Kinetics and mechanism of the aminolysis of 4-methylphenyl and 4-chlorophenyl 4-nitrophenyl carbonates in aqueous ethanol

Article abstract of DOI:10.1021/jo034008d

Reactions of 4-methylphenyl 4-nitrophenyl carbonate (MPNPC) and 4-chlorophenyl 4-nitrophenyl carbonate (CIPNPC) with a series of quinuclidines (QUIN) and the latter carbonate with a series of secondary alicyclic amines (SAA) are subjected to a kinetic investigation in 44 wt % ethanol-water, at 25.0°C and an ionic strength of 0.2 M. The reactions were followed spectrophotometrically at 330 or 400 nm (4-nitrophenol or 4-nitrophenoxide anion appearance, respectively). Under excess amine, pseudo-first-order rate coefficients (k_obsd) are found. For all these reactions, plots of k_obsd vs free amine concentration at constant pH are linear, the slope (k_N) being independent of pH. The Broensted-type plots (log k_N vs pK_a of the conjugate acids of the amines) for the reactions of the series of QUIN with MPNPC and ClPNPC are linear with slopes (β_N) 0.88 and 0.87, respectively, which are explained by a stepwise process where breakdown of a zwitterionic tetrahedral intermediate (T^±) to products is rate limiting. The Broensted-type plot for the reactions of the series of SAA with ClPNPC is biphasic with slopes β₁ = 0.2 (high pK_a region) and β₂ = 0.9 (low pK_a region) and a curvature center at pK_a⁰ = 10.6. This plot is in accordance with a stepwise mechanism through T^± and a change in the rate-determining step, from T^± breakdown to T^± formation as the basicity of the SAA increases. Two conclusions arise from these results: (i) QUIN are better leaving groups from T^± than isobasic SAA, and (ii) the nonleaving group effect on k_N for these reactions is small, since β_nlg ranges from -0.2 to - 0.3. From these values, it is deduced that ClPNPC is ca. 70% more reactive than MPNPC toward SAA and QUIN, when expulsion of the leaving group from T^± is the rate determining step.

Full text of DOI:10.1021/jo034008d

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Kin etics a n d Mech a n ism of th e Am in olysis of 4-Meth ylp h en yl a n d
4-Ch lor op h en yl 4-Nitr op h en yl Ca r bon a tes in Aqu eou s Eth a n ol
Enrique A. Castro,* Mo´nica Andu´jar, Alejandra Toro, and J ose´ G. Santos*
Facultad de Quı´mica, Pontificia Universidad Cato´lica de Chile, Casilla 306, Santiago 22, Chile
ecastro@puc.cl
Received J anuary 3, 2003
Reactions of 4-methylphenyl 4-nitrophenyl carbonate (MPNPC) and 4-chlorophenyl 4-nitrophenyl
carbonate (ClPNPC) with a series of quinuclidines (QUIN) and the latter carbonate with a series
of secondary alicyclic amines (SAA) are subjected to a kinetic investigation in 44 wt % ethanol-
water, at 25.0 °C and an ionic strength of 0.2 M. The reactions were followed spectrophotometrically
at 330 or 400 nm (4-nitrophenol or 4-nitrophenoxide anion appearance, respectively). Under excess
amine, pseudo-first-order rate coefficients (kobsd) are found. For all these reactions, plots of kobsd vs
free amine concentration at constant pH are linear, the slope (kN) being independent of pH. The
Bro¨nsted-type plots (log kN vs pKa of the conjugate acids of the amines) for the reactions of the
series of QUIN with MPNPC and ClPNPC are linear with slopes (âN) 0.88 and 0.87, respectively,
which are explained by a stepwise process where breakdown of a zwitterionic tetrahedral
intermediate (T() to products is rate limiting. The Bro¨nsted-type plot for the reactions of the series
of SAA with ClPNPC is biphasic with slopes â1 ) 0.2 (high pKa region) and â2 ) 0.9 (low pKa region)
and a curvature center at pKa0 ) 10.6. This plot is in accordance with a stepwise mechanism through
T
( and a change in the rate-determining step, from T( breakdown to T( formation as the basicity
of the SAA increases. Two conclusions arise from these results: (i) QUIN are better leaving groups
from T( than isobasic SAA, and (ii) the nonleaving group effect on kN for these reactions is small,
since ânlg ranges from -0.2 to - 0.3. From these values, it is deduced that ClPNPC is ca. 70% more
reactive than MPNPC toward SAA and QUIN, when expulsion of the leaving group from T( is the
rate determining step.
