Catalytic effects of hydrogen-bond acceptor solvent on nucleophilic aromatic substitution reactions in non-polar aprotic solvent: Reactions of phenyl 2,4,6-trinitrophenyl ether with amines in benzene-acetonitrile mixtures

Article abstract of DOI:10.1016/j.tet.2005.06.009

The effect of addition of small amounts of hydrogen-bond acceptor solvent, acetonitrile, to the benzene medium of the reactions of phenyl 2,4,6-trinitrophenyl ether with aniline and cyclohexylamine, respectively have been investigated. The addition produc

Full text of DOI:10.1016/j.tet.2005.06.009

Tetrahedron 61 (2005) 8035–8040
Catalytic effects of hydrogen-bond acceptor solvent on
nucleophilic aromatic substitution reactions in non-polar aprotic
solvent: reactions of phenyl 2,4,6-trinitrophenyl ether with amines
in benzene–acetonitrile mixtures
*
Olayinka Banjoko and Ibitola A. Babatunde
Department of Chemistry, University of Lagos, Akoka, Yaba, Lagos, Nigeria
Received 27 January 2005; revised 13 May 2005; accepted 2 June 2005
Available online 29 June 2005
Abstract—The effect of addition of small amounts of hydrogen-bond acceptor solvent, acetonitrile, to the benzene medium of the reactions
of phenyl 2,4,6-trinitrophenyl ether with aniline and cyclohexylamine, respectively have been investigated. The addition produced similar
effects in the two reactions—continuous rate increase with increasing amounts of acetonitrile. The results are interpreted in terms of the effect
of amine–solvent interaction on the nucleophilicity of the amines and are in accord with our expectations based on the effects observed for
hydrogen-bond donor solvent, methanol on the same reactions. It is also established from the results that the role of hydrogen-bond acceptor
co-solvent could be played by an added more basic non-nucleophilic amine.
q 2005 Published by Elsevier Ltd.
1. Introduction
SNAr reactions in non-polar aprotic solvents have been
reported. Surh14 studied the reaction of p-fluoronitro-
benzene with piperidine in benzene–DMSO mixtures
while Bernasconi and Zollinger15 studied the reaction of
2,4-dinitrochloro- and 2,4,-dinitrifluorobenzene in benzene–
DMSO mixtures. The latest report was by Nudelman and
Palleros16 on the reaction of 2,6-dinitroanisole with
cyclohexylamine in toluene–DMSO mixtures. As in the
case of hydrogen-bond donor co-solvent methanol, treated
in our last paper,8 we have decided to investigate in detail
the effects of addition of small amounts of another hydrogen
bond acceptor co-solvent, acetonitrile, on the reaction of
2,4,6-trinitrophenyl ether (PTPE) with aniline and cyclo-
hexylamine, respectively, the two reactions that have been
previously studied by us4,9 in pure benzene.
Some unusual findings in the kinetics of aromatic
nucleophilic substitution reactions in non-polar aprotic
solvents reported in the literature in recent times include
the sometimes observed third-order dependence of the
second-order rate constant, kA, on amine concentrations.1–7
As a result of this, a number of mechanisms have been
proposed. Attempts have thus been made by some authors to
provide support for their proposed mechanisms. Remark-
able among these attempts is the study of the effects of
added hydrogen-bond donor and hydrogen-bond acceptor
co-solvents on such reactions in benzene8–11 and toluene.12,13
Having successfully studied and rationalised the sometimes
conflicting effects of hydrogen-bond donor co-solvent,
methanol, on SNAr reactions in non-polar aprotic solvent,
benzene,8,9 and toluene,12,13 we found it necessary to
investigate also the effect of hydrogen-bond acceptor
solvent on these reactions so as to have an overall view of
the mechanisms of the reactions involving these two
different types of solvents in non-polar aprotic medium.
