Amination of phenylketenes. Substituent effect on amine-catalyzed tautomerization of amide enol

Article abstract of DOI:10.1002/poc.3162

The transient intermediates with infrared bands at 1676-1680 cm ^-1 observed for reaction of substituted phenylketenes with diethylamine in acetonitrile were suggested to be the amide enols rather than the zwitterions on the basis of the theoretical calculations. A single broad band at 1674 cm^-1 observed for reaction with the primary amines was attributed to overlap of two bands of the intermediate (amide enol) and the final product (amide). The substituent effect for the second-order rate constants of diethylamine-catalyzed tautomerization of the amide enol intermediates to give the amides was analyzed successfully by the Yukawa-Tsuno equation, giving a ρ value of 0.63 and an r^- value of 1.31. The r^- value larger than unity for pK_a of phenols indicates that the negative charge formed at an oxygen atom of the amide enol at the transition state is significantly delocalized into the aromatic π-system through the ethenyl group. This r^- value was considered to reflect an intrinsic property of β-phenylenolate skeleton. A remarkably small ρ value attributes to the cyclic transition structure where the negative charge disperses in a six-member ring. Copyright 2013 John Wiley & Sons, Ltd. The substituent effect for the reaction of the amide enol intermediate generated from phenylketene with diethylamine has been analyzed. The large r^- of 1.31 was attributed to an intrinsic property of β-phenylenolate skeleton. A remarkably small ρ of 0.63 suggested the cyclic transition structure. Copyright

Full text of DOI:10.1002/poc.3162

Special Issue Article
Received: 20 March 2013,
Revised: 6 May 2013,
Accepted: 5 June 2013,
Published online in Wiley Online Library: 10 July 2013
(wileyonlinelibrary.com) DOI: 10.1002/poc.3162
Amination of phenylketenes. Substituent
effect on amine-catalyzed tautomerization
of amide enol†
Md. Mizanur Rahman Badala, Min Zhanga, Shinjiro Kobayashia
and Masaaki Mishimaa*
The transient intermediates with infrared bands at 1676–1680 cmꢀ1 observed for reaction of substituted
phenylketenes with diethylamine in acetonitrile were suggested to be the amide enols rather than the zwitterions
on the basis of the theoretical calculations. A single broad band at 1674 cmꢀ1 observed for reaction with the primary
amines was attributed to overlap of two bands of the intermediate (amide enol) and the final product (amide). The
substituent effect for the second-order rate constants of diethylamine-catalyzed tautomerization of the amide enol
intermediates to give the amides was analyzed successfully by the Yukawa–Tsuno equation, giving a ρ value of 0.63
and an rꢀ value of 1.31. The rꢀ value larger than unity for pKa of phenols indicates that the negative charge formed
at an oxygen atom of the amide enol at the transition state is significantly delocalized into the aromatic π-system
through the ethenyl group. This rꢀ value was considered to reflect an intrinsic property of β-phenylenolate skeleton.
A remarkably small ρ value attributes to the cyclic transition structure where the negative charge disperses in a
six-member ring. Copyright © 2013 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this paper.
Keywords: phenylketene; amide enol; substituent effect; LFP-TRIR; Yukawa–Tsuno equation
amide enol (or zwitterion). The substituent effects for the
INTRODUCTION
second-order rate constants of the amination were analyzed in
terms of the Yukawa–Tsuno equation (1).[24–28]
Ketenes that possess reactive carbonyl and alkenyl groups are
highly useful and versatile organic reactive intermediates and
o
ꢀ
ꢀ
logðk=koÞ ¼ ρðσ þ r ΔσR
Þ
(1)
therefore have been utilized continuously for organic syntheses
since the first publication by Staudinger.[1] In recent years, this
field has developed remarkably to a wide variety of reac-
tions.[2–5] One of the useful reactions is nucleophilic addition of
amines to ketene, and their reaction mechanisms have been
studied extensively.[6–22] Experimental and computational inves-
tigations revealed that ketene amination reactions proceed
through a nucleophilic attack of amines across the C = O bond
of ketene leading to amide enol (or zwitterions). In a subsequent
step, tautomerization of the amide enol preferentially occurs
with catalysis of amine to give the amide. The kinetics studies
of the reactive phenylketenes with time-resolved infrared
detection (TRIR) showed only first-order dependency on the
amine concentration, indicating that an attack of the first amine
molecule is rate determining.[14] The amide enol intermediates
(or zwitterions) were detected directly, and the amine-catalyzed
tautomerization of these transients was found to be also depen-
dent on amine concentration.[14] On the other hand, the stable
ketenes follow kinetics that are dependent on both [amine]2
and [amine]3, indicating that addition of one amine molecule is
reversible and that further amine molecules are needed to
complete the reaction.[17] Thus, kinetics of amination of ketenes
is contingent upon the structure of ketenes.
