Electron-Catalyzed Cross-Coupling of Arylboron Compounds with Aryl Iodides

Article abstract of DOI:10.1002/anie.201802813

Arylboroxines in combination with zinc chloride and potassium tert-butoxide were found to undergo the electron-catalyzed cross-coupling with aryl iodides to give the corresponding biaryls without the aid of transition-metal catalysis.

Full text of DOI:10.1002/anie.201802813

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Electron-Catalyzed Cross-Coupling Reaction of Arylboron
Compounds with Aryl Iodides
Authors: Keisho Okura, Tsuyoshi Teranishi, Yuto Yoshida, and Eiji
Shirakawa
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201802813
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10.1002/anie.201802813
Angewandte Chemie International Edition
COMMUNICATION
Electron-Catalyzed Cross-Coupling Reaction of Arylboron Compounds
with Aryl Iodides**
Keisho Okura, Tsuyoshi Teranishi, Yuto Yoshida, and Eiji Shirakawa*
Abstract: Arylboroxines in combination with zinc chloride and potassium
tert-butoxide were found to undergo the electron-catalyzed cross-coupling
reaction with aryl iodides to give the corresponding biaryls with no aid of
transition metal catalysis.
Table 1: Effect of the amounts of diethylzinc and lithium chloride in
the coupling of phenylboronic acid (1a) with 4-iodoanisole (2a).[a]
I
OMe
2a
LiCl
PhB(OH)2
+
Et2Zn
Ph
OMe
THF
23 °C, 1 h
toluene/THF (1:3)
110 °C, 24 h
1a
3aa
(2.0 equiv)
The transition metal-catalyzed cross-coupling reaction of
arylmetals with aryl halides is one of the most useful and powerful
methods for biaryl synthesis.[1] Among these, the Suzuki–Miyaura
coupling reaction, employing arylboron compounds as arylmetals,
is one of the most widely used ones due to the high stability and
high availability of the reagents and the high functional group
tolerance of the reaction.[2] On the other hand, since the finding by
our group on the first electron-catalyzed cross-coupling reaction,
where aryl Grignard reagents are employed and an electron derived
from them instead of a transition metal acts as a catalyst,[3] we and
other groups have reported the transition metal-free cross-coupling
reaction of arylmetals (Zn[4]/Al[5]/Sn[6]) as well as alkyl-[7] and
alkynylzinc[8] reagents.[9,10] Here we report that arylboron
compounds are utilized as aryl nucleophiles, for the first time, in the
electron-catalyzed cross-coupling reaction with the aid of a zinc
reagent such as diethylzinc and zinc chloride/potassium tert-
butoxide.[11]
Entry
Et2Zn
LiCl
Conv. of
Yield of
(equiv)
(equiv)
2a (%)[b]
3aa (%)[b]
1
2
3
4
6
0
9
22
<1
1.5
1.5
0
0
19
95 (93)[c]
1.5
1.5
>99
3
<1
[a] A toluene solution of diethylzinc (1.2 mmol for entry 1; 0.30 mmol for
entries 2 and 3) was added to phenylboronic acid (1a: 0.40 mmol) in THF
in the presence or absence of lithium chloride (0.30 mmol) under a
nitrogen atmosphere. After stirring the resulting mixture at 23 °C for 1 h,
addition of 4-iodoanisole (2a: 0.20 mmol, 0.22 M) was followed by stirring
at 110 °C for 24 h. [b] Determined by GC based on 2a. [c] Yield of the
isolated product based on 2a.
The
electron-catalyzed
cross-coupling
reaction
of
Many reports are available on preparation of arylzinc reagents
from arylboronic acids by treating with an excess amount of
diethylzinc.[12,13] Then we anticipated that thus prepared arylzinc
reagents undergo the electron-catalyzed cross-coupling reaction.
