Rate and product studies with dimethyl phosphorochloridate and phosphorochloridothionate under solvolytic conditions

Article abstract of DOI:10.1039/b402093f

The specific rates of solvolysis of dimethyl phosphorochloridate and of dimethyl phosphorochloridothionate are very well correlated using the extended Grunwald-Winstein equation, with incorporation of the N_T solvent nucleophilicity scale and the Y_Cl, solvent ionizing power scale. The sensitivity parameters (l and m) are similar to each other and also similar to previously recorded values for solvolyses of arenesulfonyl chlorides, which were proposed to follow a concerted displacement mechanism. For solvolyses in aqueous ethanol or aqueous methanol the product selectivities (S) are close to unity. For solvolyses in aqueous 2,2,2-trifluoroethanol, the values are too small to accurately measure, showing a very large preference for product formation involving nucleophilic attack by the water component. It is concluded that the chloride and chloridothionate solvolyses, in common with the solvolyses of arenesulfonyl chlorides, follow a concerted displacement mechanism.

Full text of DOI:10.1039/b402093f

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Rate and product studies with dimethyl phosphorochloridate and
phosphorochloridothionate under solvolytic conditions†
Dennis N. Kevill* and Jeffrey S. Carver
Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb,
Illinois 60115-2862, USA. E-mail: dkevill@niu.edu
Received 10th February 2004, Accepted 27th April 2004
First published as an Advance Article on the web 21st June 2004
The specific rates of solvolysis of dimethyl phosphorochloridate and of dimethyl phosphorochloridothionate
are very well correlated using the extended Grunwald–Winstein equation, with incorporation of the NT solvent
nucleophilicity scale and the YCl solvent ionizing power scale. The sensitivity parameters (l and m) are similar to each
other and also similar to previously recorded values for solvolyses of arenesulfonyl chlorides, which were proposed to
follow a concerted displacement mechanism. For solvolyses in aqueous ethanol or aqueous methanol the product
selectivities (S) are close to unity. For solvolyses in aqueous 2,2,2-trifluoroethanol, the values are too small to
accurately measure, showing a very large preference for product formation involving nucleophilic attack by the water
component. It is concluded that the chloride and chloridothionate solvolyses, in common with the solvolyses of
arenesulfonyl chlorides, follow a concerted displacement mechanism.
Possible reasons for the difference in behavior include the
substituent variation from aryloxy to amido groups in the two
studies and the presence of aryl groups in just one of the
studies. The influence on the goodness of fit of aryl groups
Introduction
The extended Grunwald–Winstein equation1 [equation (1)]
can give information which is very helpful in assessing the
mechanism of solvolysis reactions.
can be investigated through a parallel study using the corre-
sponding methyl derivative, dimethyl phosphorochloridate
log(k/k0) = lNT ϩ mYCl ϩ c
(1)
(DMPC, frequently named as dimethyl chlorophosphate).
In the recent study of TMDAPC, the hydrolysis product was
neutral,8 and it has been proposed9 to exist as a zwitterion
(protonation on nitrogen). Fortunately, the hydrolysis product
from DMPC titrates as acid and, hence, from a study of the
position of the infinity titer between one equivalent of acid
(HCl), corresponding to all reaction with alcohol, and two
equivalents of acid, corresponding to all reaction with water,
we can indirectly and accurately determine10 the product ratio
in mixed alcohol–water solvents. Accordingly, a two-part
experimental study of the effect of solvent variation has been
carried out involving determination of both the specific rates of
solvolysis and, for aqueous alcohols, the partitioning between
the two possible solvolysis products. In addition, a parallel
study has been carried out using dimethyl phosphoro-
chloridothionate (DMPCT, frequently named as dimethyl
chlorothiophosphate) as the substrate. This is in response to
interesting trends observed on similar sulfur for oxygen substi-
tution during studies of chloroformate esters11 and carbamoyl
halides.12 The two systems studied are outlined in Scheme 1.
