Acetyl exchange between pyridine N-oxides in acetonitrile solutions: An attempt to apply the Marcus equation to acetyl transfer

Article abstract of DOI:10.1023/A:1013919523997

Forty-three (including eight identical) reactions of acetyl transfer from N-acetyloxypyridinium salts to pyridine N-oxides in acetonitrile solutions were studied. The rate constants k₂ vary in the range 10⁷-10^-1 1 mol^-1 s^-1; the equilibrium constants K, in the range 10⁷-10^-7; the activation enthalpy Δ^H≠, in the range 17-30 kJ mol^-1; the activation entropy -ΔS≠, in the range 60-85 J mol^-1 K^-1; and the heat of reaction -ΔH⁰, within ±50 kJ mol^-1. All reactions occur in a single stage by the concerted SN2 mechanism with a low degree of bond cleavage in the transition state. The rate and equilibrium of the acetyl exchange are satisfactorily described by the Bronsted equation. The quality of predicting the reactivity is substantially improved by introducing into the correlation equation a second parameter, the rates of identical reactions.

Full text of DOI:10.1023/A:1013919523997

Russian Journal of General Chemistry, Vol. 71, No. 10, 2001, pp. 1608 1615. Translated from Zhurnal Obshchei Khimii, Vol. 71, No. 10, 2001,
pp. 1696 1703.
Original Russian Text Copyright
2001 by Rybachenko, Schroeder, Chotii, Titov, Kovalenko, Leska, Grebenyuk.
Acetyl Exchange between Pyridine N-Oxides
in Acetonitrile Solutions: An Attempt to Apply the Marcus
Equation to Acetyl Transfer
V. I. Rybachenko, G. Schroeder, K. Yu. Chotii, E. V. Titov,
V. V. Kovalenko, B. Leska, and L. V. Grebenyuk
Litvinenko Institute of Physical Organic and Coal Chemistry, Ukrainian National Academy of Sciences, Donetsk, Ukraine
Mickiewicz University, Poznan, Poland
Received April 26, 2000
Abstract Forty-three (including eight identical) reactions of acetyl transfer from N-acetyloxypyridinium
salts to pyridine N-oxides in acetonitrile solutions were studied. The rate constants k2 vary in the range 107
1
1
7
10 l mol 1 s ; the equilibrium constants K, in the range 107 10 ; the activation enthalpy H , in the range
17 30 kJ mol ; the activation entropy S , in the range 60 85 J mol 1 K ; and the heat of reaction H0,
within 50 kJ mol . All reactions occur in a single stage by the concerted SN2 mechanism with a low degree
1
1
1
of bond cleavage in the transition state. The rate and equilibrium of the acetyl exchange are satisfactorily
described by the Brønsted equation. The quality of predicting the reactivity is substantially improved by
introducing into the correlation equation a second parameter, the rates of identical reactions.
Nucleophilic substitutions at the carbonyl center
are actively studied because of particular significance
of carbonyl compounds in chemistry and biology [1].
At the same time, the problem of predicting their
reactivity is still urgent [2, 3]; many questions remain
to be answered, especially concerning determination
of the reaction mechanism and choice of the analytical
procedure [3, 4]. Examples demonstrating the step
(AN + DN) mechanism of acyl transfer are well known
[5]. However, in aprotic solvents acyl transfer occurs
most frequently in a single stage as a classical SN2
reaction by the concerted (ANDN) or induced con-
certed (AN DN) mechanism [6]. This is the case, e.g.,
for transfer of the methoxycarbonyl group from acyl-
onium salts to N- and O-nucleophiles [7, 8].
between the acetyloxypyridinium salts and pyridine
N-oxides in acetonitrile solutions, including identical
reactions (Nui = Nuj), to elucidate the reaction mech-
anism, to reveal correlations between the reaction
rates and equilibria, and to examine the possibility
and assess the result of using the internal activation
barrier as the reactivity parameter.