However, treatment of phenylboronic acid (1a: 2 equiv) with Et2Zn
(6 equiv) in THF at 23 °C for 1 h followed by the reaction with 4-
iodoanisole (2a) in toluene/THF (1:3) at 110 °C for 24 h did not
give the corresponding coupling product, 4-methoxybiphenyl (3aa),
at all with a low conversion of 2a (Table 1, entry 1). In contrast,
rather surprisingly, use of a much decreased amount (1.5 equiv) of
Et2Zn promoted the coupling to some extent (19% yield of 3aa with
22% conv. of 2a) (Table 1, entry 2). Addition of LiCl (1.5 equiv),
an effective accelerator in the previous electron-catalyzed cross-
coupling reactions,[3c,4b,6] led to a full conversion of 2a to give 3aa
in a high yield (Table 1, entry 3). No reaction took place in the
absence of diethylzinc (Table 1, entry 4).
phenylboronic acid (1a) by use of Et2Zn (1.5 equiv) was found to
be applicable to iodobenzene having an alkyl or ester substituent
(Scheme 1, top). However, the coupling reaction was found to show
a low reproducibility, depending on arylboronic acids used. For
example, the reaction of 4-methoxyphenylboronic acid (1b) with
ethyl 4-iodobenzoate (2c) resulted in a low yield with a low
conversion (Scheme 1, top). Analysis of the purity of 1b used here
showed that it contains a certain amount of the corresponding
boroxine (boronic acid/boroxine = 63/37),[14] prompting us to
consider that arylboroxines are unreactive in this coupling. Actually,
the boronic acid/boroxine ratio with a phenylboron system was
found to affect the conversion and the yield, where genuine
phenylboroxine have an extremely low reactivity (Scheme 1,
bottom). The property of arylboronic acids to be transformed into
the corresponding arylboroxines by spontaneous dehydoration
potentially damages the reproducibility of this method.
I–Ar2 (2)
LiCl (1.5 equiv)
Ar1B(OH)2
+
Ar1–Ar2
Et2Zn
THF
23 °C, 1 h
toluene/THF (1:3)
110 °C, 24 h
1
(2.0 equiv)
3
(1.5 equiv)
[*]
K. Okura, Y. Yoshida
Department of Chemistry, Graduate School of Science
Kyoto University
Me
CO2Et
CO2Et
MeO
Sakyo-ku, Kyoto 606-8502 (Japan)
T. Teranishi, Prof. Dr. E. Shirakawa
Department of Applied Chemistry for Environment
School of Science and Technology
3bc
18% yield (22% conv.)
by use of ArB(OH)2/(ArBO)3 (63/37)
3ab
93% yield
3ac
95% yield
conv. of
2a (%)
yield of
3aa (%)
Kwansei Gakuin University
Sanda, Hyogo 669-1337 (Japan)
E-mail: eshirakawa@kwansei.ac.jp
This work has been supported financially in part by Grant-in-Aid for
Scientific Research (B) (25288046 to E.S.) from JSPS and Nagase
Science and Technology Foundation (E.S.).
PhB(OH)2/(PhBO)3
OMe
93/7
79/21
1/>99
>99%
57%
13%
95%
54%
7%
[**]
3aa
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
Scheme 1. Coupling of arylboronic acids with aryl iodides using
diethylzinc as an activator.
