Correlation of the rates of solvolysis of phenyl chloroformate

Article abstract of DOI:10.1039/a701140g

The specific rates of solvolysis of phenyl chloroformate in 21 solvents can be very well correlated using the extended Grunwald-Winstein equation, with incorporation of the N_T solvent nucleophilicity scale and the Y_Cl solvent ionizing scale, with sensitivities towards changes in the scale having values of 1.68 ± 0.10 and 0.57 ± 0.06, respectively. This is a solvolysis which, on the basis of several other types of evidence, is believed to follow an addition-elimination pathway with addition being rate-determining (or possibly an enforced concerted variant), and these sensitivities can be considered to be representative values for chloroformate esters following such a pathway.

Full text of DOI:10.1039/a701140g

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Correlation of the rates of solvolysis of phenyl chloroformate†
Dennis N. Kevill* and Malcolm J. D’Souza ‡
Department of Chemistry, Northern Illinois University, DeKalb, Illinois, 60115-2862, USA
The specific rates of solvolysis of phenyl chloroformate in 21 solvents can be very well correlated using the
extended G runwald–Winstein equation, with incorporation of the NT solvent nucleophilicity scale and the
YCl solvent ionizing scale, with sensitivities towards changes in the scale having values of 1.68 ± 0.10 and
0.57 ± 0.06, respectively. This is a solvolysis which, on the basis of several other types of evidence, is
believed to follow an addition–elimination pathway with addition being rate-determining (or possibly an
enforced concerted variant), and these sensitivities can be considered to be representative values for
chloroformate esters following such a pathway.
Phenyl chloroformate hydrolyzes in water to give phenol, and it
undergoes alcoholysis to give the alkyl phenyl carbonate1–3
[eqn. (1)]. The kinetics of these processes can be conveniently
closely related variant] within an addition–elimination mechan-
ism, but a direct displacement (SN 2) mechanism 17 has also been
proposed [eqn. (3)].9a,15
(3)
(1)
The unimolecular pathway involves an intermediate carboxy-
lium ion which, when R+ is a relatively stable carbenium ion,
can lose carbon dioxide [eqn. (4)] prior to product formation.
followed in terms of the hydrochloric acid which is produced in
both pathways, in the present study using a titration technique
and previously using conductivity measurements.3–7 Spectro-
scopic determination of the phenol product has also been
employed.8
Because of the ground-state stabilization present in chloro-
formate esters,1,2,6,7,9 the solvolyses of the chloroformates are
considerably slower than those of simple acyl chlorides
(RCOCl). Within the solvolyses of chloroformates, those of
phenyl chloroformate are faster than those of primary alkyl
chloroformates,7 because the electron-withdrawing influence of
the phenyl group counteracts the ground-state resonance stabil-
ization. This effect is also reflected in a lower dipole moment for
the phenyl chloroformate.10
For alkyl chloroformate esters (ROCOCl) both unimolecular
and bimolecular pathways have been observed. The tertiary 1-
adamantyl chloroformate solvolyzes by a unimolecular path-
way, which also involves loss of carbon dioxide.11 Stereochemi-
cal and kinetic evidence exists for carboxylium ion (ROCO+)
formation in the hydrolyses of secondary alkyl chlorofor-
mates.7,12,13 Methyl and ethyl chloroformates are believed to
solvolyze by the unimolecular pathway in moist formic acid
but by a bimolecular pathway in a range of less ionizing
solvents.7,12,14–16 The bimolecular pathway has usually been con-
sidered to proceed via a tetrahedral intermediate [eqn. (2) or a
R+Cl–
Products
–CO2
–Cl–
a
b
+
[
]
O
R
O
C
O
Cl
R
O
Cl–
C
(4)
SOH
R
O
C
O
OS
Queen 7 considered the hydrolysis of phenyl chloroformate
to proceed by the addition–elimination bimolecular pathway,
and this viewpoint was subsequently supported for solvolyses
in both aqueous acetone6 and ethanol.18 Recently, a direct
displacement bimolecular pathway has been suggested for
solvolyses in methanol–acetonitrile mixtures;3 this pathway
differs, however, from the Kivinen concept of an SN 2-type
mechanism 2,9a,15 in that the initial attack is believed to
resemble that leading to the addition–elimination mechanism.
