Solvolysis of N,N-dimethylthiocarbamoyl chloride: Effect of sulfur-for-oxygen substitution upon kinetics and product partitioning

Article abstract of DOI:10.1002/(SICI)1099-1395(200004)13:4<192::AID-POC228>3.0.CO;2-K

A study of the solvolyses of N,N-dimethylcarbamoyl chloride (1) was extended to the solvolyses of N,N-dimethylthiocarbamoyl chloride (2). The specific rates of solvolysis of 2 at 0.0°C are two to three orders of magnitude greater than those for 1. Analysis of the data using the extended Grunwald-Winstein equation leads to sensitivities l and m and an l/m ratio which are lower for 2 than those previously reported for 1. Product selectivities in mixtures of water with ethanol or methanol indicate a greater preference for reaction with alcohol for 2. All observations can be rationalized in terms of the formation of a more stable carbocation from 2, leading to an earlier transition state, reduced nucleophilic solvation and the possibility of extensive progression to a free carbocation prior to product formation. Copyright

Full text of DOI:10.1002/(SICI)1099-1395(200004)13:4<192::AID-POC228>3.0.CO;2-K

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2000; 13: 192–196
Solvolysis of N,N-dimethylthiocarbamoyl chloride: effect of
sulfur-for-oxygen substitution upon kinetics and product
partitioning
1
2
2
Dennis N. Kevill, * Todd M. Rudolph and Malcolm J. D'Souza
1
Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois 60115-2862, USA
Department of Chemistry, University of Wisconsin±Waukesha, Waukesha, Wisconsin 53188-2799, USA
2
Received 15 September 1999; revised 8 November 1999; accepted 10 November 1999
ABSTRACT: A study of the solvolyses of N,N-dimethylcarbamoyl chloride (1) was extended to the solvolyses of
N,N-dimethylthiocarbamoyl chloride (2). The specific rates of solvolysis of 2 at 0.0°C are two to three orders of
magnitude greater than those for 1. Analysis of the data using the extended Grunwald–Winstein equation leads to
sensitivities l and m and an l/m ratio which are lower for 2 than those previously reported for 1. Product selectivities in
mixtures of water with ethanol or methanol indicate a greater preference for reaction with alcohol for 2. All
observations can be rationalized in terms of the formation of a more stable carbocation from 2, leading to an earlier
transition state, reduced nucleophilic solvation and the possibility of extensive progression to a free carbocation prior
to product formation. Copyright  2000 John Wiley & Sons, Ltd.
KEYWORDS: N,N-dimethylthiocarbamoyl chloride; solvolysis; Grunwald–Winstein equation; selectivity
INTRODUCTION
tinued on introduction of a second sulfur atom and, for
phenyl chlorodithioformate (PhSCSCl), the ionization
mechanism was dominant over the full range of
1
,2
The extended Grunwald–Winstein equation has been
successfully applied previously to the specific rates of
solvolysis of chloroformate esters,
7
solvents. A shift towards an ionization mechanism for
3
–7,
N,N-disubstituted
and diaryl chlorophosphate
solvolysis was also observed on going from ethyl
8
–11
5
carbamoyl chlorides
chloroformate to ethyl chlorothioformate.
1
2
esters:
The solvolyses of N,N-dimethylcarbamoyl chloride (1)
9
were indicated to be S 1 in nature, but with a
N
logꢀk=k † ˆ lN ‡ mY ‡ c
ꢀ1†
0
T
Cl
pronounced nucleophilic solvation of the developing
carbocation (l = 0.56 Æ 0.05). For the solvolyses of N,N-
where k and k are the specific rates of solvolysis of a
0
diphenylcarbamoyl chloride, a much lower l value
8
substrate in a given solvent and in the standard solvent
(
0.23 ‡ 0.04) was observed and the solvolyses of N-
11
(80% ethanol), respectively, l is the sensitivity to changes
methyl-N-phenylcarbamoyl chloride gave an inter-
mediate value of 0.40 Æ 0.08.
2
,13
in solvent nucleophilicity (NT value ), m is the
sensitivity to changes in solvent ionizing power (Y
Cl
A study of product selectivities (S):
1
4,15
value
) and c is a constant (residual) term.
