Reactions of 1-Halo-4-nitroanthraquinones with C-nucleophiles

Article abstract of DOI:10.1023/A:1012444810981

1-Fluoro-4-nitroanthraquinone reacts with C-nucleophiles, yielding the corresponding fluorine replacement products. Reactions of 1-chloro-4-nitroanthraquinone with the same nucleophiles result in formation of mixtures of dechlorination and denitration pro

Full text of DOI:10.1023/A:1012444810981

Russian Journal of Organic Chemistry, Vol. 37, No. 3, 2001, pp. 410 415. Translated from Zhurnal Organicheskoi Khimii, Vol. 37, No. 3, 2001,
pp. 435 440.
Original Russian Text Copyright
2001 by Tabatskaya, Beregovaya.
Reactions of 1-Halo-4-nitroanthraquinones
with C-Nucleophiles
A. A. Tabatskaya and I. V. Beregovaya
Vorozhtsov Novosibirsk Institute of Organic Chemistry, Siberian Division, Russian Academy of Sciences,
pr. Akademika Lavrent’eva 9, Novosibirsk, 630090 Russia
Received March 17, 2000
Abstract 1-Fluoro-4-nitroanthraquinone reacts with C-nucleophiles, yielding the corresponding fluorine
replacement products. Reactions of 1-chloro-4-nitroanthraquinone with the same nucleophiles result in forma-
tion of mixtures of dechlorination and denitration products whose ratio is determined by both charge and
orbital interactions. Steric structure of the nucleophile can also affect the regioselectivity of substitution.
It is known that the chlorine atom and nitro group
in 1-chloro-4-nitroanthraquinone (I) exhibit similar
reactivities in nucleophilic substitution reactions with
amines; as a result, mixtures of aminodechlorination
and aminodenitration products are formed [1, 2].
An analogous behavior was observed for compound I
in the reaction with phenylacetonitrile anion [3]. Our
studies on the reactions of I with phenoxide and
benzenethiolate ions showed that the selectivity of
substitution is determined by the nucleophile: hard
nucleophiles (phenoxide ions) preferentially replace
the chlorine atom, while reactions with soft S-nucleo-
philes result mainly in replacement of the nitro group
[4]. On the other hand, 1-fluoro-4-nitroanthraquinone
(II) reacts with various nucleophiles, exclusively
yielding fluorine replacement products [5]. Such
behavior of fluoroanthraquinone II was explained [4]
by high electronegativity of the fluorine atom which
induces a considerable positive charge on the adjacent
carbon atom (+0.26), and nucleophilic attack by
anionic species occurs mainly at that carbon atom. In
chloronitroanthraquinone I the charge on the carbon
atom attached to chlorine is small (+0.09). Therefore,
charge-controlled reactions with O-anions lead to
preferential replacement of chlorine, whereas reactions
with soft, readily polarizable S-nucleophiles are
orbital-controlled, and nucleophilic attack is directed
at the carbon atom attached to the nitro group, whose
contribution to the lowest unoccupied molecular
orbital (LUMO) is considerably greater. In order to
elucidate whether orbital control is general for nucleo-
philic reactions of chloronitroanthraquinone I and can
it determine the regioselectivity of substitution in
fluoronitro derivative II it was necessary to extend
the series of nucleophiles. Of specific interest was
to compare the behavior of substrates I and II in
reactions with carbanions having the same kind of
anionic center but differing in charge localization and
contribution of the active atom to the highest occupied
molecular orbital (HOMO), so that both charge and
orbital control of the reaction would be favored.
It should be kept in mind that reactions of substi-
tuted anthraquinones with anions derived from CH
acids are often complicated by the presence of other
functional groups in the nucleophile and that primary
substitution products can be unstable in both basic
and acidic media [6]. Moreover, the released nitrite
ion or hydroxide ion present in the reaction mixture
could promote formation of hydroxyanthraquinones
[4, 5]. As a result, complex mixtures of products are
formed, and isolation of individual components and
their purification are not always successful. Naturally,
the presence in molecule I of two substituents with
comparable nucleophilic reactivities implies that the
reaction will take at least two pathways, and multi-
component reaction mixtures should inevitably be
obtained.