o
ꢀ
where σ is a standard substituent constant, ΔσR is a resonance
substituent constant, and rꢀ is a resonance demand parameter
that is a measure of degree of π-delocalization. The curved
substituent effects observed at a high reactivity region were
described by introducing a quadratic term of substituent param-
eters as an approximation. From these analyses, the transition
structure for an attack of amine to ketene to yield the intermedi-
ate was considered to be zwitterionic. To understand a whole
mechanism of amination of ketene (Scheme 1), it is necessary
to elucidate the second process, the base (amine)-catalyzed
tautomerization of the amide enol intermediates to give the
amides.
* Correspondence to: M. Mishima, Institute for Materials Chemistry and
Engineering, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812–8581,
Japan. E-mail: mishima@ms.ifoc.kyushu-u.ac.jp
†
This article is published in Journal of Physical Organic Chemistry as a Special
issue: In Memoriam edited by Michael Page (University of Huddersfield, Chem-
istry, Queensgate, Huddersfield, HD1 3DH, United Kingdom).
This manuscript is dedicated to the memory of Professor Rory More O'Ferrall
for his outstanding contribution to physical organic chemistry.
In the previous paper,[23] we reported the kinetic studies for
reaction of a series of phenylketene as reactive ketenes with
various amines to elucidate the transition structure of the first
process, nucleophilic addition of amine to ketene leading to
a M. M. R. Badal, M. Zhang, S. Kobayashi, M. Mishima
Institute for Materials Chemistry and Engineering, Kyushu University, Hakozaki,
Higashi-ku, Fukuoka 812-8581, Japan
J. Phys. Org. Chem. 2013, 26 1071–1076
Copyright © 2013 John Wiley & Sons, Ltd.

M. M. R. BADAL ET AL.
Scheme 1. Mechanism for reaction of phenylketenes with amines
Although theoretical studies revealed a cyclic transition
state,[15,16,22] there is no experimental evidence for the transition
structure. Therefore, we have carried out further kinetic study on
amination of m,p-substituted phenylketenes by means of a laser
flash photolysis with TRIR detection (LFP-TRIR). Diethylamine has
been chosen as a base in this study because the second-order
rate constant of the second process was reported to be much
smaller than that for the first process from phenylketene to the
amide enol.[14] This allows us to neglect practically the first
process in consideration of the kinetic equation for the second
process. The substituent effect on the second-order rate
constant for the second process has been analyzed in terms of
the Y–T equation (1), and the transition structure is discussed
in this paper.
ysis of α-diazoacetophenone showed only one transient with a
band maximum at 1674 cmꢀ1 as shown in Fig. 1(b). This band
is significantly broad compared with that observed for the inter-
mediates of the reaction with diethylamine. Moreover, the decay
of the transient at 1674 cmꢀ1 was found to be extremely slow
(kdecay < 1.0 × 103 sꢀ1, 3.0 mM benzylamine). These facts suggest
that the band at 1674 cmꢀ1 may involve two species, intermedi-
ate and final amide product. To assess this possibility, the IR spec-
tra of the authentic amides were measured. As expected, the
authentic N,N-diethyl(p-nitrophenyl)acetamide was found to
have a strong band at 1635 cmꢀ1 which is in agreement with that
observed for the final product of reaction of p-nitrophenylketene
with diethylamine. On the contrary, butyl(p-nitrophenyl)
acetamide has a strong band at 1670 cmꢀ1 (Fig. S1) which
is identical with a band at 1674 cmꢀ1 observed for reaction
of p-nitrophenylketene with butylamine, suggesting that
two bands of the intermediate and the amide overlap each
other.
RESULTS AND DISCUSSION
Amination of phenylketenes generated by photolysis of α-
diazoacetophenone leads to the corresponding amide via amide
enol (or zwitterions). Figure 1(a) shows the typical decay of the tran-
sient with a band maximum at 1680 cmꢀ1 and the formation of the
amide band at 1640 cmꢀ1 for photolysis of α-diazoacetophenone in
the presence of diethylamine. The time-resolved IR spectra are
completely in agreement with the results reported by
Wagner et al.[14] All other substituted α-diazoacetophenones
gave similar spectra.
