Fragmentation of anion radicals with elimination of aryloxy groups

Article abstract of DOI:10.1023/A:1013456112725

4-Vinylbenzyl phenyl ether, 4-phenylbenzyl phenyl ether, 1- and 2-naphthylmethyl phenyl ethers react with sodium thiophenolate under photochemical stimulation with replacement of the phenoxy group. The composition of reaction products and relation of reactivity to the structure of substrates is consistent with anion-radical mechanism. The corresponding methoxy and cyano derivatives do not undergo the reaction.

Full text of DOI:10.1023/A:1013456112725

Russian Journal of Organic Chemistry, Vol. 37, No. 10, 2001, pp. 1438 1443. Translated from Zhurnal Organicheskoi Khimii, Vol. 37, No. 10, 2001,
pp. 1507 1512.
Original Russian Text Copyright
2001 by Dneprovskii, Fedosov.
Fragmentation of Anion Radicals with Elimination of Aryloxy
Groups
A. S. Dneprovskii and I. V. Fedosov
St. Petersburg State University, St. Petersburg, 198904 Russia
Received November 25, 1999
Abstract 4-Vinylbenzyl phenyl ether, 4-phenylbenzyl phenyl ether, 1- and 2-naphthylmethyl phenyl ethers
react with sodium thiophenolate under photochemical stimulation with replacement of the phenoxy group.
The composition of reaction products and relation of reactivity to the structure of substrates is consistent
with anion-radical mechanism. The corresponding methoxy and cyano derivatives do not undergo the
reaction.
Unactivated aryl halides are known to easily enter
into substitution reactions with some nucleophile
reagents at photoinitiation or under action of solvated
electrons, and the reaction proceeds along anion-
radical chain mechanism (SRN1) [1]. This substitution
mechanism is rigorously confirmed only in some
cases. In general as a rule the direct proofs of the
presence of free radicals and anion radicals as
kinetically independent intermediates in SRN1 reac-
tions are lacking. As with the other chain processes it
is due to high reactivity and consequently to short life
time and low stationary concentration of these
species. Even if the anion-radical is detected in the
reaction mixture, it should be proved that the species
actually participates in the main and not a side
process. In this connection arises a necessity of
developing alternative procedure for proving the
participation of anion radicals in the nucleophilic
substitution.
tion proceeded via anion-radical intermediate. This
approach was used in confirming the anion-radical
mechanism of aromatic halides carbonylation in the
presence of cobalt carbonyl [3], and in detection of
anion-radical stages in reactions of nucleophilic vinyl
substitution [4]. We also showed that analogous
fragmentation is characteristic also of the other weak-
ly nucleofugal leaving groups: OAlk, OAr, SAr [5].
Our data indicated that the tendency to fragmentation
of the leaving group increased in the above series.
In the present study we investigated the fragmenta-
tion capability of anion radicals with weakly nucleo-
fugal leaving groups located in the benzyl position on
a wider range of substrates. To this end we studied
reactions of sodium thiophenolate at photochemical
stimulation with compounds containing phenoxy,
methoxy, and cyano groups in the benzyl position:
4-vinylbenzyl phenyl ether (I), 4-phenylbenzyl phenyl
ether (II), 1- and 2-naphthylmethyl phenyl ethers
(III, IV), and also the corresponding cyano deriv-
atives. It was established that under these conditions
the reactions between aryl bromides and thiophenolate
ion followed the SRN1 mechanism [6]. It was reason-
In the study of reaction between bromobenzene and
cyanomethyl anion was established that the reaction
proceeded along anion-radical mechanism furnishing
1,2-diphenylethane as the main product [2]. From
these data was made a conclusion that the inter-
mediate anion-radical of phenylacetonitrile was able
to undergo fragmentation with elimination of a
cyanide anion. The possibility of such fragmentation
we took as a base of a new confirmation procedure of
anion radicals formation as intermediates in organic
reactions [3]. The procedure consists in the reactivity
study of HlgC6H6CH2X type compounds where X is
a leaving group of low nucleofugal properties unable
to replacement by common mechanisms of nucleo-
philic substitution. The formation of products sub-
stituted at benzyl position may evidence that the reac-
Scheme 1.