As C-nucleophiles we used anions derived from
the following CH acids: malonodinitrile (III), penta-
fluorophenylacetonitrile (IV), and ethyl cyanoacetate
(V). Also, the anion derived from 2,6-di-tert-butyl-
phenol (VI) was used, in which the nucleophilic
center is located on C4 [7]. The reaction of substrate I
with 1 equiv of malonodinitrile (III) in the presence
of sodium hydride, alkali metal hydroxides, or potas-
sium carbonate as a base gave a complex mixture of
products. According to the mass spectrometric data,
1070-4280/01/3703-0410$25.00 2001 MAIK Nauka/Interperiodica

REACTIONS OF 1-HALO-4-NITROANTHRAQUINONES
411
Scheme 1.
I, Hlg = Cl; II, Hlg = F; VII, VIII, Nu = CH(CN)2; XI, XII, XIV, Nu = CH(CN)C6F5; XVII, XVIII, Nu = CH(CN)CO2Et;
XIX, XX, Nu = 4-HO-3,5-(t-Bu)2C6H2; IX, R = CN; XIII, R = C6F5; XIV, R = CH(CN)C6F5; XV, R = Cl; XVI, R = NO2.
the mixture contained both products of replacement
of chlorine or nitro group by dicyanomethyl fragment
(compounds VII and VIII; m/z 317 and 306) and
those with molecular weights of 329 and 340. In
addition, the mass spectrum always contained the
molecular ion peak of the initial compound.
derivative VII (Table 1). However, our attempt to
purify this product gave a complex mixture. Reactions
of anthraquinones II and VII with excess malonodi-
nitrile under the conditions ensuring the transforma-
tion of I into IX gave 2-amino-1,3-dicyano-6-nitro-
benzanthrone (X) which showed in the mass spectrum
ion peak with m/z 340; in the IR spectrum a strong
aromatic C N absorption band appeared instead of
weak aliphatic C N band; also, bands due to N H
stretching vibrations were observed.
A conclusion can be drawn that malonodinitrile
anion replaces in substrate II exclusively the fluorine
atom, whereas in anthraquinone I the nitro group is
replaced preferentially by the action of the same
reagent. Minor chlorine substitution products VII and
X can be detected only by mass spectrometry; the
substitution ratio Cl/NO2 is less than 0.1.
The reaction of anthraquinone I with pentafluoro-
phenylacetonitrile anion, generated from IV by the
action of sodium hydride, at room temperature led to
formation of compounds XI and XII as the major
products and 2-amino-6-chloro-1,3-bis(pentafluoro-
phenyl)benzanthrone (XIII). The latter arises from the
reaction of primary nitro-group substitution product
XII with the second nucleophile molecule. In addi-
tion, we isolated two hydroxy-substituted compounds,
1-hydroxy-4-( -cyanopentafluorobenzyl)anthra-
quinone (XIV) and 1-chloro-4-hydroxyanthraquinone
(XV). When the reaction was performed at room
temperature using potassium hydroxide as a base, no
by-products XIII XV were formed, and we obtained
a mixture of anthraquinones XI and XII at a ratio
Gorelik et al. [6] studied replacement of chlorine
or nitro group in chloro- and nitroanthraquinones by
the action of various C-anions and found that dicyano-
methylanthraquinones in the presence of bases readily
react with the second malonodinitrile molecule to
give 2-amino-1,3-dicyanobenzanthrones. Probably, in
our case primary nucleophilic substitution products
are also converted into the corresponding 6-chloro-
and 6-nitrobenzanthrones with molecular weights of
329 and 340. In fact, the reaction of anthraquinone I
with a 4 8-fold excess of malonodinitrile in the
presence of potassium hydroxide, followed by heating
of the dilute aqueous solution under reflux [7], gave
almost exclusively a product with m/z 329, namely
2-amino-6-chloro-1,3-dicyanobenzanthrone (IX). In
agreement with structure IX, in the 1H NMR spectrum
of the product (Table 1) we observed a considerable
downfield shift of the signal from one -proton of
the unsubstituted aromatic ring. The IR spectrum of
IX contained a strong absorption band from aromatic
C N group, and a long-wave absorption maximum
appeared in the UV spectrum.