Examination of the literature reveals that only few cases of
the effect of hydrogen-bond acceptor (hba) co-solvent on
2. Results and discussion
The reaction of aniline is third order in amine,4 being
catalysed by two aniline molecules while that of
cyclohexylamine is first order in amine, as it is not base-
catalysed.8 Addition of small amounts of acetonitrile to the
benzene medium of the respective reactions produced
remarkable increases in the rates of the reactions (Tables
1 and 2). Similar observations in the literature include the
reactions of 2,4-dinitrofluoro- and 2,4-dinitrochloroben-
zenes with piperidine in benzene–dimethyl sulfoxide
Keywords: Hydrogen-bond donor; Hydrogen-bond acceptor; Amine–
acetonitrile aggregate.
Corresponding author. Tel./fax: C234 15454891;
e-mail: yinkabanjoko@yahoo.com
*
0040–4020/$ - see front matter q 2005 Published by Elsevier Ltd.
doi:10.1016/j.tet.2005.06.009

8036
O. Banjoko, I. A. Babatunde / Tetrahedron 61 (2005) 8035–8040
Table 1. Second-order rate constants, kA for the reaction of phenyl-2,4,6-
trinitrophenyl ether with aniline in benzene and benzene–acetonitrile
mixtures at 25 8C
DMSO solvent system, these researchers established that
DMSO, when present as a solvent, does increase the
effective basicity of primary and secondary amines and that
this is achieved through the formation of hydrogen-bond
between amine hydrogens and DMSO thus:
[Amine]/mol dm3
0.15
% Acetonitrile (v/v)
103 kA dm3 molK1 sK1
0
5.20
6.27
7.00
7.87
8.73
10.04
12.15
0.1
0.2
0.3
0.4
0.6
0.8
K
RNH2 CSOðCH3Þ2#RHNH/OSðCH3Þ2
(1)
Such aggregates should also be possible with other
hydrogen-bond acceptor solvents, for example, acetonitrile
used in the present study as show in Eq. 2,
0.20
0.25
0.30
0
8.40
9.85
0.1
0.2
0.3
0.4
0.6
0.8
K
10.95
12.05
13.40
15.80
18.15
RNH2 CCH3CN#CH3CN/HNHR
(2)
where K is the association constant for aggregate formation.
The amine in the aggregate formed in Eq. 2 will be a better
nucleophile than the free amine because of the increased
nucleophilicity of the amine-acetonitrile aggregate. It is
therefore proposed that the amine-acetonitrile aggregate of
enhanced nucleophilicity attacks the substrate in the first
step of the SNAr reaction to produce the observed rate
increase.
0
13.10
15.22
16.72
18.50
20.04
23.92
26.70
0.1
0.2
0.3
0.4
0.6
0.8
0
19.00
21.30
23.50
25.90
28.53
32.12
36.50
0.1
0.2
0.3
0.4
0.6
0.8
2.2. Catalysis by acetonitrile
Addition of small amounts of acetonitrile (0.019–0.153 M)
to the benzene medium of the reaction of PTPE with
cyclohexylamine caused a gradual increase in the second
order rate constant, kA (Table 2) with initial value being
k1Z12.84 molK1 sK1, the rate constant for the non-base
catalysed reaction in pure benzene. These observed
increases in rate are due to the increased nucleophilicity
of the amine in the amine-acetonitrile aggregate which
attacks the substrate in the first step of the two-step SNAr
reaction, as well as the catalytic effect of acetonitrile on
the reaction involving the remaining free amine with the
substrate.
[Substrate]Z5.0!10K4 mol dmK3
.
