Mechanism and linear free energy relationships in the kinetics of formation of bicyclo[3.3.1]nonane derivatives from 1,3,5-trinitrobenzene, phenyl-substituted 1-benzyl-1-(ethoxycarbonyl)-2-propanones, and triethylamine

Article abstract of DOI:10.1002/kin.20570

The kinetics and mechanism of cyclization of the anionic sigma complex obtained from the reaction of 1,3,5-trinitrobenzene (TNB) and 1-benzyl-1-(ethoxycarbonyl)-2-propanone (BEP) in the presence of triethylamine (NEt₃) have been studied in CH₃CN-CH₃OH (50% v/v). The order of the reaction has been found to be zero in TNB and BEP, unity in NEt₃, and negative and nonintegral in triethylammonium chloride. The rate has been observed to increase slightly with an increase in the concentration of the added salt (tetraethylammonium chloride). The rate constants for the formation of bicyclic adducts from phenyl-substituted BEP and TNB in the presence of triethylamine have been correlated with γ values.

Full text of DOI:10.1002/kin.20570

Mechanism and Linear Free
Energy Relationships in the
Kinetics of Formation of
Bicyclo[3.3.1]nonane
Derivatives from 1,3,5-
Trinitrobenzene, Phenyl-
Substituted 1-Benzyl-1-
(ethoxycarbonyl)-2-
propanones, and
Triethylamine
D. KALAIVANI, M. VASUKI, S. SANTHI
Postgraduate and Research Department of Chemistry, Seethalakshmi Ramaswami College, Tiruchirappalli, 620 002, India
Received 11 February 2011; revised 18 April 2011; accepted 20 April 2011
DOI 10.1002/kin.20570
Published online 21 June 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: The kinetics and mechanism of cyclization of the anionic sigma complex obtained
from the reaction of 1,3,5-trinitrobenzene (TNB) and 1-benzyl-1-(ethoxycarbonyl)-2-propanone
(BEP) in the presence of triethylamine (NEt3) have been studied in CH3CN–CH3OH (50% v/v).
The order of the reaction has been found to be zero in TNB and BEP, unity in NEt3, and negative
and nonintegral in triethylammonium chloride. The rate has been observed to increase slightly
Correspondence to: D. Kalaivani; e-mail: kalaivbalaj@yahoo
.co.in.
c
ꢀ
2011 Wiley Periodicals, Inc.