From 1-fluoro-4-nitroanthraquinone (II) and
an equimolar amount of malonodinitrile in the pres-
ence of potassium carbonate in 30 min at room
temperature we obtained 1-dicyanomethyl-4-nitro
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 3 2001

412
TABATSKAYA, BEREGOVAYA
1
Table 1. 1H (CDCl3, , ppm) and 19F NMR ( F, ppm), IR ( , cm ), and UV ( max, nm) spectra of anthraquinones
VII, XI, XII, XIV, and XVIII XX and benzanthrones IX, X, and XIII
Comp.
no.
1H NMR spectrum
IR, UV, and 19F NMR spectra
VII
6.35 s [CH(CN)2], 7.97 m (2H, 6-H, 7-H), 8.15 8.26 m (2H, 5-H, IR: 1360 and 1540 (NO2), 2200 w (CN),
8-H), 8.18 d and 8.31 d (2-H and 3-H, J = 8.5 Hz) 1675 s (CO)
7.78 m (2H, 9-H, 10-H), 7.83 d, 8.13 d (4-H, 5-H, J = 8.5 Hz), IR: 1630 s (CO); 2220 s (CN); 3240,
IX
X
8.44 d.d and 9.03 d.d (8-H, 11-H, J = 8.0, 2.0 Hz)
3350, 3400 (NH2)
UV (DMF): 380
IR: 1625 (CO); 2220 s (CN); 3230, 3350
(NH2)
UV: 534
XI
6.96 s (CHCN), 7.83 m (2H, 6-H, 7-H), 7.86 d. 8.43 d (2-H, 3-H, 19F NMR: 2.04 (2F), 11.00, 23.22 (2F)
J = 8.5 Hz), 8.19 m (2H, 5-H, 8-H)
XII
XIII
6.85 s (CHCN), 7.78 m (2H, 6-H, 7-H), 7.90 d and 8.14 d (2-H, 19F NMR: 1.67 (2F), 10.37 and 23.02
3-H, J = 8.5 Hz), 8.07 8.23 m (2H, 5-H, 8-H)
(2F)
7.08 d and 7.61 d (4-H, 5-H, J = 9.0 Hz), 7.49 d.d and 8.37 d.d 19F NMR: 2.77 and 3.66 (2F each),
(7-H, 11-H, J = 7.5, 2.0 Hz), 7.30 7.40 m (2H, 9-H, 10-H)
10.45 and 11.49 (1F each), 24.74 and
25.10 (2F each)
XIV
7.41d and 7.66 d (2-H, 3-H, J = 8.5 Hz), 7.83 m (2H, 6-H, 7-H), 19F NMR: 0.82 (2F), 13.16 and 22.46
8.12 d and 8.29 d (5-H, 8-H, J = 8.0 Hz), 12.92 s (OH)
(2F, each)
XVIII 1.35 t (3H, CH3), 4.32 q (2H, CH2), 5.94 br.s (CHCN), 7.87 m (2H,
6-H, 7-H), 7.68 d and 7.80 d (2-H, 3-H, J = 8.5 Hz), 8.21 d.d and
8.42 d.d (5-H, 8-H, J = 7.5, 2.0 Hz)
XIX
1.46 s [18H, 3 ,5 -(t-Bu)2], 5.38 s (OH), 7.08 s (2H, 2 -H, 6 -H),
7.66 d and 7.73 d (2-H, 3-H, J = 8.5 Hz), 7.78 m (2H, 6-H, 7-H),
8.08 m and 8.20 m (5-H, 8-H)
XX
1.45 s [18H, 3 ,5 -(t-Bu)2], 5.30 s (OH), 7.05 s (2H, 2 -H, 6 -H),
7.50 d and 7.70 d (2-H, 3-H, J = 8.5 Hz), 7.82 m (2H, 6-H, 7-H),
7.99 m and 8.21 m (5-H, 8-H)
of 7:10. Apart from the products of nucleophilic
substitution by pentafluorophenylacetonitrile anion,
the mixture contained 10 15 mol% of 1-hydroxy-4-
nitroanthraquinone (XVI). The formation of hydroxy
derivatives may be explained by participation of the
released nitrite ion and hydroxide ion of the base
catalyst. Under similar conditions anthraquinone II
is converted into product XI as a result of exclusive
replacement of the fluorine atom.