CH3CONu+i X + Nuj
CH3CONu+j X + Nui,
(1)
(1),
O N
X = BPh4 ; Nui, Nuj =
Cl
(3),
CH3
O N
(2),
O N
(5),
O N
In the majority of studies devoted to nucleophilic
substitution at the carbonyl group [3, 5, 6], the reac-
tivity is analyzed in terms of the linear free energy
relationship. At the same time, recently in the field of
nucleophilic reactivity active efforts have been made
to develop and adapt new, physically more substanti-
ated models such as the Marcus equation [9] or the
method of crossing parabolas [10], the Shaik Pross
approach [11], cubic reaction diagrams [12], etc., but
examples of their practical use for interpreting the
experimental data on acyl transfer are few, and for
indentical reactions such examples are lacking at all.
CH2CH3 (4), O N
OCH3
(6),
O N
CH=CH
O N
N(CH3)2 (7),
CH=CH
O N
(8),
(9).
N(CH3)2
N
O
O N
The goal of this work was to determine the equilib-
rium and kinetic parameters of acetyl transfer (1)
1070-3632/01/7110-1608$25.00 2001 MAIK Nauka/Interperiodica

ACETYL EXCHANGE BETWEEN PYRIDINE N-OXIDES
The kinetic and equilibrium characteristics of reac- or kobs = kf + k2[Nuj].
1609
tion (1) are listed in the table. The equilibrium con-
stants (logKiej) of reaction nos. 1 6 and 9 16 in the
table were determined by UV spectroscopy in CH3CN
at 298 K. The reagents in these reactions, 4-(4 -N,N-
The constants kr were usually close to zero, with
the condition kf << k2 always fulfilled. The concen-
tration of the acetyloxypyridinium salts in the kinetic
3
experiments at i j did not exceed 3 10 M. In the
dimethylaminostyryl)pyridine N-oxide (Nu7,
max
kinetic studies of the identical reactions, under condi-
tions of stoichiometric equivalence, the salt concentra-
tions were an order of magnitude higher. However,
even under these conditions, as shown in [15], tetra-
phenylborates of the alkoxypyridinium salts are prac-
395 nm) and its O-acetyl salt (
510 nm), have
max
long-wave absorption bands which do not overlap
with each other and with the absorption bands of the
other reactants and are very strong ( 36000 and
1
50000 l mol 1 cm , respectively). This fact allowed
tically fully dissociated (
0.8). The plots for
spectrophotometric monitoring of two equilibrium
concentrations simultaneously, which considerably
improved the accuracy and reliability of determination
of the constants. This is particularly important, be-
cause, by summing in pairs reactions (2a) and (2b),
we were able to calculate the constants of reaction
Eq. (3) were always linear, i.e., the effects of ionic
association or hydrolysis were not manifested. The
characteristics of the transition states Hij and Sij
(see table) were determined from the kij values at three
temperatures in the 30 C range.
nos. 17 34 in the table for which i, j
7.
The heats of the acetyl transfer between Nui and
Nuj (see table) were calculated from the thermochem-
ical cycle (4a) + (4b).
CH3CO Nu+i + Nu7 = CH3CO Nu+7 + Nui Ke7i (2a)
CH3CO Nu+7 + Nuj = CH3CO Nu+j + Nu7 Ke7j (2b)
CH3CO Nui+ + Nuj = CH3CO Nu+j + Nui Kiej = Kie7K7ej (2)
CH3CO Nu+i Cl = CH3COCl + Nui( H0i)
CH3COCl + Nuj = CH3CO Nu+j Cl ( Hj0)
(4a)
(4b)
The optical properties of the species participating
in these reactions are so similar that reliable determi-
nation of the equilibrium concentrations is difficult.
