Equilibrium of acetyl transfer between pyridine N-Oxides and their acetylonium salts

Article abstract of DOI:10.1023/A:1012377922986

The equilibrium constants of acetyl transfer between O-acetylonium salts of a series of pyridine N-oxides and 4-(4-dimethylaminostyryl)pyridine N-oxide in acetonitrile and methylene chloride were determined. The equilibrium constants correlate with the ac

Full text of DOI:10.1023/A:1012377922986

Russian Journal of General Chemistry, Vol. 71, No. 5, 2001, pp. 786 788. Translated from Zhurnal Obshchei Khimii, Vol. 71, No. 5, 2001,
pp. 839 841.
Original Russian Text Copyright
2001 by Rybachenko, Chotii, Kovalenko, Schroeder.
Equilibrium of Acetyl Transfer between Pyridine N-Oxides
and Their Acetylonium Salts
V. I. Rybachenko, K. Yu. Chotii, V. V. Kovalenko, and H. Schroeder
Litvinenko Institute of Physical Organic Chemistry and Coal Chemistry,
National Academy of Sciences of Ukraine, Donetsk, Ukraine
Mickiewicz University, Poznan, Poland
Received December 9, 1999
Abstract The equilibrium constants of acetyl transfer between O-acetylonium salts of a series of pyridine
N-oxides and 4-(4-dimethylaminostyryl)pyridine N-oxide in acetonitrile and methylene chloride were de-
termined. The equilibrium constants correlate with the acid base characteristics of the N-oxides and with
the heats of reactions calculated quantum-chemically.
Previously we studied the kinetic characteristics of
a wide range of acyl transfer reactions (1) between
N,O-acylonium salts and heterocyclic bases (Acyl =
Me
MeO
N
N
N
N
N
N
O (Nu4)
AcO, MeOCO, Me NCO, etc.; Nu and Nu are pyri-
2
i
j
O (Nu5)
dines, their N-oxides, etc.) [1, 2].
O
O (Nu6)
K
+
+
Acyl Nu ,X + Nuj
Acyl Nu ,X + Nui (1)
j
i
Me N
O (Nu7)
2
Evidences were obtained that transfer of, in par-
ticular, acyl group in these compounds occurs by the
concerted mechanism, i.e., in one stage without for-
mation of a stable tetrahedral intermediate [3]. This
fact allows such transformations to be considered as
reactions similar to those of proton [4] and methyl [5]
transfer, extensively studied in organic chemistry and
largely controlled by the thermodynamic factor [6, 7].
For reactions (1), the values of the thermodynamic
driving forces are unknown, since in the literature data
on the equilibrium characteristics of such reactions are
lacking, except a few values of heat [8].
Me N
CH CH
O (Nu8)
2
The choice of these pairs of nucleophiles was gov-
erned by the convenience of analysis, as 4-(4-dimeth-
ylaminostyryl)pyridine N-oxide (Nu8,
395 nm)
max
and its O-acetyl salt ( max 510 nm) have strong
1
1
(
36 000 and 59 000 l mol cm , respectively) ab-
sorption bands which do not overlap with each other
and with absorption bands of the other reactants. It is
particularly important to determine the constants ac-
curately, because by summing up in pairs the reac-
tions with Nu (1) Nu (2) it is possible to estimate
In this work, using UV spectroscopy, we deter-
mined the equilibrium constants of acetyl transfer (1)
in MeCN and CH Cl at 298 K (i = 1 7, j = 8,
i
j
all the elements of the 8 8 matrix (i = 1 8, j =
1 8) many of which cannot be determined by a direct
experiment.
2
2
X = BPh ). The formulas of Nu and Nu are given
4
i
j
below:
The resulting equilibrium constants are listed in
Table 1. It is interesting that the values obtained for
system 5 in different solvents (MeCN and CH Cl )
coincide. We observed the similar trend previously
when studying the heats of acyl transfer [8]. The
insensitivity of the equlibrium constants to the nature
of the solvent is due to the fact that the differences
Nui, j = Cl
N
N
N
O (Nu1)
O (Nu2)
O (Nu3)
2
2
CH CH
1
070-3632/01/7105-0786$25.00 2001 MAIK Nauka/Interperiodica

