3504 J . Org. Chem., Vol. 61, No. 10, 1996
Castro et al.
of 4-nitrophenoxide ion (pKa ≈ 7) compared to that of
phenoxide ion (pKa ≈ 10). The rather small increase (ca.
3-fold) in k2 for NPTOC with respect to PTOC is in line
with the low sensitivity of k2 to leaving group basicity
(âlg) found in the aminolysis of S-aryl O-ethyl dithiocar-
bonates.19 This is in contrast to the large negative âlg
values for k2 found in the aminolysis of aryl acetates (âlg
) -0.5) and diaryl carbonates (âlg ) -0.7).2 This
difference in âlg can be ascribed to the higher polariz-
ability of the CdS bond compared to CdO,20 which has
been found responsible for the low sensitivity of the rate
of expulsion of an amine from a tetrahedral intermediate
(k-1 in Scheme 1) on the leaving group basicity in the
aminolysis of S-aryl dithioacetates.5,21 In contrast, âlg for
k-1 is large in the aminolysis of aryl oxyesters,1 diaryl
carbonates2 and S-aryl thioacetates.3b
The larger k1 values for the reactions of NPTOC
relative to PTOC (Table 3) are reasonable in view of the
more positively charged thiocarbonyl carbon of the former
substrate due to a larger electron withdrawal from the
4-nitrophenoxy group in NPTOC compared to phenoxy
in PTOC.
F igu r e 3. Bro¨nsted-type plots for k1 (statistically corrected),
obtained in the reactions of secondary alicyclic amines with
PTOC (O) and NPTOC (b).
On the other hand the larger k-1 values for the
reactions of PTOC compared to NPTOC (Table 3) are in
agreement with the larger “push” of the phenoxy group
in T( (Scheme 1), relative to that of 4-nitrophenoxy, to
expel the amine.1,2
in the corresponding substrates.2 Also, S-aryl dithioac-
etates21,26 are more readily attacked by alicyclic amines
to form a tetrahedral intermediate than S-aryl O-ethyl
dithiocarbonates.7
The values of k2 for the reactions of the present work
are smaller than those estimated in the aminolysis of
S-phenyl and S-(4-nitrophenyl) O-ethyl dithiocarbonates
The larger k2 and k-1 values for expulsion of phenoxide
ion and an amine from 2, respectively, relative to the
same expulsions from 1,5 are reasonable in view of the
superior “push” provided by EtO in 2 to expel the
nucleofuge and the amine, compared to that exerted by
Me in 1. This is in agreement with the results found in
the aminolyses of S-aryl O-ethyl dithiocarbonates and
S-aryl dithioacetates. Both k-1 and k2 are larger for the
former susbtrates.7,19,21 Also in line with our results is
the stepwise mechanism, through a tetrahedral inter-
mediate, found in the aminolysis of S-(2,4-dinitrophenyl)
thioacetate3b and a concerted process exhibited by the
same aminolysis of S-(2,4-dinitrophenyl) O-ethyl thio-
carbonate.22 It was argued that a tetrahedral intermedi-
ate cannot be formed in the latter reaction due to its
kinetic instability brought about by the larger values of
both k-1 and k2 concerning the “intermediate” with EtO,
compared to that with Me.22 Another example is the
stepwise acetyl transfer between pyridines23 and the
concerted methoxycarbonyl transfer between isoquinoline
and pyridines.24 Electron donation from substituted
phenyl as the “acyl” group of a potential zwitterionic
tetrahedral intermediate has been found responsible for
the kinetic destabilization of the “intermediate” in the
concerted aminolysis of benzoyl fluorides.25
(PDTC and NPDTC): k2 ) 7 × 107 and 3 × 108 s-1
,
respectively.19,21 This is in agreement with the results
in the acetate series, where k2 is larger for PhS- expul-
sion compared to PhO-, and k2 is also greater for
4-nitrobenzenethiolate ion expulsion compared to 4-ni-
trophenoxide.2,4
The k1 values for the aminolysis of PTOC and NPTOC
(Table 3) are slightly larger than those found in the same
aminolysis of PDTC and NPDTC,7 respectively. This
seems surprising in view of the larger electron-donating
resonance effect of PhO (σR ) -0.40)16 relative to PhS
(σR ) -0.23)17 and, presumably, the same larger effect
of 4-nitrophenoxy compared to the corresponding thio
group. This should be in part counterbalanced by a
greater electron-withdrawing inductive effect of PhO
than of PhS (σI ) 0.37 and 0.30, respectively)16,17 and a
larger effect of 4-nitrophenoxy than 4-nitrobenzenethio
(σI ) 0.47 and 0.36, respectively).16c,17 The slightly higher
rate of amine attack on the thionocarbonates compared
to the dithio analogues could be due to the relatively hard
nature of the series of secondary alicyclic amines used
in this study which would prefer the moderately harder
thiocarbonyl carbon of the thionocarbonates compared to
the softer thiocarbonyl carbon of the dithiocarbonates.27
The larger k-1 value for a given amine found in the
aminolysis of PTOC compared to that of PDTC7 should
be due to the greater “push” exerted by PhO compared
to PhS from the corresponding tetrahedral intermedi-
ate.28
The k1 values for the aminolysis of PTOC (Table 3) are
lower than those for the same aminolysis of O-phenyl
thionoacetate.5 This result is in line with the general
higher reactivity of acetates than carbonates, due to a
larger electron donating resonance of EtO relative to Me
(19) Castro, E. A.; Iba´n˜ez, F.; Salas, M.; Santos, J . G.; Sepu´lveda,
P. J . Org. Chem. 1993, 58, 459.
With the values of k1, k-1, and the pKa of amines of
Table 3, the Bro¨nsted-type plots for both rate microco-
efficients can be drawn (Figures 3 and 4). The plots are
(20) Cottrell, T. L. The Strength of Chemical Bonds, 2nd ed.;
Butterworth: London, 1959; pp 275-276. Kwon, D. S.; Park, H. S.;
Um, I. H. Bull. Korean Chem. Soc. 1991, 12, 93. Hill, S. V.; Thea, S.;
Williams, A. J . Chem. Soc., Perkin Trans. 2 1983, 437.
(21) Castro, E. A.; Iba´n˜ez, F.; Santos, J . G.; Ureta, C. J . Org. Chem.
1992, 57, 7024.
(26) Castro, E. A.; Iba´n˜ez, F.; Santos, J . G.; Ureta, C. J . Chem. Soc.,
Perkin Trans. 2 1991, 1919.
(22) Castro, E. A.; Iba´n˜ez, F.; Salas, M.; Santos, J . G. J . Org. Chem.
1991, 56, 4819.
(23) Fersht, A. R.; J encks, W. P. J . Am. Chem. Soc. 1970, 92, 5442.
(24) Chrystiuk, E.; Williams, A. J . Am. Chem. Soc. 1987, 109, 3040.
(25) Song, B. D.; J encks, W. P. J . Am. Chem. Soc. 1989, 111, 8479.
(27) Pearson, R. G. J . Chem. Educ. 1968, 45, 581. Pearson, R. G J .
Am. Chem. Soc. 1986, 108, 6109. Pearson, R. G. J . Chem. Educ. 1987,
64, 561. Pearson, R. G. J . Org. Chem. 1989, 54, 1423.
(28) Hupe, D. J .; J encks, W. P. J . Am. Chem. Soc. 1977, 99, 451.
J ensen, J . L.; J encks, W. P. J . Am. Chem. Soc. 1979, 101, 1476.