Kinetics and mechanism of the aminolysis of S-phenyl cyclopropanecarboxylates in acetonitrile

Article abstract of DOI:10.1039/b107403m

The kinetics and mechanism of the aminolysis of S-phenyl cyclopropanecarboxylates [cyclo-C₃H₅C(=O)- SC₆H₄Z] with benzylamines (XC₆H₄CH₂NH₂) were investigated in aceton

Full text of DOI:10.1039/b107403m

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Kinetics and mechanism of the aminolysis of S-phenyl
cyclopropanecarboxylates in acetonitrile
Hyuck Keun Oh,a Jae Yun Lee,a Hai Whang Leeb and Ikchoon Lee*b
a
Department of Chemistry, Chonbuk National University, Chonju 560-756, Korea
Department of Chemistry, Inha University, Inchon 402-751, Korea. E-mail: ilee@inha.ac.kr;
b
Fax: þ82 32 865 4855
Received (in Montpellier, France) 13th August 2001, Accepted 29th November 2001
First published as an Advance Article on the web
The kinetics and mechanism of the aminolysis of S-phenyl cyclopropanecarboxylates [cyclo-C3H5C(=O)-
SC6H4Z] with benzylamines (XC6H4CH2NH2) were investigated in acetonitrile at 35.0 ꢀC. The large magnitudes
of the Bronsted coefficients bX (1.7–2.3) and bZ (À0.9 to À1.3), together with a large positive cross-interaction
constant rXZ ¼ þ1.4, are interpreted to indicate a stepwise mechanism with rate-limiting breakdown of the
zwitterionic tetrahedral intermediate, T . The proposed mechanism is supported by adherence of the rates to
the reactivity-selectivity principle (RSP). The kinetic isotope effects involving deuterated benzylamines
(XC6H4CH2ND2) are greater than unity (kH=kD > 1.0), suggesting the possibility of a hydrogen-bonded
¼
¼
four-center transition state. The activation parameters, DH (ffi5 kcal molÀ1) and DS (¼ À 45 to À54 e.u.),
are consistent with this transition state structure.
The mechanisms of the aminolysis of aryl esters1 and carbo-
nates,2 and their thiol, thiono and dithio derivatives,3 have
been extensively studied. Curved Bronsted plots in the ami-
nolysis reactions have been interpreted in terms of a zwitter-
ionic tetrahedral intermediate, T , in the reaction path and a
change in the rate-limiting step from leaving group expulsion
to attack by the nucleophile as the nucleophile becomes more
ð1Þ
basic.4 In some of the cases, however, the aminolysis has been
found to proceed concertedly in a single step through a tetra-
hedral transition state (TS).4,5 The mechanistic change from a
stepwise mechanism through an intermediate, T , to a con-
certed one via a single TShas been reported to be caused by
destabilization of the tetrahedral intermediate, T , due to
several factors, including enhanced leaving ability of the
leaving group (LG),5d,e strong electronic push provided by the
substrate (nonleaving group)5a,b and destabilization rendered
by the amines and by substitution of SÀ by OÀ 5b in the tetra-
hedral intermediate, T . For example, the aminolyses of 2,4-
dinitrophenyl methyl carbonate with pyridines, anilines and
secondary alicyclic amines were found to proceed by a stepwise
mechanism.2d,e In contrast, however, its thiol analog (MeO
and EtO are very similar6,7) was found to react by a concerted
mechanism with secondary alicyclic amines5a,b due to desta-
The reactions have been conducted under pseudo-first-order
conditions with a large excess of amine. The purpose of this
work is to elucidate the mechanism by determining various
selectivity parameters, rX , bX , rZ and bZ , including the cross-
interaction constant8 rXZ in eqn. (2), where X and Z are
substituents in the nucleophile and leaving group, respectively.
logðkXZ=kHHÞ ¼ rXsX þ rZsZ þ rXZsXsZ
ð2Þ
We are interested in any mechanistic change that may occur
due to the change of leaving group from ArOÀ to ArSÀ.
Results and discussion
The reactions of benzylamine (BA) with S-phenyl cyclopropane
carboxylates (SPC) under the reaction conditions obey the rate
law given by eqns. (3) and (4); k0 and kN are the rate constants
for solvolysis in MeCN and aminolysis of the substrate,
respectively.
