
10
NCS
NCS
—
a) Molar ratio: 1/Pd/Oxidant = 1/0.02/0.5 or 1 atm (O2)
(Runs 1, 2, 4–10), or 1/0.02/1 (Run 3). Reaction solvent:
TMU. Reaction time: 12 h. Reaction temp: 45 °C.
b) Reaction time: 1 h.
R–m + Mn+ → 1/2 R–R + m+ + M(n−1)+
R–m+Ox catalyst→ 1/2 R–R+ m++Ox−
(1)
(2)
synthetic procedure of R–R. During the course of our study on
the synthesis and synthetic application of functionalized aryl-
zinc compounds,5 we were interested in the catalytic homo-
coupling of the compounds: although a stoichiometric amount
of Pd2+, Cu2+, Fe3+, or V5+ is known to achieve the homo-cou-
pling of organozinc compounds,6 the catalytic process has
been little explored.7 Here, we would like to report the Pd-cat-
alyzed method of converting arylzinc compounds into biaryls,
using N-chlorosuccinimide (NCS) or O2 as an oxidant.8
carbonyl, cyano, chloro, alkoxy, or acetoxy readily underwent
the Pd-catalyzed homo-coupling using O2 or NCS as an oxi-
dant in TMU to afford the corresponding biaryls in good
yields. An arylzinc compound in DMF afforded the desired
homo-coupling product in a good yield, too (Run 13). Since
this type of functionalized arylzinc compounds is readily pre-
pared by the reaction between the corresponding aryl iodides
and zinc powder in polar solvents9,10 the present catalytic reac-
tion provides a far more convenient synthetic method of func-
tionalized biaryls from aryl halides than the reported ones us-
ing aryltin, -boron, -silicon, or -bismuth compounds,4 whose
syntheses directly from aryl halides and metallic species are
generally impossible.
Organozinc compounds, prepared from organolithium com-
pounds and ZnI2 in THF, were also available for the reaction.
As shown in Table 3, desired products were readily produced
from arene (Run 1), thiophene (Runs 2 and 3), aryl bromides
(Runs 4–6), or alkyne (Runs 7 and 8) via their sequential treat-
ment with n-BuLi,11 ZnI2, NCS or O2, and Pd2+ or Pd0. It
should be noted that the reaction using phenylzinc chloride,
prepared from phenyllithium and ZnCl2, in place of arylzinc
Results and Discussion
The catalytic homo-coupling of arylzinc compounds was
first examined by the use of a reaction system composed of 2-
methoxycarbonylphenylzinc iodide 1 and various oxidants in
the presence of [PdCl2(PPh3)2] (2 mol%) in TMU (1,1,3,3-tet-
ramethylurea)9 at 45 °C for 12 h. As shown in Table 1, the de-
sired product, dimethyl biphenyl-2,2ꢀ-dicarboxylate 2, was ob-
tained in a variable yield depending on the oxidant used; the
reaction using O2 or NCS gave 2 in 86% or 87% yield, respec-
tively (Runs 1 and 2). As the catalyst, [Pd(PPh3)4] was also ef-
fective (Runs 5 and 6). As shown in Table 2, a variety of aryl-
zinc compounds containing such functional groups as alkoxy-

2416 Bull. Chem. Soc. Jpn., 74, No. 12 (2001)
Table 2. Synthesis of Functionalized Biaryls from Arylzinc Iodidesa)
Catalytic Synthesis of Biaryls from Arylzincs
Run
Oxidant
Pd catalyst
R
3-CO2CH3
3-CO2CH3
4-CO2C2H5
4-CO2C2H5
4-CO2C2H5
2-Cl
(Yield/%)b)
(94)
86 (93)
80
1
2
3
4
5
6
7
8
9
10
11
12
13c)
14
NCS
NCS
NCS
O2
[PdCl2(PPh3)2]
[Pd(PPh3)4]
[Pd(PPh3)4]
[PdCl2(PPh3)2]
[Pd(PPh3)4]
[Pd(PPh3)4]
[Pd(PPh3)4]
[Pd(PPh3)4]
[Pd(PPh3)4]
[PdCl2(PPh3)2]
[Pd(PPh3)4]
[Pd(PPh3)4]
[Pd(PPh3)4]
[Pd(PPh3)4]
(85)
(92)
86
85
62
(81)
(94)
(90)
(93)
(95)
(74)
O2
NCS
NCS
NCS
NCS
O2
2-CN
2-OAc, 5-CH3
4-OCH3
4-OCH3
4-OCH3
H
O2
NCS
NCS
Air
H
2-CO2CH3
a) Molar ratio: ArZnI/Pd/Oxidant = 1/0.02/0.5 or 1 atm (O2). Reaction solvent: TMU.
