Synthesis and oxidizing ability of p-chloranil dimer

Article abstract of DOI:10.1246/cl.121244

A p-chloranil dimer ( pCh₂) has been synthesized. The first reduction potential of pCh₂ shifted to a more positive value than that observed for the p-chloranil monomer ( pCh₁) and was more negative than that for 2,3-dichloro-5,6-dicyano-p-benzoquinone (DDQ). As expected, pCh₂ oxidized 9,10-dihydroanthracene and α-tetralol to give anthracene and α-tetralone, respectively, more efficiently than pCh1 did. The advantages of pCh₂ were observed in oxidations of 2,4-di-tert-butylphenol and 2,6-di-tertbutyl- 4-methylphenol. Although further oxidation took place in DDQ oxidations and no reaction occurred in pCh1 oxidations, initial oxidation products were solely obtained in pCh₂ oxidation because of its moderate oxidizing ability.

Full text of DOI:10.1246/cl.121244

doi:10.1246/cl.121244
398
Published on the web March 19, 2013
Synthesis and Oxidizing Ability of p-Chloranil Dimer
Naoto Hayashi,* Hiroyuki Nakagawa, Yuko Sugiyama, Junro Yoshino, and Hiroyuki Higuchi
Graduate School of Science and Engineering, University of Toyama, Gofuku, Toyama 9308555
(Received December 14, 2012; CL-121244; E-mail: nhayashi@sci.u-toyama.ac.jp)
A p-chloranil dimer ( pCh2) has been synthesized. The first
generate hydrogen cyanide. In addition, the dimer structure may
reduction potential of pCh2 shifted to a more positive value than
that observed for the p-chloranil monomer ( pCh1) and was more
negative than that for 2,3-dichloro-5,6-dicyano-p-benzoquinone
(DDQ). As expected, pCh2 oxidized 9,10-dihydroanthracene
and ¡-tetralol to give anthracene and ¡-tetralone, respectively,
more efficiently than pCh1 did. The advantages of pCh2 were
observed in oxidations of 2,4-di-tert-butylphenol and 2,6-di-tert-
butyl-4-methylphenol. Although further oxidation took place in
DDQ oxidations and no reaction occurred in pCh1 oxidations,
initial oxidation products were solely obtained in pCh2
oxidation because of its moderate oxidizing ability.
give rise to higher oxidizing ability than that of pCh1. It is well
known that the oxidizing ability of a quinonoid compound
depends on its ability to withdraw an electron from the substrate
or to abstract a hydride ion.10 Both should be significant in pCh2
because the resulting radical anion 1 or anion 2 would be highly
stabilized by the electron-withdrawing character of the quino-
noid substituent (Figure 1). Previously, we reported a significant
positive shift of the reduction potentials of benzoquinone dimers
compared with those of their monomers.11 In the present article,
the synthesis and reduction potential of pCh2 and its behavior
as an oxidizing agent in some reactions are described and
compared with those of pCh1 and DDQ.
pCh2 was synthesized as shown in Figure 2. p-Dimethoxy-
benzene dimer 3 was perchlorinated12 using sulfuryl chloride in
the presence of diisopropylamine to give 4. As direct oxidation
using cerium(IV) ammonium nitrate (CAN) failed, 4 was
demethylated to hydroquinone dimer 5 using BBr3, and then
oxidized by CAN to afford 1 in good yield. The obtained pCh2
was identified by 13C NMR, MS, IR, and elemental analyses.13
Quinones are widely used as organic oxidizing agents.1
Typically, they are employed in the dehydrogenation of hydro-
aromatic compounds2 and chromanones,3 oxidation of allyl4 and
benzyl alcohols to aldehydes or ketones,5 oxidation of the
benzylic position to give alcohols,6 oxidative cyclization,7 and
oxidative coupling of phenolic compounds.8 Unique reactivity
has been seen in some of these reactions. Among various
quinonoid oxidizing agents, 2,3-dichloro-5,6-dicyano-p-benzo-
quinone (DDQ) and tetrachloro-p-benzoquinone (p-chloranil,
pCh1) are most frequently used. DDQ has a strong oxidizing
ability; however, it decomposes to generate highly toxic
hydrogen cyanide gas in the presence of water or alcohols. In
this respect, p-chloranil is advantageous because it produces no
hydrogen cyanide; however, its oxidizing ability is rather low.