1070-4280/01/3710-1438$25.00 2001 MAIK Nauka/Interperiodica

FRAGMENTATION OF ANION RADICALS WITH ELIMINATION
1439
able to assume that the other molecules with similar
or more negative redox potentials would generate
anion radicals under the conditions in question.
Table 1. The composition of reaction mixture (mol%)
in reaction of 2-naphthylmethyl phenyl ether with sodium
thiophenolate in DMSO at photoinitiation
The reactivity of investigated substrates is
governed by the nature of the leaving group. With cyano
group attached to benzyl position the reaction failed
to occur. No reaction was also observed with (1- and
2-methoxymethyl)naphthalenes. At the same time
compounds I IV react with sodium thiophenolate
with replacement of the phenoxy group. The reaction
products obtained from compound I are shown on
Scheme 1.
Irradiation time, min
Compd.
no.
15
30
60
90
120
180
III
XI
XII
XIII
78.7 66.0 50.6 40.3
15.5 23.8 32.6 36.3
36.2
35.5
18.5
9.8
32.5
29.4
26.6
11.5
4.7
1.1
7.7 12.6 15.6
2.5 4.2 7.8
Formation of compounds V VII is consistent with
the anion-radical reaction mechanism (Scheme 2).
The thiophenolate ion attacks the naphthylmethyl
radical both at the benzyl position to yield compound
XI, and at the aromatic ring affording substance XIII.
Since the latter cannot form in a common nucleophilic
substitution, its presence is another evidence of
participation of free radicals as reaction inter-
The electron-transfer to molecule I furnishes the
corresponding anion radical A that undergoes frag-
mentation to yield 4-vinylbenzyl radical B. Its reac-
tion with thiophenolate ion followed by electron
transfer to the substrate molecule affords reaction
product V. 4-Vinyltoluene (VI) arises either due to
the hydrogen transfer from solvent or by reduction of
radical B with thiophenolate ion followed by proton
transfer.
Scheme 3.
Analogously reacts compound II (Scheme 3) as is
additionally proved by the presence among reaction
products of compound IX arising from recombination
of 4-phenylbenzyl radicals, and of a small amount
( 1%) of 4-methylbiphenyl (X), product of reduction
of these radicals.
The reaction between thiophenolate ion and com-
pound III follows the same mechanism (Scheme 4,
Table 1).
Scheme 2.
Scheme 4.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 10 2001

1440
DNEPROVSKII, FEDOSOV
mediates. The large amounts of dimer XII originate
from the increased possibility of radicals recombina-
tion with growing stability and stationary concentra-
tion of free radicals.
radicals was due to unlike localization of the unpaired
electron in the anion radicals [7]. In the former anion
radical the lone electron is placed on an antibonding
-orbital localized predominantly on the cyano group.
In the latter case the lone electron is placed on an
antibonding -orbital of the aromatic system. Rossi
et al. [7] think that in this case no fragmentation can
occur. Our results suggest that this is not the main
factor affecting the stability of the anion radical.
Actually, it is presumable that in the anion radicals
of compounds I IV the lone electron is localized on
the aromatic system of substrate and not on the leav-
ing group. Nevertheless, the fragmentation occurs in
all cases with elimination of a phenoxy anion.
Obviously the possibility of fragmentation depends
both on the stability of the initial anion radical and
on the nucleofugal properties of the leaving group.
We have previously concluded from the analysis of
relative reactivity in fragmentation reactions of anion
radicals of weakly nucleofugal leaving groups that
the reactivity thereof changes in the following order:
PhO > AlkO > CN. At sufficiently high energy of
the anion radical, as with BrC6H4CH2R, the frag-
mentation of cyano group with the lowest nucleofugal
properties is possible. For the most stable anion
radical sharply increases the selectivity toward the
change of the leaving group, and fragmentation is
then probable only with relatively labile leaving
groups OPh, SPh. It is presumable that similar rela-
tion between the anion-radical stability and selectivity
toward variation of the leaving groups will be
retained also on quantitative level.