In tr od u ction
reports concern the reactions of secondary alicyclic
amines (SAA) with phenyl 2,4-dinitrophenyl carbonate,3c
quinuclidines (QUIN) with 3-nitrophenyl, 4-nitrophenyl,
3,4-dinitrophenyl, and 2,4-dinitrophenyl phenyl carbon-
ates,5 and the reactions of SAA with 4-nitrophenyl and
2,4-dinitrophenyl 4-methylphenyl carbonates.6 Some of
these reactions have been described as stepwise, through
a zwitterionic tetrahedral intermediate (T(), due to the
biphasic Bro¨nsted-type plots obtained.5 These biphasic
plots show two linear portions, at low (with slope â ) 1)
and high (â ) 0.3) amine pKa values, which have been
assigned to rate-determining breakdown and formation
of T(, respectively.5
The kinetics and mechanism of the aminolysis of aryl
esters is well documented;1 nevertheless, the mechanism
of the aminolyses of alkyl aryl carbonates2-4 and diaryl
carbonates3c,5,6 have received little attention. The latter
(1) (a) J encks, W. P.; Gilchrist, M. J . Am. Chem. Soc. 1968, 90, 2622.
(b) Satterthwait, A. C.; J encks, W. P. J . Am. Chem. Soc. 1974, 96, 7018,
7031. (c) J encks, W. P. Chem. Soc. Rev. 1981, 10, 345. (d) Williams, A.
Adv. Phys. Org. Chem. 1992, 27, 2. (e) Williams, A. Chem. Soc. Rev.
1994, 23, 93. (f) Bennet, A. J .; Brown, R. S. In Physical Organic
Chemistry of Acyl Transfer Reactions, Comprehensive Biological
Catalysis; Academic Press: 1998; p 293. (g) Koh, H. J .; Shin, C. H.;
Lee, H. W.; Lee, I. J . Chem. Soc., Perkin Trans. 2 1998, 1329. (h)
Schmeer, G.; Six, C.; Steinkirchner, J . J . Solution Chem. 1999, 28,
211. (i) Um, I.-H.; Park, Y.-M.; Shin, E.-H. Bull. Korean Chem. Soc.
1999, 20, 392. (j) Rajarathnam, D.; Nadar, P. A. Int. J . Chem. Kinet.
2001, 33, 137. (k) Rajarathnam, D.; J eyakumar, T.; Nadar, P. A. Int.
J . Chem. Kinet. 2002, 34, 366. (l) Koh, H. J .; Han, K. L.; Lee, H. W.;
Lee, I. Bull. Korean Chem. Soc. 2002, 23, 715.
(2) (a) Bond, P. M.; Moodie, R. B. J . Chem. Soc., Perkin Trans. 2
1976, 679. (b) Castro, E. A.; Gil, F. J . J . Am. Chem. Soc. 1977, 99,
7611. (c) Castro, E. A.; Freudenberg, M. J . Org. Chem. 1980, 45, 906.
(d) Castro, E. A.; Iban˜ez, F.; Lagos, S.; Schick, M.; Santos, J . G. J .
Org. Chem. 1992, 57, 2691.
(3) (a) Castro, E. A.; Iban˜ez, F.; Saitu´a, A. M.; Santos, J . G. J . Chem.
Res., Synop. 1993, 56. (b) Castro, E. A.; Cubillos, M.; Santos, J . G. J .
Org. Chem. 2001, 66, 6000. (c) Castro, E. A.; Aliaga, M.; Campodo´nico,
P.; Santos, J . G. J . Org. Chem. 2002, 67, 8911.
Other aminolysis reactions of diaryl carbonates have
been found to be concerted, i.e., with no intermediate, in
a single step. This is the case of the reactions of SAA
with phenyl 2,4-dinitrophenyl carbonate3c and with 4-me-
thylphenyl 2,4-dinitrophenyl carbonate.6 The former
reaction exhibits a linear Bro¨nsted-type plot with slope
â ) 0.4,3c and the latter shows a slightly curved plot, with
â ) 0.1 and â ) 0.5 at high and low amine pKa,
respectively.6
With the aim to clarify the mechanism of the aminoly-
sis of diaryl carbonates, in this work we report a kinetic
investigation of the reactions of 4-methylphenyl 4-nitro-
phenyl carbonate (MPNPC) and 4-chlorophenyl 4-nitro-
phenyl carbonate (ClPNPC) with a series of QUIN and
(4) Koh, H. J .; Lee, J . W.; Lee, H. W.; Lee, I. Can. J . Chem. 1998,
76, 710.