The relative reaction rates of the dehydration of 1,2-dihydroan-
thracene by DDQ and p-chloranil have been reported to be
approximately 5500:1.9 We have designed a new oxidizing
agent, a p-chloranil dimer ( pCh2). Like pCh1, pCh2 does not
Table 1. Half-wave reduction potentials for pCh1, pCh2, and
DDQa
Compd
E1/V
E2/V
E3/V
pCh1
pCh2
DDQ
+0.03 (1e)
+0.13 (1e)
+0.54 (1e)
¹0.71 (1e)
¹0.11 (1e)
¹0.27 (1e)
¹1.08 (2e)
aElectrode; Pt (working), Pt (counter), and SCE (reference).
Supporting electrolyte; n-Bu4NClO4. Solvent; CH3CN. Scan
¹1
rate; 100 mV s
.
Cl
O
Cl
O
Cl
O
Cl
O
Cl
5
O
Cl 4' O
6
1
3'
+ H
+ e
Cl
HO Cl
Cl
Cl
Cl
Cl
5' Cl
2
2'
4
6'
O
3 Cl O 1' Cl
O
Cl
O
Cl
O
Cl
pCh2
2
1
Figure 1. Reduction and hydride abstraction of pCh2.
OCH3
Cl
OH
OCH3
OCH3
Cl
OCH3
Cl
Cl
Cl
Cl
Ce(NH4)2(NO3)6
SO2Cl2
BBr3
OCH3
OCH3
OH
OH
Cl
OCH3
Cl
pCh2
Cl
Cl
OH
CH3CN
(93%)
iPr2NH
(57%)
CHCl3
(84%)
Cl
Cl
OCH3
3
4
5
Figure 2. Synthesis of pCh2.
Chem. Lett. 2013, 42, 398-400
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399
Table 2. Oxidation using pCh1, pCh2, and DDQ as oxidizing agentsa
Oxidizing
agent
Conversion
/%
Products and Yields
/%
Entry
1
Substrate
Conditionb
pCh1
A
9
9,10-dihydroanthracene
9,10-dihydroanthracene
9,10-dihydroanthracene
anthracene (100)
anthracene (100)
anthracene (100)
2
3
pCh2
DDQ
A
A
38
69
OH
O
4
pCh1
B
2
¡-tetralol
¡-tetralol
¡-tetralol
¡-tetralone (100)
¡-tetralone (100)
¡-tetralone (100)
5
6
pCh2
DDQ
B
B
10
88
OH
7
pCh1
C
0
®
6
6
OH OH
OH
Cl
8
9
pCh2
C
C
36
7 (50)
10 (50)
O
OCH3
O
OCH3
6
DDQ
100
O
O
8 (33)
9 (67)
OH
10
pCh1
D
0
®
CH3
BHT
OH
11
12
13
BHT
BHT
BHT
pCh2
pCh2
DDQ
1
D
E
D
b
20
60
CH2OCH3
12 (100)
12 (100)
OH
100
CHO
11 (100)
aConversions and yields were based on the H NMR spectra. Reactions were performed by the literature methods except E.
A (ref 2b): the substrate:quinone = 1:1, in 1,4-dioxane, ambient temperature, 1 h; B (ref 9): the substrate:quinone = 1:1, in
methanol, ambient temperature, 30 min; C (ref 14): the substrate:quinone = 4:1, no solvent, 200-230 °C, 5 min; D (ref 8): the
substrate:quinone = 1:2, in methanol, ambient temperature, 24 h; E: the substrate:quinone = 1:2, in methanol, 45 °C, 24 h.