Table 2. Second-order rate constants, kA for the reaction of phenyl-2,4,6-
trinitrophenyl ether with cyclohexylamine in benzene–acetonitrile mixtures
at 25 8C
[Amine]/mol dm3
% Acetonitrile (v/v)
103 kA dm3 molK1 sK1
25!10K4
0
12.84
13.96
14.92
15.96
16.88
17.88
18.80
0.1
0.2
0.3
0.4
0.6
0.8
In earlier studies of SNAr reactions carried out in benzene–
DSMO mixtures by Bernasconi and Zollinger,15 and by
Suhr,14 a considerable increase in reaction rate was
observed in each case for small additions of hydrogen-
bond acceptor solvent (hba) to the reaction medium. The
rate increase was considered to exceed that expected based
on the probable increase in the dielectric constant of the
aprotic medium. Bernasconi ruled out base catalysis
because the pKa of DMSO in water is zero. Suhr, on the
other hand, attributed the increase to a sort of base catalysis.
We also rule out base catalysis for the following reason: the
reaction of PTPE with cyclohexylamine which was found
not to be base-catalysed in our last paper8 has been found to
be catalysed by the hba solvent, acetonitrile, in the present
investigation. Since base catalysis occurs in the second step
of SNAr reactions, it means that the acetonitrile catalysis
could only be taking place in the first step of the SNAr
reaction.
[Substrate]Z2.5!10K5 mol dmK3
.
(DMSO) mixtures by Bernasconi and Zollinger.15 The
reaction of the fluoro-substrate is second order in amine
while that of the chloro is first order. Nudelman and
Palleros16 also observed similar increases in the rate of the
reaction of 2,6-dinitroanisole with cyclohexylamine in
toluene–DMSO mixtures. The above observations show
that added hydrogen-bond acceptor solvents produce an
increase in rates of SNAr reactions in non-polar aprotic
medium irrespective of whether the reaction is base-
catalysed or not. This is unlike the case of hydrogen-bond
donor co-solvent where conflicting effects were observed
for base-catalysed and non-base catalysed reactions.8
2.1. Cause of rate increase
There is of course no doubt that the effect being observed is
a form of catalysis because firstly, the interaction of the hba
solvent with the amine results in increase in the rate of
reaction and secondly, the increase in rate is proportional to
the concentration of the added hba solvent. This is thus
An in-depth study of the interaction of amine with added
hydrogen-bond acceptor solvent, DMSO, was carried out by
Angella and Scott.17 Using Bronsted-acid–base studies
involving some amines with p-nitrophenol in benzene–

O. Banjoko, I. A. Babatunde / Tetrahedron 61 (2005) 8035–8040
8037
catalysis resulting from the enhanced nucleophilicity of the
aggregate formation. At low base concentration,
amine in the formed amine-solvent aggregate which will be
reflected in the enhanced value of the first step rate constant
k10 (compared to that of the free amine k1) and the catalytic
effect of the co-solvent, acetonitrile on the reaction of the
2
ð1 CK½CH3CNꢀÞ zð1 CK½CH3CNꢀÞ
in Eq. 4.
substrate with the remaining free base, aniline, which will
.
For the base-catalysed reaction, when the second step is
rate-determining, Eq. 5 holds
be reflected in the catalytic rate constant k3CH CN
3
2
k3B½Bꢀ
2.3. Mechanism of the base-catalysed reaction
kK0 1½CH3CNꢀ CkK1 [k2 C
(5)
2
1 CK½CH3CNꢀ
As in the previously studied reaction involving methanol
addition,8 SNAr reactions in non-polar aprotic solvents on
addition of a small amount of acetonitrile can be assumed to
involve the attack of the amine-acetonitrile aggregate, as
well as the free amine, on the substrate to produce the
zwitterionic intermediate. Since amine-acetonitrile aggre-
gate formation via hydrogen bonding is likely to be a very
rapid equilibrium process, two possible roots for conversion
of the zwitterionic intermediate into products are proposed.
The reaction of phenyl 2,4,6-trinitrophenyl ether with
aniline in benzene–acetonitrile mixtures (at low acetonitrile
concentrations) can be represented by Scheme 1, where S
stands for the substrate, B for the base, B/NCCH3 for the
amine-acetonitrile aggregate, [SB] for the zwitterionic
intermediate and k10 , the enhanced first-step rate constant
involving the attack of the amine-acetonitrile aggregate on
and Eq. 4 becomes Eq. 6.