3
For the solvolyses of phenyl chloroformate, it was
found that, on substitution of a sulfur atom for either
‰
esterŠprod ‰H OŠ
2
solv
S ˆ
Á
ꢀ2†
4
,7
‰amineŠ
‰ROHŠ
oxygen atom,
the addition–elimination mechanism,
prod
solv
which had been dominant over the full range of solvents
studied, now dominated only in the more nucleophilic
and less ionizing solvents. For solvents rich in fluoro
alcohol [2,2,2-trifluoroethanol (TFE) or 1,1,1,3,3,3-hexa-
fluoro-2-propanol (HFIP)], an ionization mechanism
became dominant. The trend towards ionization con-
in aqueous ethanol or aqueous methanol led for 1 to S
values that were fairly constant in value at about 0.5 in
9
10
aqueous ethanol and at about 1.1 in aqueous methanol.
These values are very similar to those observed for the
competition between alcohol and water attack in
16
solvolyses of p-methoxybenzoyl chloride and 1-ada-
17
mantyl derivatives in these solvents. These S values
have been rationalized in terms of product formation at
the solvent-separated ion pair stage, and similar behavior
*
Correspondence to: D. N. Kevill, Department of Chemistry and
Biochemistry, Northern Illinois University, DeKalb, Illinois 60115-
862, USA.
9
2
was postulated for the solvolyses of 1 and also of 4-
Copyright  2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 192–196

SOLVOLYSIS OF N,N-DIMETHYLTHIOCARBAMOYL CHLORIDE
193
1
0
(
chloroformyl)morpholine and N,N-diphenylcarbamoyl
8
chloride.
We now report parallel studies of the kinetics and
product selectivities for solvolyses of N,N-dimethylthio-
carbamoyl chloride (2). Since, for chloroformate esters,
the introduction of sulfur for oxygen brought about a
considerable shift away from addition–elimination to-
wards ionization, it will be of interest to see the effects of
a parallel substitution into 1, which already solvolyzes by
an ionization mechanism.
We found only one previous study of the kinetics of
solvolyses of N,N-disubstituted thiocarbamoyl chlorides,
reporting on hydrolyses in 70% aqueous acetone at
temperatures in the range of � 5 to 30°C for several
Scheme 1
Scheme 2
1
8
substituents. This study of substituent effects in a
constant solvent is nicely complemented by our study of
solvent effects for a constant structure.
RESULTS AND DISCUSSION
This reaction will not have any influence on either the
kinetics or product studies, as carried out in this study.
When the solvent contains both an alcohol and water, a
portion of the overall solvolysis produces the stable
thiocarbamate ester, accompanied by acid. The other
portion, reaction with water, leads to dimethylamine,
which interacts with an equivalent amount of the formed
acid to give the dimethylammonium ion, which is neutral
under the titration conditions. The kinetics of the overall
solvolysis can be obtained based on the experimental
infinity titer. This infinity titer is dependent on solvent
composition and it affords an accurate measure of the
product partitioning occurring due to attack of either
alcohol or water. These results are presented in Table 2
and are relative to the titer obtained in 100% ethanol,
The specific rates of solvolysis of 2 at 0.0°C and, so as to
allow a direct comparison through k(2)/k(1) ratios, also
those for 1 under identical conditions are reported in
Table 1, together with the ratio. It can be seen that 2
solvolyzes two to three orders of magnitude faster than 1.
The observation that the ratio is solvent dependent,
varying by about one order of magnitude, indicates that
differences will be observed in the l and/or m values. The
overall reaction scheme for solvolyses of 2 in an
aqueous–alcohol solvent can be expressed according to
Scheme 1.
The carbon oxysulfide formed during reaction invol-
ving capture by water slowly hydrolyzes to produce
1
8,19
hydrogen sulfide:
Table 1. Speci®c rates of solvolysis of N,N-dimethylthiocarbamoyl chloride (2) and N,N-dimethylcarbamoyl chloride (1) in a
variety of solvents at 0.0°C
a
4
� 1 b
6
� 1 b,c
Solvent
10 k (2) (s
)
10 k (1) (s
)
k (2) /k (1)
1
00% EtOH
0.550 Æ 0.016
4.75 Æ 0.14
12.6 Æ 0.6
26.9 Æ 0.9
2.66 Æ 0.04
10.5 Æ 0.7
20.0 Æ 1.1
1.14 Æ 0.05
6.02 Æ 0.33
26.9 Æ 0.6
48.0 Æ 1.6
42.5 Æ 1.7
16.6 Æ 0.7
6.23 Æ 0.32
1.96 Æ 0.09
0.175 Æ 0.007
1.38 Æ 0.04
5.65 Æ 0.08
17.7 Æ 0.6
314 Æ 16
344 Æ 14
223 Æ 11
152 Æ 7
157 Æ 4
217 Æ 16
120 Æ 8
0% EtOH
0% EtOH
0% EtOH
1.69 Æ 0.04
4.84 Æ 0.15
16.6 Æ 0.7
9
8
8
7
0% MeOH
0% MeOH
0% Acetone
0% Acetone
100% TFE
6.01 Æ 0.25
7.78 Æ 0.34
2.56 Æ 0.05
1.40 Æ 0.03
0.712 Æ 0.013
0.414 Æ 0.006
448 Æ 21
617 Æ 34
1660 Æ 74
1186 Æ 56
875 Æ 48
473 Æ 23
9
7% TFE
80T–20E
60T–40E
40T–60E
20T–80E
a
Since substrate was added as a solution in acetone, actually 99.2% of the indicated composition together with 0.8% acetone; concentration of
�
3
substrate, 0.00306 mol dm
.