For reaction nos. 7 and 8, the equilibrium constant
was calculated from the kinetic data: logKiej = logkij
CH3CO Nu+i Cl + Nuj=CH3CO Nu+j Cl + Nui ( Hi0j) (4)
The required heats of formation of chlorides of
acetyloxypyridinium cations from the neutral species
[reaction (4b)] in CHCl3 solutions (see table) were
determined by calorimetric titration. The values for
j = 1 3 and 8 were taken from [16], and for j = 7 the
heat of formation of the acetyloxypyridinium salt was
measured similarly in this work.
logkji. The constants for reaction nos. 3 6, 9, and 10
can be determined similarly. It can be readily seen
that the logKiej values calculated by Eq. (2) agree
within the measurement error with the constants deter-
mined from the reaction rates.
The rate constants (logkij) of reaction nos. 4, 6 8,
10, 12, 14, and 16 19 were determined for the first
time in this work in CH3CN solutions by the stopped-
flow procedure. The values for reaction nos. 1, 3, 5,
9, 23, 24, 27, and 28 were obtained similarly [13], and
those for reaction nos. 25 and 26 were determined
previously in [13] and remeasured in this work. The
other values for the reactions with i j were calcu-
Nuj
H0j , kJ/mol
1
2
3
7
8
50 3 60 3 78 3 66 2 109 2
As seen from the table, the constants of equilibri-
um (1) vary within 14 orders of magnitude, and the
rate constants, within 7 orders of magnitude, depend-
ing on structural features of the reactants. The equilib-
rium constants and the heats of reactions show a satis-
factory linear correlation (5) (Fig. 1):
lated as logkij = logKeij + logkji. The constants for the
identical (i = j) reaction nos. 35 38 and 41 43 were
determined by dynamic NMR, and for reaction no. 39
the constant was determined similarly in [14].
H0 = (0.00 2.53)
(7.48 0.69)log Kiej;
(5)
n 14, r 0.952, S0 0.48.
All the experimentally studied reactions (1) follow
second-order kinetics. The constants of the non-
identical reactions were determined by Eq. (3) under
The fact that an isothermodynamic relationship of
type (5) is obeyed indicates [2] that the transition
states are similar for all the members of the reaction
series, i.e., the reaction mechanism is the same. The
activation parameters of reaction (1) are typical of
spontaneous (noncatalytic) bimolecular nucleophilic
pseudo-first-order conditions at
a
10-fold and
larger excess of one of the reactants for its 3 6 analyt-
ical concentrations.
kobs = kf + k2[Ac Nu+i X ],
(3)
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 10 2001

1610
RYBACHENKO et al.
Sij) and thermodynamic (log Kiej, Hiej) characteristics of reaction (1)
Kinetic (logkij, Hij,
Run
no.
Nui
Nuj
log kij
log Kiej
Hiej,
kJ mol
Hij,
kJ mol
Sij,
log (kii kjj)
pKi
pKj
1
1
1
J mol 1 K
1
2
3
4
5
6
7
8
9
1
7
2
7
3
7
4
7
5
7
6
7
8
7
9
7
3
4
5
9
9
9
2
2
3
4
1
2
8
5
5
5
3
3
1
2
3
4
5
6
7
8
9
7
1
7
2
7
3
7
4
7
5
7
6
7
8
7
9
9
9
9
3
4
5
8
5
5
5
3
3
2
2
3
4
1
2
1
2
3
4
5
6
7
8
9
4.31
1.59a
4.14
2.24
3.46
3.00
3.35
3.05
3.24
4.13
3.4a
2.72
2.72
1.76
1.76
0.48
0.48
0.30b
0.30b
1.35
1.35
0.86
0.86
5.26
5.26
3.80
3.80
4.28c
4.1c
2.45c
4.28c