The reduction behavior of pCh2 was examined using cyclic
voltammetry (Table 1). pCh2 showed three reversible waves in
acetonitrile, which could be attributed to one-, one-, and two-
electron reductions. The first reduction potential (E1) of pCh2
shifted to a more positive value by 0.10 V compared with that
observed for pCh1, but it was more negative than that for
DDQ. Compared with 5,5¤-di-tert-butyl-2,2¤-bi-p-benzoquio-
none (quinone dimer), whose first reduction shifted to a more
positive value than that of 2,5-di-tert-butyl-p-benzoquinone
(monomer) by 0.36 V in CH2Cl2,11a the effect of dimerization of
the perchlorinated quinone was unexpectedly small. This could
be because of the large dihedral angle between the two quinone
Chem. Lett. 2013, 42, 398-400
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

400
planes, giving little electronic communication between the ³
systems. In fact, B3LYP/6-31G(d,p) calculations revealed that
the dihedral angle was 84° [for (Cl-)C-C-C-C(-Cl)] at the
energy minimum as a result of steric repulsion between (C-)Cl
and (C=)O atoms at the 1(3) and 1¤(3¤) positions. This indicates
that the negative charge in 1 should be localized in half of the
molecule, as depicted in Figure 1, and the other half essentially
exerts an inductive effect. The hypothesis is further supported by
a small gap between E1 and E2 in pCh2 (0.24 V) compared with
those in pCh1 (0.74 V) and DDQ (0.81 V). The second reduction
of pCh2 should proceed in a facile manner because it is virtually
the reduction of the quinone moiety of 1, whereas in those of
pCh1 and DDQ, the radical anions undergo reductions, which
occur less easily as a result of the on-site Coulomb repulsion.
The oxidizing ability of pCh2 was investigated. Since the
purpose of this article is to compare pCh2 with pCh1 and DDQ,
the reactions were conducted using literature methods, as noted
in Table 2 and, except for Entry 12, were not optimized.
Dehydrogenation of 9,10-dihydroanthracene at ambient temper-
ature2b yielded anthracene as the sole product in all cases
(Entries 1-3). The conversion of the pCh2 oxidation was
between those of the DDQ and pCh1 oxidations, as expected
from the order of the first reduction potentials E1. Similar results
were obtained in the oxidation of ¡-tetralol, giving ¡-tetralone
in methanol (Entries 4-6).5 Note that DDQ showed high
reactivity but gradually decomposed to generate hydrogen
cyanide gas under the present conditions, whereas the reactivity
of p-chloranil was too low. In such a case, pCh2 may be a better
choice with respect to safety and efficiency.
Next, oxidation of 2,4-di-tert-butylphenol (6) was exam-
ined. In DDQ oxidation, 6 is known to undergo oxidative
coupling to give 2,2¤-biphenol derivative 7 and is then further
oxidized to yield furan derivatives 8 and 9 (Entry 9).14 However,
because of its low oxidizing ability, the pCh1 oxidation did not
yield any oxidative products (Entry 7). In contrast, when pCh2
was used as the oxidizing agent, 7 and 10 were afforded in a 1:1
molar ratio (Entry 8). This can be rationalized in terms of the
enhanced oxidizing ability of pCh2, which facilitated oxidative
coupling of 6, and its weaker oxidizing ability compared to that
of DDQ, which depressed further oxidation of 7 and 10. A
chlorine atom of 10 should originate from 1. Although 10 is
known to undergo oxidative coupling by K3[Fe(CN)6] to give
bis(cyclohexadienone),15 this reaction was also depressed in the
pCh2 oxidation.
undergo one-electron oxidation than it was for BHT. The unique
reactivity of pCh2 in this reaction is again attributed to its
moderate oxidizing ability.