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10.1002/anie.201802813
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COMMUNICATION
In order to develop a more reliable cross-coupling system using
arylboron compounds, we investigated what kind of species is
generated at the starting point of the cross-coupling reaction by
analyzing the mixture (A in Scheme 2) after treatment of
phenylboronic acid (1a) with Et2Zn (0.75 equiv) in the presence of
lithium chloride (0.75 equiv) in toluene-d8–THF-d8 (1:2) at 23 °C
As we expected, arylboroxine-derived species underwent the
coupling reaction, though some modification was required. After
stirring a mixture of phenylboroxine (4a: 1.5 equiv), ZnCl2 (1.5
equiv) and KOt-Bu (3.0 equiv) in THF at 23 °C for 1 h, the
resulting mixture was treated with 4-iodoanisole (2a) at 110 °C for
24 h to give 4-methoxybiphenyl (3aa) in 15% yield with 19%
for 1 h (cf. Table 1, entry 3). In the 1H NMR spectrum of mixture A, conversion of 2a (Table 2, entry 1). An increased amount (2.4
no peaks of Et–B and Et–Zn but Et–H were observed as those
derived from the ethyl group of Et2Zn (Scheme 2, top), showing
that no Zn–B transmetalation takes place at this point, where each
Et–Zn unit of Et2Zn works as a base to abstract protons from the
hydroxyl groups of PhB(OH)2 to give arylboron species I having a
Ph–B–O–Zn sequence (Scheme 2, top right). The observation that
the C–B bond of 1a is not cleaved at this stage was further
confirmed by the outcome that the phenylboron unit was trapped as
the corresponding boronate in 84% yield upon treatment of a
mixture, prepared in the same way as above (mixture A) but in non-
deuterated solvents, with 2,2-dimethyl-1,3-propanediol (3 equiv)
and ammonium chloride (1.5 equiv) followed by MgSO4 (Scheme 2,
bottom). However, after treatment of 1a with Et2Zn and LiCl for 1
h at 110 °C instead of 23 °C, the phenylboron unit remained only in
20%, showing that, under the cross-coupling conditions at a high
temperature, the phenylboron species is converted to some others,
which are likely to be the real active species for the cross-coupling
reaction. We anticipated that, if we prepare a species of type I
having a Ph–B–O–Zn sequence in whatever way, it should be
converted at a high temperature into some active species that can
undergo the coupling with aryl iodides. We expected that treatment
of arylboroxines with ZnCl2 and KOt-Bu as shown in Scheme 3
gives species II that is suitable for this purpose. This method will
be advantageous over the previous one because no organometallic
reagents such as Et2Zn are required and the boronic acid/boroxine
equilibrium does not matter under anhydrous conditions.
equiv) of ZnCl2 enhanced the reactivity, where reduction of the
amount (1.8 equiv) of KOt-Bu did not affect the yield (Table 2,
entries 2 and 3). Use of N-methylpyrrolidone (NMP) as a solvent
instead of THF drastically improved the conversion and the yield
(Table 2, entry 4), where reduction of the amounts of the reagents
keeping the ratio among these (4a/ZnCl2/KOt-Bu = 1.2/2.1/1.5) also
worked (Table 2, entry 5). Use of a weaker base was ineffective
(Table 2, entries 6 and 7).[15] No coupling product was obtained in
the absence of ZnCl2, where anisole was the major product (60%
yield) (Table 2, entry 8).[16]
Table 2: Effect of the amounts of zinc chloride and potassium tert-
butoxide in the coupling of phenylboroxine with 4-iodoanisole.[a]
Entry 4a
ZnCl2 Base
Solvent Conv. of Yield of
(equiv) (equiv) (equiv)
2a (%)[b] 3aa (%)[b]
1
2
3
4
5
6
7
8
1.5
1.5
1.5
1.5
1.2
1.2
1.2
1.2
1.5
2.4
2.4
2.4
2.1
2.1
2.1
0
KOt-Bu (3.0)
KOt-Bu (3.0)
KOt-Bu (1.8)
KOt-Bu (1.8)
KOt-Bu (1.5)
K3PO4 (1.5)
K2CO3 (1.5)
KOt-Bu (1.5)
THF
THF
THF
NMP
NMP
NMP
NMP
NMP
19
46
42
>99
>99
3
15
38
40
99
98
1
8
2
98
<1
OH
OH
O
Zn
LiCl (0.75 equiv)
+
mixture A
[a] To a mixture of phenylboroxine (4a), zinc chloride and potassium tert-
butoxide was added THF (1.0 mL for entries 1–3) or NMP (1.0 mL for
entries 4–8). After stirring the resulting mixture at 23 °C for 1 h, addition
of 4-iodoanisole (2a: 0.50 mmol, 0.50 M) was followed by stirring at
110 °C for 24 h. [b] Determined by GC.