½CH CNꢀ ꢀ
ꢁ
k10 K½CH3CNꢀ
2
kCH
CN
3
k1
k3½Bꢀ
3
C
C
k2 C
1CK½CH3CNꢀ
1CK½CH3CNꢀ
1CK½CH3CNꢀ
1CK½CH3CNꢀ
kA Z
kK0 1½CH3CNꢀ CkK1
(6)
If we assume that acetonitrile increases the nucleophilicity
of the amine considerably, k10 [k1 and correspondingly, it
can also be assumed that kK0 1 [kK1
½CH CNꢀ ꢀ
ꢁ
k10 K½CH3CNꢀ
1CK½CH3CNꢀ
2
kCH
CN
3
k1
k3½Bꢀ
3
C
C
k2 C
1CK½CH3CNꢀ
1CK½CH3CNꢀ
1CK½CH3CNꢀ
rkA
Z
kK0 1½CH3CNꢀ
(7)
On expanding and re-arranging, Eq. 7 becomes Eq. 8.
the substrate, k3CH CN, the rate constant for the acetonitrile-
3
k10 k2K k1k2K k2k3CH CN
3
catalysed reaction of the substrate with the remaining free
amine and k3B, the amine catalytic rate constant for the
conversion of the zwitterionic intermediate into products.
kA Z
C
C
kK0 1
kK0 1
kK0 1
!
k10 k2K2 k2k3CH CN
kK0 1
K
3
C
C
½CH3CNꢀ
kK0 1
!
k10 k3BK k3Bk3CH3CN
2
C
C
½Bꢀ
(8)
kK0 1
kK0 1
At constant acetonitrile concentration, this equation reduces
to Eq. 9.
2
kA Z k0 Ck00½Bꢀ
(9)
Scheme 1.
where k0 and k00 are defined by Eqs. 10 and 11, respectively.
Since the amine may exist in free or hydrogen-bonded forms
as given by Eq. 2, the stoichiometric measured concen-
tration, BStoich will be related to the free base [B]Free by
Eq. 3.
k10 k2K k1k2K k2k3CH CN
3
k0 Z
C
C
kK0 1
kK0 1
kK0 1
½B/NCCH3ꢀ C½BꢀFree Z ½BꢀStoich (3)
From Eqs. 2 and 3, the unmeasurable quantities
ꢂ
ꢃ
k10 k2K2
kK0 1
3
C
Ck2kCH CNK ½CH3CNꢀ
(10)
(11)
3
[B/NCCH3] and [B]Free are derived in terms of the
measurable quantity [B]Stoich
.
k10 k3BK k3Bk3CH3CN
kK0 1
k00 Z
C
kK0 1
Application of the steady-state hypothesis to Scheme 1 in
terms of the stoichiometric base concentration leads to Eq. 4
for the observed overall rate-constant kA,
The plots of kA against [aniline]2 gave straight lines giving
credence to Eq. 8. The values of intercepts and slopes are
listed in Table 3.
½CH CNꢀ ꢀ
ꢁ
k10 K½CH3CNꢀ
1CK½CH3CNꢀ
2
kCH
CN
3
k1
k3½Bꢀ
3
C
C
k2 C
k3B½Bꢀ
2
1CK½CH3CNꢀ
1CK½CH3CNꢀ
kK0 1½CH3CNꢀ CkK1 Ck2 C
ð1CK½CH3CNꢀÞ
kA
Z
2
When the amine concentration is kept constant, while the
acetonitrile concentration is varied, Eq. 12, becomes
applicable.