b
With associated standard deviation, based on all of the integrated rate coefficients from duplicate runs.
These values were determined by A. G. Sell and I. Zdravkovic (University of Wisconsin–Waukesha).
c
Copyright  2000 John Wiley & Sons, Ltd.
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1
94
D. N. KEVILL, T. M. RUDOLPH AND M. J. D’SOUZA
Table 2. Percentage of the overall reaction of N,N-
dimethylthiocarbamoyl chloride (2) in aqueous±organic
solvents which is accompanied by development of acid and
selectivity values (S)
[amine]prod proportional to the difference between the
acid titer for solvolysis in 100% ethanol and the
corresponding titer when the 100% ethanol is replaced
by the solvent under consideration. In contrast to the
values for solvolyses of 1, the S values are appreciably
greater than unity, showing a marked preference for
reaction with alcohol rather than water.
a
b
c
Solvent
Acid (%)
S (2)
S (1)
1
00% EtOH
100.0
91.8
88.3
86.4
100.0
95.2
82.6
0.3
9
8
7
0% EtOH
0% EtOH
0% EtOH
3.9
5.9
8.5
0.53
0.51
0.50
Before moving on to a consideration of the mechan-
istic implications of the analyses of the solvent effect on
kinetic and product studies, we shall also outline the
100% MeOH
9
8
8
0% MeOH
0% MeOH
0% Acetone
5.0
2.7
1.14
1.05
18
previously reported analysis of substituent effects in
20
terms of the Taft equation:
a
b
c
à Ã
See footnote a in Table 1.
As defined in Eqn. (2).
From Refs 9 and 10.
logꢀk=k † ˆ % ꢀ ‡ ꢁE
ꢀ3†
0
s
where k and k are the rate coefficients in the presence of
0
a given substituent and a methyl substituent, respectively,
%* is the sensitivity to changes in the polar parameter s*
and ꢁ is the sensitivity to changes in the steric parameter
when all reaction leads to acid production. It will be
noted that, in 80% acetone, all reaction will be with water
and, as expected, essentially zero acidity develops.
The two kinetic runs in aqueous acetone were carried
out by adding portions of solution at appropriate time
intervals into excess methanol, such that the subsequent
solvolysis produces an amount of acid proportional to the
amount of unreacted 2 at the time of sampling. That this
affords an accurate way of following the kinetics is
indicated by the observation that our specific rate of 6.02
E . Using the summation over the two substituents for s*
s
18
and E , values were obtained at 15°C in 70% acetone of
s
� 1.73 for %* and 0.002 for ꢁ, indicating essentially no
sensitivity to changes in the steric environment and an
appreciable sensitivity to changes in the polar environ-
ment, with faster reaction in the presence of electron-
supplying substituents.
We can now consider the three types of evidence
available for the solvolyses of 2 in terms of reaction
mechanism, based in part on a comparison with the
�
4
� 1
(
Æ0.33) Â 10
with a value of 6.03 Â 10
extrapolation of values at 5–30°C, obtained by measur-
s
at 0.0°C is in excellent agreement
�
4
� 1
s
resulting from a modest
9,10,21
corresponding parameters for the solvolyses of 1,
1
8
ing changes in electrical conductivity.
where a rate-determining ionization with a pronounced
nucleophilic solvation of the developing cation, followed
by product formation predominantly at the solvent-
A treatment of the 15 specific rates of solvolysis of 2
(Table 1) leads in terms of the simple Grunwald–
9
Winstein equation [Eqn. (1) without the lN term] to
values for m of 0.34 Æ 0.04 and for c of � 0.25 Æ 0.25,
with a correlation coefficient of 0.9053 and F-test value
of 59. Using the full equation, values are obtained of
separated ion-pair stage, was proposed.
The evidence obtained in this and previous work is all
consistent with the mechanism presented in Scheme 1.