468
KALAIVANI, VASUKI, AND SANTHI
with an increase in the concentration of the added salt (tetraethylammonium chloride). The
rate constants for the formation of bicyclic adducts from phenyl-substituted BEP and TNB in
ꢀ
C
the presence of triethylamine have been correlated with σ values. 2011 Wiley Periodicals,
Inc. Int J Chem Kinet 43: 467–473, 2011
1-(X-benzyl)-2-propanones were prepared from corre-
sponding substituted benzyl chlorides and the sodium
salt of ethyl acetoacetate [19].
INTRODUCTION
Various electron-deficient aromatic compounds have
been reported to form amazingly stable crystalline
metabridged bicyclic adducts containing bicy-
clo[3.3.1]nonane skeleton with the anions of acetyl-
acetone, dicarbomethoxyacetone, ethylacetoacetate,
cyclohexanone, dibenzylketone etc. [1–4]. Previous
studies in our laboratories have proved that α-bromo
and ω-bromo acetoacetanilides do not form stable bi-
cyclic adducts with 1,3,5-trinitrobenzene (TNB) in the
presence of NEt3 due to the presence of the bulky
bromine atom at the site of condensation [5]. The grow-
ing interest in examining the steric effects caused by
the substituents during the cyclization of 1:1 sigma
adduct prompted us to isolate a series of crystalline
adducts containing the bicyclo[3.3.1]nonane skeleton
from the carbanions of beta keto esters and TNB in our
laboratories [6]. Although a number of carbon-bonded
sigma complexes and bicyclic adducts have been iso-
lated in the crystalline form and characterized [2–9],
there are relatively few [3,10–15] comprehensive ki-
netic and thermodynamic studies of their formation
because some carbon nucleophiles are difficult to gen-
erate under conditions suitable for studies. Structure–
reactivity correlation studies, which are effective tools
in deducing the reaction mechanism in their formation
and decomposition, are still more limited. A search of
the literature indicates that only a few reports have
so far appeared on the structure–reactivity correla-
tions in the conversion of sigma complexes to bi-
cyclo[3.3.1]nonane derivatives [3,10,13,14]. Attempts
have now been made to expand the scope of correlation
analysis in the formation of bicyclic adducts.
Kinetic Measurements
Stock solutions of TNB, NEt3, 1-benzyl-1-
(ethoxycarbonyl)-2-propanone (BEP), and the
salts were prepared in CH3CN CH3OH (50% v/v),
and the required quantities for each run were pipetted
into volumetric flasks and diluted to volume. TNB
and BEP were taken in one flask, and NEt3 and the
salts were in another flask. After injection of the
appropriate solutions into the reaction chamber of
the spectrophotometer thermostated at 298 K, the
decrease in the absorbance as a function of time
was recorded at 570 nm. In all the experiments,
pseudo-first-order conditions are maintained and good
kinetic plots resulted. The results are reproducible
within 5% error. Kinetic measurements were made
employing a Carl-Zeiss Vsu2-P spectrophotometer.
RESULTS AND DISCUSSION
Addition of NEt3 to a solution of BEP and TNB in
CH3CN CH3OH (50% v/v) gives rise immediately
upon mixing to a visible spectrum with two maxima at
450 and 570 nm characteristic of 1:1 Meisenheimer
adduct, and this rapidly changes to give a peak at
500 nm owing to the formation of the bicyclic adduct.
As reported earlier [10,14], in the present study, the
slow disappearance of 1:1 adduct during the formation
of a bicyclic adduct has been followed by observing
the decrease in optical density at 570 nm.
EXPERIMENTAL
Effect of the TNB Concentration
The slope of a log–log plot of kobs versus the con-
centration of TNB (Fig. 1, line A) is ∼ −0.08, which
indicates that TNB has no significant effect on the rate
of the cyclization reaction (see Table I).
CH3CN (Merck, India) was dried over molecular
sieves, type 4A, distilled, and used. Methanol was pu-
rified by the known procedure [16]. TNB was pre-
pared from p-nitrotoluene [17] and was repeatedly re-
crystallized from ethanol (mp 396 K). Triethylamine
(analar grade) was distilled from small quantities of
phenyl isocyanate [18]. Triethylammonium chloride
was recrystallized from absolute ethanol and dried un-
der vacuum. Tetraethylammonium chloride (Fluka AG,
Mumbai, India) was used as such. 1-(Ethoxycarbonyl)-
Effect of BEP and NEt3 Concentrations
Log–log plots of kobs versus BEP concentration have
slopes very close to zero (Fig. 2, lines A–D). This
implies that BEP is not involved in the rate-determining
International Journal of Chemical Kinetics DOI 10.1002/kin

KINETICS AND MECHANISM OF CYCLIZATION OF THE ANIONIC SIGMA COMPLEX
469
+
+
Figure 1 Effect of TNB, N Et4, and H N Et3 concentrations of the rate of cyclization: (A) 1–5, (B) 26–29, and (C) 22–25.
Table I Effect of the TNB Concentration on Cyclization in CH3CN/CH3OH (50% v/v) at 298 K
+
−
+
−
[TNB] (× 104
[BEP] (×101
[NEt3]
(×101 mol dm−3
[H N Et3C l ]
[N Et4C l ]
kobs (×103
Run
mol dm−3
)
mol dm−3
)
)
(× 104 mol dm−3
)
(×102 mol dm−3
)
min−1
)
1
2
3
4
5
2.50
5.00
7.50
10.00
15.00
4.56
4.56
4.56
4.56
4.56
4.03
4.03
4.03
4.03
4.03
8.54
8.54
8.54
8.54
8.54
5.22
5.22
5.22
5.22
5.22
4.20
4.06
3.84
3.70
3.62
step. A log–log plot of kobs versus NEt3 concentrations
has slope close to unity (Fig. 2, line E), indicating that
the cyclization is catalyzed by NEt3 (see Table II).
For the sequence of reactions under stage II outlined
in Scheme 1, two rate-limiting expressions can be de-
rived. On the basis of steady-state treatment,
d[1]
dt
d[3]
dt
k1k2[1][NEt3]
Effect of Salt Concentrations
−
= +
=
+
+
−
k
−1[H N Et3] + k2
A log–log plot of kobs versus H N Et3Cl concentration
is nonlinear with an approximate slope of −0.4 in the
high-concentratio+n regio−n (Fig. 1, curve C). When the
concentration of N Et4Cl is varied, little effect on the
rate is seen (Fig. 1, line B; see Table III). This is ex-
pected for a reaction where charge is neither created
nor destroyed.
On the basis of preequilibrium treatment,
d[1]
dt
d[3]
dt
K1k2[1][NEt3]
−
= +
=
+
[H N Et3]
The kinetic results lead to the establishment of the
mechanism outlined in Scheme 1. The rate of formation
of 3 can be given as
According to the steady-state and preequilibrium
treatment, the cyclization rate should be zero order
in TNB, zero order in BEP, and first order in NEt3.
In t+he present investigation, a plot of log kobs versus
d[1]
dt
d[3]
dt
−
= +
= k2[2]
[H N Et3] is nonlinear with slope ∼−0.4 and hence
International Journal of Chemical Kinetics DOI 10.1002/kin

