Regioselective halogenation of biphenyls for preparation of valuable polyhydroxylated biphenyls and diquinones

Article abstract of DOI:10.1016/j.tet.2005.10.009

Electron-rich biphenyls were selectively oxyfunctionalised through a halogenation-methoxylation sequence. The obtained biphenyl methyl ethers were then oxidised to the corresponding quinones. This strategy transforms commercially available biphenyls into both natural and bioactive oxidised compounds.

Full text of DOI:10.1016/j.tet.2005.10.009

Tetrahedron 62 (2006) 635–639
Regioselective halogenation of biphenyls for preparation
of valuable polyhydroxylated biphenyls and diquinones
a
a
b,
Paolo Bovicelli,a, Roberto Antonioletti, Antonella Onori, Giovanna Delogu,
*
*
Davide Fabbrib and Maria Antonietta Dettorib
a
`
C.N.R. Istituto di Chimica Biomolecolare-Sezione di Roma, c/o Dipartimento di Chimica, Universita di Roma ‘La Sapienza’,
p.le A. Moro 5, Box 34 Roma 62, I-00185 Roma, Italy
bC.N.R. Istituto di Chimica Biomolecolare-Sezione di Sassari, Traversa La Crucca, 3, I-07040 Sassari, Italy
Received 28 July 2005; revised 23 September 2005; accepted 6 October 2005
Available online 2 November 2005
Abstract—Electron-rich biphenyls were selectively oxyfunctionalised through a halogenation–methoxylation sequence. The obtained
biphenyl methyl ethers were then oxidised to the corresponding quinones. This strategy transforms commercially available biphenyls into
both natural and bioactive oxidised compounds.
q 2005 Elsevier Ltd. All rights reserved.
1. Introduction
From all of the useful substances, hydroxylated biphenyls
are peculiar compounds. There is an increasing interest
towards hydroxylated biphenyl structures as building block
to prepare biomolecules involved in human pathologies
because hydroxylated biphenyls are present in plants and
marine organisms, which manifest important pharmaco-
logical activities.1–3 The hydroxylated biphenyl unit is
present in important bioactive molecules of pharmacologi-
cal interest such as vancomycin, biphenomycin and
ellagitannins, the latter present in Nature in more than 500
derivatives.4 Moreover, many vegetables and fruits contain
hydroxylated biphenyls,5 being so included in the human
diet.6
Free radical processes are involved both in many diseases
and in degenerative phenomena of food species. Oxygen,
essential for the life in some conditions can produce radical
species, which attack the cellular membranes causing their
degradation. In the organism the amount of free radicals is
controlled by enzymatic systems, superoxide dismutase,
catalase, peroxidase, and a number of antioxidant com-
pounds are introduced with the diet such as ascorbic acid,
tocopherol, carotenoids and phenols. In the food field,
radicals are at the basis of degradation processes since these
reactive species attack lipids, constituents of the cellular
membranes, causing deterioration of food and a loss in
economy.
One of the main features of hydroxylated biphenyls is the
possibility to generate diquinones. These species often
possess pharmacological activity7 and are involved in the
synthesis of bioactive biphenyls.8 The most common
approach for the synthesis of quinones involves the
oxidation of a phenol9 or the corresponding O-methyl
derivative.10 As part of our ongoing studies toward the
synthesis of bioactive compounds, we planned to prepare
biphenoquinones starting from suitable biphenyl halides.
Today, the most common strategies for food conservation,
together with packing under vacuum or in controlled
atmosphere, consist in the addition of preserving com-
pounds. With this aim, antioxidant compounds such as
polyphenols, quinones and biphenols are seen with
particular interest by scientists and more and more new
compounds are synthesised and biologically tested every
year.
In the last years, we successfully used the systems
dimethyldioxirane (DMD)/NaX/H2SO4 and oxone/
acetone/NaX11 to introduce a halogen selectively in
activated aryl compounds (Scheme 1).
Keywords: Halogenation; Hydroxylated biphenyls; Biphenoquinones;
Dimethyldioxirane.
*
Corresponding authors. Tel.: 39 06490422; fax: C39 0649913628 (P.B.);
e-mail addresses: paolo.bovicelli@uniromal.it; giovanna.delogu@
icb.cnr.it
With the aim of introducing halides selectively in electron-
rich biphenyls starting from available biphenyls we
0040–4020/$ - see front matter q 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2005.10.009