This was also the mechanism proposed for the solvolyses
of the oxygen-containing analog 1. There are, however,
appreciable quantitative differences between the solvo-
lytic behavior of 1 and 2 in terms of both sensitivities
within the linear free energy relationships and product
selectivities. These can all be rationalized in terms of the
carbocation from 2 being more stable than that formed
from 1. If the oxygen or sulfur is represented by Z, we can
write the following resonance structures:
0
.31 Æ 0.07 for l, 0.57 Æ 0.06 for m and � 0.07 Æ 0.16 for
c, with a multiple correlation coefficient of 0.9674 and
F-test value of 87. Inspection of the data shows a good
correlation for all of the data, except the two water–
acetone mixtures. That this is not experimental error,
associated with the different procedure used to follow
these two runs, is indicated by the excellent agreement of
the value obtained in 70% acetone with a previous
1
8
measurement. When these two data points are excluded
from the treatment using the full equation, correlation
values are obtained of 0.29 Æ 0.03 for l, 0.55 Æ 0.03 for
m and � 0.03 Æ 0.07 for c, with a multiple correlation
coefficient of 0.9932 and F-test value of 362. The
experimental specific rates of solvolysis in 80% and 70%
acetone are, respectively, 3.0 and 2.5 times slower than
the values calculated using these parameters. In Fig. 1,
the plot is given based on the 13 solvents and the
aqueous–acetone points are added to show the deviations.
The selectivity values (Table 2) are calculated using
Eqn. (2), with [ester]prod proportional to the acid titer and
Scheme 3
The third contributor will be expected to make a larger
contribution when Z = S, because sulfur can much better
carry a positive charge in onium-type structures than
oxygen. In turn, this will lead to an enhanced stability for
the resonance hybrid.
Copyright  2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 192–196

SOLVOLYSIS OF N,N-DIMETHYLTHIOCARBAMOYL CHLORIDE
195
Figure 1. Plot of log (k/k ) for solvolyses of N,N-dimethylthiocarbamoyl chloride (2) against 0.29N ‡ 0.55Y
0
T
Cl
The %* value of � 1.73 for hydrolysis of thiocarbamoyl
chlorides in 70% acetone at 15°C is considerably lower
in magnitude than the � 4.11 for hydrolysis of carbamoyl
consistent with the idea of a more stable (less reactive)
cation being formed, with the reduced reactivity provid-
ing the opportunity for progression to the free-ion stage
before capture by solvent. The magnitude of S observed
for previous systems believed to involve capture at the
free-ion stage is illustrated by the following examples.
2
1
chlorides in 50% acetone at 50°C. This is consistent
with a reduced dependence on electron supply from
nitrogen when it is also readily available from the sulfur
atom.
23
Ta-Shma and Jencks
found, for capture of 4-
YC H CHCH ions in a 50:40:10 water–TFE–ethanol
‡
The l and m values are both lower than the
6
4
3
9
corresponding values for solvolyses of 1 of 0.56 for l
mixture, S values of 11.7 when Y is CH O and 27 when Y
3
and 0.74 for m. This is consistent with an earlier
transition state associated with the formation of a more
stable intermediate, as is also indicated by the consider-
ably faster solvolyses of 2 than 1. The lower l/m ratio
is (CH ) N. In solvolyses of benzhydryl chloride in
3
2
24
ethanol–water mixtures, Bentley and Ryu found S
values of 2.5–4.6 and, for solvolyses in methanol–water
mixtures, S values of 4.2–7.7. Corresponding values for
solvolyses of p-methoxybenzyl chloride were 1.7–4.8
(0.53 for 2 and 0.76 for 1) can be considered to reflect a
25
reduced need for stabilization of the cation by solvation
when an additional important source of internal stabiliza-
tion is present. This reduced solvation can also explain
and 3.6–7.2. Richard et al. have found, for capture of
several a-substituted 1-(4-methoxyphenyl)ethyl carboca-
tions in 50% methanol, values in the range 5–22.
the lower sensitivity to changes in the steric parameter E
for solvolyses of 2, relative to 1 (0.002 and 0.80).
s
18,21
The product selectivities relate to step 2 of Scheme 1.
EXPERIMENTAL
For solvolyses of 1, the values of about 0.5 for S [Eqn.
(2)] for competition between water and ethanol were
The N,N-dimethylthiocarbamoyl chloride (Aldrich, 97%)
considered to reflect capture at the solvent-separated ion-
pair stage, based on the similarity in value and the
constancy of the value both mimicking the behavior for
systems generally believed to involve capture at this
was used without further purification. References for
13
solvent purification have been presented previously.