(5) Gresser, M. J .; J encks, W. P. J . Am. Chem. Soc. 1977, 99, 6963.
(6) Castro, E. A.; Andu´jar, M.; Campodo´nico, P.; Santos, J . G. Int.
J . Chem. Kinet. 2002, 34, 309.
10.1021/jo034008d CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/29/2003
3608
J . Org. Chem. 2003, 68, 3608-3613

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Aminolysis of Aryl 4-Nitrophenyl Carbonates
TABLE 1. Exp er im en ta l Con d ition s a n d kobsd Va lu es for Rea ction s of a Ser ies of QUIN w ith 4-Meth ylp h en yl
4-Nitr op h en yl Ca r bon a te (MP NP C)a
number
of runs
b
amine
quinuclidine
pH
FN
103 [N]tot (M)c
103 kobsd (s-1
)
10.36
10.66
10.96
9.0
9.3
9.6
7.5
7.8
8.1
6.8
0.33
0.50
0.67
0.169
0.289
0.448
0.33
0.50
0.67
0.33
0.50
0.67
1.24-15.0
1.25-15.0
1.26-12.0
0.667-8.00
1.00-12.0
1.33-16.5
4.20-50.0
14.1-85.6
16.6-56.8
87.6-150
50.0-175
50.0-170
2.80-44.9
4.88-72.8
6.71-89.0
0.512-5.70
0.900-10.7
2.10-15.0
0.0867-0.812
0.375-1.96
0.475-1.52
0.182-0.311
0.218-0.559
0.270-0.745
6
7
7
7
6
7
7
6
6
7
6
6
3-hydroxyquinuclidine
3-chloroquinuclidine
3-quinuclidinone
7.1
7.4
a
b
In 44 wt % ethanol-water, at 25.0 °C, ionic strength ) 0.2 M (KCl). Free amine fraction. c Concentration of total amine (free base
plus protonated forms).
coefficients (kobsd) were found throughout, the reactions being
followed for at least 5 half-lives.
For all of the reactions, the pH was maintained constant
(three pH values for each amine) by the buffer formed by
partial protonation of the amine.
The kobsd values and the experimental conditions for the
reactions of MPNPC with the series of QUIN are exhibited in
Table 1. Those for the reactions of ClPNPC with the series of
SAA and QUIN are shown in Table 2.
the latter carbonate with a series of SAA. By comparison
of the kinetic results obtained in this work with those
for the aminolysis of other carbonates, we evaluate the
effect of the nonleaving group of the substrate and the
amine nature on the kinetics and mechanism of these
reactions.
P r od u ct Stu d ies. 4-Nitrophenoxide anion (and/or its con-
jugate acid in the reactions at pH lower than ca. 7) was
identified as one of the products of the aminolyses of MPNPC
and ClPNPC. This was carried out by comparison of the UV-
vis spectra after completion of these reactions with that of an
authentic sample of sodium 4-nitrophenoxide, under the
kinetic conditions. The other product of the reactions of
MPNPC and ClPNPC with 3-hydroxyquinuclidine was identi-
fied as the corresponding carbamate. This was achieved by
comparison of the UV-vis spectra after completion of these
reactions with those at the end of the reactions of 3-hydroxy-
quinuclidine with 4-methylphenyl and 4-chlorophenyl chloro-
formates, respectively. The other product of the reaction of
ClPNPC with morpholine was identified as 1-(4-chlorophen-
oxycarbonyl)morpholine, by comparison of the HPLC retention
time of the product after completion of the reaction with that
of an authentic sample. HPLC conditions: column, Supelcosil
LC-18-DB (25 cm, 5 µm); eluant, acetonitrile/water) 50/50;
isocratic mode, 0.4 mL/min.
Exp er im en ta l Section
Ma ter ia ls. The SAA and QUIN series of amines were
purified as reported.5,7 The substrates, MPNPC6 and ClPNPC,8
were synthesized as described in the literature.6,8 One of the
products of the reaction of ClPNPC with morpholine, 1-(4-
chlorophenoxycarbonyl)morpholine, was synthesized by the
reaction of 4-chlorophenyl chloroformate with morpholine, in
acetonitrile, according to a general method.9 This product
melted at 76.8-77.3 °C (lit.10 mp 80.0 °C) and was identified
1
by H NMR, 13C NMR, and IR analyses.