The structure of the intermediate formed in reaction of ketene
with amine has been considered to be amide enol or zwitterionic
ylide. A band maximum at 1680 cmꢀ1 would arise from the
stretch of the carbon–carbon double bond of these species.
Most of the intermediates assigned as the amide enols until
now are stabilized kinetically by steric congestion.[20,21] The
amide enol generated from amination of 2-pyridylketene is sta-
bilized by a strong intramolecular hydrogen bond to the pyridyl
nitrogen.[18] The existence of zwitterions as transients was also
reported in the literature.[29–33] There is little experimental infor-
mation on the structures of the intermediates formed from
On the other hand, in the presence of the primary amines
such as butylamine and benzylamine the TRIR spectra for photol-
0.004
0.003
0.002
0.001
0
0.002
0.0015
0.001
0.0005
0
-0.0005
-0.001
-0.001
-0.002
-0.0015
1600
1620
1640
1660
1680
1700
1625 1647 1668 1690 1712 1733
(a)
(b)
Figure 1. TRIR spectra in the carbonyl region observed at 10 μs intervals after 266 nm irradiation of α-diazoacetophenone in acetonitrile. (a) In the
presence of 3.0 mM diethylamine. Arrows indicate the change of the IR absorbance with time. (b) In the presence of 3.0 mM benzylamine
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Copyright © 2013 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2013, 26 1071–1076

SUBSTITUENT EFFECT ON AMINE-CATALYZED TAUTOMERIZATION OF AMIDE ENOL
highly reactive ketenes (unstable) such as phenylketene. It is
difficult to differentiate experimentally between the amide enols
and zwitterions; therefore, we carried out the theoretical calcula-
tions for the amide enols, zwitterions, and the final products
(amides) in gas phase and in acetonitrile by using Tomasi's Polar-
ized Continuum Model (PCM). The frequencies calculated at the
B3LYP/6-311 + G(d,p) level of theory are listed in Table 1 along
with the experimental frequencies. The calculated frequencies
of zwitterions in gas phase are 90 – 100 cmꢀ1 higher than the
corresponding amide enols for all substituted ones. Comparison
of the calculated frequencies between in gas phase and in aceto-
nitrile shows that the effect of solvent on both species lowers
their frequencies significantly and that the effect is larger in
the zwitterion than that in the amide enol. This is consistent with
a large dipole moment of the zwitterions. The amides are also
influenced moderately by solvation. Interestingly, two strong
bands in the region of 1600 to 1700 cmꢀ1 were obtained for
zwitterions and some amide enols in acetonitrile different from
the results in gas phase. A band with a higher frequency would
arise from the stretch of carbon–carbon double bond or the
asymmetric stretch of the C = C–Oꢀ moiety and a lower band
would be due to the stretch of the benzene ring. The differences
between two bands are 10–30 cmꢀ1 and 60–90 cmꢀ1 for amide
enols and zwitterions, respectively. Considering the resolution
(12–18 cmꢀ1) of the TRIR measurement under the present condi-
tions, two bands of amide enol should merge into a single broad
band, and the latter would be observed as split two bands or
significantly broadened band. However, the band shape
recorded for the intermediates is similar to that for the final
product amide as shown in Fig. 1. Table 1 shows another impor-
tant result that the frequencies with stronger absorbance in
acetonitrile calculated for several amide enols and zwitterions
are practically identical with those calculated for the correspond-
ing amides. This is also in disagreement with the experimental
observations. In conclusion, these findings suggest that the
PCM method is not suitable to predict frequencies in acetonitrile
for highly polarized amide enols and zwitterions. That is, the ef-
fect of acetonitrile that weakens the carbon–carbon double
bond character seems to be overestimated. This would also lead
to the erroneous estimation of energy of the highly polarized
species. Therefore, we pay here attention to the theoretical
calculations in gas phase.