À
bilization of the TSby the enhanced leaving ability of ArS
relative to ArOÀ. For aminolysis of ethyl S-aryl thiolcarbo-
nates, the mechanism also changes from stepwise to concerted
when Ar is changed from 4-NO2Ph to 2,4-(NO2)2Ph and 2,4,6-
(NO2)3Ph with quinuclidine bases,5d and also when Ar is
changed from 2,4-(NO2)2Ph to 2,4,6-(NO2)3Ph with anilines.5e
These results show that although the EtO (and MeO) group is
conducive to a concerted process, the mechanism is also sen-
sitive to the leaving group ability.
Rate ¼ kobs Á ½SPCꢁ
kobs ¼ k0 þ kN Á ½BAꢁ
ð3Þ
ð4Þ
Plots of kobs against benzylamine concentration, [BA], were
linear, according to eqn. (4) with a negligible intercept
(k0 ffi 0.0) and a slope kN (MÀ1
s
À1), summarized in Table 1.
In this work, we investigate the kinetics and mechanism of
the aminolysis of S-Z-phenyl cyclopropanecarboxylates (1)
with X-benzylamines in acetonitrile at 35.0 ꢀC, eqn. (1), where
X ¼ p-OMe, p-Me, H, p-Cl and m-Cl, and Z ¼ p-Me, H, p-Cl
and p-Br.
No third-order or high-order terms were detected, and no
complications were found in the determination of kobs nor in
the linear plots of eqn. (4). This suggests that there is no base-
catalysis or noticeable side reactions and the overall reaction
DOI: 10.1039/b107403m
New J. Chem., 2002, 26, 473–476
473
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Table 1 The second-order rate constants, kN ꢂ 103 MÀ1 sÀ1 of the reactions of S-Z-phenyl cyclopropanecarboxylates with X-benzylamines in
acetonitrile at 35.0 ꢀC
Z
a
b
X
p-Me
H
p-Cl
117
p-Br
rZ
bZ
p-OMe
20.1
41.7
150
105
75.6
111
51.6
24.3
17.7
12.6
10.7
2.12 0.08
À0.86 0.06
14.2c
9.94d
p-Me
H
p-Cl
11.4
26.1
12.0
4.51
89.4
38.7
18.2
2.43 0.12
2.50 0.06
2.83 0.11
À1.00 0.05
À1.02 0.08
À1.16 0.07
4.78
1.68
1.21c
0.883d
0.581
À2.32 0.09
m-Cl
a,e
rX
1.75
À2.09 0.08
8.71
À1.76 0.07
3.14 0.17
À1.29 0.05
À1.74 0.08
f
rXZ ¼ 1.44 0.31
g
bX
2.26 0.13
2.04 0.12
1.71 0.11
1.69 0.12
a
The s and sÀ values were taken from C. Hansch, A. Leo and R. W. Taft, Chem. Rev., 1991, 91, 165. Correlation coefficients were better than
0.997 in all cases. The pKa values were taken from Dictionary of Organic Chemistry, ed. J. Buckingham, Chapman and Hall, New York, 5th edn.,
b
c
1982. An interpolated value of pKa ¼ 5.87 was used for p-BrC6H4SH. Correlation coefficients were better than 0.994 in all cases. At
d
e
f
g
25.0 ꢀC. At 15.0 ꢀC. Correlation coefficients were better than 0.997 in all cases. Correlation coefficient was 0.997. The pKa values were
taken from A. Fischer, W. J. Galloway and J. Vaughan, J. Chem. Soc., 1964, 3588. Correlation coefficients were better than 0.994 in all cases.
pKa ¼ 9.67 was used for X ¼ p-CH3O (H. K. Oh, J. Y. Lee and I. Lee, Bull. KoreanChem. Soc. , 1998, 19, 1198).
follows the route given by eqn. (1). The Hammett coefficients,
rX and rZ , and the Bronsted coefficients, bX (bnuc) and bZ
(blg), are also collected in Table 1, together with the cross-
interaction constant rXZ
.