636
P. Bovicelli et al. / Tetrahedron 62 (2006) 635–639
OH
OH
OH
Cl
NaBr / DMD
NaCl / DMD
H2SO4, acetone, 62%
H2SO4, acetone, 96%
OH
OH
OH
Br
Br
1
2
3
Scheme 1. Selective halogenation of activated aryls.
projected to explore the potential of the selective
halogenation method in this field.
and 12 to bromination reactions (Scheme 2). Selective
symmetrical dibromination was observed for all of the
biphenyls even in the case of biphenyl 10, which possesses
an acid-sensitive group. The yields were high and in all
cases no fully halogenated compounds were obtained, even
with an excess of reagents. The presence of a C2-symmetry
axis in the products gave simplified NMR spectra and
allowed us to assign the positions of the bromo substituents.
When 2,20-dihydroxybiphenyl and 4,40-dihydrobiphenyl
were submitted to chlorination, a mixture of halogenated
products was obtained whereas bromination of 2,
20-dihydroxybiphenyl gave mainly the 5,50-dibromo deriva-
tive, in accordance with the different reactivity of the two
halogen cations (Table 1, entries 1). As observed in the case
of phenols, the O-methylation of the phenol moiety results
in a lower activation of the aromatic ring and it allows
the chlorination reaction to be more selective.11 In fact,
starting from the corresponding O-methyl-derivatives,
5,50-dichloro-2,20-dimethoxy-biphenyl and 5,50-dichloro-
4,40-dimethoxy-biphenyl were both obtained in good yields
(Table 1, entries 2, Table 2, entry 1).
Br
H3CO
H3CO
OCH3
OCH3
H3CO
H3CO
OCH3
OCH3
NaBr, oxone
acetone, water
Br
8
9 (85%)
Table 1. Halogenation of 2,20-dihydroxy and 2,20-dimethoxy biphenyls
Br
Br
X
O
O
O
O
NaBr, oxone
OR
OR
acetone, water
OR
OR
NaX / DMD
H2SO4, acetone
X
4a-b
Entry
5a-c
10
11 (90%)
Br
Substrate
Product, halide, yield (%)
H3CO
H3CO
H3CO
H3CO
1
2
3
a
RZH
b RZCH3
b RZCH3
5a, XZBr, 45
5b, XZCl, 83
5c, XZBr, 80
OCH3
OCH3
OCH3
OCH3
NaBr, oxone
acetone, water
Table 2. Halogenation of 4,40-dimethoxy biphenyls
OCH
OCH
3
Br
3
X
13 (80%)
12
NaX / DMD
Scheme 2. Selective halogenation of biphenyls.
H2SO4, acetone
Halogenated biphenols can be precursor of bisbenzo-
quinones by substitution of the halide with a methoxyl
group. The reaction, carried out at 110–120 8C in a mixture
of DMF/H2O 95:5 using MeONa in large excess and CuBr
as catalyst, was previously optimised.14 If specific reaction
conditions (see Section 2) are strictly used, the procedure is
of general value. We obtained good yields of products
starting from all dibromo-biphenol methyl ethers that we
used for our investigations (Scheme 3). In contrast,
compounds with free hydroxyl groups decomposed
completely.
X
OCH
OCH
3
3
6
7
a-b
Entry
Substrate
Product, halide, yield (%)
1
2
6
6
7a, XZCl, 90
7b, XZBr, 83
The system oxone/NaBr/acetone/H2O (method A), was very
efficient in the case of O-methyl protected phenols
compared to the system NaX/DMD/H2SO4 (method B),
which gave better results with biphenols. According to these
results, we submitted electron-rich biphenyls 8,12 1013
Finally, the biphenyl methyl ethers obtained by this
procedure were oxidized to give the corresponding o- or
p-diquinones. Among all methods reported in literature10,15