EQUILIBRIUM OF ACETYL TRANSFER
787
Table 1. Equilibrium constants K of acetyl transfer in MeCN (Nu = Nu Nu , Nu = Nu , 298 K)
i
1
7
j
8
System
i
pKBH
+
(Nui)a
K
System
5
i
pKBH
+
(Nu )a
K
i
2
1
2
3
4
1
2
3
4
0.38
0.79
1.10
1.29
535 30
5
2.05
(4.5 0.3) 10
(4.5 0.4) 10
(1.6 0.1) 10
(5.5 0.3) 10
2
4
6
b
58
4
7.3 0.4
3.0 0.2
6
7
6
7
3.25
3.88
a
The values of pK
+
in water were taken from [9]. The value of pK
+
for Nu in water at 298 K was determined spectro-
BH
BH 6
b
photometrically in our laboratory by N.G. Korzhenevskaya, similarly to [10]. In CH Cl .
2
2
Table 2. Thermal effects of acetyl transfer calculated by the AM1 and PM3 methods and the free energies G determined
0
1
from the equilibrium constants at 298 K (kJ mol )
Determination method
PM3
Determination method
PM3
System
System
AM1
G0
AM1
G0
1
2
3
4
59.0
49.0
21.8
37.2
41.8
40.0
13.4
28.9
15.5
10.1
4.94
2.72
5
6
7
23.0
4.6
14.2
14.2
10.9
7.1
7.70
21.7
30.0
Table 3. Equilibrium constants (K) of acetyl transfer between pyridine N-oxides Nu , Nu in MeCN (i, j = 1 8, 298 K)
i
j
Nuj
Nui
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
1
9.1
1
72
1.8 102
19
1.2 104
3.3 106
9.5 107
525
58
3
5
7
0.11
7.9
1
0.41
1.3 10
3.6 10
1.1 10
2
3
5
7
8
3
1.6 102
4.6 104
1.3 106
1.4 10
5.7 10
8.6 10
3.0 10
1.0 10
1.9 10
0.13
2.4
7.3
2
4
6
8
2
4
5
5.2 10
7.8 10
2.8 10
9.5 10
1.7 10
1
67
1
1.9 10
5.5 10
3.0
4.5 10
1.6 10
5.5 10
1
3
5
7
1
1.5 10 2
5.3 10 5
1.8 10 6
0.33
2.8 102
1
8.3 103
2
4
6
6.2 10
2.2 10
7.5 10
1.4 10
3.6 10 3
1.2 10 4
22
29
1
2
3.4 103
6.3 10
1.8 105
between the energies of solvation and of ionic bonds
in the ion pairs for the initial species and products in
Eq. (1) are insignificant, i.e., the thermodynamic char-
acteristics of reaction (1) mainly depend on the struc-
tural factors. It is seen that, depending on the structure
of the salt cation, the equilibrium constants vary in a
very wide range (up to 8 orders of magnitude). There
is a good linear correlation (2) between log K and
atom of the dimethylamino group is protonated first,
with pK [NMe ] 4.30 [9]. For Nu log K = 0,
BH+
2 8
and pKBH (N O) = 1.43 0.06.
+
The structural similarity between the initial species
and products in reaction (1) suggests that in this reac-
tion the entropy factor does not affect significantly the
position of the equilibrium. Indeed, the entropies of
reactions in systems 1 7, calculated quantum-chemi-
pKBH
+
of the leaving group in the salt cation:
1
1
cally, vary within the range 3 5 J mol deg , i.e.,
are practically constant. In this connection, it is inter-
esting to compare the free energies and heats of reac-
tion (1). Calorimetric measurement of the heats of
reactions (1) for i = 1 7 is difficult because of the
poor solubility of the reactants in aprotic solvents and
low thermal effects [8]. Therefore, we calculated the
heats of acetyl transfer quantum-chemically. The
results of the AM1 and PM3 calculations of the H
values of the reactions listed in Table 1 are given in
Table 2. The calculated heats of the reactions and the
log K = (3.92 0.14)
(2.74 0.06)pKBH ;
+
n 7, r 0.998, s0 0.21.
(2)
From relation (2) it follows that the equilibrium of
acyl transfer, as compared to proton transfer, is
considerably more sensitive to structural features. This
correlation allows evaluation of the basicity of the
N-oxide group in 4-(4-dimethylaminostyryl)pyridine
N-oxide in which, according to [10], the nitrogen
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 5 2001