470
KALAIVANI, VASUKI, AND SANTHI
Figure 2 Effect of BEP and NEt3 concentrations on the rate of cyclization. Line A: runs 6–9; line B: runs 10–13; C: runs
14–17; D: runs 18–21; E: runs 6,10,14, and 18.
Table II Effect of the BEP and NEt3 Concentration on Cyclization in CH3CN/CH3OH (50% v/v) at 298 K
+
+
[TNB] (×104
[BEP] (×101
[NEt3] (×101
[H N Et3C −
]
[N Et4C −
l
]
kobs
(×103 min−1
l
Run
mol dm−3
)
mol dm−3
)
mol dm−3
)
(×104 mol dm−3
)
(×102 mol dm−3
)
)
6
7
8
9
5.00
5.00
5.00
5.00
5.00
5.00
5.00
5.00
5.00
5.00
5.00
5.00
5.00
5.00
5.00
5.00
4.56
6.50
9.12
13.60
4.56
6.50
9.12
13.60
4.56
6.50
9.12
13.60
4.56
6.50
9.12
13.60
2.88
2.88
2.88
2.88
4.32
4.32
4.32
4.32
5.76
5.76
5.76
5.76
7.20
7.20
7.20
7.20
4.69
4.69
4.69
4.69
4.69
4.69
4.69
4.69
4.69
4.69
4.69
4.69
4.69
4.69
4.69
4.69
9.14
9.14
9.14
9.14
9.14
9.14
9.14
9.14
9.14
9.14
9.14
9.14
9.14
9.14
9.14
9.14
2.40
2.39
2.41
2.42
3.72
3.72
3.68
3.73
5.20
5.30
5.31
5.32
5.60
5.62
5.65
5.70
10
11
12
13
14
15
16
17
18
19
20
21
steady-state treatment is applicable. A very slight
anionic intermediate. The mechanistic picture of the
present work is comparable with that of Strauss et
al. [10], who have worked with symmetrical ketone
(dibenzylketone).
increase in the rate with increasing concentration of
+
N Et4 can be attributed to the stabilization of in-
creasing charge in the transition state leading to di-
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KINETICS AND MECHANISM OF CYCLIZATION OF THE ANIONIC SIGMA COMPLEX
471
Table III Salt Effect on Cyclization in CH3CN/CH3OH (50% v/v) at 298 K
+
+
[TNB] (×104
[BEP] (×101
[NEt3] (×101
[H N Et3C −
]
[N Et4C −
l
]
kobs
(×103 min−1
l
Run
mol dm−3
)
mol dm−3
)
mol dm−3
)
(×104 mol dm−3
)
(×102 mol dm−3
)
)
22
23
24
25
26
27
28
29
5.00
5.00
5.00
5.00
5.00
5.00
5.00
5.00
4.56
4.56
4.56
4.56
4.56
4.56
4.56
4.56
5.76
5.76
5.76
5.76
4.03
4.03
4.03
4.03
23.30
70.10
93.50
114.00
10.16
10.16
10.16
10.16
30.60
7.22
4.31
3.72
3.24
2.59
3.83
3.90
4.00
4.30
–
–
–
–
14.40
28.80
Figure 3 Substituent effect in cyclization process. Line A: 4 + log kobs σ at 308 K, line B: 3 + log kobs σ at 303 K, line C: 4
+ log kobs σ at 298 K.
Table IV Substituent Effect in the Cyclization Process
kobs (×103 min−1
)
Ea
ꢀH= (kJ mol−1) ꢀS= (kJ mol−1) ꢀG= (kJ mol−1
)
Serial No. Substituent 298 K 303 K 308 K (kJ mol−1
)
at 303 K
at 303 K
at 303 K
σ
1
2
3
4
5
6
7
4-OCH3
4-CH3
4-C2H5
H
4-Cl
4-Br
0.91 1.25 1.67
1.26 1.69 2.24
1.29 1.74 2.35
1.71 2.49 3.49
4.31 5.01 5.76
4.67 5.28 5.90
1.38 1.97 2.70
47.86
43.84
46.48
51.58
22.09
17.53
56.27
45.31
41.33
43.93
51.54
19.53
15.02
53.76
−151.08
−161.75
−152.88
−124.85
−224.59
−239.11
−119.49
91.12
90.37
90.29
89.41
87.65
87.52
89.99
−0.268
−0.170
−0.150
0
0.227
0.232
–
2-NO2
+
[TNB] = 5 × 10−4 mol dm−3, [BEP] = 4.56 × 10−1 mol dm−3, [NEt3] = 3.31 × 10−1 mol dm−3, [HN Et C−] = 5.75 × 10−4 mol dm−3
,
l
3
+
[N Et C−] = 2.51 × 10−2 mol dm−3, and solvent CH3CN/CH3OH (50% v/v).
l
4
ous phenyl-substituted BEP in the presence of NEt3
Substituent Effect
in CH3CN:CH3OH (50% v/v) are presented in Ta-
ble IV. It is evident from the magnitude of the rate
constants that the reaction is susceptible to polar
The rate constants for the formation of bicyclic adducts
from the 1:1 Meisenheimer adducts of TNB and vari-
International Journal of Chemical Kinetics DOI 10.1002/kin