Although originally parameterized using attack at sp3-hybrid-
ized carbon, it has been found to also be applicable to attack at
the sp2-hybridized carbon of acyl2 and carbamoyl3 halides and
chloroformate esters,4 and to attack at the sulfur of sulfonyl
halides.5 In equation (1), k and k0 are the specific rates of
solvolysis in the solvent under consideration and the standard
solvent (80% ethanol), respectively; l is the sensitivity to
changes in the solvent nucleophilicity value (NT); m is the
sensitivity to changes in the solvent ionizing power value (YCl);
c is a constant (residual) term.
When it was applied to solvolytic displacement of chloride
ion from phosphorochloridates the situation was found to be
less clear cut. Initially, diaryl phosphorochloridates were
studied6 and, while evidence for significant nucleophilic
participation was provided, equation (1) was found to hold
rather poorly, with significant dispersion and variation in slope
for different mixed solvent systems in linear free energy
relationship plots. In contrast, we found7 quite good corre-
lations in a study of the solvolyses of N,N,NЈ,NЈ-tetramethyl-
diamidophosphorochloridate (TMDAPC) in a wide variety
of pure and mixed solvents, the only major deviation being
the often observed lower log (k/k0) values for 2,2,2-trifluoro-
ethanol (TFE)–ethanol mixtures relative to those calcu-
lated based on correlation using results from the other
solvents used in the study. The nature of these deviations was
discussed.7
The cause of the differences in the quality of the correlation
in the two studies6,7 is not clear. Further, it is not possible to
know whether one of the solvolyses can be considered as typical
and the other as atypical as regards analyses in terms of the
extended Grunwald–Winstein equation for solvolyses involving
attack at phosphorus.
† Presented, in part, at the Tenth Kyushu International Symposium on
Physical Organic Chemistry, Fukuoka, October 2003. Abstracted in
part from the MS thesis of J. S. C., Northern Illinois University,
December 1998.
Scheme 1
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 2 0 4 0 – 2 0 4 3
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
2040

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Table 1 Specific rates of solvolysis (k) of dimethyl phosphorochlor-
idatea and dimethyl phosphorochloridothionateb at 25.0 ЊC in a variety
of pure and mixed solvents and solvent nucleophilicity (NT) and solvent
ionizing power (YCl) values
Table 2 Percentages of reaction with acid as producta and product
selectivities (S) for solvolyses of dimethyl phosphorochloridate and
dimethyl phosphorochloridothionate in aqueous alcohol solvents at
25.0 ЊC
105k, sϪ1
(CH3O)2POClb
(CH3O)2PSClc
e
f
Solventc
(CH3O)2POCl
(CH3O)2PSCl
NT
YCl
Solventd
% acid
Se
% acid
Se
100% EtOH
90% EtOH
80% EtOH
70% EtOH
60% EtOH
50% EtOH
40% EtOH
20% EtOH
100% H2O
29.6 1.2
0.86 0.04
2.90 0.06
5.11 0.17
8.46 0.18
12.7 0.3
21.7 0.5
40.1 1.2
76.6 3.3
0.37
0.16
0.00
Ϫ2.52
Ϫ0.94
0.00