Deter m in a tion of p Ka . The pKa values of the conjugate
acids of quinuclidine and 3-hydroxyquinuclidine were deter-
mined spectrophotometrically, and those of 3-chloroquinucli-
dine and 3-quinuclidinone were measured potentiometrically,
all of them in 44 wt % ethanol-water solution, at 25.0 ( 0.1
°C and an ionic strength 0.2 (KCl).
Kin etic Mea su r em en ts. The kinetics of the reactions were
analyzed through a diode array spectrophotometer in 44 wt
% ethanol-water, at 25.0 ( 0.1 °C and an ionic strength of
0.2 M (maintained with KCl). The reactions were followed at
400 nm (appearance of 4-nitrophenoxide anion), except the
reaction of ClPNPC with piperazinium ion, which was carried
out at a low pH, following at 330 nm the appearance of
4-nitrophenol.
Resu lts a n d Discu ssion
The pseudo-first-order rate constants (kobsd) obtained,
under excess amine, for all of the reactions obey eq 1,
where k0 and kN are the rate coefficients for solvolysis
and aminolysis of the substrates, respectively. The values
of k0 and kN show no dependence on pH within the pH
range employed. These values were obtained as the
intercept and slope, respectively, of linear plots of kobsd
against free amine concentration ([free amine]) at con-
stant pH.
All reactions were studied under at least a 12-fold amine
excess over the substrate, the initial concentration being 2.5
× 10-5 M. Under these conditions, pseudo-first-order rate
kobsd ) k0 + kN[free amine]
(1)
For the reactions of MPNPC, the k0 values were
negligible, except in the reactions with quinuclidine and
3-hydroxyquinuclidine (ca. 0.0001 s-1), although they
were subjected to large errors. For the reactions of
ClPNPC, the k0 values were significant in the reactions
with piperidine, piperazine, quinuclidine and 3-hydroxy-
quinuclidine (ca. 0.001 s-1) and in the reaction with
(7) Castro, E. A.; Ureta, C. J . Org. Chem. 1989, 54, 2153.
(8) Castro, E. A.; Pavez, P.; Santos, J . G. J . Org. Chem. 2002, 67,
4494.
(9) Dannley, R. L.; Lukin, M.; Shapiro, J . J . Org. Chem. 1955, 20,
92. Broxton, T. J .; Deady, L. W.; Lim, R. H. K. Aust. J . Chem. 1981,
34, 1993.
(10) Union Chim. Belge, Belg. P. 532057, 1954; Beilstein 27/1, p 292,
1983.
J . Org. Chem, Vol. 68, No. 9, 2003 3609

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Castro et al.
TABLE 2. Exp er im en ta l Con d ition s a n d kobsd Va lu es for Rea ction s of a Ser ies of SAA a n d QUIN w ith 4-Ch lor op h en yl
4-Nitr op h en yl Ca r bon a te (ClP NP C)a
number
of runs
b
amine
pH
FN
103 [N]tot (M)c
103 kobsd (s-1
)
piperidine
piperazine
10.52
10.82
11.12
9.41
9.71
10.01
8.79
9.09
9.39
8.18
8.48
8.78
7.33
7.63
7.93
5.07
5.37
5.67
10.36
10.66
10.96
9.0
0.33
0.50
0.67
0.33
0.50
0.67
0.33
0.50
0.67
0.33
0.50
0.67
0.33
0.50
0.67
0.33
0.50
0.67
0.33
0.50
0.67
0.169
0.289
0.448
0.33
0.50
0.67
0.33
0.50
0.67
0.30-3.0
0.30-3.0
0.30-3.0
1.0-10.0
1.0-10.0
1.0-10.0
6.0-60.0
6.0-60.0
6.0-60.0
10.0-100
10.0-100
10.0-100
10.0-100
10.0-100
10.0-100
10.0-90.0
10.0-100
10.0-100
1.24-15.0
1.25-15.0
1.26-12.0
0.667-8.00
1.00-10.0
1.33-15.9
8.40-50.0
14.1-100
16.6-71.2
87.6-150
25.0-175
25.0-170
18.0-100
23.0-140
28.0-200
7.10-83.0
13.0-130
19.0-170
12.0-110
19.0-190
28.0-250
13.0-97.0
18.0-170
23.0-220
1.20-9.20
1.70-15.0
2.70-19.0
9
10
10
10
10
10
10
9
14
10
10
10
10
10
10
9
10
10
6
7
6
7
6
6
7
1-(2-hydroxyethyl)piperazine
morpholine
1-formylpiperazine
piperazinium ion
quinuclidine
0.053-0.240
0.062-0.340
0.094-0.510
7.58-71.5
10.5-110
7.72-101
3-hydroxyquinuclidine
3-chloroquinuclidine
3-quinuclidinone
0.766-9.49
1.09-11.0
9.3
9.6
7.5
7.8
8.1
6.8
7.1
7.4
2.21-19.3
0.231-1.17
0.671-3.58
0.824-3.13
0.329-0.571
0.144-0.788
0.174-0.997
7
7
7
7
7
a
b
In 44 wt % ethanol-water, at 25.0 °C, ionic strength ) 0.2 M (KCl). Free amine fraction. c Concentration of total amine (free base
plus protonated forms).
