Um et al.
The Yukawa-Tsuno equation (eq 4) has been applied
to numerous reactions in which a partial positive charge
develops in the transition state (TS) of the RDS.21-24 The
magnitude of the r value represents the resonance
demand of the reaction center or the extent of resonance
contribution, and the term (σ+ - σ°) is the resonance
substituent constant that measures the capacity for
π-delocalization of the π-electron donor substituent.21
k1 value is smaller for the reactions with primary amines
than for those with secondary amines of similar basicity,
while the k-1/k2 ratios are almost the same for both
reactions with primary and secondary amines. Accord-
ingly, the smaller k1 value has been suggested to be fully
responsible for the lower reactivity of primary amines.
(3) The Hammett plots for the aminolyses of 1a -f are
nonlinear, but the corresponding Yukawa-Tsuno plots
are linear. The nonlinear Hammett plots have been
ascribed to resonance stabilization of the GS of the
substrate as illustrated by the resonance structures I T
II.
kX
log
) F[σ° + r(σ+ - σ°)]
(4)
(
)
kH
The r value has been found to be as large as 1.53 for
solvolysis of 1-aryl-2,2,2-trifluoroethyl tosylates, in which
the resonance demand is significantly high due to the
strong electron-withdrawing ability of the R-CF3 group.23
On the other hand, the r value decreases when the
coplanarity of the aryl moiety and the reaction center is
hindered by twisting the aryl moiety.21-24 However, it has
been reported that a large r value always accompanies a
large F value or vice versa for SN1 reactions.21-24
Exp er im en ta l Section
Ma ter ia ls. Compounds (2,4-dinitrophenyl X-substituted
benzoates, 1a -f) were prepared from the reaction of X-
substituted benzoyl chlorides with 2,4-dinitrophenol in anhy-
drous ether in the presence of triethylamine as reported
previously.9 Their purity was checked by means of melting
point and spectral data such as IR and 1H NMR characteris-
tics. Amines and other chemicals were of the highest quality
available. Distilled water was further boiled and cooled under
nitrogen just before use.
In contrast, the r value in the present reaction in-
creases as the F value decreases, i.e., the r value increases
from 0.49 to 0.58 and 0.60 as the F decreases from 1.56
to 1.38 and 1.19 for the reaction of 1d with ethylamine,
glycylglycine, and trifluoroethylamine, respectively (see
Figure 5). Such an inverse relationship between r and F
is more significant for the reactions with secondary
amines, i.e., the r value increases from 0.36 to 0.65 and
0.89 as the F decreases from 1.48 to 1.04 and 0.71 for
the reactions of 1d with piperidine, morpholine and
piperazinium ion, respectively (see Figure S2 in Sup-
porting Information). The same trend in the magnitude
of the r and F values has been found for reactions of
4-nitrophenyl-substituted benzoates with alicyclic sec-
ondary amines, i.e., the r values are 0.75, 1.05, 1.20, 1.29,
and 1.38 while the F values are 0.75, 0.54, 0.51, 0.44, and
0.42 for the reaction with piperidine, piperazine, 1-(2-
hydroxyethyl)piperazine, morpholine, and 1-formylpip-
erazine, respectively. It is noted that the r value increases
while the F value decreases as the basicity of amines
decreases, indicating that the magnitude of the r and F
values is associated with the RDS of ester aminolyses.
Kin etics. The kinetic study was performed with a UV-vis
spectrophotometer for the slow reactions (t1/2 > 10 s) or a
stopped-flow spectrophotometer for the fast reactions (t1/2
e
10 s) equipped with a constant temperature circulating bath
at 25.0 ( 0.1 °C. All solutions were transferred by gastight
syringes. The reactions were followed by monitoring the
appearance of the leaving 2,4-dinitrophenoxide ion. All reac-
tions were carried out under pseudo-first-order conditions in
which amine concentrations were at least 100 times greater
than the substrate concentration. The reaction medium was
80 mol % H2O/20 mol % DMSO to eliminate the problem of
low solubility of the substrates in pure H2O.