Et2Zn
Ph
B
Ph
B
toluene-d8–THF-d8 (1/2)
23 °C, 1 h
O
Y
(0.75 equiv)
1a
No Et–B or Et–Zn units
Et–H δ 0.82 (s)
Y = Zn, B, Li, H
δ 1.24 (t, J = 8.2 Hz)
δ 0.12 (q, J = 8.2 Hz)
plausible structure I
HO
OH
(3 equiv)
Et2Zn (0.75 equiv)
LiCl (0.75 equiv)
The coupling of arylboroxines with aryl iodides using the
combination of ZnCl2 and KOt-Bu showed a wide scope (Table 3).
Phenyl iodides having an electron-donating or -withdrawing group
at the para- or meta-position underwent coupling with
phenylboroxine (4a) in high yields (Table 3, entries 1–6). ortho-
Substituted phenyl iodides also reacted with 4a (Table 3, entries 7
and 8). The coupling is applicable also to heteroaryl iodides (Table
3, entries 9–11). As for arylboroxines, both electron-donating and -
withdrawing substituents at para-position are tolerant (Table 3,
entries 12–18). In addition to arylboroxines, heteroarylboroxines
underwent coupling (Table 3, entries 19 and 20).
O
NH4Cl (1.5 equiv)
23 °C, 10 h
MgSO4
Ph
23 °C, 12 h
1a
B
toluene–THF(1/2)
temperature, 1 h
O
temperature
yield
23 °C
110 °C
84%
20%
Scheme 2. Elucidation of the species generated by mixing an
arylboronic acid and diethylzinc.
Ph
O
O
B
B
O
ZnOt-Bu
t-Bu
+
+
ZnCl2
2 KOt-Bu
1/3 Ph
B
O
As described above, some active species are considered to be
generated at a high temperature from (Ar1BO)3/ZnCl2/KOt-Bu via
arylboron species having a Ar1–B–O–Zn sequence such as II. Such
an active species, designated as Ar1–Y here, is likely to undergo
coupling with an aryl iodide (I–Ar2) through the same mechanism
as the previous electron-catalyzed cross-coupling reaction of aryl
magnesium and -zinc reagents as shown in Scheme 4.[3,4b] Namely,
Ph
B
O
expected structure II
Ph
4a
– 2 KCl
O
+
Zn(Ot-Bu)2
Ph
B
single electron transfer (SET) from Ar1–Y to I–Ar2 gives [I–Ar2] • –
,
Scheme 3. Working hypothesis to generate arylboron species ready
which reacts with Ar1–Y to give [Ar1–Ar2] • –. SET from [Ar1–Ar2] •
for the cross-coupling.
–
• – [17,18,19]
to I–Ar2 gives Ar1–Ar2 and regenerates [I–Ar2]
.
The
involvement of the anion radical intermediates is strongly supported
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10.1002/anie.201802813
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COMMUNICATION
Table 3: Coupling of arylboroxines with aryl iodides using zinc
chloride and potassium tert-butoxide as activators.[a]
Entry Ar1 in 4
Ar2 in 2
Time Yield Prod.
(h)
(%)[b]
1
2
3
4
5
6
Ph (4a)
Ph (4a)
Ph (4a)
Ph (4a)
Ph (4a)
Ph (4a)
4-MeOC6H4 (2a)
4-MeC6H4 (2b)
24
48
24
24
48
24
48
24
48
24
95
87
83
91
93
92
92
92
81
93
90
87
96
92
88
90
95
86
91
97
3aa
3ab
3ac
3ad
3ae
3af
4-EtO2CC6H4 (2c)
4-CF3C6H4 (2d)
4-ClC6H4 (2e)
3-EtO2CC6H4 (2f)
2-PhC6H4 (2g)
7[c] Ph (4a)
8[c] Ph (4a)
9[c] Ph (4a)
10[c] Ph (4a)
11[c] Ph (4a)
3ag
3ah
3ai
2-(3-butenyl)C6H4 (2h)
6-quinolyl (2i)
Scheme 5.
Verification of the involvement of anion radical
intermediates.