788
RYBACHENKO et al.
experimentally determined (from the equilibrium con-
stants) free energies of acetyl transfer are poorly
consistent but satisfactorily correlate with each other
1-Acetyloxy-4-(4-dimethylaminostyryl)pyridini-
um tetraphenylborate. To a solution of 0.075 g of
4-(4-N,N-dimethylaminostyryl)pyridine N-oxide in
3 ml of acetic anhydride, we added in the dark a four-
fold excess of sodium tetraphenylborate ( 0.42 g) and
vigorously stirred for 10 15 min. Then about 100 ml
of diethyl ether was added, and after 30 40 min
the dark violet crystalline precipitate was filtered
off; yield 0.14 g ( 80%). Found, %: C 81.5; H 6.7;
N 4.8. C H BN O . Calculated, %: C 81.7; H 6.5;
[
Eqs. (3), (4)], which allows equilibrium (1) to be
classed with isoentropic processes.
AM1:
PM3:
H = ( 7.2 0.7) + (1.5 0.2) G0;
n 7, r 0.961, s0 1.9.
(3)
(4)
4
1
39
2
2
H = ( 5.2 0.8) + (1.2 0.2) G0;
n 7, r 0.931, s0 2.0.
N 4.6. The salt is light-sensitive and was handled in
the dark.
The equilibrium constants of reactions (1) for all
the combinations of Nu and Nu , determined by sum-
mation of the reactions below, are listed in Table 3.
ACKNOWLEDGMENTS
i
j
The authors are grateful for the support to the Mia-
nowski Foundation for Support of Science (Poland).
Ac Nu+ + Nu8 = Ac Nu+ + Nui, K ,
i
8
i8
REFERENCES
Ac Nu+ + Nu = Ac Nu j+ + Nu8, K ,
8
j
8j
1
. Rybachenko, V.I., Schroeder, H., Titov, E.V., Cho-
tii, K.Yu., Semenova, R.G., and Makarova, R.A.,
Zh. Obshch. Khim., 1996, vol. 66, no. 6, pp. 1007
Ac Nu+ + Nu = Ac Nu j+ + Nui, Kij = K K .
i
j
i8 8j
1
011.
2
3
. Rybachenko, V.I., Schroeder, H., Chotii, K.Yu., Se-
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Kovalenko, V.V., Teor. Eksp. Khim., 1998, vol. 34,
no. 3, pp. 96 102.
EXPERIMENTAL
The electronic absorption spectra were taken on
a Specord UV-Vis spectrophotometer. The optical
densities of solutions were measured on an SF-26
spectrophotometer in a temperature-controlled cell
4. Bell, R.P., The Proton in Chemistry, Ithaca: Cornell
(298 0.1 K).
Univ., 1973, 2nd ed.
5
6
. Shaik, S.S., Schlegel, H.B., and Wolfe, S., Theoretical
Aspects of Physical Organic Chemistry. The SN2
Mechanism, New York: Wiley, 1992.
. Marcus, R.A., Faraday Symp. Chem. Soc., 1975,
no. 10, pp. 60 68.
Quantum-chemical calculations were performed by
the AM1 and PM3 methods using the MOPAC 97
program.
+
8
All the salts except AcNu BPh were prepared
4
and purified as in [11]. The N-oxides were distilled
or recrystallized before use. Acetonitrile and methyl-
ene chloride (Aldrich) were stored over molecular
sieves (3 ).
7. Lewis, E.S., Kukes, S., and Slater, C.D., J. Am. Chem.
Soc., 1980, vol. 102, no. 5, pp. 1619 1622.
8. Rybachenko, V.I., Titov, E.V., Gol’dshtein, I.P.,
Grebenyuk, L.V., Chotii, K.Yu., and Semenova, R.G.,
Zh. Fiz. Khim., 1997, vol. 71, no. 2, pp. 248 252.
The equilibrium constants for systems 1 5 were
calculated from the equilbrium concentrations of two
reagents in reaction (1), which, in turn, were deter-
9
. Sawada, M., Yukawa, Ya., Hanafusa, T., and Tsu-
no, Yu., Tetrahedron Lett., 1980, vol. 21, no. 41,
pp. 4013 4016.
mined from the optical densities of the solutions at
1
0. Korzhenevskaya, N.G. and Rybachenko, V.I.,
+
3
95 [Ac Nu ] and 510 nm [Nu ]. For systems 6 and 7
Zh. Obshch. Khim., 1999, vol. 69, no. 3, pp. 437 439.
8
8
we determined only the equilibrium concentrations of
Nu8, 510 nm.
11. Titov, E.V., Chotii, K.Yu., and Rybachenko, V.I.,
Zh. Obshch. Khim., 1981, vol. 51, no. 3, pp. 682 688.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 71 No. 5 2001