0.78
1.38
2.02
2.75
4.09
4.57
Ϫ1.17
Ϫ0.18
0.67
1.46
2.07
3.25
4.10
Ϫ2.39
Ϫ0.83
0.17
90% EtOH
80% EtOH
70% EtOH
60% EtOH
40% EtOH
20% EtOH
90% MeOH
80% MeOH
70% MeOH
60% MeOH
40% MeOH
20% MeOH
90% TFE
50.3
63.6
71.0
0.36
0.46
0.56
38.8
49.4
57.8
64.8
81.5
89.1
19.9
30.0
39.0
46.8
61.8
82.0
0.57
0.83
1.00
1.16
1.10
1.59
1.00
1.31
1.51
1.70
2.09
1.98
161
285
3
7
416 13
Ϫ0.20
Ϫ0.38
Ϫ0.58
Ϫ0.74
Ϫ1.16
Ϫ1.38
0.17
Ϫ0.01
Ϫ0.06
Ϫ0.40
Ϫ0.54
Ϫ0.87
Ϫ1.23
Ϫ0.35
Ϫ0.37
Ϫ0.42
Ϫ0.52
Ϫ0.70
Ϫ0.83
Ϫ1.11
22.9
40.1
0.85
0.84
156
7
100% MeOH
90% MeOH
80% MeOH
70% MeOH
60% MeOH
40% MeOH
20% MeOH
90% Acetone
80% Acetone
70% Acetone
60% Acetone
50% Acetone
40% Acetone
20% Acetone
90% Dioxane
80% Dioxane
70% Dioxane
60% Dioxane
50% Dioxane
97% TFEg
113
309 10
496
2
3.74 0.14
9.17 0.17
15.3 0.4
25.3 0.8
39.0 1.3
78.1 2.1
8
f
f
f
f
100.0
99.2
102.8
98.8
80% TFE
70% TFE
50% TFE
116
5
49.1 0.9
0.75 0.03
2.32 0.05
4.74 0.09
8.77 0.09
15.1 0.2
30.1 0.4
96.4 1.3
0.73 0.02
2.78 0.05
6.54 0.11
13.3 0.2
24.7 0.3
a Based on (CH3O)2PS(O)OH production of 0% in pure alcohol and
100% in 60% acetone. b Concentration of 0.0070 M. c Concentration of
0.0037 M. d Solvents contain 0.6% acetone and are on a volume–
volume basis, except for the 2,2,2-trifluoroethanol–H2O mixtures,
which are on a weight–weight basis. e For ester formation relative to
acid formation, defined as in eqn. 2. f Only small amounts of
(CH3O)2P(O)OCH2CF3 are formed and the S value is too low to deter-
mine by this technique.
172
322
2
8
506 10
0.95
1.73
2.46
3.77
67.2 1.1
233 10
425 23
Ϫ0.46
Ϫ0.37
Ϫ0.54
Ϫ0.66
Ϫ3.30
Ϫ2.55
Ϫ2.19
Ϫ1.98
Ϫ1.73
Ϫ1.76
Ϫ0.94
Ϫ0.64
Ϫ0.34
0.08
were then used to calculate product selectivity values (S) using
equation (2); these values are also presented in Table 2.
0.36 0.01
1.79 0.06
11.9 0.5
34.8 1.7
2.83
2.85
2.90
2.96
3.16
1.89
0.63
0.16
90% TFEg
80% TFEg
0.48 0.01
1.31 0.02
5.99 0.07
70% TFEg
(2)
50% TFEg
186
8
80T–20Eh
0.81 0.03
4.59 0.14
60T–40Eh
Discussion
50T–50Eh
0.36 0.01
0.51 0.01
0.71 0.01
0.49 0.02
1.56 0.03
40T–60Eh
13.1 0.6
25.3 1.4
7.06 0.25
29.5 0.7
Ϫ0.48
Ϫ1.42
3.83
The specific rates of solvolysis of DMPC and DMPCT were
studied in a wide variety of pure and mixed solvents, including
the TFE- and HFIP-containing systems, which are important
components of meaningful extended Grunwald–Winstein
correlations. The data points for solvolyses in TFE–ethanol
mixtures fell below the best fit line, as is often observed in
treatments of this type. This phenomenon was very recently
discussed7 and it will not be considered again in this report.
Correlations were carried out both with and without the
TFE–ethanol data.
20T–80Eh
70% HFIPg
50% HFIPg
Ϫ2.94
Ϫ2.49
3.80
a Concentration of ca. 0.010 mol dmϪ3
.
b Concentration of ca. 0.0070
mol dmϪ3
.
c Unless otherwise stated, on a vol–vol basis, at 25.0 ЊC, with
the other component water. d With associated standard deviations.
e From refs. 13 and 14. f From refs. 15 and 16. g Solvent prepared
on weight–weight basis. h T–E are 2,2,2-trifluoroethanol–ethanol
mixtures.