TABLE 3. Va lu es of p Ka for Con ju ga te Acid s of
Qu in u clid in es (QUIN) a n d kN Va lu es for Rea ction s of
QUIN w ith 4-Meth ylp h en yl 4-Nitr op h en yl Ca r bon a te
(MP NP C) a n d 4-Ch lor op h en yl 4-Nitr op h en yl Ca r bon a te
(ClP NP C)a
102kN (s-1 M-1
)
amine
quinuclidine
3-hydroxyquinuclidine
3-chloroquinuclidine
3-quinuclidinone
pKa
MPNPC
ClPNPC
10.66
9.7
7.8
930 ( 30
136 ( 3
4.4 ( 0.2
0.62 ( 0.002
1360 ( 60
170 ( 5
7.0 ( 0.2
0.80 ( 0.03
7.1
a
Both the pKa and kN values were determined in 44 wt %
ethanol-water, at 25.0 °C, ionic strength ) 0.2 M (KCl).
piperazinium ion (ca. 0.00002 s-1). These k0 values were
much smaller than the aminolysis term (kN[free amine]
in eq 1) and also subjected to large errors.
F IGURE 1. Bro¨nsted-type plots for the quinuclidinolysis of
MPNPC (O) and ClPNPC (b) in 44 wt % ethanol-water, at
25.0 °C, ionic strength ) 0.2 M (KCl). The slopes (â) are 0.88
and 0.87, respectively.
The kN values obtained for the quinuclidinolysis of
MPNPC and ClPNPC are shown in Table 3; the corre-
sponding Bro¨nsted plots can be seen in Figure 1.
The values of kN for the reactions of SAA with ClPNPC
are shown in Table 4. These values, as well as those of
the pKa of the conjugate acids of the amines, were
statistically corrected with q ) 2 for piperazine (q ) 1
for all other SAA) and p ) 2 for all the conjugate acids of
the amines, except that for piperazinium ion with p )
4.11,12 The parameter q is the number of equivalent basic
sites on the free amine, and p is the number of equivalent
dissociable protons on the conjugate acid of the amine.12
With these corrected values, the Bro¨nsted-type plot was
drawn (see Figure 2). For comparison, Figure 2 also
shows the Bro¨nsted-type plot obtained for the reactions
of the same amines with MPNPC in the same solvent.6
The reactions of MPNPC and ClPNPC with the series
of QUIN show linear Bro¨nsted-type plots (Figure 1) of
slopes â ) 0.88 ( 0.08 and 0.87 ( 0.06, respectively. From
the magnitude of these slopes, it can be deduced that
(11) Castro, E. A.; Ureta, C. J . Chem. Soc., Perkin Trans. 2 1991,
63.
(12) Bell, R. P. The Proton in Chemistry; Methuen: London, 1959;
p 159.