2
1CK½CH3CNꢀ
(4)
where [B] is the total (stoichiometric) base concentration
and K is the association constant for amine-acetonitrile
kA Z k0 Ck00½CH3CNꢀ
(12)

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O. Banjoko, I. A. Babatunde / Tetrahedron 61 (2005) 8035–8040
Table 3. Values of intercepts and slopes of the plots of kA against [aniline]2
at constant acetonitrile concentrations for the reaction of phenyl 2,4,6-
trinitrophenyl ether with aniline in benzene–acetonitrile mixtures
rate-determining and the inequality in Eq. 16 holds.
k2 [kK0 1½CH3CNꢀ CkK1
(16)
(17)
104 k0 /dm3 molK1 sK1
102 k00 dm6/molK2 sK1
% Acetonitrile
k10 K½CH3CNꢀ Ck1 CkCH3CN½CH3CNꢀ
3
0.1
0.2
0.3
0.4
0.6
1.09
1.32
1.59
1.85
2.75
2.24
2.46
2.69
2.94
3.29
rkA Z
1 CK½CH3CNꢀ
At low acetonitrile concentration, 1CK[CH3CN]z1
rkA Z k1 Cðk10 K Ck3CH3CNÞ½CH3CNꢀ
The reaction of PTPE with cyclohexylamine in benzene is
not base-catalysed but is catalysed by acetonitrile and so
conforms with Scheme 2 and Eq. 17 derived from it.
where k0 and k00 are given by Eqs. 13 and 14, respectively.
k10 k2K k1k2K k2k3CH CN
3
k0 Z
C
C
kK0 1
kK0 1
kK0 1
A plot of the second-order rate constant kA against
[acetonitrile] thus gives a straight line with the intercept
k1 being 12.84!10K3 dm3 molK1 sK1 and slope given by
!
k1k3BK k3Bk3CH3CN
2
k10 KCk3CH CN, showing that the reaction is catalysed by
C
C
½Bꢀ
(13)
(14)
3
kK0 1
kK0 1
acetonitrile.
k10 k2K2 k2k3CH CN
kK0 1
K
3
When no acetonitrile is present in the reaction medium
Eq. 17 reduces to Eq. 18.
k00 Z
C
kK0 1
kA Z k1
(18)
The plots of kA against [acetonitrile] gave straight lines in
accordance with Eq. 12. The values of the intercepts and
slopes with the correlation coefficients are listed in Table 4.
The observed rate constant then becomes equal to the rate
constant for the formation of the zwitterionic intermediate
complex in the first step of the reaction in pure benzene, as is
usually the case with non base-catalysed reactions.
Table 4. Values of intercepts and slopes of the plots of kA against
[acetonitrile] at constant aniline concentrations for the reaction of phenyl
2,4,6-trinitrophenyl ether with aniline in benzene–acetonitrile mixtures at
25 8C
[Aniline]/molK1
r
102 k0/molK1 sK1 102 k00 dm6/molK2 sK1
2.5. Other hydrogen-bond acceptors
0.15
0.20
0.25
0.30
0.538
0.8581
1.347
1.933
4.30
6.25
8.76
11.25
0.999
0.998
0.9998
0.9988
Other hydrogen-bond acceptors such as triethylamine or
pyridine, being stronger bases with high pKa values, 11.01
and 5.58, respectively, can easily act as hydrogen-bond
acceptors by forming aggregates with weaker amines like
aniline (pKa, 4.61). These aggregates will be similar to that
proposed for acetonitrile and aniline (Eq. 2). The formed
amine–amine aggregate [R3N/HNHR] can further use the
second proton on the aniline to engage in similar hydrogen-
bond formation with a second tertiary amine molecule thus:
2.4. Mechanism of the reaction that is not base-catalysed
For the reaction of PTPE with cyclohexylamine which is not
base-catalysed, but is catalysed by acetonitrile, Scheme 2
applies.
K
2R3N CRNH2#R3N/HNRH/NR3
(19)
where K is the association constant for the aggregate
formation.