As we have pointed out previously, the Bronsted slopes
obtained by plotting log kN(MeCN) vs. pKa(H2O) are justi-
fied9 since the pKa(MeCN) values are found to vary in par-
allel with the pKa(H2O) values for structurally similar
amines. Since we found that the Bronsted slopes for the plots
of log kN(MeCN) vs. pKa(H2O) of pyridines are somewhat
larger than those for the plots of log kN(MeCN) vs. pKa-
(MeCN) of pyridines in the phosphoryl transfer reactions,10
the reported magnitude of the bX values may be somewhat
larger than the correct values using pKa(MeCN). However,
the comparison of bX values in Table 1 with the corre-
sponding values determined using the similar pKa(H2O) may
be approximately applicable.9c
Rates for the aminolysis of S-phenyl (–SAr) propane-
carboxylates are much faster than the corresponding values
for the aminolysis of the phenolate (–OAr) analogs;1j for
example, for the reaction of the phenolate analog with X ¼ H
and Z ¼ 4-CH3CO (sp ¼ þ0.50) kN is 4.17 ꢂ 10À3 MÀ1 sÀ1 at
55.0 ꢀC while for thiophenolate with X ¼ H and Z ¼ 4-Me
(sp ¼ À0.17) kN is 4.78 ꢂ 10À3 MÀ1 sÀ1 at 35.0 ꢀC. Note that
the former has a lower rate at a higher reaction temperature
(by 20 degrees) despite its electron acceptor substituent. Since
the nucleophile is the same, benzylamine in both cases, the
faster rates of thiophenolates indicate the importance of bond
cleavage in the TS, since the thiophenolates used in the
present work are weakly basic relative to the phenolates used
in the studies and hence are much better leaving groups.
The bX (bnuc) values are large, ranging from 1.7 to 2.3, which
are quite larger than the bX 5 0.8–1.0 normally expected for a
stepwise mechanism with rate-limiting expulsion of the leaving
group (ArSÀ) from T .1,2 Similarly the bZ (blg) values have
also large magnitudes (bZ ¼ À0.9 to À1.3), which are again
greater than those normally expected from such a mechanism,
albeit the exact comparison is difficult due to the use of
pKa(H2O) for the bZ determination.1a,2a These bX and bZ
values strongly suggest a stepwise mechanism with rate-
limiting breakdown of the T intermediate, kb in eqn. (5),
ð5Þ
where kN ¼ (ka/kÀa)kb ¼ KÁkb . Close examination of these
values, however, shows that the magnitude of bX is somewhat
larger with ArSÀ (bX ¼ 1.7–2.3) than that with ArOÀ
(bX ¼ 1.3–2.1), while that of bZ is smaller (bZ ¼ À0.9 to À1.3)
than that for ArOÀ 1j (bZ ¼ À1.1 to À1.4). The differences are
small, but the trends are clear. These trends reflect that the TS
with ArSÀ is somewhat tighter than that with ArOÀ, as evi-
denced by a larger positive cross-interaction constant rXZ (1.4
vs. 1.1).8 We therefore conclude that the enhanced leaving
ability from substitution of ArOÀ by ArSÀ causes no
mechanistic change but leads to a somewhat tighter TS.
Table 2 Kinetic isotope effects in the reactions of S-Z-phenyl cyclo-
propanecarboxylates with deuterated X-benzylamines in acetonitrile
at 35.0 ꢀC
X
Z
kH ꢂ 103=MÀ1 sÀ1 kD ꢂ 103=MÀ1 sÀ1 kH=kD
p-OMe p-Me 20.1 0.2
p-OMe 41.7 0.9
p-OMe p-Cl
p-OMe p-Br
p-Cl
p-Cl
p-Cl
p-Cl
14.6 0.2
31.8 0.7
93.6 0.2
1.38 0.02a
1.31 0.04
1.25 0.03
1.21 0.04
1.43 0.02
1.39 0.02
1.32 0.03
1.26 0.03
H
117
150
2
3
124
2
p-Me 1.68 0.02
4.51 0.05
1.17 0.01
3.24 0.03
13.8 0.2
19.3 0.3
H
p-Cl 18.2 0.2
p-Br 24.3 0.3
a
Standard deviations.
474
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Table 3 Activation parametersa of the reactions of S-Z-phenyl cyclo-
propanecarboxylates with X-benzylamines in acetonitrile
groups, Me and Et, are stronger electron donors, the lone
pair level on the oxygen of the alkoxy group, EtO, is raised.
PhO is in this sense not as efficient since Ph is an acceptor.