2-dibenzothiophenyl (2j)
3aj
3-(9-phenyl)carbazolyl (2k) 24
3ak
3bc
3bd
3ca
3cc
3dl
12
13
14
15
16
4-MeOC6H4 (4b) 4-EtO2CC6H4 (2c)
4-MeOC6H4 (4b) 4-CF3C6H4 (2d)
24
consisting of SET to 2m and the reaction of the resulting anion
radical (III) with Ph–Y, undergoes reaction with another Ph–Y,
leading to 3’am, faster than passes an electron to another 2m,
leading to 3am.[20] The possibility that 4-bromobiphenyl (3am)
possesses an extraordinary reactivity and thus production of 3’am
is predominated was excluded by the result that the coupling of
3am with 4a under the same conditions did not give 3’am at all
even after 24 h (Scheme 5, eq. 2).
24
48
24
48
48
24
48
24
4-CF3C6H4 (4c) 4-MeOC6H4 (2a)
4-CF3C6H4 (4c) 4-EtO2CC6H4 (2c)
4-ClC6H4 (4d)
Ph (2l)
17[c] 4-ClC6H4 (4d)
18[c] 4-ClC6H4 (4d)
19[d] 2-thiophenyl (4e) 4-EtO2CC6H4 (2c)
4-MeOC6H4 (2a)
3da
3dc
3ec
3fc
4-EtO2CC6H4 (2c)
In conclusion, we have developed the electron-catalyzed cross-
coupling reaction of arylboroxines with aryl iodides by use of zinc
chloride and potassium tert-butoxide as activators, where use of a
transition metal and even an organometal is not required.
20 3-furyl (4f) 4-EtO2CC6H4 (2c)
[a] To a mixture of an arylboroxine (4: 1.2 equiv), zinc chloride (2.1 equiv)
and potassium tert-butoxide (1.5 equiv) was added NMP (1.0 mL). After
stirring the resulting mixture at 23 °C for 1 h, addition of an aryl iodide (2:
0.50 mmol, 0.50 M) was followed by stirring at 110 °C for 24 or 48 h. [b]
Yield of the isolated product based on 2. [c] An arylboroxine (4: 1.5 equiv),
zinc chloride (2.4 equiv) and potassium tert-butoxide (1.8 equiv) were
used. [d] At 140 °C.
Received: ((will be filled in by the editorial staff))
Published online on ((will be filled in by the editorial staff))
Keywords: electron catalysis · cross-coupling · arylboron
compounds · biaryls • radical reaction
by the experiments shown in Scheme 5. The reaction of
phenylboroxine (4a) with 4-bromo(iodo)benzene (2m) using ZnCl2
and KOt-Bu (4a/ZnCl2/KOt-Bu = 1.2/2.1/1.5) in a short reaction
period (30 min) gave p-terphenyl (3’am), the bisphenylation
product of 2m, in preference to 4-bromobiphenyl (3am), a
monophenylation product of 2m (Scheme 5, eq. 1). The result is
rationally understood as follows: anion radical IV of 4-
bromobiphenyl, generated from 2m through the usual pathway
[1]
For reviews, see: a) Metal-Catalyzed Cross-Coupling Reactions and
more, Vol. 1–2, 2nd ed. (Eds.: A. de Mijere, S. Bräse, M. Oestreich),
Wiley-VCH, Weinheim, 2014; b) J.-P. Corbet, G. Mignani, Chem.
Rev. 2006, 106, 2651–2710.
[2]
[3]
For reviews, see: a) N. Miyaura, A. Suzuki, Chem. Rev. 1995, 95,
2457–2483; b) F.-S. Han, Chem. Soc. Rev. 2013, 42, 5270–5298.
a) E. Shirakawa, Y. Hayashi, K. Itoh, R. Watabe, N. Uchiyama, W.
Konagaya, S. Masui, T. Hayashi, Angew. Chem. 2012, 124, 222–225;
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Shirakawa, T. Hayashi, Chem. Commun. 2013, 49, 364–366; c) E.
Shirakawa, K. Okura, N. Uchiyama, T. Murakami, T. Hayashi, Chem.