Table 1 shows that the calculated frequencies in gas phase for
amide enols and zwitterions are higher than the observed ones
for the intermediates. This is true for the amide frequencies. In
general, the calculated frequencies must be corrected by a
scaling factor depending on theory and a basis set used.[34–36]
A scaling factor of 0.967 was evaluated as an averaged value
from the experimental and calculated frequencies for a series
of the final product amides. Interestingly, this scaling factor is
practically identical to that obtained for phenylketenes by the
same way although the PCM calculations for the acetonitrile
solution reveal different solvent effects on frequency between
ketene and amide, i.e. the solvent effect of acetonitrile lowers
frequency by ca. 50 cmꢀ1 for ketenes while that for amides is
ca. 100 cmꢀ1 (Tables 1 and S1). The agreement of a scaling factor
between ketene and amide may be accidental, but this scaling
factor would be used as a practical approximation to estimate
frequencies in acetonitrile from the calculated values in gas
phase because these scaling factors must involve already solvent
effects, Thus, the corrected frequencies of 1641–1666 cmꢀ1 were
obtained for amide enols, and 1749–1762 cmꢀ1 for zwitterions
by using a scaling factor of 0.97. The experimental frequencies
are obviously close to those for the amide enols rather than
that for the zwitterions. In addition, the calculated energies
(ΔH and ΔG) in gas phase indicated that the amide enols are
thermodynamically more stable than the corresponding zwit-
terions by 3–6 kcal molꢀ1 (Table S2). Although these theoretical
calculations do not provide decisive evidences for amide enol
as the intermediate for lack of appropriate consideration of
solvent effects on frequency, we can assume at the present
stage that a plausible structure of a species with a band max-
imum at 1680 cmꢀ1 is amide enol as well as those from highly
stabilized ketenes.
Table 1. Calculated frequencies of amide enols, zwitterions,
and amides in gas phase and acetonitrile at the B3LYP/6-
311 + G(d,p) // B3LYP/6-311 + G(d,p)a
Frequency/cmꢀ1
Subst.
Amide enol Zwitterion
Amide
In gas phase
p-MeO
H
p-Cl
m-Cl
m-CF3
p-CF3
m-NO2
p-CN
p-NO2
3,5-(CF3)2
In MeCNa
p-MeO
H
(1676)
(1680)
(1676)
(1676)
(1680)
(1680)
(1680)
(1676)
(1680)
(1680)
1718
1713
1711
1706
1709
1718
1707
1700
1694
1701
1815 (1636) 1693
1817 (1640) 1694
1809 (1636) 1695
1811 (1636) 1694
1808 (1640) 1696
1809 (1640) 1693
1808 (1640) 1691
1817 (1636) 1694
1803 (1640) 1700
1802 (1640) 1694
1663
1653
1631, 1598
1601
1665
1669, 1612
1667
1598
1612
1600
1596
1602
1601
1600
1603
1601
1605
p-Cl
m-Cl
1669, 1600
m-CF3
p-CF3
m-NO2
p-CN
p-NO2
3,5-(CF3)2
1611. 1589 1669, 1615
1628
1612
1566
1678, 1606
1671, 1613
1677, 1589
Substituent effect on the second-order rate constant for
reaction of N,N-diethylamino-β-phenylethenol with
diethylamine
The decay of the band at 1680 cmꢀ1 and growth of the band at
1640cmꢀ1 obeyed the first-order rate law. The observed first-order
rate constants were determined by least-squares fitting of a single-
exponential function. The first-order rate constants provided to be
linear functions of diethylamine concentration, and the observed
rate constants were therefore given by a linear function of added
amine concentration shown as Eqn (2).
1572, 1560 1680, 1593
1587 1670, 1605
Values in parentheses are the observed frequencies of the
intermediates and the final products (amides).
aFrequencies in acetonitrile calculated using Tomasi's
Polarized Continuum Model (PCM). Underline denotes a
band with stronger absorbance than the other.
J. Phys. Org. Chem. 2013, 26 1071–1076
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M. M. R. BADAL ET AL.
kobs ¼ k0 þ k2½amineꢁ
(2)
where k0 is the observed decay or growth rate constant in the ab-
sence of amine, and k2 is the second-order rate constant. The
values of k2 were obtained from the slopes of the plots of kobs vs
[amine] (Table S3). The second-order rate constants k2 for all N,N-
diethylamino-β-phenylethenols (intermediates) are summarized
in Table 2.
It is found that the second-order rate constant for the decay of
the 1680 cmꢀ1 band is practically identical with that for the
growth of the 1640 cmꢀ1 band for all derivatives, indicating that
this kinetics is consistent with the process in Scheme 2. The av-
eraged k2 values for the decay of amide enol and growth of am-
ide have been used for analysis of substituent effect on the
diethylamine-catalyzed tautomerization of the amide enol inter-
mediates to give the amides.