The aggregate formed in Eq. 19 will be a better nucleophile
than that of the free amine molecule. It is this new aggregate
with much enhanced nucleophilicity that now attacks the
substrate in the first step of the SNAr reaction to give the
intermediate complex shown in Scheme 3, where S stands
for the substrate, B for the base, and SB for the zwitterionic
intermediate.
Scheme 2.
Application of steady-state hypothesis to Scheme 2,
working in terms of the stoichiometric base concentration
gives the observed overall second order rate constant kA as
Scheme 3 is similar to Scheme 1 except that two molecules
of hydrogen-bond acceptor (non-nucleophilic amine) are
involved in aggregate formation with the nucleophilic
amine.
ꢀ
ꢁ
k3CH ½CH3CNꢀ
CN
3
k10 K½CH3CNꢀ
1CK½CH3CNꢀ
k1
k2
C
C
1CK½CH3CNꢀ
1CK½CH3CNꢀ
kA Z
(15)
kK0 1½CH3CNꢀ CkK1 Ck2
Since the reaction is not base-catalysed, the first step is
Application of the steady-state hypothesis to Scheme 3 with
the necessary assumptions gives the observable second-
order rate constant kA as shown in Eq. 20.

O. Banjoko, I. A. Babatunde / Tetrahedron 61 (2005) 8035–8040
8039
heated under reflux for 6 h and then distilled. The process
was repeated twice and the middle fraction distilling at
132 8C was collected (lit. 132–133 8C).19 Analar acetonitrile
(500 cm3) was poured over phosphorous pentoxide in a
1-dm3 round bottomed flask, refluxed for 3 h and then
distilled. The process was repeated twice and the fraction
that distilled at 81 8C was collected and stored in a
dessicator (lit.19 bp 81 8C). Reaction products were prepared
by the reaction of the substrate with twice its molar
concentration of the appropriate amine in benzene. The
volume of each reaction was reduced to about a third to
allow the precipitation of the product.
Scheme 3.
k10 k2K k1k2K k2kR N
kK0 1
3
kA Z
C
C
kK0 1
kK031
N-(2,4,6-Trinitrophenyl)aniline was crystallised from
glacial acetic acid and then toluene, mp 181 8C (lit.20
181–182 8C), lmax (C6H6) 370 nm.
!
k10 k2K2 k2k3R N
kK0 1
K
3
2
C
C
½R3Nꢀ
kK0 1
N-(2,4,6-Trinitrophenyl)cyclohexylamine was crystallised
from toluene, mp 90–91 8C (lit.20 181–182 8C), lmax (C6H6)
370 nm.
!
k10 k3BK k3Bk3R3N
kK0 1
2
C
C
½Bꢀ
(20)
kK0 1
Kinetic procedure. The reactions were studied spectro-
photometrically under conditions of excess nucleophile over
substrate by measuring the increase in absorbance of the
product of the reaction of each amine at the respective
absorption maximum. The reaction of aniline with the
substrate was carried out using pipette procedure. Solutions
of PTPE (25 cm3, 1.0!10K3 mol dmK3) and aniline
(50 cm3, 1.5!10K1 to 3.0!10K1 mol dmK3) were allowed
separately to attain 29 8C in a thermostated bath. The aniline
solution (25 cm3) was quickly transferred into the substrate
solution and thoroughly mixed. A 2 cm3 aliquots of the
reaction mixture was immediately pipetted and added to
20 cm3 of quenching mixture (1 mol dmK3 H2SO4/metha-
nol solution) in a small container. The instant of addition of
the aliquot to the quenching mixture was noted as the initial
time (zero time) for the reaction. Ten of such aliquots were
afterwards pipetted at regular time intervals, t, and each
added to 20 cm3 of the quenching mixture. The absorbance
of each quenched reaction mixture was determined. The
reaction of cyclohexylamine with the substrate (which was
much faster) was monitored directly in the spectrophoto-
meter. For reactions in mixed solvents, the acetonitrile
content (v/v) refers to its final volume in the reaction
mixture. In all cases the absorption spectrum of the reaction
mixture at ‘infinity time’ corresponded within 2% of the
‘mock’ infinity prepared by using the respective N-(2,4,6-
trinitrophenyl)amine obtained as a product of the reaction.