The results of the correlations are presented in Figs. 1 and 2
and reported in Table 3, together with the corresponding
parameters obtained in the analyses of earlier kinetic studies of
attack at a variety of phosphorus and sulfur centers. The
specific rates measured in dioxane–water mixtures could not be
included in the correlations due to a lack of YCl values. The
l and m values [equation(1)] for DMPC and DMPCT are
similar to each other, with the variation within the combined
standard errors. The values are also very similar to those
previously derived7 for the solvolyses of N,N,NЈ,NЈ-tetra-
methyldiamidophosphorochloridate. The values obtained5
from an analysis of literature values17–19 for the solvolyses of
arenesulfonyl chlorides are also very similar. Only the l values
of 1.7 to 1.8 for the solvolyses of diphenyl phosphorochloridate
and the di-para-chloro-substituted derivative show appreciable
deviation from the other values reported in Table 3; these values
are in the range anticipated for an addition-elimination
mechanism but, because of the poor correlations, one should
not read too much into these values.
Results
The specific rates of solvolysis were determined at 25.0 ЊC in
25 solvents for DMPC and in 36 solvents for DMPCT. The
solvents consisted of ethanol, methanol and water, binary
mixtures of TFE with ethanol, and binary mixtures of
water with ethanol, methanol, acetone, dioxane, TFE, and
1,1,1,3,3,3-hexafluoro-2-propanol (HFIP). The specific rates
of solvolysis are reported in Table 1, together with the
available solvent nucleophilicity (NT)13,14 and ionizing power
(YCl)15,16 values.
The percentages of dimethyl hydrogen phosphate [(CH3O)2-
P(O)OH] within the product were calculated by taking the
excess titer over that for reaction in 100% alcohol, relative to
the excess titer for solvolysis in 60% acetone over that for the
reaction in 100% alcohol. Values for duplicate experiments were
recorded at 10, 15, and 20 half lives. No trends were observed
and the values reported in Table 2 are based on the averages of
the six determinations for each solvent composition. The
percentages of acid formation (x), resulting from hydrolysis,
The other substrates of Table 3 all have solvolyses with l
values in the 1.1 to 1.3 range and m values in the 0.45 to 0.7
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 2 0 4 0 – 2 0 4 3
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Table 3 Coefficients from the extended Grunwald–Winstein correlations of the specific rates of solvolysis of dimethyl phosphorochloridate and
dimethyl phosphorochloridothionate at 25.0 ЊC and a comparison with corresponding values for other phosphorochloridates and for arenesulfonyl
chlorides
Substrate
na
lb
mb
cb
Rc
Fd
(MeO)2POCl
22
18
31
28
31
27
38
31
33
37
1.36 0.23
1.24 0.14
1.21 0.10
1.16 0.08
1.20 0.07
1.14 0.05
1.72 0.18
1.79 0.20
1.25 0.15
1.10 0.17
0.54 0.13
0.45 0.08
0.60 0.04
0.55 0.03
0.69 0.04
0.63 0.03
0.68 0.06
0.58 0.08
0.62 0.04
0.61 0.04
Ϫ0.02 0.17
0.18 0.11
0.22 0.07
0.30 0.06
0.03 0.32
0.17 0.21
0.42 0.15
0.11 0.18
0.21 0.20
0.22 0.23
0.844
0.941
0.943
0.966
0.958
0.982
0.885
0.863
0.967
0.959
24
(MeO)2POCle
54
112
154
155
320
(MeO)2PSCl
(MeO)2PSCle
(Me2N)2POClf
(Me2N)2POCle, f
(PhO)2POClg
(p-ClC6H4O)2POClg
p-MeC6H4SO2Clh
p-MeOC6H4SO2Clh
216
194
a Number of data points. b From eqn. 1.. c Correlation coefficient. d F-test value. e Data points for TFE–ethanol mixtures excluded. f From ref. 7.
g From ref. 6. h From ref. 5.
unlike the situation for phenyl chloroformate and its three
thio-derivatives,4a,11,21 the similar l and m values for (MeO)2-
POCl and (MeO)2PSCl, with the values essentially constant
across the full range of solvent compositions studied, give no
evidence for any changeover in mechanism to one involving
ionization assisted by nucleophilic solvation of the developing
cation.