However, the reaction was considered to be concerted, due to
the belief that the tetrahedral intermediate had an insignificant
lifetime.19 A direct pathway of this type would be expected to
show in kinetic investigations most of the characteristics
of an addition–elimination process, including general-base
catalysis by a second molecule of solvent.20
That the mechanism for phenyl chloroformate solvolysis,
despite several studies, is not firmly established is suggested by
the observation that a study involving solvolyses in aqueous
dioxane and aqueous tetrahydrofuran was considered to give
support to a unimolecular mechanism (pathway 4b, followed by
loss of carbon dioxide to give phenol).4
(2)
In the present study, the extended Grunwald–Winstein equa-
tion [eqn. (5)]21 is applied to the solvolyses of phenyl chloro-
log (k/k0) = lNT ϩ mYCl ϩ c
(5)
formate in a wide range of solvent types. This is the first time
that this equation has been used as a tool in studies of chloro-
formate ester solvolysis. In eqn. (5), k and k0 are the specific rates
of solvolysis of the substrate in a given solvent and in 80%
† Abstracted, in part, from the PhD dissertation of M. J. D., Northern
Illinois University, August 1992.
‡ Current address: Department of Chemistry, University of Wisconsin
Center-Waukesha County, Waukesha, Wisconsin, 53188-2799, USA.
J. Chem. Soc., Perkin Trans. 2, 1997
1721

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Table 1 First-order rate coefficients for the solvolysis of phenyl chloro-
formate a at 25.0 ЊC
Solvent b
k/10Ϫ5 sϪ1 c
NT
YCl
d
e
100% EtOH
90% EtOH
80% EtOH
70% EtOH
60% EtOH
100% H2O
100% MeOH
100% TFE i
97% TFE i
90% TFE i
260 ± 3 f
389 ± 6
503 ± 11
546 ± 9
658 ± 10
1338g
ϩ0.37
ϩ0.16
0.00
Ϫ2.52
Ϫ0.94
0.00
0.78
1.38
4.57
Ϫ1.17
2.79
2.83
2.85
1.89
0.63
4.31
3.83
3.80
Ϫ0.80
0.60l
4.28
2.97
3.71
4.23
Ϫ0.20
Ϫ0.38
Ϫ1.38
ϩ0.17
Ϫ3.93
Ϫ3.30
Ϫ2.55
Ϫ1.76
Ϫ0.94
Ϫ3.84
Ϫ2.94
Ϫ2.49
Ϫ0.37
Ϫ0.48l
Ϫ1.23
Ϫ0.98l
Ϫ1.12
Ϫ1.25l
695 ± 9h
0.001 32 ± 0.000 30
0.0570 ± 0.0030
1.15 ± 0.08
2.43 ± 0.21
17.5 ± 0.5
0.166 ± 0.004
10.5 ± 0.3
31.6 ± 0.6
68.8 ± 0.8
168 ± 1k
j
80T–20E
60T–40E
90% HFIP i
70% HFIP i
50% HFIP i
80% Acetone
65% Acetone
10% Acetone
30% Dioxane
20% Dioxane
10% Dioxane
Fig. 1 Plot of log (k/k0) for solvolyses of phenyl chloroformate at
25.0 ЊC against (1.68NT ϩ 0.57YCl)
1198 ± 9k
780m
910m
Discussion
1090m
The data presented in Table 1 are sufficient in and of themselves
to refute the claim4 that the solvolyses of phenyl chloroformate
in aqueous dioxane and aqueous tetrahydrofuran are SN 1 in
character. The 100% TFE and 90% TFE solvents have higher
and essentially identical ionizing powers but differ in solvent
nucleophilicity by 1.4 NT units. The specific rate of solvolysis in
the more nucleophilic 90% TFE is higher by a factor of 870,
corresponding to an approximate (two point) sensitivity to
changes in solvent nucleophilicity [l value of eqn. (5)] of 2.1.
Such a very large value is clearly incompatible with an SN 1
process.
a
Substrate concentration of 0.008–0.012 mol dmϪ3 b Unless otherwise
.
stated, binary solvents are on a volume–volume basis at 25.0 ЊC. c With
associated standard deviation. d From ref. 22. e From ref. 23. f From ref.