3610 J . Org. Chem., Vol. 68, No. 9, 2003

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Aminolysis of Aryl 4-Nitrophenyl Carbonates
SCHEME 1
TABLE 4. Va lu es of p Ka for Con ju ga te Acid s of
Secon d a r y Alicyclic Am in es (SAA) a n d kN Va lu es for
Rea ction s of SAA w ith 4-Ch lor op h en yl 4-Nitr op h en yl
Ca r bon a te (ClP NP C)a
since there seems to be little influence of the substituents
4-Cl and 4-Me on the k2/k-1 ratio (see below).
log kN (ClPNPC) ) log kN (MPNPC) + 0.24 (2)
amine
pKa
kN (s-1 M-1
)
piperidine
piperazine
1-(2-hydroxyethyl)piperazine
morpholine
1-formylpiperazine
piperazinium ion
10.82
9.71
9.09
8.48
7.63
5.37
89 ( 3
With the kN values found in the present reactions of
QUIN together with the pKa values of the conjugate acids
of the nucleophiles, and the nonleaving groups (10.1 and
9.4 for 4-methylphenol and 4-chlorophenol in water,
respectively), eq 3 can be deduced by dual regression
analysis (n ) 8, R2 ) 0.992). In this expression, N and
nlg refer to the nucleophile and the nonleaving group,
respectively; the pKa(N) and pKa(nlg) coefficients (âN and
26 ( 0.3
6.1 ( 0.1
3.1 ( 0.1
0.28 ( 0.01
0.0072 ( 0.0003
a
Both the pKa and kN values were determined in 44 wt %
ethanol-water, at 25.0 °C, ionic strength ) 0.2 M (KCl).
â
nlg, respectively) are both subjected to an error of ( 0.1.
log kN ) -6.2 + 0.87pKa(N) - 0.20pKa(nlg) (3)
The sensitivity of log kN to the nonleaving group (ânlg
) -0.2) is similar to that obtained for log k1 for the
reactions of the series of SAA with aryl 4-nitrophenyl
thionocarbonates (ânlg ) -0.2 ( 0.1).13 Assuming that the
latter value can be extended to carbonates, and since this
value measures the sensitivity on pKa(nlg) of log k1 and
the former that of logkN (where kN ) k1k2/k-1), the value
of ânlg for k2/k-1 must be 0 ( 0.2. Namely, there seems to
be little or no influence of the above nonleaving group
substituents on the k2/k-1 ratio. It is reasonable that all
these values of ânlg are small (for both rate-limiting steps)
since there should be little charge development on the
oxygen atom of the nonleaving group in going from the
substrate to the corresponding transition state,5 since the
bond between the oxygen (nlg) and the central carbon is
not being broken.
The above result is in accordance with the finding of
Gresser and J encks for the quinuclidinolysis of diaryl
carbonates in water.14 They found that there is little
effect of the nonleaving group substituents on the k2/k-1
ratio (ânlg ) 0.2 ( 0.04).14,15 Since it is known that the
inductive effect is the most important one for groups
attached to the central carbon of a T( intermediate,16 this
means that inductively electron-donating substituents
attached to the nonleaving group of T( slightly favor
aryloxide leaving from T( relative to amine expulsion.14
F IGURE 2. Bro¨nsted-type plots (statistically corrected, see
text) for the reactions of SAA with MPNPC (O, ref 6) and
ClPNPC (b, this work) in 44 wt % ethanol-water, at 25.0 °C,
ionic strength ) 0.2 M (KCl).
these reactions proceed by a stepwise mechanism, i.e.,
through the formation of a zwitterionic tetrahedral
intermediate (T() whose breakdown to products is the
rate-limiting step (see Scheme 1).2,5-7,11
A plot (not shown) of log kN for the quinuclidinolysis
of ClPNPC vs log kN for the same aminolysis of MPNPC
is linear, according to eq 2 (n ) 4; R2 ) 0.995). The
magnitude of this intercept indicates that ClPNPC is ca.
70% more reactive toward a given QUIN than its
4-methyl derivative. Since for these reactions the k2 step
of Scheme 1 is rate determining, it follows that kN ) k1k2/
k-1, where k1/k-1 is the equilibrium constant for the first
step of this scheme. The larger kN for the reaction of a
given QUIN with ClPNPC than that with MPNPC can
be attributed to a larger k1 value for the former reaction
(13) Castro, E. A.; Leandro, L.; Quesieh, N.; Santos, J . G. J . Org.
Chem. 2001, 66, 6130.
(14) Gresser, M. J .; J encks, W. P. J . Am. Chem. Soc. 1977, 99, 6970.
(15) Error of ânlg for k2/k-1 is not reported in ref 14. This was
calculated by us from the data in Table 1 of ref 14.
(16) Sayer, J . M.; J encks, W. P. J . Am. Chem. Soc. 1973, 95, 5637.
Fox, J . P.; J encks, W. P. J . Am. Chem. Soc. 1974, 96, 1436.
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