Communication
the oxidizing ability of the quinonediimine. N,N’-Diphenyl-p-
benzoquinonediimine (2) is a redox-active unit of polyaniline,
which is one of the most famous conductive and redox-active
on the oxidant in the homocoupling of phenylmagnesium bro-
mide (3). To the THF solution of 2 (60 mol% to 3), compound
3 was added dropwise at room temperature, and the orange
color was immediately changed to dark greenish black. After
work-up, the mixture was purified by silica-gel column chroma-
tography. In the chromatographic analysis, the desired biphen-
yl (4) was eluted with dichloromethane/hexane (quantitative,
Table 1, entry 1), and then the reduced form 2-red was eluted
with ethyl acetate/hexane. In this way, the purification of 4
and recovery of 2-red is quite easy. Because 2-red can be cata-
lytically oxidized to give 2 in a good yield as described above,
p-benzoquinonedimine 2 is formally recyclable. On the other
hand, use of diphenoquinone 1 as an oxidant resulted in some
products, in which the yield for 4 is only 14% (Table 1, entry 2).
Stronger oxidant, such as 2,3,5,6-tetrachloro-p-benzoquinone
(chloranil) and 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ),
also gave the complex mixture (Table 1, entries 3 and 4). N,N’-
[
5]
p-conjugated polymers. The quinonediimine moiety can
accept two electrons and two protons to give the reduced
form, N,N’-diphenyl-p-phenylenediamine (2-red; Scheme 1). To
date, we have demonstrated the catalytic aerobic oxidative
coupling reaction of 2,6-di-tert-butylphenol by using quinone-
[
6]
diimine compounds as an organic catalyst. Similarly, polyani-
line exhibits the efficiency for the catalytic aerobic dehydro-
[
7]
genation of amines. We have also developed the hybrid
redox catalyst with transition metals and metal nanoparticles
based on the redox function of polyanilines and p-benzoquino-
[
8]
nediimine derivatives. Herein, we report the p-benzoquinone-
diimine-induced oxidative homocoupling of aryl- and vinyl-
magnesium reagents. Notably, this p-benzoquinonediimine oxi-
dant 2 exhibited superior activity for typical Grignard reagents
than diphenoquinone 1.
[10]
Diphenyl-(2,3,5,6-tetramethylbenzoquinone) 1,4-diimine (5)
p-Benzoquinonediimine 2 was prepared from the commer-
induced the homocoupling in a moderate yield (58% yield,
Table 1, entry 5) without any by-products (except benzene).
Neither the desired reaction nor any side reactions proceeded
II
cially available 2-red by Cu -catalyzed aerobic oxidation in
[
9]
a high yield (see the Supporting Information). This reaction is
easy to operate and scalable. Pure 2 can be easily obtained by
recrystallization as orange crystals. Thus-prepared 2 was used
for the following investigation. Table 1 shows the dependence
[11]
well in the case of bis-dimethylamino derivative 6 and an-
[12]
thraquinonediimine 7 (Table 1, entries 6 and 7). First reduc-
tion potential of some oxidants, which was obtained based on
differential pulse voltammetry measurement, is also shown in
Table 1. The trend of the reactivity might reflect the reduction
potential of quinonediimine-type oxidants. Therefore, these re-
sults suggest that the reaction might be controlled depending
on the redox potential tuned by the substituent.
Table 1. Oxidative homocoupling of phenylmagnesium bromide (3) by
using various oxidants.
Some phenylmagnesium derivatives and phenyl-lithium
were investigated in Table 2. Phenylmagnesium chloride homo-
coupled in 98% yield (Table 2, entry 1). Diphenylmagnesium
was also quantitatively transformed to 4 (Table 2, entry 2).
[
b,c]
Entry
Oxidant
first reduction potential [V] vs. Fc/Fc )
Yield [%]
+
[a]
(
À
+
However, the reaction of the ate complex [Ph Mg Li ·LiBr]
3
[
d]
1
2
2
100 (100)
14
(
1
À1.50)
(
À1.09)
Table 2. Oxidative homocoupling of phenylmagnesium derivatives and
phenyl-lithium by using 2 as an oxidant.
3
4
chloranil
DDQ
9
trace
5
6
58
2
[a,b]
Entry
Yield [%]
98
(
(
À1.69)
À1.66)
1
2
3
4
[d]
100
[f]
3
6
[
d]
7
6
1
[
a] Yield determined by H NMR analysis by using 1,3,5-trimethoxyben-
zene as an internal standard. [b] Calculated as follows: (mole of 4)ꢁ2/
mole of PhMg)ꢁ100. [c] Prepared by addition of dry dioxane to the THF
solution of 3. [d] The molar ratio of Ph Mg/2 is 1:1.2 in this entry. The
(
(
À1.87)
2
[
a] Reduction potential was obtained based on differential-pulse voltam-
metry measurement (0.5 mm in THF, [Bu NClO ]=0.1m, Pt electrode).
b] Yield determined by H NMR analysis by using 1,3,5-trimethoxyben-
zene as an internal standard. [c] Calculated as follows: (mole of 4)ꢁ2/
mole of 3)ꢁ100. [d] Isolated yield.
yield was calculated as follows: (mole of 4)/(mole of Ph Mg)ꢁ100. [e] Pre-
2
4
4
pared by addition of two equivalents of phenyl-lithium to the THF solu-
1
À
+
[
tion of 3. [f] The molar ratio of Ph Mg Li ·LiBr/2 is 1:1.8 in this entry. The
3
À
+
yield was calculated as follows: (mole of 4)/[(mole of Ph Mg Li ·LiBr)ꢁ
3
(
1.5]ꢁ100.
Chem. Eur. J. 2014, 20, 653 – 656
654
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim