Rate and equilibrium constants of benzoyl group transfer between pyridine N-oxides

Article abstract of DOI:10.1007/s11178-005-0244-5

Kinetic characteristics of 19 transfer reactions of benzoyl group from N-benzoyloxypyridinium salts to pyridine N-oxides and 4-dimethylaminopyridine were studied in acetonitrile by the stopped-flow method. The rate of an identical reaction for 4-methoxypyridine was measured by dynamic NMR spectroscopy. For 5 other identical reactions the rates were estimated from Bronsted correlations. Equilibrium constants were estimated with the use of UV spectrophotometry (6), IR spectroscopy (2), from kinetic data (K _ij = k _ij /k _ji ) (2), and in one case as logK _i-j = logK _i-x - logK _j-x . The second order rate constants (k _ij ) varied in the range 10²-10⁵ l mol ^-1 s^-1, the equilibrium constants (K _ij ) in the range 10²-10^-2; the activation parameters (ΔH ^≠) were within 15-50 kJ mol^-1, (-ΔS ^≠) -20-110 J mol^-1 K^-1. The reactions under study occur in a single stage following the concerted S_N2 mechanism through an early associative transition state. The benzoyl groups exchange rate and equilibrium are well described by simplified Marcus equation (omitting the quadratic term). 2005 Pleiades Publishing, Inc.

Full text of DOI:10.1007/s11178-005-0244-5

Russian Journal of Organic Chemistry, Vol. 41, No. 5, 2005, pp. 774-778. Translated from Zhurnal Organicheskoi Khimii, Vol. 41, No. 5, 2005,
pp. 788-792.
Original Russian Text Copyright Ó 2005 by Rybachenko, Schroeder, Chotii, Lenska, Red’ko, Kovalenko.
.
Rate and Equilibrium Constants of Benzoyl Group Transfer
between Pyridine N-Oxides
V.I. Rybachenko1, G. Schroeder2, K.Yu. Chotii1, B. Lenska2,A.N. Red’ko1, and V.V. Kovalenko1
1Litvinenko Institute of Physical Organic and Coal Chemistry, National Academy of Sciences of Ukraine
Donetsk, 83114 Ukraine
fax: (0622) 558 524
2Faculty of Chemistry, Adam Mickiewicz University, Poznan, Poland
Received March 23, 2004
Abstract—Kinetic characteristics of 19 transfer reactions of benzoyl group from N-benzoyloxypyridinium salts to
pyridine N-oxides and 4-dimethylaminopyridine were studied in acetonitrile by the stopped-flow method. The rate
of an identical reaction for 4-methoxypyridine was measured by dynamic NMR spectroscopy. For 5 other identical
reactions the rates were estimated from Bronsted correlations. Equilibrium constants were estimated with the use of
/
UV spectrophotometry (6), IR spectroscopy (2), from kinetic data (Kij = kij/kji) (2), and in one case as logKi–j
=
logKi–x – logKj–x. The second order rate constants (kij) varied in the range 102–105 l mol-1 s-1, the equilibrium
¹
¹
–2
constants (Kij) in the range 102–10 ; the activation parameters (DH ) were within 15–50 kJ mol-1, (–DS )
–20–110 J mol-1 K-1. The reactions under study occur in a single stage following the concerted SN2 mechanism
through an early associative transition state. The benzoyl groups exchange rate and equilibrium are well described
by simplified Marcus equation (omitting the quadratic term).
Benzoylation is considerably accelerated in the pres-
ence of pyridine N-oxides, highly basic pyridines, and
azoles [1, 2]. This fact is rationalized as formation on the
reaction coordinate of the corresponding acylonium salt
[3]. We formerly investigated the structure of some
benzoylonium salts [4, 5] and estimated the kinetic and
thermodynamic characteristics of formation reactions for
a series of N-benzoyloxypyridinium and N-benzoyl-
pyridinium salts [5, 6]. The present study concerns the
reactivity of these compounds in the exchange processes
of the type (1).
2
%3K
1X
1X
N
N
2
%3K
1X
1X
(1)
Kinetic and in some cases also equilibrium data were
measured in acetonitrile solutions for reaction series where
as nucleophiles and/or departing groups (Nui, Nuj) oper-
ated the following compounds:
1X
1X
1X
ꢀ
2
2
2
ꢀ
&+
1
1
&Oꢀ
1
1X
1X
2
2
1
&+
1
2&+ ꢀ
&+
1ꢁ&+ ꢂ
ꢀ
1X
1X
2
1
1ꢁ&+ ꢂ ꢀ
1
1ꢁ&+ ꢂ ꢃ
salt concentration (cAOC) in the kinetic experiments did
The experimentally determined rates of reactions (1)
follow the second order kinetics. The measurements were
carried out by the stopped-flow method under pseudofirst
order conditions where (CNu/CAOC > 10). The acylonium
–4
not exceed 3 ´ 10 mol l-1 thus excluding the ion associa-
tion effects on the measured magnitudes [7]. The second
order rate constants (kij) were calculated by the least-
1070-4280/05/4105-0774Ó2005 Pleiades Publishing, Inc.