P. Bovicelli et al. / Tetrahedron 62 (2006) 635–639
637
investigations in this field to explore the potentialities of
the method.
Br
Br
CH3O
O
O
O
O
MeONa / MeOH,CuBr
DMF, 2h, reflux
2. Experimental
2.1. General
CH3O
11
14 (75%)
OCH3
1H and 13C NMR spectra were performed in CDCl3 or
DMSO-d6 using either Varian XL-300 or Varian ‘Gemini’
200-MHz spectrometers. IR spectra were recorded in CCl4
and CHCl3 using a Shimadzu IR-740 instrument. MS
spectra were recorded by an HP5971A/MS detector coupled
with an HP5890 gas chromatograph. Column chromato-
graphy, unless otherwise stated, was carried out on
Kieselgel Merck (70–230 mesh and 230–400 mesh).
Analytical TLC: Kiesegel F254 (Merck), detection UV
absorption (lZ254 nm) or H2SO4. GC: HP 5890.
OCH3
Br
OCH3
MeONa / MeOH, CuBr
DMF, 2h, reflux
OCH3
Br
OCH3
7b
OCH3
15 (70%)
Typical procedures for the bromination of activated
aromatic rings. Method A. The substrate was dissolved in
a 1:1 solution of water and acetone (sol. 0.1 M) and 2 equiv
of NaX were added. Oxone was then added in portions until
completion of the reaction. Method B. To the substrate,
dissolved in acetone (sol. 0.1 M), were added 2 equiv of
NaX and H2SO4 in catalytic amount, then a 0.08 M solution
of dimethyldioxirane in acetone was added until completion
of the reaction. The reactions were monitored by TLC
and GC–MS.
Br
CH3O
MeONa / MeOH,CuBr
DMF, 2h, reflux
OCH3
OCH3
OCH3
OCH3
Br
CH3O
5d
12 (60%)
Scheme 3. Methoxylation of halogenated biphenyls.
we found that cerium ammonium nitrate (CAN) was the best
oxidizing reagent for our purposes, giving in few minutes
the best yields of products compared to other oxidants such
as MnO2, AgO or stronger chromium based reagents16
(Table 3).
Typical procedure for the methoxylation of aromatic
bromines. To a hot solution (80 8C) of the substrate
(0.1 mmol) in a DMF/H2O mixture 95:5 (0.2 mL),
16 equiv of sodium methylate (0.34 mL, 25% sol. in
methanol) were added in four portions, leaving the solvent
to evaporate every time. The temperature was raised to
110 8C, then the catalyst CuBr (0.02 mmol, 2.8 mg) was
added. All reactions were monitored by TLC and GC until
completeness. The reaction was cooled and quenched with
0.5 mL of water, then extracted with three portions (5 mL)
of dichloromethane. The organic layer was washed with
water (two portions of 5 mL), dried with anhydrous sodium
sulfate and the solvent evaporated under vacuum. The crude
products were purified by flash chromatography eluting with
a mixture of hexanes and ethyl acetate (typically in 80:20
ratio).
The synthetic strategy reported in this paper transforms
simple commercially available biphenyls to higher oxidised
compounds that could be useful in the synthesis of both
natural and bioactive biphenyls as an alternative to classic
coupling reaction based strategies.17 Among them, the
preparation of tetra- and trimethoxy-biphenyls and their
corresponding quinones appear to be of high interest as
antitubulin colchicine-like compounds.18 Functionalised
biphenoquinones can also find application in the preparation
of biomaterials.19 Our main interest is to continue
Table 3. Oxidation of biphenyl methyl ethers to diquinones
OCH3
OCH3
O
O
O
H3CO
H3CO
O
CH3O
CH3O
OCH3
OCH3
ox
O
O
O
O
ox
ox
OCH3
OCH3
O
O
17
12
16
14
15
Substrate
OxZCAN
OxZMnO2
OxZAgO
tZ300 yield 90%
tZ1 h yield 92%
tZ400 yield 80%
12
15
14
tZ2 h yield 57%
tZ2 h yield 63%
tZ2 h yield 55%
tZ2 h yield 68%
tZ2 h yield 88%
tZ2 h yield 70%

638
P. Bovicelli et al. / Tetrahedron 62 (2006) 635–639
2.1.1. 5,50-Dibromo-2,20-dihydroxy-biphenyl (5a). Light
yellow oil, 1H NMR (DMSO-d6), d (ppm): 7.35 (2H, d, JZ
8.4 Hz), 7.30 (2H, d, JZ2.5 Hz), 6.88 (2H, dd, JZ2.5,
8.4 Hz). 13C NMR, d (ppm): 142.8, 135.1, 133.5, 125.8,
118.8, 113.2. IR (CH2Cl2, cmK1) 3350, 1495, 765, 550.
Anal. Calcd for C12H8Br2O2: C 41.90, H 2.34. Found
C 41.65, H 2.70.
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(2H, d, JZ2.2 Hz), 7.38 (2H, dd, JZ2.2, 8.7 Hz), 7.00
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