4.1c
2.45c
7.0c
3.1c
1.83c
1.65c
2.2c
1.3c
7.0c
3.1c
1.83c
1.65c
2.2c
1.3c
0
16
16
6
6
12
12
6.59
6.59
6.85
6.85
7.15
7.15
7.16
7.16
7.61
7.61
7.02
7.02
8.31
8.31
8.09
8.09
7.92
7.93
8.38
7.92
7.93
8.38
7.84
7.14
7.44
7.45
6.42
6.68
7.84
7.14
7.44
7.45
6.42
6.68
0.38
1.43
0.79
1.43
1.29
1.43
1.34
1.43
2.05
1.43
1.10
1.43
3.88
1.43
3.25
1.43
1.29
1.34
2.05
3.25
3.25
3.25
0.79
0.79
1.29
1.34
0.38
0.79
3.88
2.05
2.05
2.05
1.29
1.29
0.38
0.79
1.29
1.34
2.05
1.10
1.43
3.88
3.25
1.43
0.38
1.43
0.79
1.43
1.29
1.43
1.34
1.43
2.05
1.43
1.10
1.43
3.88
1.43
3.25
3.25
3.25
3.25
1.29
1.34
2.05
3.88
2.05
2.05
2.05
1.29
1.29
0.79
0.79
1.29
1.34
0.38
0.79
0.38
0.79
1.29
1.34
2.05
1.10
1.43
3.88
3.25
30
17
29
62
83
82
17
81
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
2.54
1.3a
43
43
6.53
1.61a
5.41
5.97
6.07
5.81
1.69a
1.97a
3.36a
7.5
49
5.31
4.46
4.89
4.40
4.13
0.5a
25.5
66.2
28
18
49
2.2a
2.63a
3.24
2.2a
28
18
2.8a
2.93
3.19
3.49
3.5
3.95
3.36d
3.66
4.65
4.43
0
0
0
0
0
0
0
0
22
82
a
ij
ij ji b
ij
ij ji c
ij
i7 7j
d
Calculated from k = K k . Determined from K = k /k . Determined by summation of two reactions (K = K K ). Calcu-
lated by Eq. (9).
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 10 2001

ACETYL EXCHANGE BETWEEN PYRIDINE N-OXIDES
1611
H,
kJ mol
7
1
log k
14
14
23
40
20
6
27
28
16
(a)
5
4
1
8
1
3
10
2
0
20
40
60
4
5
6
7
5
11
2
9
3
34
8
33
(b)
12
4
29
2
1
13
15
2
12
8
4
0
4
8 log Ke
0
1
2
3
4
pKBH+
Fig. 1. Correlation between the thermal effect of reac-
tion (1) and the equilibrium constant. The reaction
numbering is the same as in the table; the same for
Figs. 2 5.
Fig. 2. Correlation between the rate of the acetyl
exchange and basicity of the (a) nucleophile and
(b) leaving group.
+
substitutions [17]. The low activation enthalpies H
group is protonated first, pKBH [N(CH3)2] = 4.30
1
(17 30 kJ mol ) and entropies
S
(from 60 to
[20]. However, the basicity can be estimated from the
correlation between logKe and pKBH
1
82 J mol 1 K ) suggest an associative transition
state with a low extent of cleavage of the bond with
the leaving group [11]. As shown in [18], nucleophilic
substitutions proceeding via stable tetrahedral interme-
diates are usually characterized by negative activation
enthalpies. Therefore, our values of H can be con-
sidered as evidence in favor of a concerted, rather
than a multistage, reaction mechanism.
+
.
+
;
log Ke = (3.92 0.14)
(2.74 0.06)pKBH
(8)
r 0.998, n 7, S0 0.21.
+
Then for Nu7 logKe = 0 and pKBH (N O) is, cor-
respondingly, 1.43. According to [6, 19], the lack of
a break in the point for the identical reaction (in the
+
vicinity of pKBH 1.43 in Fig. 2) is a necessary condi-
Recently there has been a considerable progress in
identification of concerted mechanisms of organic
reactions [19]. Figure 2 shows the correlations be-
tween the rate constants and basicities of the nucleo-
phile and leaving group for the reactions in which Nu7
is either a nucleophile or a leaving group. The corre-
sponding correlation equations are as follows:
tion and evidence of the concerted mechanism.