RATE AND EQUILIBRIUM CONSTANTS OF BENZOYL GROUP TRANSFER
775
¹
¹
Table 1. Kinetic (logkij, DHij , –DSij ) and thermodinamic (log Kij) characteristics of reaction (1) (CH3CN, 298 K)
S.
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log K2–5 = log K2–4 – logK5–4; b log K3–5 = log (k3–5/k5–3).
a
ꢀ
K35 were obtained as the ratio of the rate constants k53
and k35. In event i = 2 and j = 5 the equilibrium constant
squares procedure from the relation of kapp to three or
four analytical concentrations of the nucleophilea by equa-
tion (2).
was found as K2–5 = K2–4/K5–4
.
The rate and equilibrium constants logarithms, and also
(2)
kapp = kji + kij[Nuj]
¹
¹
values DH and –DS of reaction (1) at T 298 K are com-
piled in Table 1. Here are also given the pKBH+ values of
bases in water at T 298 K, and the stretching vibration
frequencies of the benzoyl groupAOC in acetonitrile so-
lutions measured in [5, 9].
The thermodynamic parameters of the transition state
were calculated from kij values obtained at three tem-
peratures in the range 298–308 K. The equilibrium con-
stants (Kij) for reactions of Nu4 were evaluated from
UV spectrophotometric measurements as in [8] (for
Nu4 lmax 395 nm, e 36000 l mol-1 cm-1, in N-oxybenzoyl
salt of this nucleophilea lmax 510nm, e59000lmol-1 cm-1).
The equilibrium constants K23, K32 were measured in
CH3CN with the use of IR spectroscopy. Here as an
analytical band was chosen that of the stretching vibra-
tions of the heterocyclic ring skeleton n8a of the
1-benzoyloxy-4-methylpyridinium tetraphenylborate
As seen from Table 1, the variation in rate and
equilibriom constants of reaction (1) depending on struc-
tural factors reaches ~3-4 orders of magnitude. In the
series of reactions under study an isokinetic relation is
valid:
¹
¹
DH ij = (42.8 ± 1.7) + (0.25 ± 0.02)DS
,
(3)
ij
n 12, r 0.970, S0 2.52,
(n8a 1628.0 cm , e 185 l mol-1 cm-1). This absorption
–1
evidencing that the process occurs along a single-type
mechanism [10]. The activation parameters of reaction
(1) are typical of spontaneous (noncatalyzed) bimolecu-
lar reactions of nucleophilic substitution [1, 11]. Inasmuch
band considerably overlapped with the corresponding ab-
sorption band of the 1-benzoyloxypyridinium tetraphenyl-
borate (n8a 1613.5 cm-1 [4]) hampering the precise evalu-
ation of the equilibrium concentrations. Therefore in these
experiments we applied perdeuterated in the heterocy-
clic ring analog of the 1-benzoyloxypyridinium tetra-
¹
as in all cases the DH values are positive the mechanism
of the reaction in question should be concerted [12] and
not stepwise [13]. The proper values of the activation
–1
¹
1
¹
phenylborate where n8a was 1570 cm . The values K53,
enthalpy DH (~15–50 kJ mol- ) and entropy DS
RUSSIAN JOURNALOF ORGANIC CHEMISTRYVol. 41 No. 5 2005