Mixtures containing both chlorine and nitro group
substitution products were also obtained by reactions
of anthraquinone I with the anion derived from
2,6-di-tert-butylphenol (VI). In this case, the replace-
ment of chlorine was the predominant reaction path-
way: the ratio of 4-nitro- and 4-chloroanthraquinones
XIX and XX in the reaction mixtures was (5 6):1.
The reaction was always accompanied by formation
of hydroxynitroanthraquinone XVI (12 15 mol %).
The reaction of chloronitroanthraquinone I with
ethyl cyanoacetate (V) in the presence of alkali metal
hydroxide or potassium carbonate both at room tem-
perature and at 45 C yields approximately equal
amounts of products XVII and XVIII as a result of
replacement of the chlorine atom and nitro group,
respectively. 1-[(Cyano)ethoxycarbonylmethyl]-4-
nitroanthraquinone (XVII) was obtained previously as
the sole product of the reaction of fluoronitroanthra-
quinone II with the same nucleophile [4].
The reaction of fluoronitroanthraquinone II with
an equimolar amount of phenol VI in the presence of
potassium hydroxide at 20 45 C afforded a mixture
1
of products, in which we identified by H NMR spec-
troscopy compounds XIX and XVI and also initial
reagent VI. No denitration product was detected, for
ion peak with m/z 430 was not found in the mass
1
spectrum and the H NMR spectrum lacked signals
from OH proton and 2-H and 6-H of hydroxyphenyl
fragment, belonging to a compound other than XIX.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 3 2001

REACTIONS OF 1-HALO-4-NITROANTHRAQUINONES
413
Presumably, the formation of product XVI in the
reactions of I and II with anions derived from CH
acids IV and VI is not the result of transformations
of products XI and XIX; therefore, it does not affect
the reactivity ratio of the chlorine atom and the nitro
group in these processes.
Table 2. Charges on the anionic centers (q), their orbital
coefficients in the HOMO (c), and HOMO energies of
nucleophiles (EHOMO, EV) and Cl/NO2 substitution ratios
in reactions with anthraquinone I
Nucleophile
q
c
EHOMO Cl/NO2
Table 2 contains the ratios of products formed by
replacement of the chlorine atom and the nitro group
in 1-chloro-4-nitroanthraquinone, determined from the
1H or 19F NMR spectra, and the calculated HOMO
energies, charges on the donor centers, and the corre-
sponding orbital coefficients for carbanions. The
results obtained previously for the reactions of anthra-
quinone I with phenylacetonitrile anion [3] and phen-
oxide and benzenethiolate ions [4] are also given for
comparison. It is seen that the calculated parameters
of malonodinitrile anion (CN)2CH , such as orbital
coefficient c and others, are very similar to those of
benzenethiolate ion PhS . Both these nucleophiles
exhibit very high selectivity for replacement of the
nitro group in I. On the other hand, the anion derived
from 2,6-di-tert-butylphenol (VI) has the lowest
(among those considered) orbital coefficient of the C4
atom, and it preferentially replaces the chlorine atom
in I (as in the case of PhO ion).
The highest selectivity for replacement of chlorine
was observed for 4-nitrophenoxide ion which is
characterized by low orbital coefficient of the oxygen
atom in the HOMO and large energy difference
between its HOMO and LUMO of substrate I
( 1.78 eV) [4]. These parameters are unfavorable for
orbital control of the reaction. However, the results
obtained for C6F5(CN)CH (Table 2) contradict the
above relation. Pentafluorophenylacetonitrile anion is
characterized by a low orbital coefficient of the
anionic center and low HOMO energy. Therefore, the
reaction should be charge-controlled, and replacement
of chlorine should predominate. Nevertheless, the
selectivity of the reaction with C6F5(CN)CH is low:
the ratio of substitution products Cl/NO2 is 0.7.