The selectivity values (S) for the solvolytic substitution of
chloride ion, either by nucleophilic attack of an alcohol
molecule or by nucleophilic attack of a water molecule, are
presented in Table 2 for a wide range of composition of the
binary solvent mixture. One must avoid a simplistic view
of these values since it is well established that, for attack at
carbonyl carbon,22 phosphorus,6,23 or sulfur,18,23 general-base
catalysis involving both of the components of the binary
mixture is frequently operative.
The values of 0.36 to 0.91 for the solvolyses of DMPC in
Fig. 1 Plot of log (k/k0) for solvolyses of dimethyl phosphoro-
chloridate at 25.0 ЊC against (1.24 NT ϩ 0.45 YCl).
aqueous ethanol or aqueous methanol (5 determinations)
indicate only a very slight preference for a general-base cata-
lyzed attack by water. It could well be that a slightly higher
nucleophilicity for an alcohol molecule is counterbalanced by
slightly less steric hindrance to nucleophilic attack by a water
molecule. Some support for this view comes from a comparison
with S values previously obtained for the same five solvent
compositions for solvolyses of diphenyl phosphorochloridate
[(PhO)2POCl]6 and ethyl phenylphosphonochloridate [Ph-
(EtO)POCl].23 The lowest set of S values (0.15 to 0.46) are
with (PhO)2POCl as the substrate, with the S values for
Ph(EtO)POCl (0.20 to 0.68) intermediate in value. Values
relative to (PhO)2POCl for the 5 common solvents are
from 1.3 to 1.6 for Ph(EtO)POCl and, except for a value of
3.0 for 90% MeOH, of from 1.8 to 2.4 for (MeO)2POCl.
This indicates that the S values fall with increased steric
hindrance, with Ph(EtO)2POCl apparently being intermediate
in character.
For the thio-derivative (DMPCT), the S values in the five
solvents are somewhat larger (0.57–1.31). The values rise with
further increases in water content, reaching 1.59 for 20%
ethanol and 1.98 for 20% methanol. For the five solvents used
in both studies, the S values are about 50% higher than the
values for solvolyses of DMPC. These could be coupled with a
lower l value for DMPCT being indicative of a reduced steric
hindrance effect, but it is probably best to consider the 50%
difference as being too small for a detailed discussion of its
source. For the solvolyses of 2,4,6-trimethylbenzenesulfonyl
chloride18 in the five solvents, the S values of 1.9 to 3.2 are
noticeably higher than for the reactions at the phosphorus of a
phosphorochloridate or a phosphonochloridate.
Fig. 2 Plot of log (k/k0) for solvolyses of dimethyl phosphoro-
chloridothionate at 25.0 ЊC against (1.16 NT ϩ 0.55 YCl).
range. It is well-established that displacement at the sulfonyl
group is a concerted process,20 and these similarities can be
considered to give indirect evidence in support of a parallel
process for solvolytic displacement of chloride ion from
the phosphorus of phosphorochloridates and phosphoro-
chloridothionates.
The slower reaction with the thio-derivative is consistent with
the findings for phenyl chloroformate that progressive intro-
duction of sulfur for oxygen leads to a reduction of rate for
the component of the solvolyses believed to follow a mech-
anism with a rate-determining bimolecular attack. However,
For solvolyses of DMPC in 90%–50% TFE the percentages
of (MeO)2POOH produced were in excess of 99% of the theor-
etical and no appreciable amount of reaction with the TFE
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 2 0 4 0 – 2 0 4 3
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component could be taking place. This is consistent with the
corresponding product study with (PhO)2POCl as the substrate,
when only small amounts (<1%) of the trifluoroethyl ester were
observed in the products by reversed-phase high-performance
liquid chromatography.6
Acknowledgements
Acknowledgement is made to the donors of the American
Chemical Society Petroleum Research Fund for partial support
of this research. D. N. K. thanks the Royal Society of Chem-
istry for an International Authors Grant and Dr T. W. Bentley
(University of Wales, Swansea) for many helpful discussions
and hospitality during the time that this manuscript was being
prepared.