The coefficients of Eqs. (6) and (7) (
= 1.37
and
= 0.87) are higher than those for tNhue similar
log
reaction series studied previously [17, 21], i.e., ex-
change of the CH3CO group between pyridine N-ox-
ides is, apparently, one of the most structure-sensitive
acetyl transfer reactions. Comparison of the coeffi-
cients of Eqs. (6) and (7) shows that the nucleophilic
substitution at the carbonyl group is more sensitive to
structural changes in the nucleophile as compared to
the leaving group. This result can be checked.
j
log k7j = (1.15 0.08) + (1.37 0.04)pKBH
(6)
+
;
n 8, r 0.997, S0 0.13.
i
log ki7 = (4.64 0.15)
(0.87 0.07)pKBH
(7)
+
;
Figure 3 illustrates the dependence of the rate on
the basicity of the nucleophile and/or leaving group
for a set of identical reaction; the corresponding equa-
tion is the first example of such correlations for acyl
transfer:
n 8, r 0.980, S0 0.23.
As seen, in both cases there are good linear corre-
lations. It should be noted that for 4-(4 -N,N-dimethyl-
aminostyryl)pyridine N-oxide (Nu7) it is impossible
to evaluate experimentally the basicity of the N-oxide
group, since the nitrogen atom of the dimethylamino
i
log kii = (2.83 0.05) + (0.49 0.02)pKBH
(9)
+
;
n 7, r 0.994, S0 0.08.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 10 2001

1612
RYBACHENKO et al.
dynamic NMR spectra. However, another fact is very
5
important. It can be readily shown [22] that within a
reaction series the coefficients of the correlation equa-
tions are related as follows:
log k
42
42
4
= 1/2
= 1/2
+ 1/2
1/2
,
(10)
(11)
(12)
e
ij
ji
ii
ii
ji
39
,
e
+
=
.
ii
ij
37
36
Summation of the coefficients in Eqs. (6) and (7)
(1.37 0.87 = 0.50) leads to a result that excellently
agrees with a slope of 0.49 in the identical series;
this fact proves the reliability of the coefficients in
Eqs. (6) and (7).
38
35
0
1
2
3
4
pKBH+
Figure 4 shows the correlation between the rate and
equilibrium [Eq. (1)] constants, which is described by
the equation
Fig. 3. Correlation between the rate constants of the
identical acetyl transfer reactions and the basicity of the
nucleophile.
log kij = (3.62 0.07) + (0.499 0.025)log Ki4j; (13)
n 43, r 0.950, S0 0.47.
Using Eq. (9), we can estimate, in particular, the
rate constant of the identical reaction for i, j = 6,
logk66 = 3.36, which was difficult to determine ex-
perimentally because of large errors in analysis of the
This equation satisfactorily reproduces the correla-
tion between the kinetic and equilibrium characteris-
log k
23
7
14
18
6
5
17
19
24
16
26
25
42
43
27
1
41
39
3
4
3
37, 38
40
28, 10
5
32
11
22
7
36
8
34
31
12
30
15
35
4
21
20
2
33
2
13
1
29
0
8
6
4
2
0
2
4
6
8
log Ke
Fig. 4. Correlation between the rates and equilibrium constants of acetyl exchange.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 10 2001

ACETYL EXCHANGE BETWEEN PYRIDINE N-OXIDES
1613
logkcalc
8
23
14
17
16
1 25
6
4
2
18
19
10
3
24
26
11
27
28
5
8
6
7
9
12
31
22
4
34
33
30
15
2
13
32
31
20
29
0
2
4
6
8
20
log kobs
Fig. 5. Experimental (logk ) and calculated by Eq. (13) (logk ) rate constants of reaction (1).
obs
calc
tics of reactions (1); Fig. 4 reveals no curvature of the
Brønsted plot. However, identical reactions cannot be
described in the same coordinates; their rate is vari-
able [Eq. (9)], whereas the position of the equilibri-
um is the same, which determines the location of the
corresponding points in Fig. 4.