In summary, the p-chloranil dimer pCh2 was synthesized
from dimethoxybenzene dimer 3 in good yield. Since the first
reduction potentials were found to be more negative in the order
pCh1, pCh2, and DDQ, the yields of the oxidation of 9,10-
dihydroanthracene and ¡-tetralol to give anthracene and ¡-
tetralone, respectively, increased in this order. In contrast,
oxidations of 2,4-di-tert-butylphenol and BHT yielded different
products when pCh2 was used as the oxidizing agent instead of
DDQ. This is thought to be because no further reaction occurred
in the pCh2 oxidation as it is a less strong oxidizing agent than
DDQ. Apparently, pCh2 is a safe and moderately strong
oxidizing agent and is thus suitable for reactions where a rather
strong oxidizing agent is required but further oxidation should
be avoided. Further investigations of the oxidizing ability of
pCh2, including optimization of the reaction conditions, are
currently underway.
References and Notes
1
2
3
H.-D. Becker, A. B. Turner, in The Chemistry of Quinonoid
Compounds, ed. by S. Patai, Z. Rappoport, Wiley,
Chichester, 1988, Vol. 2, p. 1351.
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C. G. Shanker, B. V. Mallaiah, G. Srimannarayana, Synthesis
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4
5
B. A. McKittrick, B. Ganem, J. Org. Chem. 1985, 50, 5897.
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61, 4439.
6
7
H. Lee, R. G. Harvey, J. Org. Chem. 1988, 53, 4587.
M. Tsukayama, A. Oda, Y. Kawamura, M. Nishiuchi, K.
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H.-D. Becker, J. Org. Chem. 1965, 30, 982.
L. M. Lackman, Adv. Org. Chem. 1960, 2, 329.
8
9
10 a) E. S. Lewis, J. M. Perry, R. H. Grinstein, J. Am. Chem.
Soc. 1970, 92, 899. b) P. J. van der Jagt, H. K. de Haan, B.
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Ohnuma, Y. Saito, H. Higuchi, K. Ninomiya, Tetrahedron
2009, 65, 3639.
Similarly, 2,6-di-tert-butyl-4-methylphenol (BHT) is report-
ed to give aldehyde 11 quantitatively in DDQ oxidation at
ambient temperature (Entry 13).8 In contrast, no reaction took
place in the pCh1 oxidation (Entry 10). Unlike DDQ and pCh1,
using pCh2 as the oxidizing agent yielded 12 as the sole product,
with 20% conversion (Entry 11). When the reaction temperature
was increased (45 °C), the conversion increased to 60%
(Entry 12). Note that 12 is an intermediate product to give 11
in the DDQ oxidation. Both BHT and 12 undergo one-electron
oxidation in the DDQ oxidation,8 and should also do so in the
pCh2 oxidation. However, 12 was hardly oxidized in the latter
case. This is because the methoxymethyl group behaves as a
weakly electron-withdrawing group [the Hammett constant (·p)
is +0.0116] because of the large electronegativity of the oxygen
atom, whereas the methyl group is a moderately electron-
donating group (¹0.17), so it was more difficult for 12 to
12 Readers who are obscurely anxious about the toxicity of
polychlorinated compounds should read the following book.
IUPAC White Book of Chlorine as Pure and Applied
Chemistry, 1996, Vol. 68.
13 Compound 1: 13C NMR (150 MHz, CDCl3): 172.66, 169.95,
143.49, 141.58, 141.18, 135.85. MS (EI): m/z 418 (M+),
420 ([M + 2]+), 422 ([M + 4]+), 424 ([M + 6]+), 426
([M + 8]+), 428 ([M + 10]+), 430 ([M + 12]+). IR (Nujol):
¹1
¯
1671 cm
(C=O). Anal. Calcd for C12O4Cl6: C,
max
34.23%. Found: C, 34.11%.
14 H.-D. Becker, J. Org. Chem. 1969, 34, 1198.
15 M. Tashiro, H. Yoshiya, G. Fukata, J. Org. Chem. 1981, 46,
3784.
16 C. Hansch, A. Leo, R. W. Taft, Chem. Rev. 1991, 91, 165.
Chem. Lett. 2013, 42, 398-400
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/