Lett. 2014, 43, 922–924. For the coupling with alkenyl halides, see:
d) E. Shirakawa, R. Watabe, T. Murakami, T. Hayashi, Chem.
Commun. 2013, 49, 5219–5221.
[4]
a) H. Minami, X. Wang, C. Wang, M. Uchiyama, Eur. J. Org. Chem.
2013, 7891–7894; b) E. Shirakawa, F. Tamakuni, E. Kusano, N.
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[5]
[6]
Scheme 4. A plausible mechanism.
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10.1002/anie.201802813
Angewandte Chemie International Edition
COMMUNICATION
[7]
[8]
K. Okura, E. Shirakawa, Eur. J. Org. Chem. 2016, 3043–3046.
K. Okura, H. Kawashima, F. Tamakuni, N. Nishida, E. Shirakawa,
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contained in the reagents because K3PO4 and K2CO3 are known to be
effective bases for the palladium-catalyzed Suzuki–Miyaura coupling
reaction. For a recent example of the Suzuki–Miyaura coupling that
was initially considered to be transition metal-free but later was
revealed to be catalyzed by a trace amount of palladium contained in
the reagents, see: a) K. Inamoto, C. Hasegawa, K. Hiroya, Y. Kondo,
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2012, 53, 3147–3148.
[9]
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[16] An enolate derived from NMP and KOt-Bu is known to work as a
single electron donor toward aryl halides in the reduction of aryl
halides into arenes. S. Zhou, E. Doni, G. M. Anderson, R. G. Kane, S.
W. MacDougall, V. M. Ironmonger, T. Tuttle, J. A. Murphy, J. Am.
Chem. Soc. 2014, 136, 17818–17826.
[17] No involvement of aryl radical species is supported by a radical clock
experiment using 2-(3-butenyl)phenyl iodide (2h), which is often
used for a radical clock reaction and the radical derived from which is
known to readily cyclize (kc = 5 x 10–8 s–1 at 50 °C). The coupling of
2h did not give any cyclization product but the coupling product in a
high yield (Table 3, entry 8). For radical clock reactions using 2h,
see: a) A. L. J. Beckwith, W. B. Gara, J. Am. Chem. Soc. 1969, 91,
5691–5692; b) H.-X. Zheng, X.-H. Shan, J.-P. Qu, Y.-B. Kang, Org.
Lett. 2017, 19, 5114–5117. See also Refs. [3b] and [4b].
[18] A DFT calculation study on the coupling of aryl Grignard reagents
with aryl iodides (cf. Ref. [3a]) is reported to show that the coupling
proceeds through an aryl radical intermediate derived from an aryl
iodide in a unique situation, where the aryl radical does not behave as
a usual σ-radical species. B. E. Haines, O. Wiest, J. Org. Chem. 2014,
79, 2771–2774.
[10] For reviews on "electron catalysis", see: a) A. Studer, D. P. Curran,
Nat. Chem. 2014, 6, 765–773; b) A. Studer, D. P. Curran, Angew.
Chem. 2016, 128, 58–106; Angew. Chem. Int. Ed. 2016, 55, 58–102.
[11] The transition metal-free coupling of arylboronic acids with allylic,
propargylic and benzylic halides has been reported, where a non-
radical mechanism is proposed, see: a) A. Scrivanti, V. Beghetto, M.
Bertoldini, U. Matteoli, Eur. J. Org. Chem. 2012, 264–268; b) M.
Ueda, K. Nishimura, R. Kashima, I. Ryu, Synlett 2012, 23, 1085–
1089; c) M. Ueda, K. Nishimura, I. Ryu, Synlett 2013, 24, 1683–
1686; d) M. Ueda, D. Nakakoji, Y. Kuwahara, K. Nishimura, I. Ryu,
Tetrahedron Lett. 2016, 57, 4142–4144.
[19] Cation radical [Ar1–Y] • +, which is generated by SET in the initiation
step, reacts with another Ar1–Y to give [Ar1–Ar1] • –, from which SET
to I–Ar2 takes place to give Ar1–Ar1 and [I–Ar2] • –. Actually, a small
amount of Ar1–Ar1 derived from an arylboroxine (4) is observed in
each reaction. For example, the coupling reaction of phenylboroxine
(4a) with 4-iodoanisole (2a) gave biphenyl in 1.1% yield based on 2a
(Table 2, entry 5). See also Refs. [3b] and [4b].