In Fig. 2 are plotted log (k/k0) of the reaction of the amide enol
with diethylamine against σo (closed circles). There is no simple
linear relationship covering all substituents. However, limited to
meta substituents and p-chloro substituent, there is a good linear
correlation. Strong π-acceptors such as p-nitro and p-cyano
groups deviate significantly upward from the line determined
by above non-conjugative substituents. Such upward deviations
of strong para-π-acceptors were observed whenever the nega-
tive charge developed at the reaction site is delocalized into
the aromatic π-system.[26–28] The present result seems to be
the case because the negative charge develops on an oxygen
atom of amide enol at the transition state when basic
diethylamine abstracts a hydrogen atom of the hydroxyl group
of the amide enol. Therefore, application of the Yukawa–Tsuno
equation (1) to the present substituent effect is suitable to eluci-
Scheme 2. Base-catalyzed reaction of the amide enol
date the properties of the transition state. Indeed, application of
the Y–T equation provides a ρ value of 0.63 and an rꢀ value of
1.31 with satisfactory precision (R = 0.997) as shown in Fig. 2.
The rꢀ value larger than unity for pKa of phenols indicates that
the negative charge formed at an oxygen atom of the amide
enol at the transition state is significantly delocalized into the
aromatic π-system through the ethenyl group. Such a large rꢀ
value is not surprising because the rꢀ value for the gas-phase
acidity of acetophenone oxime which has an iso-electronic struc-
ture of β-phenylethenol was found to be 1.4 times larger value
than that for gas-phase acidity of phenol while the ρ value is
0.7 times smaller than that for phenol.[37,38] The larger rꢀ with
a smaller ρ than those for phenol was interpreted as follows.
Since a polar effect is in general dependent on a distance
between a reaction site and the substituent, a longer distance
between a deprotonation site and the substituent in the
acetophenone oxime skeleton than that for phenol would result
in a reduction of ρ value. On the contrary, the transmission of
resonance effect in the conjugated framework is less sensitive
to a distance, i.e. ρ × rꢀ would be nearly constant. As a result, a
large rꢀ value with a small ρ value was observed for the gas-
phase acidity of acetophenone oxime. The present system is also
the case. Accordingly, the observed rꢀ value larger than unity
must reflect an inherent property of β-phenylenolate skeleton.
This interpretation is consistent with our previous conclusion
that a resonance demand for a reaction of interest is inherent
in a molecular structure regardless of a position of the transition
state while an amount of the developed charge at the reaction
site influences a ρ value.[39–42] From these results, the transition
structure for the process from amide enol to amide is proposed
as illustrated in Scheme 3.
Table 2. Second-order rate constants for reaction of amide
enols with diethylamine measured by bands at 1680 and
1640 cmꢀ1 in acetonitrile at 23°C, and substituent constants
k2/107 Mꢀ1 sꢀ1
Substituent constants d
This is quite in agreement with the proposed structure based
on theoretical studies.[6,15,17,22] The alternative mechanism may
be considered for this reaction, i.e. diethylamine abstracts a
hydrogen atom of amide enol at the rate-determining step and
proton transfer subsequently takes place from diethylammonium
ꢀ
ΔσR
Subst.
1680 cmꢀ1 a 1640 cmꢀ1 b
σo
p-OCH3
H
p-Cl
1.2
2.1
3.0
3.8
4.2
5.6
6.7
8.7
8.8
17.0
1.4
ꢀ0.10 (ꢀ0.276)f 0.000
2.5
0.000
0.281
0.373
0.493
0.510e
0.709
0.640e
0.98
0.000
0.000
0.000
0.000
0.191e
0.000
0.303e
0.000
0.459e
c
m-Cl
3.9
3.4
5.8
6.0
9.1
m-CF3
p-CF3
m-NO2
p-CN
3,5-(CF3)2
p-NO2
7.8
c
0.802e
aSecond-order rate constants for decay of the band at
1680 cmꢀ1
bSecond-order rate constants for formation of the band at
.
1640 cmꢀ1
.
cIntensity of the band at 1640 cmꢀ1 was too weak to
determine the accurate rate constant of formation of the
amide.
dRef. 26.
eRef. 27.
fRef. 28.