18; also value of 320 (± 30) × 10Ϫ5 sϪ1 (ref. 5). g Interpolated from data
at other temperatures (ref. 7). h A value of 694 × 10Ϫ5 sϪ1 has been
reported (ref. 3). i On weight–weight basis. j T–E are TFE–ethanol mix-
tures. k Value from ref. 6. l Interpolated values. m Values from ref. 8.
Table
2
First-order rate coefficients for the solvolysis of p-
methoxyphenyl chloroformatea at 25.0 ЊC and comparison with the
values for the corresponding solvolyses of phenyl chloroformate
A comprehensive analysis, giving also the sensitivity to
changes in solvent ionizing power (m value), has been carried
out using all of the 21 specific rates of solvolysis listed in Table
1. An analysis in terms of the simple (original)24 Grunwald–
Winstein equation [eqn. (5) without the lNT term] leads to an
extremely poor correlation with a negative m value of
Ϫ0.17 ± 0.18, c value of Ϫ0.73 ± 1.67, F-test value of only 0.96
and a correlation coefficient of 0.219. The correlation is enor-
mously improved by the use of the full eqn. (5), with an l value
of 1.68 ± 0.10, m value of 0.57 ± 0.06, c value of 0.12 ± 0.41,
F-test value of 159, and a correlation coefficient of 0.973.
The very large sensitivity (l value) to changes in solvent
nucleophilicity suggests a very pronounced involvement of the
solvent as a nucleophile in the rate-determining step, consistent
with the first step of an addition–elimination mechanism being
rate-determining.
If eqn. (2), or a closely related variant, represents the mech-
anism, the nucleophilic attack by the solvent will be accom-
panied not by heterolysis of the carbon–chlorine bond (which
will merely elongate slightly, due to the rehybridization of the
carbon) but by the π electrons of the carbonyl group migrating
to the oxygen, which formally now carries a full negative charge.
The relatively large m value (0.57 ± 0.06) can be rationalized in
terms of the stabilization (dispersion) by the solvent of the
appreciable increase in negative charge on the oxygen at the
transition state.
One very compelling type of evidence for a rate-determining
addition, within an addition–elimination pathway, involves a
study of chlorine versus fluorine leaving-group effects. This
approach has been used very successfully in studies of the
addition–elimination pathway involved in nucleophilic aro-
matic substitution.25 When the carbon–halogen bond is broken
in a nucleophilic displacement reaction, the chloro-derivative
reacts appreciably faster than the fluoro-derivative, as much as
105 to 107 times faster for a unimolecular ionization 26,27 and
some 101 to 103 times faster in concerted bimolecular displace-
ments at a saturated carbon.28 The previous observations5,29–31
Solvent
k/10Ϫ5 sϪ1
kp-M eO/kH
100% EtOH
100% MeOH
90% HFIP d
50% HFIP d
65% Acetonee
10% Acetonee
153 ± 4b
414c
0.172 ± 0.007
24.9 ± 0.5
103 ± 1f
999 ± 9 f
0.59
0.60
1.04
0.79
0.61
0.83
Substrate concentration of 0.008–0.010 mol dmϪ3
.
b From ref. 18.
a
c From ref. 3. d On weight–weight basis. e On volume–volume basis at
25.0 ЊC. f From ref. 6.
ethanol, respectively; l is the sensitivity towards changes in
solvent nucleophilicity (the NT scale based on S-methyl-
dibenzothiophenium ion solvolyses22 being used);
m is
the sensitivity towards changes in solvent ionizing power (YCl
values based on 1-chloroadamantane solvolyses23 being used)
and c is a constant (residual) term.
Results
The specific rates of solvolysis of phenyl chloroformate, at
25.0 ЊC, were determined in methanol, 2,2,2-trifluoroethanol
(TFE) and in binary aqueous solvents with the other com-
ponent being ethanol, acetone, TFE, or 1,1,1,3,3,3-
hexafluoropropan-2-ol (HFIP). Determinations were also made
in TFE–ethanol mixtures. Values for 100% ethanol18 and
water 7 (interpolated from studies at other temperatures), 65
and 10% acetone,6 and several aqueous dioxane compositions8
were available from the literature. In Table 1 are presented the
21 data points used in correlations involving both simple and
extended [eqn. (5)] forms of the Grunwald–Winstein equation.
A more limited set of data was obtained either by experiment or
from the literature for corresponding solvolyses of the p-
methoxy derivative (Table 2).