[12] Aryl(ethyl)zinc reagents, prepared from arylboronic acids and
diethylzinc, have often been used for the asymmetric arylation of
aldehydes. For the first example, see: a) C. Bolm, J. Rudolph, J. Am.
Chem. Soc. 2002, 124, 14850–14851. For reviews, see: b) F. Schmidt,
R. T. Stemmler, J. Rudolph, C. Bolm, Chem. Soc. Rev. 2006, 35,
454–470; c) M. W. Paixão, A. L. Braga, D. S. Lüdtke, J. Braz. Chem.
Soc. 2008, 19, 813–830.
[20] Bromo- or chloro-substituted iodobenzenes are often used for the
probe of the involvement of radical anion intermediates in SRN
1
reaction, where bissubstitution predominantly takes place. a) J. F.
Bunnett, X. Creary, J. Org. Chem. 1974, 39, 3611–3612. b) J. F.
Bunnett, X. Creary, J. Org. Chem. 1974, 39, 3612–3614. For an
effective utilization of this system, see: c) B. Janhsen, C. G. Daniliuc,
A. Studer, Chem. Sci. 2017, 8, 3547–3553.
[13] For a mechanistic study on transmetalation between boron and zinc,
see: a) C. Jimeno, S. Sayalero, T. Fjermestad, G. Colet, F. Maseras,
M. A. Pericàs, Angew. Chem. 2008, 120, 1114–1117; Angew. Chem.
Int. Ed. 2008, 47, 1098–1101. Transmetalation between arylboronic
acids and diethylzinc is reported to give not only aryl(ethyl)zinc
species but also aryln(ethyl)4–nborates (n = 0–4). b) R. B. Bedford, N.
J. Gower, M. F. Haddow, J. N. Harvey, J. Nunn, R. A. Okopie, R. F.
Sankey, Angew. Chem. 2012, 124, 5531–5534; Angew. Chem. Int. Ed.
2012, 51, 5435–5438.
[14] Electron-rich arylboronic acids such as 1b are reported to have high
tendency to form the corresponding boroxines. Y. Tokunaga, H.
Ueno, Y. Shimomura, T. Seo, Heterocycles 2002, 57, 787–790.
[15] ZnCl2 (99.999% trace metals basis, Sigma-Aldrich Co., product
number 229997) and KOt-Bu (99.99% trace metals basis, Sigma-
Aldrich Co., product number 659878) were used in all entries in
Table 2. The reaction using ZnCl2 and KOt-Bu purchased from a
different supplier (Wako Pure Chemical Industries) under the
conditions of entry 5 scored a comparable yield (96%) of 3aa with a
full conversion of 2a. For the details, see Supporting Information. In
addition to this result, the outcome that no coupling took place by use
of K3PO4 or K2CO3 instead of KOt-Bu (entries 6 and 7 of Table 2)
supports to exclude the possibility that the present cross-coupling is
catalyzed by a trace amount of a transition metal such as palladium
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10.1002/anie.201802813
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Keisho Okura, Tsuyoshi Teranishi, Yuto
Yoshida and Eiji Shirakawa*
ZnCl2
KOt-Bu
R1
R1
R2
R2
+
BO
I
3
Page No. – Page No.
R1
Electron-Catalyzed Cross-Coupling
Reaction of Arylboron Compounds with
Aryl Iodides
O
ZnOt-Bu
B
via
Ot-Bu
An electron was found to catalyze the coupling of arylboron compounds with aryl
iodides with the aid of zinc chloride and potassium tert-butoxide to give the
corresponding biaryls. The transformation requires no transition metal catalysts.
This article is protected by copyright. All rights reserved.

10.1002/anie.201802813
Angewandte Chemie International Edition
COMMUNICATION
This article is protected by copyright. All rights reserved.