The observed pseudo-first-order rate constants were
obtained by the least squares method as the slope of the
correlation log(ANKAt) against t, where AN is the optical
density of the reaction solution measured at ‘infinity’ time
(more than 10 half lives). In all cases, the reaction followed
pseudo-first-order kinetics well to at least 70% reaction. The
second-order rate constants kA were obtained by dividing
the pseudo-first-order rate constants by the amine concen-
trations. All rates were accurate to within G2%.
A plot of kA against [R3N]2 should therefore give a straight
line thus giving credence to the fact that two molecules of
the non-nucleophilic amine as well as two molecules of the
nucleophilic amine (aniline) are involved in the catalysis of
the reaction.
This thus explains the kinetic behaviour observed by
Nudelman and Montsserat in their reaction of 2,4-
dinitrofluorobenzene with aniline in toluene when a non-
nucleophilic amine such as pyridine was added as catalyst.13
It is thus obvious that these authors’ assertions12 that such
kinetic behaviour could not be explained by the mechanism
of Banjoko et al. but by only the ‘dimer nucelophile’
mechanism is clearly erroneous as already established in our
previous paper.8
3. Conclusion
Addition of hydrogen-bond acceptor solvent to SNAr
reactions involving a substrate and an amine in non-polar
aprotic solvent results in the formation of amine-solvent
aggregates of increased nucleophilicity thus causing an
increase in the rate of reaction in addition to its catalytic
effect. The role of the hydrogen-bond acceptor co-solvent
could, however, be played by the addition of a more basic
non-nucleophilic amine, that is, one having a higher pKa
than the nucleophilic amine. The resulting amine–amine
aggregate will be a better nucleophile than the nucleophilic
amine.
4. Experimental
Phenyl 2,4,6-trinitrophenyl ether (PTPE) was prepared by
the reaction of potassium phenolate with picril chloride in
aqueous ethanol. The product was precipitated with water
and recrystallised from ethanol.18 Aniline was dried over
potassium hydroxide for 3 days and twice distilled over Zn
powder (bp 182–1838C, lit.19 1848C). Cyclohexylamine was
Acknowledgements
Grateful acknowledgement is made by O.B, for a grant-in-

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O. Banjoko, I. A. Babatunde / Tetrahedron 61 (2005) 8035–8040
F and NO2 in position 6 of the phenyl rings deleted. [B] in Eq.
(12) should have been deleted.
aid of research from the Central Research Committee of the
University of Lagos.
9. Banjoko, O.; Bayeroju, I. A. J. Chem. Soc., Perkin Trans. 2
1988, 1853–1857.
10. Nudelman, N. S.; Palleros, D. J. J. Chem. Soc., Perkin Trans. 2
1984, 1277–1280.
References and notes
11. Nudelman, N. S.; Marder, M.; Gurevich, A. J. Chem. Soc.,
Perkin Trans. 2 1997, 229–233.
1. Bernasconi, C. F.; Zollinger, H. Helv. Chim. Acta 1966, 49,
2570–2581.
12. Nudelman, N. S.; Montserrat, J. M. J. Chem. Soc., Perkin
Trans. 2 1990, 1073–1076.
2. Bernasconi, C. F.; de Rossi, R. H. J. Org. Chem. 1976, 41,
44–49.
13. Nudelman, N. S.; Alvaro, C. E. S.; Yankelevich, Jeanette S.
J. Chem. Soc., Perkin Trans. 2 1997, 2125–2130.
14. Suhr, H. Chem. Ber. 1963, 67, 893–896.
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