Conclusion
Unlike the previously studied solvolyses of (PhO)2POCl, the
solvolyses of (MeO)2POCl and of (MeO)2PSCl give good corre-
lations when analyzed using the extended Grunwald–Winstein
equation. The goodness of fit in each case is comparable
to that observed previously for (Me2N)2POCl. As has been
found to be frequently the case, the correlations are improved
upon omission of the data points for TFE–ethanol mixtures.
The l values (1.27 0.14 and 1.17 0.07) and m values (0.47
0.08 and 0.55 0.03) are very similar to each other, indicating
that no appreciable change of mechanism occurs on the
replacement of the oxygen by sulfur. The values are very
similar to those previously recorded for the solvolyses of
arenesulfonyl chlorides. This is taken as support for the con-
cept of a concerted displacement reaction,24 similar to that put
forward5,20 for nucleophilic attack at the sulfur of the sulfonyl
chlorides.
The selectivity values (S) in aqueous ethanol and aqueous
methanol are similar for the two substrates and, for relatively
low water content, slightly favor reaction with the water. The
values are somewhat lower than those for arenesulfonyl
chloride solvolyses, which moderately favor reaction with
the alcohol. Comparison with values for solvolyses of Ph-
(EtO)POCl and (PhO)2POCl suggests that a balance between
higher nucleophilicity for the alcohol and lower steric
hindrance to water attack may be a major factor influencing
the variation of the S values with substrate structure.
References
1 S. Winstein, E. Grunwald and H. W. Jones, J. Am. Chem. Soc., 1951,
73, 2700.
2 D. N. Kevill and M. J. D’Souza, J. Phys. Org. Chem., 2002, 15, 881.
3 (a) M. J. D’Souza, D. N. Kevill, T. W. Bentley and A. C. Devaney,
J. Org. Chem., 1995, 60, 1632; (b) D. N. Kevill, A. J. Oldfield and
M. J. D’Souza, J. Chem. Res. (S), 1996, 122; (c) D. N. Kevill,
A. J. Cassamassa and M. J. D’Souza, J. Chem. Res. (S), 1996, 472;
(d ) D. N. Kevill, B. J. Best and M. J. D’Souza, Org. React. (Tartu),
1997, 31, 55.
4 (a) D. N. Kevill and M. J. D’Souza, J. Chem. Soc., Perkin Trans. 2,
1997, 1721; (b) D. N. Kevill and M. J. D’Souza, J. Org. Chem., 1998,
63, 2120; (c) D. N. Kevill, J. C. Kim and J. B. Kyong, J. Chem. Res.
(S), 1999, 150; (d ) J. B. Kyong, Y.-G. Kim, D. K. Kim and D. N.
Kevill, Bull. Korean Chem. Soc., 2000, 21, 662; (e) J. B. Kyong, B.-C.
Park, C.-B. Kim and D. N. Kevill, J. Org. Chem., 2000, 65, 8051.
5 D. N. Kevill and M. J. D’Souza, Collect. Czech. Chem. Commun.,
1999, 64, 1790.
6 T. W. Bentley, D. Ebdon, G. Llewellyn, M. H. Abduljaber, B. Miller
and D. N. Kevill, J. Chem. Soc., Dalton Trans., 1997, 3819.
7 D. N. Kevill and B. Miller, J. Org. Chem., 2002, 67, 7399.
8 D. F. Heath and P. Casapieri, Trans. Faraday Soc., 1951, 47, 1093.
9 E. W. Crunden and R. F. Hudson, J. Chem. Soc., 1962, 3591.
10 I. Dostrovsky and M. Halmann, J. Chem. Soc., 1953, 502.
11 D. N. Kevill and M. J. D’Souza, Can. J. Chem., 1999, 77, 1118.
12 D. N. Kevill, T. M. Rudolph and M. J. D’Souza, J. Phys. Org. Chem.,
2000, 13, 192.
