Kinetics and mechanism of the aminolysis of aryl propanedithioates in acetonitrile

Article abstract of DOI:10.1039/b006974o

The kinetics and mechanism of the aminolysis of aryl proanedithioates with benzylamines are investigated in acetonitrile at - 35.0 °C. A large magnitude of the Hammett (ρx and ρz) and Broensted (βx and βz) coefficients and exceptionally large cross-interaction constant ρxz(= 3.5) are consistent with a stepwise mechanism in which leaving group expulsion from an intermediate, T^±, is the rate-determining step. The faster rates observed for dithio esters (I1) than for thio esters (I), and the validity of the reactivity-selectivity principle (RSP) are also in line with the mechanism proposed. The k_H/k_D values (= 1.0-1.8) determined with deuterated benzylamines (XC₆H₄CH₂ND₂) and the activation parameters, ΔH^≠(?8 kcal mol^-1) and ΔS^≠(= -16 to -23 e.u.), suggest that proton transfer occurs concurrently with leaving group departure in the transition state.

Full text of DOI:10.1039/b006974o

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Kinetics and mechanism of the aminolysis of aryl propanedithioates in
acetonitrile¤
Hyuck Keun Oh,a Sun Kyung Kim,a Hai Whang Leeb and Ikchoon Lee*b
a Department of Chemistry, Chonbuk National University, Chonju, 560-756, Korea
b Department of Chemistry, Inha University, Inchon, 402-751, Korea. E-mail: ilee=inha.ac.kr;
Fax: ]82 32 865 4855
Received (in Montpellier, France) 24th August 2000, Accepted 9th October 2000
First published as an Advance Article on the web 22nd December 2000
The kinetics and mechanism of the aminolysis of aryl proanedithioates with benzylamines are investigated in
acetonitrile at [35.0 ¡C. A large magnitude of the Hammett (o and o ) and Bronsted (b and b ) coefficients and
X
Z
X
Z
exceptionally large cross-interaction constant o (\3.5) are consistent with a stepwise mechanism in which leaving
XZ
group expulsion from an intermediate, TB, is the rate-determining step. The faster rates observed for dithio esters
(II) than for thio esters (I), and the validity of the reactivity-selectivity principle (RSP) are also in line with the
mechanism proposed. The k /k values (\1.0È1.8) determined with deuterated benzylamines (XC H CH ND )
H
D
6
4
2
2
and the activation parameters, *HE(^8 kcal mol~1) and *SE(\[16 to [23 e.u.), suggest that proton transfer
occurs concurrently with leaving group departure in the transition state.
In the aminolysis of thio esters (I), it has been shown that
replacing them by dithio esters (II) [eqn. (1)] results in a
amine nucleophile is increased.3 However, the mechanistic
change from a stepwise one for S-aryl phenylethanethioates4
(Ic) to a concerted one for aryl phenylethanedithioates5 (IIc)
was not observed with benzylamine nucleophiles in acetonit-
rile. The change of R \ CH (a) to C H CH (c) in I (and II)
weaker push, provided by the S~, to expel the amine, R NH,
2
from the zwitterionic tetrahedral intermediate, TB. This leads
to a lowering of pKo ,1 where k \ k , by decreasing k
a
~a
b
~a
3
6
5
2
more than k , thus, decreasing the ratio k /k . This lowering
appears to raise the pKo value due to an electron-withdrawing
b
~a
b
a
of pKo was found to change the mechanism of the aminolysis
e†ect of the phenyl ring in the latter compounds.
a
of S-aryl ethanethioates (Ia) with benzylamines (pK P 9.0)
In view of the sensitive variation of the aminolysis mecha-
nism depending on the stereoelectronic e†ects of the R group,
we extend our work in this paper to the aminolysis with ben-
zylamines of the dithio series involving R \ CH CH (IIb), for
a
from a stepwise one with rate-limiting expulsion of the leaving
group, ArS~ from TB (k ) with a b (\b ) of 1.36 to a rate-
b
X
nuc
limiting formation of TB for aryl ethanedithioates (IIa) in ace-
tonitrile (b \ 0.55), since in the latter case the pKo is lower
3
2
which a stepwise mechanism with rate-limiting expulsion of
X
a
than the pK s of the benzylamines used,2 which may be
caused by the destabilization of TB due to the solvent, aceton-
the leaving group (ArS~) has been reported for the S-aryl
a
propanethioate analogues, (Ib), with b \ 2.11 in acetonitrile.6
X
itrile. It is well-known that a mechanistic change occurs at the
One of the reasons for choosing R \ CH CH (IIb) in the
3
2
pKo where the Bronsted b (b ) value changes from a large
present work is that in the aminolysis of the three thio esters
(Ia–c) with benzylamines in acetonitrile,4,6,7 the magnitudes of
o (\o ), b (\b ) and o were found to be the largest and
a
nuc
nuc X
(b P 0.8) to a small (b O 0.3) value as the basicity of the
nuc
X
nuc
X
nuc
XZ
also the rate k , [see eqn. (3) below] was the fastest for Ib
N
among the three substrates. o is deÐned by eqn. (2)8 where
XZ
X
and
Z
are the substituents in the nucleophile
(XC H CH NH ) and leaving group (ZC H S~).
6
4
2
2
6 4
log(k /k ) \ o p ] o p ] o p p
(2a)
(2b)
XZ HH
X X
Z Z
XZ X Z
o
\ do /dp \ do /dp
XZ
Z
X
X
Z
The aminolysis of all three thio esters (Ia–c) with benzyl-
amines was found to proceed by a stepwise mechanism with
rate-limiting expulsion of the leaving group from TB, and the
rate constants (k ) observed for these reactions can be
N
expressed as:
k \ (k /k ) É k \ K É k
~a b
(3)
N
a
b
It is therefore of some interest to examine whether the same
trends also hold for the dithio esters (IIa–c) and if so, what
could be the causes for this seemingly odd behavior exhibited
by the compounds with R \ CH CH in the aminolysis of the
3
2
two series of esters, Ia–c and IIa–c.
Results and discussion
(1)
The aminolysis of aryl propanedithioates (IIb) with an excess
amount of benzylamines (Scheme 1) in acetonitrile obeyed the
¤ Non-SI units employed: 1 kcal \ 4.184 kJ.
DOI: 10.1039/b006974o
New J. Chem., 2001, 25, 313È317
313
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2001

