Aminolysis of Diaryl Carbonates
TABLE 1. Experimental Conditions and kobsd Values for
the Reactions of Anilines with Bis(4-nitrophenyl)
Carbonate (BNPC)a
the slightly curved Brønsted plot found in the reactions
of the former carbonate,3a and the linear Brønsted plot
of slope 0.44 obtained in the SA aminolysis of ClPDNPC.3d
On the other hand, the quinuclidinolysis of MPNPC
and ClPNPC show linear Brønsted plots of slopes ca. 0.9,
consistent with a stepwise mechanism where T( decom-
position is the rate-determining step.3c
Recently, we have investigated the reactions of anilines
with MPDNPC and ClPDNPC in aqueous ethanol, where
linear Brønsted plots of slopes 0.68 and 0.66 were
obtained. These, together with other results, were ex-
plained by a concerted mechanism.3d
To clarify the mechanisms of the aminolysis of diaryl
carbonates, in this work we study kinetically the reac-
tions of anilines with bis(4-nitrophenyl) carbonate (BNPC),
MPNPC and ClPNPC in aqueous ethanol. The main goal
is to evaluate the influence of the amine nature and that
of the leaving and nonleaving groups of the carbonate
on the kinetics and mechanisms. This will be possible
through a comparison of the kinetic results obtained in
the title reactions in aqueous ethanol with those found
in similar aminolyses in the same solvent.3
no. of
runs
amine
pH 103[N]totb/M 103kobsd/s-1
4-phenylenediamine
7.0
7.2
7.5
5.00-42.5 2.11-13.5
5.00-42.5 2.50-15.2
5.00-50.0 3.11-19.3
6
6
7
7
7
7
7
7
7
6
7
7
6
7
7
5
5
4-methoxyaniline
4-methylaniline
aniline
7.0 10.0-100
7.2 10.0-100
7.5 10.0-100
7.0 10.0-100
7.2 10.0-100
7.5 10.0-100
7.0
7.2
7.5
0.899-5.83
0.920-6.35
1.05-6.56
0.632-3.28
0.683-3.39
0.729-3.62
5.00-85.0 0.396-1.44
5.00-100
5.00-100
0.431-1.45
0.435-1.45
3-aminoacetophenone 7.0
2.50-25.0 0.411-0.455
2.50-25.0 0.450-0.520
2.50-25.0 0.484-0.540
0.446-0.472
7.2
7.5
3-aminobenzonitrile
6.8 12.5-85.0
7.0 50.0-170
0.394-0.449
a In 44 wt % ethanol-water, at 25.0 °C, ionic strength 0.2 (KCl),
in the presence of phosphate buffer 0.01M. b Concentration of total
amine (free base plus protonated forms).
Product Studies. 4-Nitrophenoxide anion was identified
as one of the products of the reactions, by comparison of the
UV-vis spectra at the end of the reactions with an authentic
sample of 4-nitrophenol, at the kinetic conditions. 4-Meth-
ylphenyl and 4-chlorophenyl N-phenylcarbamates were found
as the other product of the reactions of MPNPC and ClPNPC
with unsubstituted aniline, respectively, as shown by HPLC,
using as reference the corresponding carbamate. HPLC condi-
tions: column Eurospher C-18 (10 cm, 7 µm), eluant acetoni-
trile/water ) 70/30, isocratic mode 0.5 mL/min.
Experimental Section
Results and Discussion
Materials. The anilines were purified either by distillation
or recrystallization.4 BNPC was
a commercial product.
The title reactions were found to be ruled by the rate
equations depicted in eqs 1 and 2, where NPO- and S
represent 4-nitrophenoxide and the substrate, respec-
tively, and kobsd, k0, and kN are the pseudo-first-order, the
solvolysis, and the anilinolysis rate coefficients, respec-
tively.
MPNPC and ClPNPC were synthesized as reported.3a,5 4-
Methylphenyl and 4-chlorophenyl N-phenyl carbamates, which
are the products of the reactions of MPNPC and ClPNPC with
unsubstituted aniline, were synthesized by the reactions of the
corresponding chloroformate with aniline in acetonitrile.3d
Determination of pKa. The pKa value of the conjugate acid
of phenylenediamine was determined spectrophotometrically
in 44 wt % ethanol-water, at 25.0 °C and an ionic strength of
0.2 (KCl).
Kinetic Measurements. The kinetics of the reactions were
measured by a diode array spectrophotometer in 44 wt %
ethanol-water solution, at 25.0 ( 0.1 °C and an ionic strength
of 0.2 M (maintained with KCl), by monitoring (400 nm) the
formation of 4-nitrophenoxide anion. The initial substrate
concentration was 5 × 10-5 M. To maintain constant pH,
phosphate buffer 0.01 M was employed in all reactions.
Under excess amine, pseudo-first-order rate coefficients
(kobsd) were found throughout. For the reactions of BNPC with
the series of anilines the kobsd values were obtained through
the kinetic software (for first-order reactions) of the spectro-
photometer. On the other hand, for the reactions of ClPNPC
and MPNPC with the series of anilines, the kobsd values were
determined by the initial rate method,6 due to the fact that
these reactions are slow. Tables 1-3 show the experimental
conditions of the reactions and the kobsd values.
d[NPO-]
dt
kobsd ) k0 + kN[amine]
) kobsd[S]
(1)
(2)
The values of k0 and kN were obtained as intercept and
slope, respectively, of the linear plots of kobsd vs [amine],
at constant pH. The kN values show independence of pH.
For the slowest reactions, where the k0 term was impor-
tant compared with kN [amine] in eq 2, the k0 values are
dependent on pH.
Table 4 shows the values of kN for the reactions of
anilines with BNPC, MPNPC, and ClPNPC, along with
the pKa values of the conjugate acids of these amines.
The kN and pKa values were statistically corrected with
q ) 1 for all the anilines, except phenylenediamine with
q ) 2, and p ) 3 for the conjugate acids of all anilines.3d,7
The statistical parameter q is the number of equivalent
basic sites of the amine and p is the number of equivalent
(4) Castro, E. A.; Iban˜ez, F.; Saitu´a, A. M.; Santos, J. G. J. Chem.
Res., Miniprint 1993, 317.
(5) Castro, E. A.; Pavez, P.; Santos, J. G. J. Org. Chem. 2002, 67,
4494.
(6) (a) Sattherwait, A. C.; Jencks, W. P. J. Am. Chem. Soc. 1974,
96, 7019. (b) Ba-Saif, S.; Luthra, A. K.; Williams, J. Am. Chem. Soc.
1987, 109, 6362.
(7) Bell, R. P. The Proton in Chemistry; Methuen: London, 1959; p
159.
J. Org. Chem, Vol. 70, No. 20, 2005 8089