Reaction time: 2–16 h. Reaction temp: 35–45 °C.
b) Isolated yields. GLC yields are in parentheses.
c) Reaction solvent: DMF.
Table 3. Synthesis of Homo-Coupling Products from Organozinc Iodides via
Corresponding Organolithium Intermediatesa)
Run Starting Material
Organolithium
Pd catalyst
Oxidant
Product
(Yield/%)b)
1
2
3
4
5
6
7
8
C6H5OCH3
C4H4Sc)
2-CH3OC6H4Li
2-C4H3SLi
2-C4H3SLi
4-ClC6H4Li
4-ClC6H4Li
3-CH3OC6H4Li
C6H5CCLi
[Pd(PPh3)4]
[Pd(PPh3)4]
[Pd(PPh3)4]
[Pd(PPh3)4]
[PdCl2(PPh3)2]
[PdCl2(PPh3)2]
[PdCl2(PPh3)4]
[PdCl2(PPh3)2]
NCS
NCS
O2
NCS
NCS
NCS
NCS
O2
(84)
90
80
85
87
83
81
95
C4H4Sc)
4-ClC6H4Br
4-ClC6H4Br
3-CH3OC6H4Br
C6H5CCH
C6H5CCH
C6H5CCLi
a) Lithiation was carried out at 0 °C (Runs 2 and 3) or −78 °C (Runs 1 and 4–8) to
room temp using 1 equiv of n-BuLi in THF under N2. The resulting organolithium
intermediate was allowed to react with THF solution of ZnI2 (1 equiv) and then with
THF suspension of NCS (0.45 equiv) or O2 (1 atm) and Pd catalyst (0.02 equiv), fol-
lowed by the stirring of the resulting solution at room temp (Runs 2–6), 40 °C (Run 8),
or 45 °C (Runs 1 and 7) for 10–15 h.
b) Isolated yields. GLC yield is in parenthesis.
c) C4H4S = thiophene.
iodides did not afford the homo-coupling product at all, when
NCS was used as an oxidant. Instead, chlorobenzene was pro-
duced predominantly (Scheme 1). When O2 was used as an
oxidant, however, the desired product was obtained in a good
yield from the same zinc compound (Scheme 1).
In order to elucidate the unexpected difference in reactivity
between two oxidants towards phenylzinc chloride, and to see
the reaction path involved in the Pd-catalyzed homo-coupling
of arylzinc compounds, the progress of catalytic reaction was
followed by the measurement of the amount of each compo-
nent present in the solution, using the reaction systems com-
posed of 1, NCS or O2, and [Pd(PPh3)4] as arylzinc compound,
oxidant, and catalyst, respectively, in TMU. As shown in Fig.
Scheme 1. Effect of ZnX2 and oxidant in the reaction.

K. M. Hossain et al.
Bull. Chem. Soc. Jpn., 74, No. 12 (2001) 2417
Fig. 1. The progress with time of reaction of 1 using NCS as
an oxidant. The mixture composed of 1 (0.25 mmol),
[Pd(PPh3)4] (0.005 mmol), NCS (0.13 mmol), and TMU
was stirred at room temperature for 5 min and then at 45
°C for given periods of time.