(b) On the other hand, an electron withdrawing group such
as p-NO2 or p-CN on the leaving group depresses the s*C–LG
level, while the s*C–S level is significantly lower than the
s*C–O level.13a (c) The amine expulsion from T in 3 is faster
in the order5d,14 pyridine < aniline < secondary alicyclic ami-
nes < primary amines, so that provided other conditions are
equal, pyridines are more likely to lead to a stable T and
hence a stepwise mechanism, while the secondary alicyclic
and primary amines are more likely to lead to a concerted
mechanism. (d) The thiono intermediate, with Y ¼ Sin 3, is
more stable than the carbonyl counterpart, Y ¼ O, since the
C=Sbond is weaker than the C =O bond.3,5b
¼
¼
X
Z
DH =kcal molÀ1
ÀDS =cal molÀ1 KÀ1
p-OMe
p-OMe
p-Cl
p-Me
p-Br
p-Me
p-Br
5.6
5.4
5.1
5.2
48
45
54
49
p-Cl
a
Calculated by the Eyring equation. The maximum errors calculated
(by the method of K. B. Wiberg, Physical Organic Chemistry, Wiley,
New York, 1964, p 378) are 0.5 kcal molÀ1 and 2 e.u. for DH and
¼
¼
DS , respectively. The levels of confidence are better than 95%.
Temperature range is 15.0–35.0 ꢀC.
In conclusion, a concerted aminolysis is favored by (i) an
alkoxy group, R ¼ CH3O, EtO, etc. in 3, (ii) by a leaving group
with a lower s*C–LG level, that is s*C–S < s*C–O , and leaving
group (LG) with strong electron-withdrawing substituents,
(iii) by primary or secondary amines rather than pyridines, and
(iv) by a carbonyl (C=O) rather than thiono (C=S) compound.
The strongest contribution comes from a nO-s*C–LG type vic-
inal charge transfer interaction in the tetrahedral intermediate
T , due to a strong destabilization caused by the weakening of
the C–LG bond. Alkyl groups with s ¼ À0.04 to s ¼ À0.21 in 3
cannot destabilize the T enough to cause a concerted ami-
nolysis,1j,k,15 although they are electron donors, due to the lack
of n-s*C–LG type vicinal charge transfer (or anomeric effect),
although the amine (benzylamine) and leaving group (ArSÀ)
are highly conducive to such a single-step nucleophilic
displacement.
The kinetic isotope effects (kH/kD) in Table 2 involving
deuterated benzylamine (XC6H4CH2ND2) nucleophiles in
acetonitrile are greater than unity (kH=kD ¼ 1.3–1.4), indica-
ting that the N–H proton transfer takes place in the rate-
determining step8b so that a four-center-type TS, 2, is invol-
ved.8b In this type of TS, hydrogen bonding of an amine
hydrogen atom to the departing thiophenoxide facilitates the
rate-limiting bond cleavage step, forming a rather constrained
four-membered ring. This is reflected in the low activation
¼
enthalpies (DH ffi 5 kcal molÀ1) and large negative activation
Experimental
¼
entropies (DS ¼ À45 to À54 e.u.) in Table 3. The expulsion of
the ArSÀ anion in the rate-determining step (an endoergic
process) is assisted by the hydrogen bonding with an amino
hydrogen of the benzylammonium ion within the intermediate,
Materials
Merck GR acetonitrile was used after three distillations. The
benzylamine nucleophiles, Aldrich GR, were used without
further purification. Thiophenols and cyclopropanecarbonyl
chloride were Tokyo Kasei GR grade. Preparation of deuter-
ated benzylamine: benzylamine was dissolved in excess D2O
under nitrogen atmosphere and left for 5 h. The deuterated
benzylamine was extracted with dry ether and dried again over
MgSO4 and then the solvent was removed. This procedure was
repeated three times. The anlysis (NMR) of the dried deuter-
ated benzylamine has more than 99% deuterium content and
thus the kH=kD values were not corrected for the deuterium
content.
¼
T . This will lower the DH value, but the TSbecomes
structured and rigid (low entropy process), which should lead
to a large negative DS value.
¼
It is also notable that the trends in rates and selectivity
parameters in Table 1 conform to the reactivity-selectivity
principle,11 which supports our proposed mechanism.1j,8,12
Here it is appropriate to comment on the factors that
destabilize the zwitterionic tetrahedral intermediate, 3 (T ),
and hence are conducive to a concerted aminolysis reaction.