Typically, a reaction was initiated by adding 5 µL of a 0.01
M solution of 2,4-dinitrophenyl benzoate in acetonitrile by
syringe to a 10-mm quartz UV cell containing 2.50 mL of the
thermostated reaction mixture made up of solvent and an
aliquot of the amine stock solution. Generally, the amine
concentration was varied over the range (1-100) × 10-3 M,
while the substrate concentration was 1 × 10-5 M. Pseudo-
first-order rate constants (kobsd) were calculated from the well-
known equation ln(A∞ - At) ) - kobsdt + c. The plots of ln(A∞
- At) vs time were linear over ca. 90% reaction. Usually, five
different amine concentrations were employed, and replicate
values of kobsd were determined to obtain the second-order rate
constants (kN) from the slope of linear plots of kobsd versus
amine concentrations.
Con clu sion s
P r od u cts An a lysis. 2,4-Dinitrophenoxide was liberated
quantitatively and identified as one of the products in the
aminolysis of 1a -f by comparison of the UV-vis spectra after
completion of the reactions with those of the authentic sample
under the same reaction conditions. Other products such as
N-ethylbenzamide and N-(n-propyl)benzamide have been iden-
tified by GC-MS.
The present study has allowed us to conclude the
following: (1) The reaction of 1d with primary amines
proceeds through T( in which the RDS changes from
breakdown of T( to its formation as the basicity of amines
increases (e.g., pKa° ) 9.2). (2) Primary amines are less
reactive than isobasic secondary amines toward 1d . The
Ack n ow led gm en t. This work was supported by
Korea Research Foundation (KRF-2002-070-C00061).
I.H.U. is also grateful for a J SPS fellowship (L-03709).
(21) (a) Tsuno, Y.; Fujio, M. Adv. Phys. Org. Chem. 1999, 32, 267-
385. (b) Tsuno, Y.; Fujio, M. Chem. Soc. Rev. 1996, 25, 129-139. (c)
Yukawa, Y.; Tsuno, Y. Bull. Chem. Soc. J pn. 1959, 32, 965-970.
(22) (a) Fujio, M.; Rappoport, Z.; Uddin, M. K.; Kim, H. J .; Tsuno,
Y. Bull. Chem. Soc. J pn. 2003, 76, 163-169. (b) Nakata, K.; Fujio, M.;
Nishimoto, K.; Tsuno, Y. J . Phys. Org. Chem. 2003, 16, 323-335. (c)
Uddin, M. K.; Fujio, M.; Kim, H. J .; Rappoport, Z.; Tsuno, Y. Bull.
Chem. Soc. J pn. 2002, 75, 1371-1379.
(23) Murata, A.; Sakaguchi, S.; Mishima, M.; Fujio, M.; Tsuno, Y.
Bull. Chem. Soc. J pn. 1990, 63, 1129-1137.
(24) (a) Nakashima, T.; Fujiyama, R.; Kim, H. J .; Fujio, M.; Tsuno,
Y. Bull. Chem. Soc. J pn. 2000, 73, 429-438. (b) Nakashima, T.;
Fujiyama, R.; Fujio, M.; Tsuno, Y. Bull. Chem. Soc. J pn. 1999, 72,
1043-1047. (c) Nakashima, T.; Fujiyama, R.; Fujio, M.; Tsuno, Y.
Tetrahedron Lett. 1999, 40, 539-542.
Su p p or tin g In for m a tion Ava ila ble: Method to deter-
mine the k-1/k2 ratios for the reactions of 1d with primary and
secondary amines; Table S1 for the second-order rate constants
for the reactions of 1d with secondary amines taken from
reference 9; Figures S1 and S2 for Hammett and Yukawa-
Tsuno plots, respectively, for the reactions of 1d with second-
ary amines; and Tables S2-S21 of the kinetic results for the
reactions of 1a -f with primary amines in 80 mol % H2O/20
mol % DMSO at 25.0 ( 0.1 °C. This material is available free
J O049694A
3942 J . Org. Chem., Vol. 69, No. 11, 2004