Probably, in this case other factors are responsible for
the reaction output. One of these may be difference
in steric requirements of carbanions. It is known that
the cyano group is characterized by minimal steric
requirements and that its effective size is similar to
the size of CO2R [8]. Then, the ratios of substitution
products given in Table 2 for carbanions derived from
malonodinitrile (III) and ethyl cyanoacetate (V)
properly reflect the contribution of orbital interaction
to the reaction with substrate I. The reactions with
bulky C6F5(CN)CH , Ph(CN)CH , and probably
2,6-(t-Bu)2C6H3O ions involve some steric
hindrances while accessing the reactive centers in
(CN)2CH
0.51
0.36
0.38
0.50
0.80
0.58
0.67
0.83
2.83
3.58
2.54
3.26
2.90
2.52
3.86
2.63
0.1
0.7
0.5 [3]
1.0
5.5
4.3 [4]
10 [4]
0.15 [4]
C6F5CH(CN)
PhCH(CN)
EtOCOCH(CN)
2,6-(t-Bu)2C6H3O
PhO
0.30a 0.56a
0.53
0.45
0.54
0.44
0.34
0.75
4-NO2C6H4O
PhS
a
C4 atom.
anthraquinone I, and the product ratio changes. In
particular, strong steric requirements for pentafluoro-
phenylacetonitrile anion hinder its approach to the
substrate to a distance ensuring effective Coulomb
interaction with the C4 atom attached to chlorine.
On the other hand, orbital interaction with C1 can
occur at a longer distant; as a result, the nitro group
is replaced mainly in the reaction with 2,6-di-tert-
butylphenoxide ion.
We can conclude that nucleophilic substitution of
the halogen or nitro group in 1,4-halonitroanthra-
quinones I and II is controlled by charge or orbital
interactions, depending on the nucleophile structure.
Naturally, a combination of steric and electronic
factors could vary the type of predominant interaction
and hence the predominant SNAr reaction pathway of
Table 3. Melting points and molecular weights of com-
pounds VII, IX XIV, and XVIII XX
M
Comp.
no.
mp, C
Formula
found calculated
VII
IX
X
200 203 C17H7N3O4
>350 C19H8ClN3O3
>350 C19H8N4O3
317.0430 317.0436
329.0375 329.0356
340.0591 340.0596
458.0325 458.0326
XI
228 230 C22H7F5N2O4
XII
XIII
XIV
XVIII
XIX
XX
193 195 C22H7ClF5NO2 447.0076 447.0085
>340 C29H8ClF10NO 611.0127 611.0135
247 249 C22H8F5NO3
57 59 C19H12ClNO4
266 269 C28H27NO5
213 216 C28H27ClO3
429.0436 429.0424
353.0458 353.0455
457.1886 457.1889
446.1653 446.1649
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 3 2001

414
TABATSKAYA, BEREGOVAYA
aromatic compounds with several possible leaving
groups. An example can be obtained by comparing the
mobilities of the para-chlorine and meta-fluorine
atoms in substituted nitrobenzenes under the action of
benzenethiolate and methoxide ions [9]. Reactions of
4-chloro-3-fluoro-5-X-nitrobenzenes with sodium
benzenethiolate result exclusively in chlorine replace-
ment, whereas charge-controlled reaction with hard
methoxide ion involves both chlorine and fluorine
atoms at a ratio depending on the 5-substituent. One
more example of the nucleophile effect on the direc-
tion of fluorine replacement in perfluoroquinoline and
perfluoroisoquinoline is given in [10]: soft S-anions
replace fluorine atoms in positions 4 and 6, while
hard O-anions attack ortho-positions with respect to
the nitrogen atom.
1-Dicyanomethyl-4-nitroanthraquinone (VII)
and 2-amino-2,3-dicyano-6-nitrobenzanthrone (X).
To a solution of 0.3 0.4 mmol of compound II and
an equimolar amount of malonodinitrile in 6 10 ml
of DMF we added excess (0.7 1.2 mmol) of freshly
calcined potassium carbonate or powdered potassium
hydroxide. The mixture was stirred for 2 or 0.5 h,
respectively, at room temperature and poured into
dilute hydrochloric acid. The precipitate, mp 200
203 C, was compound VII. An attempt to purify it
by chromatography on silica gel resulted in formation
of a complex mixture of products, which we failed to
separate. The reaction of fluoronitroanthraquinone II
with excess malonodinitrile under the conditions for
synthesis of benzanthrones gave compound X. Found,
%: N 16.47. C19H8N4O3. Calculated, %: N 16.46.
1-( -Cyanopentafluorobenzyl)-4-nitroanthra-
quinone (XI). A mixture of anthraquinone II and
pentafluorophenylacetonitrile (IV) (0.33 mmol each)
in 4 ml of DMSO containing 2 mmol of potassium
hydroxide was stirred for 30 min at room temperature
and was then treated as described above for the syn-
thesis of VII. We isolated 126 mg of almost pure
compound XI. Tables 1 and 3 give the parameters of
product XI after additional chromatographic purifica-
tion on silica gel using benzene as eluent.