472
KALAIVANI, VASUKI, AND SANTHI
Scheme 1 Mechanism of cyclization of anionic sigma complex.
effects. The rate data (Table IV) have been correlated
with σ [20] (Fig. 3). The plot reflects good correla-
tion (line A; r = 0.9999; s = 0.035; ρ = 1.25). As
noticed in our previous work [12], the rate increases
with an increase in the electron-withdrawing power of
the substituent. The observed rates of the various se-
lected substituents follow the following order: p-OCH3
< p-CH3 < p-C2H5 < P-H < P-Cl < p-Br. For these
substituents, proton abstraction (k1; Scheme 1) is rate
determining. However, when a nitro group is intro-
duced at the 2-position, the rate observed is much less
than that of p-bromo compound. As the reaction site
is one atom away from the ring-carrying substituent,
the retardation in the rate observed could not be due to
steric hindrance. The large I effect of the nitro group
makes the proton abstraction (k1; Scheme 1) faster
and consequently cyclization (k2; Scheme 1) becomes
rate determining in this case. When ꢀH= values are
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KINETICS AND MECHANISM OF CYCLIZATION OF THE ANIONIC SIGMA COMPLEX
473
plotted versus ꢀS=(Table IV; the isokinetic relation-
ship), a straight line is obtained for all substituents
including the o-NO2 group, indicating the operation of
same mechanism in all cases. As p-nitrobenzyl chlo-
ride is a serious contaminant of 1-(ethoxycarbonyl)-1-
(p-nitrobenzyl)-2-propanone, kinetic studies could not
be attempted with this substance. The entropy of ac-
tivation is negative under all experimental conditions,
indicating that the freedom of motion in the side chain
of the sigma complex is hindered in the transition state
due to the cyclization.
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International Journal of Chemical Kinetics DOI 10.1002/kin