Fig. 2. The progress with time of reaction of 1 using O2 as an
oxidant. The mixture composed of 1 (0.25 mmol),
[Pd(PPh3)4] (0.005 mmol), O2 (1 atm), and TMU was
stirred at 35 °C for given periods of time.
synchronous decrease in 1, and the presence of 3 was not de-
tected through the whole course of reaction, as shown in Fig. 2.
These results clearly suggest that each of the two oxidants
takes its own reaction pathway yielding the same product from
the same starting material. As to the former oxidant, path A
(Scheme 3, Negishi reaction13) might be plausible, wherein the
key intermediate abbreviated as ArPd+ is formed by the oxida-
tive addition of Pd0 to aryl iodides, generated in situ by the re-
action between NCS and ArZnI, followed by the transmetalla-
tion with ArZnI to yield ArPdAr, which yields a homo-cou-
pling product Ar–Ar to regenerate Pd0 catalyst. In general,
aryl chlorides are inert to Pd0 under cited conditions.14 As to
the latter oxidant, path B (Scheme 3) might be plausible, which
is essentially the same as that which had been proposed in Pd-
Scheme 2. Reaction of 1 with NCS.
1, in NCS reaction, methyl 2-iodobenzoate 3 was formed rap-
idly in a quantitative yield based on NCS, and 2 was formed
gradually in compensation for the simultaneous decreases in 1
and 3 in equal amounts. The control run ascertained that the
reaction between 1 and NCS takes place readily in TMU at
room temperature to yield 3 in a quantitative yield (Scheme
2).12 In O2 reaction, however, 2 was formed gradually with the
Scheme 3. Reaction path.

2418 Bull. Chem. Soc. Jpn., 74, No. 12 (2001)
Catalytic Synthesis of Biaryls from Arylzincs
catalyzed homo-coupling of organometallic compounds (B,
Sn, Si, or Bi) by the use of auxiliary oxidants including O2.4
That is, key intermediate ArPd+ is formed by the transmetalla-
tion between ArZnX and Pd2+, and the oxidant O2 plays the
role to convert Pd0 into Pd2+.15 Here, because of the presence
of Zn2+ between Ar− and X− fragments in ArZnX, the differ-
ence in reactivity of aryl fragment towards Pd2+ species be-
tween ArZnCl and ArZnI must be small, which probably ex-
plains the comparable reactivity of phenylzinc chloride to the
corresponding iodide in O2 reaction.
As mentioned above, both NCS and O2 are equally available
for the Pd-catalyzed homo-coupling of arylzinc compounds,
albeit operating in different ways (Scheme 3). From the practi-
cal viewpoint, each possesses an advantage over the other. In
the utility of O2, one need not take care of the molar ratio of
ArZnX/oxidant, since the oxidant reacts not with ArZnX but
with Pd. Accordingly, the catalytic reaction may be carried out
by the attachment of balloon filled with O2 or air (Run 14, Ta-
ble 2) to the reaction flask, which makes the reaction proce-
dure simple. On the other hand, NCS has to be added, but is
able to be added to the reaction solution in an accurate amount.
Therefore, when one intends to stop the reaction at certain
points of conversion including 100%, NCS is the oxidant of
choice. Indeed, by the use of a given amount of NCS, the
homo-coupling reaction not only was suspended at the expect-
ed conversion (Scheme 4 and see Experimental) but also was
succeeded by the second reaction, e.g., cross-coupling, in one
pot (Scheme 5 and see Experimental).
In conclusion, two oxidants, NCS and oxygen, are disclosed
to be efficient for the homo-coupling of arylzinc compounds in
the presence of a catalytic amount of Pd; such a result disclos-
es a new synthetic method of biaryls from aryl halides or are-
nes via arylzinc intermediates.
Experimental
General. Melting points were determined on a Yanaco
MP-3 micro melting point apparatus and are uncorrected.