It was remarked earlier [3 5] that anion radicals
with a thiophenoxy group in the benzyl position also
were capable of fragmentation. This is supported by
the character of changes in reaction products com-
position with duration of the process (Table 1). At
the start of the reaction accumulates compound XI.
Further due to reversibility of its formation its con-
centration decreases, and as a result arises the
stable dimer XII. 1-Naphthylmethyl phenyl ether (IV)
reacts with thiophenolate ion in a similar way
(Scheme 5), and the composition of the reaction
products is also well consistent with the mechanism
under consideration.
Scheme 5.
EXPERIMENTAL
The reaction rate grows in the series I< II< III<
IV. Within 90 min the conversion of the initial ethers
amounts o 33, 54, and 60% for ethers I, II, and III
respectively. This reactivity series is in agreement
with the relative stability of anion-radicals character-
ized by the LUMO energy values of the correspond-
ing molecules. The stability of anion-radicals from
2-naphthylmethyl and 1-naphthylmethyl phenyl ethers
(III, IV) is virtually the same. However the concur-
rent reaction of sodium phenolate performed with
equimolar mixture of these substrates resulted in
conversion of 63% for ether IV and 43% for ether
III within 40 min. This difference is apparently due
to significant difference in stability of 1- and
2-naphthylmethyl radicals. Therefore the fragmenta-
tion of anion radical in 1-naphthylmethyl system is
more exothermic, and the rate of the reaction should
be higher.
NMR spectra of compounds were registered from
5% solutions in CDCl3 and (CD3)2SO on spectrometer
Bruker DPX-300 (300 MHz).
The reactions were carried out in DMSO at
photoinitiation (mercury lamp of moderate pressure
DRL-400, power 400 W) at 35 40 C. The distance
from the lamp to ampules was 30 cm.
The reaction mixtures were analyzed by GLC on
chromatograph Chrom-5 (flame-ionization detector,
carrier gas argon, flow rate 25 ml/min), equipped
with glass columns packed with a carrier Chromaton-
N-Super with the following characteristics: (1)
1200 3 mm, stationary phase 5% SE-30; (2) 1200
3 mm, stationary phase 5% XE-60; (3) 2500 3 mm,
stationary phase 5% SE-30.
The qualitative analysis was carried out by
comparison of products retention times with those of
It was formerly presumed that different behavior
of phenylacetonitrile and naphthylacetonitrile anion
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FRAGMENTATION OF ANION RADICALS WITH ELIMINATION
1441
authentic samples prepared by independent syntheses
om several columns of different polarity. The quantit-
ative analysis was carried out by the method of
internal reference.
Table 2. The composition of reaction mixtures (mol%)
in reactions of ethers I, II, IV with sodium thiophenolate
in DMSO at photoinitiation
Initial
compd.
no.
Reaction products
(mol%)
Conver- Time,
In kinetic measurements was used DMSO of
chemically pure grade after double distillation in a
vacuum (7 mm Hg) on calcium hydride through a
Vigreux column. The extraction of reaction products
was performed with dichloromethane of chemically
pure grade.
sion, %
min
I
II
IV
V (78), VI (14), VII (8)
VIII (83), IX (16), X (1)
XIV (59), XV (26), XVI (14)
33
54
35
90
90
20
Sodium thiophenolate was prepared under argon
from thiophenol (5% excess) and sodium ethylate in
ethanol. The residue after evaporation of ethanol was
boiled with hexane (3 ml of hexane per 1 g of sodium
thiolate), filtered, and dried in a vacuum.
acetic acids, of 1- and 2-(methoxymethyl)naphthalene
with sodium thiophenolate. Reaction time 3 h, con-
version 0%, material balance 99%.