RYBACHENKO et al.
776
tions between the reaction rate and equilibrium [17, 18].
The comparison of rate and equilibrium constants we mea-
sured gives a correlation of the same statistical validity
as (4).
logkij = (3.19 ± 0.14) + (0.51 ± 0.11)logKij,
n 9, r 0.873, S0 0.370
(5)
We believe that the low predictability of the reactivity
in reaction (1) by applying equations (4) and (5) is due to
the fact that the fixed term in these equations is not really
constant.
The modern interpretation of Bronsted and Evans-
/
The rate constants of benzoyl transfer as a function of the
Polanyi [19] equations regards this as the kinetic charac-
teristic (barrier) of the identical reaction. We showed
however experimentally [16, 20] that the rate of identical
(Nui = Nuj) acyl transfer reactions depended both on the
character of the acyl group and on the structure of the
nucleophile (departing group).Actually the measuring of
rates of indentical reactions is a labor-consuming and not
always soluble problem. To a certain extent this state-
ment is also valid for the identical reactions of benzoyl
transfer. However we have a possibility to estimate the
difference between basicity of nucleophile and departing group
(the numeration same as that presented in Table 1).
(~ –20–110J mol-1 K-1] suggest an early associative tran-
sition state where the extent of rupture and formation of
bonds is small. In such cases the variations of reactivity
usually follow well the structural characteristics of the
reagents [14].
We measured for all reactions studied the frequencies
of the stretching vibrations of C=O group in reactants
and products of reaction (1), and basicity constants of
the nucleophile (Nuj) and the departing group (Nui). As
seen from Table 1 the vibration bands nC=O are split for a
number of salts apparently due to the interaction between
the vibrations [15], and the overall pattern of their changes
is poorly interpretable. As a result we did not find any
logk values from the Bronsted correlations for the reac-
/
ii
tion series (1) where either Nui = const or Nuj = const.
The parameters of equations obtained are presented in
Table 2. Here the correlation factors are mainly consid-
erably higher than for equations (4) and (5). The pre-
dicted rate constants of identical benzoyl transfer reac-
tions are given in Table 3. In the case of 4-methoxypyridine
N-oxide the experimental value of identical transfer rate
constant ia also presented. The latter finding was obtained
in the acetonitrile solution by dynamic NMR spectroscopy
[19, 20]. Therewith as indicator signals were chosen the
proton signals from the methoxy group of the 4-methoxy-
pyridine N-oxide (d 3.81 ppm) and of 1-benzoyloxy-4-
methoxypyridinium tetraphenylborate (d 4.12 ppm).
The close values of experimental logk55 and that obtained
from the correlation permit a conclusion that the logkii
found from the correlation actually represent the experi-
mental values. Another argument supporting the above
statement is the nonlinearity of the logkii (Table 3) rela-
correlations between logkij and nC=O
.
The correlation in the coordinates of Bronsted equa-
/
+
tion is achieved better using simultaneously both pKBH
values of the nucleophile and the departing group. The
+
relationship between logkij and values pKBH
=
pK(Nui) – pK(Nuj) is presented on the figure. The great
scatter of the points on this correlation should be indi-
cated, especially the deviation of the point 19, namely,
when the nucleophile and the departing group are of dif-
ferent nature. This phenomenon we frequently observed
investigating, for instance, transfer of acetyl [16] and
dimethylcarbamoyl moieties [17]. This is caused by the
failure of agreement between the basicity and the nu-
cleophilicity at the change of the nucleophile [6]. How-
ever even without point 19 the quality of correlation (4)
cannot be regarded as fair.
+
tion to pKBH in keeping with the theoretical predictions
[21]. Note that in the transfer reaction of dimethylcar-
+
bamoyl group [17] the values logkii and pKBH well fit to
a parabola.
logkij = (3.16 ± 0.23) – (0.67 ± 0.16)pKi + (0.76 ± 0.08)pKj,
Now let us test the possibility to apply logkii for the
analysis of reactivity in reactions (1). In an explicit form
these values enter the Marcus equation [22], that for the
rate and equilibrium can be written [22] as expression
(6).
n 18, r 0.853, S0 0.370
(4)
The other way to analyze the reactivity in one-step
reactions of nucleophilic substitution is the use of rela-
RUSSIAN JOURNALOF ORGANIC CHEMISTRYVol. 41 No. 5 2005