Runs in the presence of an alcohol component in the
13
solvent were carried out as described previously;
a
9
,16,17,22
stage.
The very different S values for solvolyses
stock solution of 3.10 g of the substrate in 65 ml of
acetone was prepared and, at 0.0°C, 0.200 ml was added
to 25.0 ml of pure or mixed solvent under carefully
controlled conditions. In this way, the infinity titer taken
at 10 half-lives reflected accurately the partitioning
between reaction with alcohol and, when present, water.
Runs were performed in duplicate and two infinity titers
were taken for each run.
of 2, which also vary considerably more with solvent
composition than for 1, of 3.9–8.3 for ethanol–water
mixtures and 2.7–5.0 for methanol–water mixtures,
strongly suggest that the capture is not at the solvent-
separated ion-pair stage.
A survey of the literature shows that values of these
magnitudes have been observed when capture by solvent
is at the free-ion stage. Such a proposal is, of course,
The reactant solutions for runs in water–acetone
Copyright  2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 192–196

1
96
D. N. KEVILL, T. M. RUDOLPH AND M. J. D’SOUZA
mixtures were made up in identical fashion but the 2 ml
portions removed at appropriate time intervals were
added to 10.0 ml of methanol and allowed to stand for in
excess of 5 h before addition of 15 ml of acetone and
9. Kevill DN, Oldfield AJ, D’Souza MJ. J. Chem. Res. (S) 1996; 122.
0. Kevill DN, Casamassa AJ, D’Souza MJ. J. Chem. Res. (S) 1996;
1
1
4
72.
1. Kevill DN, Best BJ, D’Souza MJ. Org. React. (Tartu) 1997; 31: 55.
12. Bentley TW, Ebdon D, Lllewellyn G, Abduljaber MH, Miller B,
1
0
Kevill DN. J. Chem. Soc., Dalton Trans. 1997; 3819.
titration in the usual manner.
1
1
1
3. Kevill DN, Anderson SW. J. Org. Chem. 1991; 56: 1845.
4. Bentley TW, Llewellyn G. Prog. Phys. Org. Chem. 1990; 17: 121.
5. Bentley TW, Carter GE. J. Am. Chem. Soc. 1982; 104: 5741.
REFERENCES
16. Bentley TW, Harris HC, Koo IS. J. Chem. Soc., Perkin Trans. 2
988; 783.
1
1
7. Kevill DN, Hawkinson DC. J. Org. Chem. 1990; 55: 5394, and
references cited therein.
1
. Winstein S, Grunwald E, Jones HW. J. Am. Chem. Soc. 1951; 73:
2
700.
1
1
2
8. Bacaloglu R, D a˘ escu C. Rev. Roum. Chim. 1977; 22: 977.
9. Thompson HW, Kearton CF, Lamb SA. J. Chem. Soc. 1935; 1033.
0. Lowry TH, Richardson KS. Mechanism and Theory in Organic
Chemistry (3rd edn). Harper and Row: New York, 1987; 152–153.
1. Bacaloglu R, D a˘ escu C, Ostrogovich G. J. Chem. Soc., Perkin
Trans. 2 1972; 1011.
2
. Kevill DN. In Advances in Quantitative Structure–Property
Relationships, vol. 1, Charton M (ed). Jai Press: Greenwich, CT,
1
996; 81–115.
. Kevill DN, D’Souza MJ. J. Chem. Soc., Perkin Trans. 2 1997;
721.
3
2
1
4. Kevill DN, Bond MW, D’Souza MJ. J. Org. Chem. 1997; 62: 7869.
5. Kevill DN, D’Souza MJ. J. Org. Chem. 1998; 63: 2120.
6. Kevill DN, Kim JC, Kyong JB. J. Chem. Res. (S) 1999; 150.
7. Kevill DN, D’Souza MJ. Can. J. Chem. 1999; 77: 1118.
8. D’Souza MJ, Kevill DN, Bentley TW, Devaney AC. J. Org. Chem.
¯
2
2. Oki M, Ikeda H, Toyota S. Bull. Chem. Soc. Jpn. 1998; 71: 749.
23. Ta-Shma R, Jencks WP. J Am. Chem. Soc. 1986; 108: 8040.
24. Bentley TW, Ryu ZH. J. Chem. Soc., Perkin Trans. 2 1994; 761.
25. Richard JP, Lin S-S, Buccigross JM, Amyes TL. J. Am. Chem. Soc.
1996; 118: 12603.
1
995; 60: 1632.
Copyright  2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 192–196