GLC analyses were carried out with a Hitachi G-3000 with
flame ionization detectors equipped with an OV-17 column us-
ing nitrogen as carrier gas. The GLC yields were determined
using appropriate aromatics as internal standards. The isola-
tion of pure products was carried out with column chromatog-
raphy on SiO2. IR data were obtained with a Hitachi 260-10
spectrophotometer. 1H and 13C NMR were recorded with a
Jeol FX90A spectrometer and a Varian VXR200 spectrometer
using CDCl3 as a solvent with tetramethylsilane used as an in-
ternal standard. Elemental analyses were performed on a Per-
kin-Elmer 2400 Series ꢁ.
Materials. TMU was distilled under N2 and stored over
Molecular Sieve 2A. Arylzinc compounds were prepared by
the reaction of aryl iodides with zinc powder in TMU or DMF,
following the reported procedure9 or the reaction of aryllithi-
um compounds11 with ZnI2 or ZnCl2 in THF. The aliquot of
the solutions were used in the homo-coupling reaction, after
the concentration of arylzinc compounds was determined by
quenching the aliquot of the solution with iodine, followed by
the GLC analysis of the amount of aryl iodides formed. The
other chemicals were commercial products and were used
without purification.
Reaction of 3-Methoxycarbonylphenylzinc Iodide.
A
typical procedure using arylzinc compounds prepared from
aryl iodides and zinc powder: To the mixture of [Pd(PPh3)4]
(0.005 mmol) and NCS (0.13 mmol), 0.89 mL of TMU-solu-
tion of 3-methoxycarbonylphenylzinc iodide (0.25 mmol) was
added under nitrogen. The resulting solution was stirred at
room temperature for 30 min. and then at 45 °C for 16 h. After
the usual post-treatment of the resulting solution with ether/aq
HCl, purification by chromatography on silica-gel column us-
ing hexane–ethyl acetate (9:1) as an eluent, followed by the re-
crystalization from EtOH, afforded 29 mg of dimethyl biphe-
nyl-3,3ꢀ-dicarboxylate(86%): mp 104 °C (Ref. 16, mp 103–
104 °C); IR (CDCl3) 1732 cm−1; 1H NMR δ 3.96 (s, 6H), 7.54
(t, J = 7.8 Hz, 2H), 7.82 (dt, J = 7.8 , 1.6 Hz, 2H), 8.06 (dt, J
= 7.8, 1.6 Hz, 2H), 8.31 (t, J = 1.6 Hz, 2H).
Scheme 4. Homo-coupling using a limited amount of NCS.
5,5ꢀ-Dimethylbiphenyl-2,2ꢀ-diyl Diacetate: mp 90.5–91
°C; IR (CDCl3) 1760, 1370, 1221, 1204 cm−1; 1H NMR δ 2.0
(s, 6H), 2.4 (s, 6H), 6.9–7.2 (m, 6H); 13C NMR δ 20.7, 20.8,
130.9, 137.8, 137.9, 143.6, 144.0, 156.0, 169.4. Anal. Calcd
for C18H18O4: C, 72.5; H, 6.0%. Found: C, 72.35; H, 6.20%.
Reaction of 2-Thienylzinc Iodide. A typical procedure
using arylzinc compounds prepared from aryllithium com-
pounds and zinc salts: ZnI2 (1.0 mmol) was dried by air-gun
heating for 5 min under vacuum, then was dissolved in THF (1
mL). The solution was added by cannula into a solution of 2-
thienyllithium, which was prepared by mixing a THF solution
(1 mL) of thiophene (1.0 mmol) and 0.65 mL of 1.54 M hex-
ane solution of butyllithium (1.0 mmol) at 0 °C for 3 h. The
Scheme 5. Consecutive homo- and cross-coupling in one
pot.

K. M. Hossain et al.
Bull. Chem. Soc. Jpn., 74, No. 12 (2001) 2419
Chem., 63, 5497 (1998); X-C. Li, H. Sirringhaus, F. Garnier, A. B.
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resulting solution was warmed gradually to room temperature,
followed by the addition to the THF suspension (0.5 mL) of
NCS (0.45 mmol) and [PdCl2(PPh3)2] (0.019 mmol) by the use
of cannula. Then, the reaction solution was stirred at the tem-
perature for 15 h. After the usual post-treatment of the result-
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mg of 2,2ꢀ-bithiophene (90%). mp 30 °C (Ref. 17, mp 30 °C);
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1
IR (CDCl3) 830 cm−1; H NMR δ 6.94 (dd, J = 5.0, 3.6 Hz,
2H), 7.1–7.2 (m, 4H).