13 D. N. Kevill, in Advances in Quantitative Structure-Property
Relationships, ed. M. Charton, JAI Press, Greenwich, CT, 1996,
vol. 1, pp. 81–115.
Experimental
14 D. N. Kevill and S. W. Anderson, J. Org. Chem., 1991, 56, 1845.
15 T. W. Bentley and G. Llewellyn, Prog. Phys. Org. Chem., 1990, 17,
121.
16 (a) T. W. Bentley and G. E. Carter, J. Am. Chem. Soc., 1982, 104,
5741; (b) D. N. Kevill and M. J. D’Souza, J. Chem. Res. (S), 1993,
174; (c) J. S. Lomas, M. J. D’Souza and D. N. Kevill, J. Am. Chem.
Soc., 1995, 117, 5891.
17 I. Lee, I. S. Koo and H. K. Kang, Bull. Korean Chem. Soc., 1981, 2,
41.
18 (a) I. S. Koo, T. W. Bentley, D. H. Kang and I. Lee, J. Chem. Soc.,
Perkin Trans. 2, 1991, 175; (b) I. S. Koo, T. W. Bentley, G. Llewellyn
and K. Yang, J. Chem. Soc., Perkin Trans. 2, 1991, 1175.
19 R. M. Forbes and H. Maskill, J. Chem. Soc., Chem. Commun., 1991,
854.
20 (a) I. M. Gordon, H. Maskill and M.-F. Ruasse, Chem. Soc. Rev.,
1989, 18, 123; (b) A. Williams, Concerted Organic and Bioorganic
Mechanisms, CRC Press, Boca Raton, 2000, pp. 181–189.
21 D. N. Kevill, M. W. Bond and M. J. D’Souza, J. Org. Chem., 1997,
62, 7869.
22 T. W. Bentley and I. S. Koo, J. Chem. Soc., Perkin Trans. 2, 1989,
1385.
23 T. W. Bentley and D. N. Ebdon, J. Phys. Org. Chem., 2001, 14, 759.
24 A. Williams, Concerted Organic and Bioorganic Mechanisms, CRC
Press, Boca Raton, 2000, pp. 172–177.
25 Safety: The Sigma-Aldrich Library of Chemical Safety Data, vol. 1,
2nd edn., ed. R. E. Lenga, Sigma-Aldrich, Milwaukee, Wisconsin,
1988, p. 1362.
Both of the substrates (Aldrich, 97%) were purified by
distillation of 25 g under reduced pressure using a 10 cm
Vigreaux column. A forerun of 5 g and a pot residue of
about 5 g were discarded. The distillation temperature was
40–50 ЊC. CAUTION!: Dimethyl phosphorochloridothionate
must be distilled under reduced pressure because it undergoes
a violent autocatalytic decomposition when heated above
120 ЊC.25 A comparison of weights of substrate taken and
infinity titers after solvolysis in 60% acetone (2 equivalents
of acid produced) indicated 99.3
0.4% purity for
(MeO)2POCl and 100.4 0.6% purity for (MeO)2PSCl.
The solvents were purified and the kinetic runs carried out
as previously described.14 The product ratios could have been
determined from the infinity titers of the kinetic runs but, to
maximize the accuracy, the values reported were determined
independently. A 0.47 M solution of the substrate in acetone
was prepared and 0.300 mL was added to 20.0 mL of the binary
solvent mixture under investigation. All determinations were in
duplicate at 25.0 ЊC and 5.00 mL portions were titrated in the
usual manner14 at 10, 15, and 20 half lives.
The multiple regression analyses were performed using
the ABSTAT statistical package (Anderson-Bell, Arvada,
Colorado, USA).
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 2 0 4 0 – 2 0 4 3
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