(a) The strongest influence comes from the nonleaving group,
R in 3; if this group has an electron donor the leaving group
(LG) expulsion is facilitated. However, the enhanced
nucleofugality by an electron donor R alone is not enough
normally to enforce a concerted aminolysis. When the R
group has an oxygen (or nitrogen) with a lone pair (n),
preferably of the p type, directly attached to the carbonyl
carbon as in R ¼ MeO, EtO and PhO, the strong nO-s*C–LG
vicinal charge transfer interaction13 in 3 causes extensive
weakening of the C–LG bond to the extent that the inter-
mediate cannot exist. This n-s* lone pair-antibond, or bond-
antibond interaction, is stronger (i) when the lone pair level is
higher and the s* level is lower, and (ii) the n and s* have an
antiperiplanar conformation,12 as in 4. Since the alkyl
Preparations of S-phenyl cyclopropanecarboxylates
Thiophenol derivatives and cyclopropanecarbonyl chloride
were dissolved in anhydrous ether and pyridine carefully
added, keeping the temperature at 0–5 ꢀC. Ice was then added
to the reaction mixture and the ether layer was separated, dried
on MgSO4 and distilled under reduced pressure to remove the
solvent. IR (Nicolet 5BX FT-IR) and 1H and 13C NMR
(JEOL 400 MHz) data are given below. A Hewlett Packard
quadrupole mass spectrometer with electron impact ionization
mode was used to measure MSdata and a Perkin Elmer
elemental analyzer was used for microanalysis. The yields
ranged from from 70 to 95%.
S-p-Tolyl cyclopropanecarboxylate. Liquid; IR (KBr) 3013
(C–H, aromatic), 2932 (C–H, CH3), 1695 (C=O), 1578 (C=C,
aromatic) cmÀ1; 1H NMR (400 MHz, CDCl3) d 0.88–1.25 (4H,
m, CH2), 2.07 (1H, t, CH), 2.35 (3H, s, CH3), 7.21 (2H, d,
J ¼ 8.30 MHz, meta H), 7.29 (2H, d, J ¼ 8.30 MHz, ortho H);
13C NMR (100.4 MHz, CDCl3) d 207.6 (C=O), 139.4, 134.3,
New J. Chem., 2002, 26, 473–476
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130.0, 129.2, 22.1, 21.4, 9.14; MS, m=z 192 (Mþ). Anal. calcd.
for C11H12OS: C, 68.7; H, 6.31. Found: C, 68.5; H, 6.33.
Acknowledgement
This work was supported by a Korea Research Foundation
Grant (KRF-2000-015-DP0209).
S-Phenyl cyclopropanecarboxylate. Liquid; IR (KBr) 3013
(C–H, aromatic), 1695 (C=O), 1578 (C=C, aromatic) cmÀ1; 1H
NMR (400 MHz, CDCl3) d 0.88–1.25 (4H, m, CH2), 2.05 (1H,
t, CH), 7.34–7.43 (5H, m, aromatic); 13C NMR (100.4 MHz,
CDCl3) d 197.2 (C=O), 134.3, 129.1, 128.8, 127.7, 21.0, 14.2;
MS m=z 178 (Mþ). Anal. calcd. for C10H10OS: C, 67.4; H,
5.71. Found: C, 67.6; H, 5.69.
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CH2), 2.08 (1H, t, CH), 7.32 (2H, d, J ¼ 8.78 MHz, meta H),
7.38 (2H, d, J ¼ 9.22 MHz, ortho H); 13C NMR (100.4 MHz,
CDCl3) d 196.6 (C=O), 135.6, 135.5, 129.1, 126.2, 22.3, 11.4;
MS m=z 212 (Mþ). Anal. calcd. for C10H9ClOS: C, 56.5; H,
4.32. Found: C, 56.7; H, 4.34.
B.-S. Lee, H. J. Koh and I. Lee, J. Chem. Soc., PerkinT rans. 2
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3013 (C–H, aromatic), 1695 (C=O), 1578 (C=C, aro-
matic) cmÀ1; 1H NMR (400 MHz, CDCl3) d 0.88–1.25 (4H, m,
CH2), 2.07 (1H, t, CH), 7.27 (2H, d, J ¼ 8.30 MHz, meta H),
7.53 (2H, d, J ¼ 8.30 MHz, ortho H); 13C NMR (100.4 MHz,
CDCl3) d 196.4 (C=O), 135.7, 132.1, 126.8, 123.8, 22.4, 11.4;
MS m=z 257 (Mþ). Anal. calcd. for C10H9BrOS: C, 46.7; H,
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Kinetic measurements
Rates were measured conductometrically at 35.0 0.05 ꢀC.
The apparatus used in this work was a home-built computer
automatic A=D converter conductivity bridge. Pseudo-first-
order rate constants, kobs , were determined by the Guggen-
heim method16 with
a
large excess of benzylamine.