1722
J. Chem. Soc., Perkin Trans. 2, 1997

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that, when a bimolecular attack at an acyl carbon is operative,
fluoroformate and chloroformate esters solvolyze at very simi-
lar rates, frequently with the fluoroformate ester the faster, have
been uniformly rationalized in terms of the addition step of an
addition–elimination mechanism being rate-determining. In
particular, at 25.0 ЊC, the specific rates of ethanolysis of phenyl
chloroformate and phenyl fluoroformate are identical.5 Swain
and Scott26 rationalized the wide range of RCl:RF rate ratios in
substitution reactions in terms of the replacement of chlorine
by fluorine making both the heterolysis of the C–Hal bond
more difficult and, by increasing the positive charge on the α-
carbon, the formation of Nu–C bonds easier.
observed 3,6,7 for the solvolyses of phenyl chloroformate and
ring-substituted derivatives are also indicative37 of a bimolecu-
lar mechanism.
Conclusions
A very good correlation of the specific rates of solvolysis of
phenyl chloroformate has been observed in a wide variety of
solvent types. The extended Grunwald–Winstein equation was
used, with incorporation of the NT scale of solvent nucleo-
philicity and the YCl scale of solvent ionizing power [eqn. (5)].
The very large sensitivity to changes in the NT value (l value)
of 1.68 ± 0.10 is clearly inconsistent with a recent discussion 4
of the mechanism of solvolysis of phenyl chloroformate in
aqueous dioxane or tetrahydrofuran in terms of a rate-
determining unimolecular ionization. It is consistent with the
high degree of participation by the solvent which is to be
expected if the addition step of an addition–elimination path-
way is rate-determining. Such a mechanism has been given
strong support based on an F/Cl leaving group effect of unity in
the ethanolysis of phenyl haloformate, Hammett treatments of
substituent effects, solvent isotope effects, and large negative
values for the entropies of activation. Very recently, it has been
suggested 3 that the tetrahedral intermediate involved may be
so unstable that an enforced concerted variant pathway is
followed. However, the F/Cl leaving-group effect suggests
that, if followed, such a pathway must closely resemble the
traditional addition–elimination pathway.
A recent study3 of the solvolyses of the parent phenyl chloro-
formate and four ring-substituted derivatives in methanol and
six methanol–acetonitrile mixtures led in each solvent to
Hammett plots with a discontinuity. In the analyses, two lines
were drawn, intersecting at the central (unsubstituted) data
point, with slopes (ρ values) of ca. 0.8 and ca. 1.6. It was con-
cluded that an associative SN 2-type mechanism was operative,
with a transition state similar to a tetrahedral intermediate.
However, to account for the discontinuity, a transition-state
structure which varied with the identity of the substituent was
proposed. A previous study8 in aqueous dioxane was inter-
preted in terms of a linear plot. However, inspection of the plot
shows that the point for the solvolysis of the p-methoxy deriva-
tive lies some distance above the best fit line.
The most thorough study is presented in two papers by
Bacaloglu and co-workers.6,18 For the parent phenyl chlorofor-
mate and eleven ring-substituted derivatives, they obtained a
good linear plot against the traditional Hammett σ values for
solvolyses in 10% aqueous acetone,6 except that the data points
for the p-methoxy and, especially, the p-benzyloxy substituents
lay above the plot. Noting that similar behaviour in related
solvolyses of diaryl carbonates had been corrected32 by use of
the ‘normal’ σ0 values of Taft 33 (established using substrates
with isolated substituted phenyl groups, with no resonance
interaction 33–35), they also substituted σ0 values for the original
σ values and found that a good linear relationship (ρ = 1.03)
then resulted. Parallel behaviour was found in 65% acetone6
(ρ = 1.59) and ethanol18 (ρ = 1.73). Therefore, it appears that,
with use of the appropriate substituent constants, one good
linear relationship is obtained for solvolyses over the full range
of substituted phenyl chloroformates (substituents ranging
from p-benzyloxy to p-nitro), consistent with an essentially con-
stant reaction mechanism not only over a wide range of sol-
vents but also over a wide range of ring substituents.
Accordingly, the l value and the m value of 0.57 ± 0.06 can be
considered to be representative values for a mechanism involv-
ing rate-determining addition within an addition–elimination
pathway, which will be very useful as reference values in
extended Grunwald–Winstein treatments of the solvolyses of
other chloroformate esters.