the edges of reaction series (13) may reach 0.5 log
unit, which is comparable with the errors in experi-
mental determination of the rates of fast reactions and
with the consequences of neglecting logB [23]. There-
fore, it is appropriate to restrict the consideration to
the first two terms in Eq. (14), as it was done, e.g.,
for methyl transfer reactions [22, 25, 26]. By a two-
parameter treatment of the data in the table, we ob-
tained a correlation equation with the coefficients
virtually identical to those in Eq. (14):
At the same time, characteristics of identical reac-
tions are key parameters in predicting the reactivity
[4, 11]. Therefore, let us analyze our results with the
Marcus equation [9]. This equation, when written for
the rates and equilibria, has the following form [22]:
log kij = (0.51 0.02)(log kii + log kjj)
+ (0.50 0.03)log Kiej;
(15)
log kij = 1/2(log kii + log kjj) + 1/2log Kij
(log Kij)2/{16[log B
1/2(log kii + log kjj)]}. (14)
n 34, r 0.987, S0 0.21.
Here logB corresponds to the working term in the
common notation. In solutions this term is neglected
since, on the one hand, it is apparently small [23] and,
on the other hand, there are no verified procedures for
its determination [24]. The last term in Eq. (14) is
responsible for deviations from linearity in the Brøn-
sted coordinates, observed in some cases. Estimation
of the maximal values of this term from the available
data (see table) shows that at (logKij)2 < 25 and
s(logkii + logkjj) 50 the quadratic deviations at
The rate constants calculated by Eq. (14) well
reproduce (Fig. 5) the experimental values.
ij
ij
log k = (1.02 0.02)log k ; n 34, r 0.983, S0 0.29. (16)
obs
calc
As seen, the use of the thermoneutral activation
barrier not only considerably improves the quality of
the correlation treatment but also offers a way of
simple prediction by Eq. (14) of the rates of reactions
that have not been studied experimentally. Compari-
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 10 2001

1614
RYBACHENKO et al.
son of Eqs. (13) and (15) shows that the acetyl trans-
fer in reaction (1) is controlled not only by the ther-
modynamic factor as it could be concluded from
Eq. (13) considered separately, but also from the in-
ACKNOWLEDGMENTS
The authors are grateful to the Mianowski Founda-
tion for Science Support (Poland) for assistance.
ternal activation barrier
= 1/2(logkii + logkjj).
This reactivity parameter was introduced for the first
time by Marcus in analysis of the simplest one-
stage reactions [9], and its role and significance in
nucleophilic substitution are only outlined [22, 26],
especially for reactions at the carbonyl center [27, 28],
and require further study.
REFERENCES
1. Jencks, W.P., Catalysis in Chemistry and Enzymology,
New York: McGraw-Hill, 1969.
2. Palm, V.A., Osnovy kolichestvennoi teorii organi-
cheskikh reaktsii (Principles of the Quantitative
Theory of Organic Reactions), Leningrad: Khimiya,
1977.
EXPERIMENTAL
3. Litvinenko, L.M. and Oleinik, N.M., Mekhanizmy
deistviya organicheskikh katalizatorov (Mechanisms
of the Action of Organic Catalysts), Kiev: Naukova
Dumka, 1984.
All the salts were prepared and purified as de-
scribed in [13, 29]. 4-(4 -N,N-Dimethylaminostyryl)-
pyridine N-oxide was prepared by the procedure given
in [30]. The nucleophiles were distilled or recrystal-
lized before use. Acetonitrile of anh grade (Aldrich)
4. Trushkov, I.V., Chuvylkin, N.D., Koz’min, A.S., and
Zefirov, N.S., Izv. Ross. Akad. Nauk, Ser. Khim., 1995,
no. 5, pp. 804 826.
was stored over 3
molecular sieves before use.
5. Litvinenko, L.M. and Oleinik, N.M., Organicheskie
katalizatory i gomogennyi kataliz (Organic Catalysts
and Homogeneous Catalysis), Kiev: Naukova Dumka,
1981.
The reaction rate constants were determined with
an Applied Photophysics stopped-flow apparatus
equipped with a temperature-controlled unit ( 0.1 K).
The observed pseudo-first-order rate constants were
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