Special experiments showed that compounds I IV
are stable under reaction conditions without photo-
initiation, and also at irradiation without nucleophile.
Compounds VI, IX, X, XII, XIII, XV, and XVI are
stable in the course of reaction. Compounds V, VIII,
XI, and XIV are unstable in the presence of the
nucleophile.
Reactions of 4-vinylbenzyl phenyl ether (I),
4-phenylbenzyl phenyl ether (II), 1-naphthylmethyl
phenyl ether (IV) with sodium thiophenolate at
photoinitiation.
Precisely
weighed
portions
(0.4 mmol) of substrate, freshly prepared sodium
thiophenolate (1 mmol), and internal reference
(1,2-diphenylethane, 0.17 mmol) were dissolved in
4 ml of DMSO. A weak flow of argon was passed
through the mixture till the reagents dissolved
completely (10 min). Into 4 ampules of Pirex glass
were charged 1 ml portions of the solution, and a
weak flow of argon was passed through the ampules
4-Vinylbenzyl phenyl ether (I) was prepared by
procedure [5]. mp 111 112 C (from ethanol) [5].
1H NMR spectrum (CDCl3, , ppm): 7.54 7.38 m
(4H), 7.36 7.23 m (2H), 7.09 6.92 m (3H), 6.76 d.d
3
(1H, JHH 10.9, 17.5 Hz), 5.79 d (1H, 3JHH 17.5 Hz),
1
3
( 15 ml min ). Three ampules were sealed and after
5.29 d (1H, JHH 10.5 Hz), 5.08 s (2H).
keeping in a thermostat at 35 C they were irradiated
with UV light under constant shaking. With every
substrate one ampule remained as control, three
others were irradiated. Intermittently the ampules
were opened, each time 6 ml of water were added,
the reaction products were extracted into dichloro-
methane (3 2 ml). The combined extracts were dried
with a little magnesium sulfate and analyzed by GLC.
The content of the control ampule was treated in the
same way, and the solution was used for comparison.
Material balance was over 95%. The results of
analyses are listed in Table 2.
4-Phenylbenzyl phenyl ether (II) was prepared
along the common procedure for synthesis of ethers
of ArCH2OAr type. To a solution of sodium
phenolate obtained by short heating to 90 C of
1.128 g (12 mmol) of phenol and 0.48 g (12 mmol)
of NaOH powder in 20 ml of DMSO till complete
dissolution of the solid was added in one portion
2.41 g (10 mmol) of 4-bromomethylbiphenyl [8]. In
30 min the reaction mixture was poured into 100 ml
of 1% water solution of NaOH. The precipitate was
filtered off. Yield 78%, mp 117 118 C (from
ethanol). Found, %: C 87.63; H 6.09. C19H16O.
Calculated, %: C 87.66; H 6.19. The other ethers
and their thioanalogs were also obtained along this
procedure.
Reaction of 2-naphthylmethyl phenyl ether III
with sodium thiophenolate at photoinitiation.
Precisely weighed portions of substrate (168 mg,
0.7 mmol), freshly prepared sodium thiophenolate
(231 mg, 1.75 mmol), and internal reference (1,2-di-
phenylethane, 54.6 mg, 0.3 mmol) were dissolved in
7 ml of DMSO. Reaction and workup of the reaction
mixtures were carried out as described for compounds
I IV. Material balance was over 95%.
2-Naphthylmethyl phenyl ether (III) was pre-
pared from 2-bromomethylnaphthalene [9] and
sodium phenolate. Yield 68%, mp 119 120 C (from
1
ethanol) [10]. H NMR spectrum (CDCl3, , ppm):
7.96 7.82 m (4H), 7.62 7.46 m (3H), 7.39 7.26 m
(2H), 7.11 6.95 m (3H), 5.26 s (2H).