Experimental
Phenyl chloroformate (Aldrich, 99%) and p-methoxyphenyl
chloroformate (Aldrich, 98%) were used without further purifi-
cation. Solvents were purified and the kinetic runs carried out
as previously described.22a The simple and multiple regression
analyses were performed using the ABSTAT statistical package
(Anderson–Bell, Arvada, Colorado, USA).
Acknowledgements
A reinspection of the figure presented by Lee and co-
workers3 shows good linear plots for four of the five data
points, with, as in the earlier studies,6,8,18 the p-methoxy sub-
stituent lying above the plot. We believe, therefore, that these
results are also best explained by the need to use ‘normal’ σ0
values33 (or σn values34), and there is no convincing evidence for
appreciable variations in transition state structure3 for these
solvolyses. In support of this view, we have tabulated in Table 2
the kp-MeO/kH ratios for solvolyses in six solvents and find a fairly
constant ratio which is not far removed from unity, consistent
with the small negative, somewhat solvent dependent,6,33 ‘nor-
mal’ substituent constants for the p-methoxy group.33,34
D. N. K. thanks Dr T. W. Bentley (University of Wales, Swan-
sea) and Professor H. Mayr (Universität München) for hospi-
tality during the time that this manuscript was being prepared.
References
1 M. Matzner, R. P. Kurkjy and R. J. Cotter, Chem. Rev., 1964, 64,
645.
2 D. N. Kevill, in The Chemistry of Acyl Halides, ed. S. Patai, Inter-
science, New York, 1972, ch. 12.
3 K. H. Yew, H. J. Koh, H. W. Lee and I. Lee, J. Chem. Soc., Perkin
Trans. 2, 1995, 2263.
4 M. A. Mousa, E.-H. M. Diefallah, H. A. Dessouki and S. T. Atwa,
Thermochim. Acta, 1989, 141, 1.
5 S. I. Orlov, A. L. Chimishkyan and M. S. Grabarnik, J. Org. Chem.
USSR (Engl. Transl.), 1983, 19, 1981.
6 G. Ostrogovich, C. Csunderlik and R. Bacaloglu, J. Prakt. Chem.,
1975, 317, 62.
7 A. Queen, Can. J. Chem., 1967, 45, 1619.
8 A. R. Butler, I. H. Robertson and R. Bacaloglu, J. Chem. Soc.,
Perkin Trans. 2, 1974, 1733.
9 (a) A. Kivinen, in The Chemistry of Acyl Halides, ed. S. Patai, Inter-
science, New York, 1972, ch. 6; (b) D. P. N. Satchell, Q. Rev. Chem.
Soc., 1963, 17, 160.
Solvent isotope effects (kH O/kD O), which have been meas-
2
2
ured, include values for aryl chloroformates of 2.19 to 2.24 in
10% acetone6 and 1.79 for phenyl chloroformate in 100%
water.7 Similar values have also been obtained for solvolyses of
chloroformate esters containing a primary alkyl group.7,9 These
values are within the range predicted for a bimolecular solvoly-
sis which is accompanied by general-base catalysis.27,36 The
solvent isotope effect of 2.3–2.5 for methanolysis (kM eOH /kM eOD
)
of a series of substituted phenyl chloroformates was also con-
sidered 3 to represent evidence for a bimolecular attack involv-
ing general-base catalysis.17 The small activation energies and,
especially, the negative entropies of activation which have been
10 E. Bock and D. Iwacha, Can. J. Chem., 1967, 45, 3177.
11 D. N. Kevill, J. B. Kyong and F. L. Weitl, J. Org. Chem., 1990, 55,
4304.
J. Chem. Soc., Perkin Trans. 2, 1997
1723

View Article Online
12 E. W. Crunden and R. F. Hudson, J. Chem. Soc., 1961, 3748.
13 M. Green and R. F. Hudson, J. Chem. Soc., 1962, 1076.
14 R. Leimu, Chem. Ber., 1937, 70, 1040.
27 B. D. Song and W. P. Jencks, J. Am. Chem. Soc., 1989, 111, 8470.
28 (a) D. N. Glew and E. A. Moelwyn-Hughes, Proc. R. Soc. London,
Ser. A, 1952, 211, 254; (b) I. Fells and E. A. Moelwyn-Hughes,
J. Chem. Soc., 1959, 398; (c) R. H. Bathgate and E. A. Moelwyn-
Hughes, J. Chem. Soc., 1959, 2642; (d) R. E. Robertson, Prog. Phys.