EXPERIMENTAL
The 1H and 19F NMR spectra of solutions in
CDCl3 were recorded on Bruker WP 200-SY and
AC-200 spectrometers (200 MHz). The IR spectra
were obtained in KBr on a UR-20 istrument. The UV
spectra were measured on a Specord UV-Vis spectro-
photometer. The elemental compositions were deter-
mined from the exact m/z values in the high-resolution
mass spectra which were obtained on a Finnigan MAT
spectrometer. The compositions of the reaction mix-
Reaction of anthraquinone I with pentafluoro-
phenylacetonitrile (IV). Following the procedure
described above for the reaction with anthraquinone
II, from 0.68 mmol of I and 0.68 mmol of IV in
the presence of excess KOH (3.3 mmol) we obtained
1
tures (mol %) were determined from the H and 19F
NMR spectra.
Quantum-chemical calculations with full geometry
optimization of planar structures were performed by
the MNDO method (MNDO-92 program [11]).
1
294 mg of a mixture containing (according to the H
and 19F NMR data) compounds XI and XII at a ratio
of 7:10. The products were separated by chromato-
graphy on silica gel. In some cases, the reaction
mixture contained up to 10 mol % of 1-hydroxy-4-
nitroanthraquinone (XVI). The reaction of equimolar
amounts of I and IV in DMSO in the presence of
sodium hydride in 5 h gave a complex mixture of
products from which, apart from the major com-
ponents, compounds XIII XV were isolated.
Dimethylformamide of pure grade was dried over
zeolite, DMSO of chemically pure grade was dried
over zeolite and distilled under reduced pressure at
50 60 C. Compounds III VI were commercial prod-
ucts which were used without additional purification.
The synthesis of anthraquinones I and II was reported
in [4, 5]. The spectra of hydroxyanthraquinones XIV
and XVI were given in [12]. Table 3 contains the
melting points and molecular weights of the newly
synthesized compounds.
2-Amino-6-chloro-1,3-dicyanobenzanthrone
(IX). To a solution of 0.45 mmol of anthraquinone I
in 7 ml of DMSO we added 3.5 mmol of malonodi-
nitrile (III) and 4.3 mmol of powdered potassium
hydroxide. The mixture was stirred for 0.5 h at 50 C,
diluted with 25 ml of water, refluxed for 1 h, cooled,
and acidified with dilute hydrochloric acid. The pre-
4-Nitro- and 4-chloro-1-[cyano(ethoxycarbonyl)-
methyl]anthraquinones XVII and XVIII obtained
by reaction of I with ethyl cyanoacetate in DMF with
potassium carbonate as a base (20 min, 45 C; yield
0.42 mmol) were separated first by column and then
by thin-layer chromatography on silica gel using
benzene as eluent.
1-(3,5-Di-tert-butyl-4-hydroxyphenyl)-4-nitro-
anthraquinone (XIX) and 1-chloro-4-(3,5-di-tert-
butyl-4-hydroxyphenyl)anthraquinone (XX) obtain-
cipitate was filtered off, washed with water, dried, ed by reaction of equimolar amounts of anthraquinone
and recrystallized from acetic acid. Yield 73%.
I and phenol VI in the presence of potassium
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 37 No. 3 2001

REACTIONS OF 1-HALO-4-NITROANTHRAQUINONES
415
hydroxide (1 h at room temperature or 20 min at
50 C) were separated by chromatography on silica
gel using chloroform as eluent. Under the same condi-
tions, the reaction of anthraquinone II with phenol
gave 62% of compound XIX.
5. Tabatskaya, A.A. and Vlasov, V.M., Russ. J. Org.
Chem., 1997, vol. 33, no. 5, pp. 700 704.
6. Gorelik, M.V., Titova, S.P., and Kanor, M.A.,
Zh. Org. Khim., 1992, vol. 28, no. 11, pp. 2294
2300.
7. Stahly, G.P., J. Org. Chem., 1985, vol. 50, no. 17,
pp. 3091 3094; Buncel, E. and Manderville, R.A.,
J. Phys. Org. Chem., 1993, vol. 6, no. 2, pp. 71 82.
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