RATE AND EQUILIBRIUM CONSTANTS OF BENZOYL GROUP TRANSFER
Table 2. Parameters of Bronsted equation for the benzoyl transfer (1) in CH CN
777
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ꢀ
logkij = 1/2(logkii + logkjj) + 1/2 logKij
– (logKij)2/8(logkii + logkjj)
Table 3. Kinetic characteristics of identical (i = j) reactions (1)
1X ꢀꢀLꢀ ꢄ
S.
ꢄ
ORJN ꢄ
(6)
ꢁꢄ
ꢂꢄ
ꢃꢄ
ꢄꢄ
ꢅꢄ
ꢆꢄ
ꢅꢃꢆꢆꢄ
ꢆꢃꢇꢅꢄ
ꢆꢃꢇꢊꢄ
ꢆꢃꢋꢈ ꢄ
The data from Tables 1 and 3 show that the value of
ꢅꢃꢈꢉꢄ
ꢋꢃꢇꢉꢄ
ꢋꢃꢌꢆꢄ
ꢇꢃꢅꢊꢄ
ꢆꢃꢍꢍꢄ
the “quadratic” term in equation (6) is less than 0.1 loga-
rithmic unit. Therefore we do not go beyond the two first
terms in equation (6) as has been done for methyl [22],
acetyl [16], and dimethylcarbamoyl transfer [17]. The
correlation processing of the data from Tables 1 and 3
afforded the equation:
ꢆꢃꢅꢍ ꢄ
ꢆꢃꢆꢌꢄꢁꢆꢃꢊꢉꢄ“ꢄꢅꢃꢋꢇꢂ ꢄ
ꢌꢃꢅꢅꢄ
ꢄ
a Average value determined from correlations 3 and 5.
b Average value determined from correlations 4 and 6.
c Experimental value.
logkij = (0.987 ± 0.030)(logkii + logkjj) + (0.964 ± 0.152)logKij,
(7)
n 9, r 0.994, S0 0.266
were obtained on a spectrometer Gemini-200 at opera-
tion frequency 200 MHz. The concentration values were
used in calculation of constants. The reproducibility of
rate constants measured by the stopped-flow method was
±5–7%; that of the rate of identical reaction was better
than ±15%; that of equilibrium constants was no worse
than ±5%.
As compared to equation (5) equation (7) significantly
refines the quality of prediction for benzoyl transfer re-
actions (1). Hence the benzoyl transfer like the other acyl
transfer reactions of type (1) we have previously investi-
gated [16, 17] is governed alongside the thermodynamic
factor, logKij, also by a purely kinetic, 1/2 (logkii + logkjj),
internal barrier [22, 23]. It may be concluded in general
that the characteristic of identical reaction is a useful and
indispensable part of the analysis and prediction of nu-
cleophilic substitution reactions at the carbonyl center.
1-Benzoyloxy-4-(4'-dimethylaminostyryl)pyri-
dinium tetraphenylborate. To a dispersion of 0.240 g
(1 mmol) of 4-(4'-dimethylaminostyryl)pyridine N-oxide,
and 0.342 g (1 mmol) of sodium tetraphenylborate in
a small amount of acetonitrile (~4–5 ml) was slowly added
at stirring a solution of 0.281 g (2 mmol) of benzoyl chloride
in 1 ml of acetonitrile. The reaction mixture was stirred in
the dark for 2 h and then filtered. The filtrate was diluted
with ~100–150 ml of dry ethyl ether. In 40–50 min the
separated precipitate was filtered off and purified by ad-
ditional reprecipitation from the acetonitrile solution with
anhydrous ethyl ether. Yield 0.53 g (80%), dark-violet
crystals. 1H NMR spectrum (acetone-d6), d, ppm: 8.75 d
[2H, Py(a), J 7.3 Hz], 8.27 d [2H, Ps(b), J 7.3 Hz],
8.11 d [2H, Bz(a)], 7.97 d (2H, CH=CH, J 16.4 Hz),
7.74 d (2H, Ph, J 7.1 Hz), 7.63 d [1H, Bz(g)], 7.49 t [2H,
Bz(b)], 7.39 d (2H, CH=CH, J 16.4 Hz), 7.34 m [8H,
EXPERIMENTAL
All reagents were prepared and purified as described
in [1, 2, 4]. Acetonitrile of Acros for HPLC grade was
dried befor use on molecular sieves 3A. The rates of fast
reactions were measured on stopped-flow installation
Applied Photophysics equipped with a temperature-con-
trol device (±0.1 K). Electronic absorption spectra were
registered on a spectrophotometer Specord 200; the opti-
cal densities were measured in a cell under temperature
control at T 298 ± 0.1 K. IR spectra were recorded on a
spectrophotometer Perkin-Elmer Spectrum BX (resolu-
tion 4 cm , Beer–Norton apodization). 1H NMR spectra
–1
RUSSIAN JOURNALOF ORGANIC CHEMISTRYVol. 41 No. 5 2005

RYBACHENKO et al.
778
10. Shaik, S.S., Schlegel, H.B., and Wolfe, S., Theoretical
Aspects of Physical Organic Chemistry. The SN2
Mechanism, New York: J. Wiley & Sons, Inc., 1992, 285 p.
11. Jedrzejczak, M., Motie, R.E., Satchel, D.P.N., Satchel, S.S.,
and Wassef, W.N., J. Chem. Soc., Perkin Trans. II, 1994,
p. 1471.
12. Carroll, F.A., Perspectives on Structure and Mechanism in
Organic Chemistry, California: ITP, 1998, 450 p.
13. Bellamy, L.J., Advances in Infra-red Group Frequencies,
London: Methuen, 1966. Translated under the title Novye
dannye po IK spektram slozhnykh molekul, Moscow: Mir,
1971.
BPh4(o-H)], 7.05 d (2H, Ph, J 7.1 Hz), 6.92 m [8H,
BPh4(m-H)], 6.76 m [8H, BPh4(ï-H)], 2.85 c [6H,
N(CH3)2]. Found, %: C 83.5; H 6.3; N 4.1 C46H41BN2O2.
Calculated, %: C 83.1; H 6.2; N 4.2.
The authors are grateful for help to I. Mianowsky
Foundation for Support of Science (Poland).
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