Org. Chem., 1967, 4, 213.
29 M. Green and R. F. Hudson, J. Chem. Soc., 1962, 1055.
30 A. Queen and T. A. Nour, J. Chem. Soc., Perkin Trans. 2, 1976, 935.
31 D. N. Kevill and J. B. Kyong, J. Org. Chem., 1992, 57, 258.
32 G. D. Cooper, H. T. Johnson and B. Williams, J. Org. Chem., 1965,
30, 3989.
33 (a) R. W. Taft, Jr., J. Phys. Chem., 1960, 64, 1805; (b) R. W. Taft, Jr.,
S. Ehrenson, I. C. Lewis and R. E. Glick, J. Am. Chem. Soc., 1959,
81, 5352.
34 (a) A. J. Hoefnagel and B. M. Wepster, J. Am. Chem. Soc., 1973, 95,
5357; (b) H. van Bekkum, P. E. Verkade and B. M. Wepster, Rec.
Trav. Chim. Pays-Bas, 1959, 78, 815.
15 A. Kivinen, Acta Chem. Scand., 1965, 19, 845.
16 H. K. Hall, Jr., J. Am. Chem. Soc., 1955, 77, 5993.
17 T. W. Bentley, G. Llewellyn and J. A. McAlister, J. Org. Chem., 1996,
61, 7927, and references cited therein.
18 C. Csunderlik, R. Bacaloglu and G. Ostrogovich, J. Prakt. Chem.,
1975, 317, 81.
19 W. P. Jencks, Chem. Soc. Rev., 1981, 10, 345.
20 (a) D. N. Kevill and F. D. Foss, J. Am. Chem. Soc., 1969, 91, 5054;
(b) T. W. Bentley and H. C. Harris, J. Org. Chem., 1988, 53, 724.
21 (a) S. Winstein, E. Grunwald and H. W. Jones, J. Am. Chem. Soc.,
1951, 73, 2700; (b) F. L. Schadt, T. W. Bentley and P. v. R. Schleyer,
J. Am. Chem. Soc., 1976, 98, 7667.
22 (a) D. N. Kevill and S. W. Anderson, J. Org. Chem., 1991, 56, 1845;
(b) D. N. Kevill, in Advances in Quantitative Structure–Property
Relationships, ed. M. Charton, JAI Press, Greenwich, Connecticut,
1996, vol. 1, pp. 81–115.
23 (a) T. W. Bentley and G. E. Carter, J. Am. Chem. Soc., 1982, 104,
5741; (b) T. W. Bentley and G. Llewellyn, Prog. Phys. Org. Chem.,
1990, 17, 121; (c) D. N. Kevill and M. J. D’Souza, J. Chem. Res. (S),
1993, 174; (d) I. S. Koo, T. W. Bentley, D. H. Kang and I. Lee,
J. Chem. Soc., Perkin Trans. 2, 1991, 296.
35 C. D. Ritchie, Physical Organic Chemistry: The Fundamental Con-
cepts, Marcel Dekker, New York, 2nd edn., 1990, pp. 98–100.
36 B. D. Song and W. P. Jencks, J. Am. Chem. Soc., 1987, 109, 3160.
37 C. K. Ingold, Structure and Mechanism in Organic Chemistry, 2nd
edn., Cornell University Press, Ithaca, New York, 1969, pp. 471–
473.
24 E. Grunwald and S. Winstein, J. Am. Chem. Soc., 1948, 70, 846.
25 (a) J. Miller, Nucleophilic Aromatic Substitution, Elsevier, Amster-
dam, 1968; (b) J. F. Bunnett, E. W. Garbisch, Jr., and K. M. Pruitt,
J. Am. Chem. Soc., 1957, 79, 385.
Paper 7/01140G
Received 18th February 1997
Accepted 14th April 1997
26 C. G. Swain and C. B. Scott, J. Am. Chem. Soc., 1953, 75, 246.
1724
J. Chem. Soc., Perkin Trans. 2, 1997