1-Naphthylmethyl phenyl ether (III) was prepar-
ed from 1-chloromethylnaphthalene [9] and sodium
In a similar way were carried out reactions of
nitriles of 1- and 2-naphthylacetic and 4-biphenyl-
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 10 2001

1442
DNEPROVSKII, FEDOSOV
phenolate. Yield 78%, mp 74 75 C (from ethanol)
thiophenolate. Yield 77%, mp 72 73 C (from
ethanol) [17]. H NMR spectrum (CDCl3, , ppm):
7.89 7.75 m (3H), 7.71 s (1H), 7.56 7.44 m (3H),
7.40 7.17 m (5H), 4.31 s (2H).
1
1
[11]. H NMR spectrum (CDCl3, , ppm): 8.11 d
3
(1H, JHH 8.8 Hz), 7.96 7.87 m (2H), 7.68 7.46 m
(4H), 7.41 7.34 m (2H), 7.15 7.02 m (3H), 5.56 s
(2H).
1,2-Di(1-naphthyl)ethane (XV) was prepared by
4-Vinylbenzyl phenyl sulfide (V) was prepared by
procedure [5]. mp 80 81 C (from ethanol) [12].
1H NMR spectrum (CDCl3, , ppm): 7.42 7.15 m
method [11]. Yield 49%, mp 162 163 C.
4-Methyl-1-phenylsulfonylnaphtalene (XVI) was
prepared as compound XIII from 8.5 g of 1-bromo-4-
methylnaphthalene. Yield 56%, bp 140 C (0.2
0.4 mm Hg). Colorless thick oily substance. Found,
%: C 81.49; H 5.51. C17H14S. Calculated, %: C
81.56; H 5.64.
3
(9H), 6.71 d.d (1H, JHH 17.6, 11.9 Hz), 5.74 d (1H,
3JHH 17.6 Hz), 5.25 d (1H 3JHH 11.9 Hz), 4.12 s (2H).
(4-Phenylsulfonylmethyl)biphenyl (VIII) was
obtained from 4-bromomethylbiphenyl [8] and sodium
thiophenolate. Yield 80%, mp 127 128 C (from
ethanol). Found, %: C 82.41; H 5.77. C19H16S. Cal-
culated, %: C 82.57; H 5.83.
2-Naphthylmethyl methyl ether was prepared as in
[18]. Yield 70%, bp 155 (26 mm Hg), 113 C
(1.3 mm Hg) [18].
1,2-Bis(4-phenylphenyl)ethane (IX) was obtained
by boiling of 4.82 g of 4-bromomethylbiphenyl and
0.5 g of sodium in 50 ml of octane for 4 days. Yield
41%, mp 200 202 C (from ethanol) [13].
1-Naphthylmethyl methyl ether was prepared as
above. Yield 85%, bp 152 C (27 mm Hg), 106
107 C (3 mm Hg) [19].
2-Naphtylacetic acid nitrile was prepared by heat-
ing to 60 C for 2 h of 3.315 g (15 mmol) of 2-bromo-
methylnaphthalene in 30 ml DMSO with 1.27 g
(19.5 mmol) of powdered KCN. On diluting with
water (100 ml) the precipitate was filtered off and
recrystallized from hexane chloroform (1: 1) mix-
ture. Yield 72%, mp 85 86 C [20].
4-Methylbiphenyl (X) was prepared by procedure
[14]. Yield 20%, mp 51 C (from methanol).
(2-Phenylsulfonylmethyl)naphthalene (XI) was
obtained from 2-bromomethylnaphthalene [9] and
sodium thiophenolate. Yield 69%, mp 102 103 C
1
(from ethanol). H NMR spectrum (CDCl3, , ppm):
7.89 7.75 m (3H), 7.71 s (1H), 7.56 7.44 m (3H),
7.40 7.17 m (5H), 4.31 s (2H). Found, %: C 81.51;
H 5.71. C17H14S. Calculated, %: C 81.56; H 5.64.
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RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 10 2001

FRAGMENTATION OF ANION RADICALS WITH ELIMINATION
1443
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