[Substrate] ffi 10À3 M; [BA] ¼ 0.2–7 ꢂ 10À1 M. Second-order
rate constants, kN , were obtained from the slope of a plot of
kobs vs. benzylamine with more than five concentrations and
were reproducible to within 3%.
6
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Chem. Commun., 1999, 64, 1529.
(a) I. Lee, C. K. Kim, I. S. Han, H. W. Lee, W. K. Kim and Y. B.
Kim, J. Phys. Chem. B, 1999, 103, 7302; (b) W. J. Spillane,
G. Hogan, P. McGrath, J. King and C. Brack, J. Chem. Soc.,
PerkinT rans. 2 , 1996, 2099; (c) H. J. Koh, K. L. Han and I. Lee,
J. Org. Chem., 1999, 64, 4783.
Product analysis
S-Phenyl cyclopropanecarboxylate (0.05 mole) and p-
chlorobenzylamine (0.5 mole) were added to acetonitrile and
reacted at 35.0 ꢀC under the same conditions as for the
kinetic measurements. After more than 15 half lives, the
solvent was removed under reduced pressure and the pro-
duct separated by column chromatography (silica gel, 10%
ethylacetate–n-hexane). Analysis of the product gave the
following results.
10 H. W. Lee, A. K. Guha, C. K. Kim and I. Lee, unpublished data.
11 (a) A. Pross, Adv. Phys. Org. Chem., 1977, 14, 69; (b) E. Buncel and
H. Wilson, J. Chem. Educ., 1987, 64, 475.
12 I. Lee, B.-S. Lee, H. J. Koh and B. D. Chang, Bull. KoreanChem.
Soc., 1995, 16, 277.
13 (a) N. D. Epiotis, W. R. Cherry, S. Shaik, R. Yates and
F. Bernaidi, Structural T heory of Organic Chemistry, Springer-
Verlag, Berlin, 1997, ch. IV; (b) R. E. Reed, L. A. Curtiss and
F. Weinhold, Chem. Rev., 1998, 88, 899; (c) E. D. Glendening
and F. Weinhold, J. Comput. Chem., 1998, 19, 610; (d ) E. D.
Glendening, J. K. Badenhoop and F. Weinhold, J. Comput.
Chem., 1998, 19, 628.
cyclo-C3H5C(=O)NHCH2C6H4Cl. m.p. 147–149 ꢀC; IR
(KBr) 3284 (N–H), 3001 (C–H, CH2), 2975 (C–H, benzyl), 2943
(C–H, CH3), 1685 (C=O), 1549 (C=C, aromatic), 825 (C–H,
aromatic) cmÀ1; 1H NMR (400 MHz, CDCl3) d 0.764 (2H, dt,
cyclopropane), 1.01 (2H, dt, cyclopropane), 1.31–1.36 (1H, m,
CH), 4.40 (2H, d, NH–CH2), 7.21–7.20 (2H, d, aromatic ring,
meta, J ¼ 8.30 Hz), 7.23–7.27 (2H, d, aromatic ring, ortho,
J ¼ 8.30 Hz); 13C NMR (100.4 MHz, CDCl3) d 175.8 (C=O),
173.4, 136.9, 133.2, 129.1, 128.7, 43.1, 14.8, 7.42; MS m=z 209
(Mþ). Anal. calcd. for C11H12ClNO: C, 63.0; H, 5.81. Found:
C, 63.2; H, 5.83.
14 H. K. Oh, S. K. Kim, I. H. Cho, H. W. Lee and I. Lee, J. Chem.
Soc., PerkinT rans. 2 , 2000, 2306.
15 (a) H. K. Oh, J. H. Yang, H. W. Lee and I. Lee, Bull. Korean
Chem. Soc., 1999, 20, 1418; (b) H. K. Oh, J. H. Yang, I. H. Cho,
H. W. Lee and I. Lee, Int. J. Chem. Kinet., 2000, 32, 485; (c) H. K.
Oh, C. Y. Park, J. M. Lee and I. Lee, Bull. KoreanChem. Soc. ,
2001, 22, 383; (d) H. K. Oh, S. K. Kim and I. Lee, Bull. Korean
Chem. Soc., 1999, 20, 1017.
16 E. A. Guggenheim, Philos. Mag., 1926, 2, 538.
476
New J. Chem., 2002, 26, 473–476