Figure 2. Y–T plot for reaction of amide enol intermediates with
diethylamine in acetonitrile solution. Closed circle; σ°, open circle; σꢀ,
square; σ at rꢀ = 1.31
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SUBSTITUENT EFFECT ON AMINE-CATALYZED TAUTOMERIZATION OF AMIDE ENOL
J = 6.7 Hz), 3.63 (s, 2H), 5.41 (br s, 1H), 7.46 (d, 2H, J = 8.5 Hz), 8.19 (d, 2H,
J = 8.5 Hz). N,N-Diethyl-2-(4-nitrophenyl)acetamide: Viscous orange oil;
1H NMR (CDCl3) : δ (ppm) 1.14 (t, 3H, J = 7.1 Hz), 1.18 (t, 3H, J = 7.1 Hz),
3.34 (q, 2H, J = 7.1 Hz), 3.41 (q, 2H, J = 7.1 Hz), 3.79 (s, 2H), 7.44 (d, 2H,
J = 8.7 Hz), 8.19 (d, 2H, J = 8.7 Hz).
Scheme 3. Transition state of amide enol – amide tautomerization
TRIR measurements
ion to the ethenyl group. In this mechanism, the negative charge
developed at an oxygen atom at the transition state is stabilized
solely by the phenylethenyl group. This would result in a large
effect of the ring-substituent. However, the observed ρ value is
remarkably small. This is consistent with the cyclic transition struc-
ture because the negative charge disperses in a six-member ring in
addition to the aryl group.
TRIR experiments were conducted with a JASCO TRIR-1000 dispersive-
type IR spectrometer. This method allowed access to the entire mid IR
spectrum (4000–800 cmꢀ1) with high sensitivity and frequency resolution
(12–18 cmꢀ1) to probe a wide range of transient species in solution.
Solutions were flowed through a CaF2 cell, the path length of which
was adjustable from 0.25 to 1 mm. The broad-band output of a newly
developed MoSi2 infrared source (JASCO) was crossed with excitation
pulses (266 nm) from a Continuum cw Q-sw Nd:YAG laser. Changes in
IR intensity were monitored by an MCT photovoltaic IR detector ampli-
fied and digitized with a Tektronix TDS520A oscilloscope. The TRIR
spectrum was analyzed by the IGOR PRO program (Wavemetrics Inc.) in
the form of a difference spectrum.
CONCLUSIONS
The band maximum at 1676–1680 cmꢀ1 observed for the
intermediates formed in the reaction of m,p-substituted
phenylketenes with diethylamine were identified as the corre-
sponding amide enols rather than the zwitterions on the basis
of the theoretical calculations. In addition, a single broad band
at 1674 cmꢀ1 observed for the reaction with the primary amines
was attributed to overlap of two bands of intermediate (amide
enol) and final product (amide).
The substituent effect for the second-order rate constant of
reaction of the amide enol with diethylamine was analyzed
successfully by the Yukawa–Tsuno equation, giving a ρ value of
0.63 and an rꢀ value of 1.31. The rꢀ value larger than unity
indicates that the negative charge formed at an oxygen atom
of the amide enol at the transition state is significantly
delocalized into the aromatic π-system through the ethenyl
group. This rꢀ value must reflect an inherent property of
β-phenylenolate skeleton. A remarkably small ρ value is
consistent with the cyclic transition structure where the
negative charge disperses in a six-member ring.
Theoretical calculations
The geometries were fully optimized at the B3LYP/6-311 + G(d,p) level of
theory with normal convergence using the Gaussian 09 program.[43]
Vibrational normal mode analyses were performed at the same level to
ensure that each optimized structure was a true minimum on the
potential energy surface and to calculate the thermal correction needed
to obtain H, G, and S. The zero point energies used for the thermal
correction were unscaled. Geometries in acetonitrile were fully optimized
at the same level of theory using Tomasi's PCM with the United Atom for
Hartree–Fock model to build the cavity, and frequency calculations were
conducted.
SUPPORTING INFORMATION
Details of the kinetic experiments, calculated frequencies, calcu-
lated energies, optimized geometries, and infrared spectra.
Acknowledgements
We are indebted to the Joint Project of Chemical Synthesis Core
Research Institutions and the Ministry of Education, Culture,
Sport, Science, and Technology, Japan for financial support.
EXPERIMENTAL
1
The H NMR spectra were recorded on a JEOL JNM-EX 400 or JNM ECA-
500 FT-NMR spectrometer in CDCl3 or DMSO-d6 and chemical shifts were
given in ppm (δ) downfield from TMS as an internal standard.
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