The Progress with Time of Reaction of 2-Methoxycar-
bonylphenylzinc Iodide (1). To the mixture of [Pd(PPh3)4]
(0.005 mmol) and NCS (0.13 mmol), 0.89 mL of TMU-solu-
tion of 1 (0.25 mmol) was added under nitrogen. The resulting
solution was stirred at room temperature for 5 min and then at
45 °C for 0, 2, 4, 6, 8, 10, or 12 h. After 0.024 mL of octylben-
zene was added to the solution, the aliquot was withdrawn and
treated with aq HCl to measure the amounts of 2 and 3 or with
I2 to measure the amount of 1 in the solution by GLC analysis.
Homo-Coupling Using Limited Amount of NCS. To the
mixture of [Pd(PPh3)4] (0.006 mmol) and NCS (0.10 mmol),
0.31 mL of TMU-solution of 4-bromophenylzinc iodide (0.30
mmol) was added under nitrogen and the solution was stirred
at room temperature for 4 h. The GLC analysis showed that
0.10 mmol of 4,4ꢀ-dibromobiphenyl (63% based on arylzinc
compound; 95% based on NCS) and 0.11 mmol of remaining
arylzinc compound (36%) was present in the resulting solu-
tion.
Consecutive Reactions Using Limited Amount of NCS.
To the mixture of [Pd(PPh3)4] (0.016 mmol) and NCS (0.10
mmol), 0.51 mL of TMU-solution of 4-ethoxycarbonylphen-
ylzinc iodide (0.80 mmol) was added under nitrogen and the
solution was stirred at 45 °C for 5 h. GLC analysis using the
aliquot of the solution showed that 0.09 mmol of diethyl bi-
phenyl-4,4ꢀ-dicarboxylate (22% based on arylzinc compound;
90% based on NCS) was present in the resulting solution, to
which 0.20 mL of TMU solution of 1,4-diiodobenzene (0.10
mmol) was added; this was stirred at 45 °C for 10 h. GLC
analysis showed that 0.09 mmol of diethyl 1,1ꢀ:4ꢀ,1ꢂ-terphen-
yl-4,4ꢂ-dicarboxylate (21% based on arylzinc compound) and
0.22 mmol of remaining arylzinc compound (28%) were
present in the resulting solution along with diethyl biphenyl-
4,4ꢀ-dicarboxylate.
3
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B. Deacon, Synlett, 1997, 131; F. Babudri, A. R. Cicciomessere,
G. M. Farinola, V. Fiandanese, G. Marchese, R. Musio, F. Naso,
and O. Sciacorelli, J. Org. Chem, 62, 3291 (1997); T. Ohe, T.
Tanaka, M. Kuroda, C. S. Cho, K. Ohe, and S. Uemura, Bull.
Chem. Soc. Jpn., 72, 1851 (1999).
Diethyl Biphenyl-4,4ꢀ-dicarboxylate: mp 114 °C (Ref.
18, 144°C); IR (CDCl3) 1720 cm−1; 1H NMR δ 1.42 (t, J = 7.2
Hz, 6H), 4.41 (q, J = 7.2 Hz, 4H), 7.69 (d, J = 8.4 Hz, 4H),
8.14 (d, J = 8.4 Hz, 4H).
Diethyl 1,1ꢀ:4ꢀ,1ꢁ-Terphenyl-4,4ꢁ-dicarboxylate: mp
1
260 °C (Ref. 19, mp 259.8 °C); IR (CDCl3) 1709 cm−1; H
NMR δ 1.42 (t, J = 7.1 Hz, 6H), 4.42 (q, J = 7.1 Hz, 4H), 7.71
(d, J = 8.2 Hz, 8H), 7.74 (s, 4H), 8.14 (d, J = 8.2 Hz, 8H).
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