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for the dithio esters, II, the rates increase in the order R \
CH (a)2 \ C H CH (c)2 \ CH CH (b) (given that at [25 ¡C
3
6
5
2
3
2
the rate constant for IIb should be more than twice the value
at [35 ¡C). It is notable that the rate-limiting formation of TB
was found only for IIa and d, and for all other esters (Ia–d and
IIb, c, e, f) the stepwise mechanism with rate-limiting expul-
sion of the leaving group (LG), ArS~, was found to apply.
Furthermore, the rates of reaction of the dithio esters, II, are
much faster than those of the thio esters, I. These trends are in
fact in line with the stepwise mechanism, eqn. (1), with rate-
Scheme 1
limiting expulsion of the leaving group (k ). If the aminolysis
mechanism of esters I and II (excluding IIa and d) were con-
certed or stepwise with rate-limiting formation of the interme-
diate, TB, the rates of reaction of the thiol esters (Ia–c) should
have been faster since mixing of the C2O p* orbital and the
r* LUMO of the breaking CÈLG bond is stronger due to the
narrower energy gap between p* and r* than the correspond-
simple rate law given by eqn. (4) and (5), where [S] and [Nu]
are the concentrations of substrate (IIb) and benzylamine
nucleophile, respectively.
b
Rate \ k [S]
(4)
(5)
obs
k
\ k [Nu]
obs
N
ing mixing between p* and r*
, which has been shown to
C/S
ChLG
The k values were determined from the slopes of linear
lead to a greater possibility of the tetrahedral species becom-
ing a TS rather than a stable adduct9 for the carbonyl (I) than
the thiocarbonyl (II) esters.
There are examples of aminolysis reactions that are in
agreement with these arguments: Castro et al., reported on
N
plots of k vs. [Nu], eqn. (5) and are summarized in Table 1
obs
together with the Hammett (o and o ) and Bronsted (b and
X
Z
X
b ) coefficients. The rates (k ) and selectivity parameters,
Z
N
including the cross-interaction constants o [eqn. (2)] for the
XZ
reactions of thio (I) and dithio esters (II) with benzylamines in
the
aminolyses
of
O-ethyl
S-aryl
thiocarbonates
acetonitrile are compared together with those for the reactions
of other related R groups in Table 2. Surprisingly, the rates
are the fastest with R \ CH CH among various esters with
[C H OC(2O)SAr; Ig]10 and aryl O-ethyl dithiocarbonates
2
5
[C H OC(2S)SAr; IIg]11 with substituted pyridines
2
5
(XC H N) in aqueous solution. For both dinitrophenyl
3
2
5 4
di†erent R groups in each series of compounds, Ia–d and
IIa–f. For example, for the thio esters, I, the rates increase in
the order R \ C H CH (c)4 \ CH (a)7 \ CH CH (b)6, and
[Ar \ 2,4-(NO ) C H ] and trinitrophenyl [Ar \ 2,4,6-
2 2 6
3
(NO ) C H ] compounds, rates with dithio (IIg) esters were
2 3 6
2
faster below the pKo (^7 for DNPDTC and ^6 for
6
5
2
3
3
2
a
Table 1 The second order rate constants, k (M~1 s~1) for the reactions of aryl propanedithioates [EtC(2S)SC H Z] with benzylamines
N
6 4
(XC H CH NH ) in acetonitrile at [35.0 ¡C
6
4
2
2
Z
X
p-Me
H
p-Cl
p-Br
o a
b b
z
z
p-OMe
p-Me
H
23.2
9.68
14.9c
7.25d
42.9
20.3
111
70.9
116
84.5
202c
98.2d
1.74 ^ 0.05
2.29 ^ 0.16
[0.73 ^ 0.04
[0.93 ^ 0.02
3.42
0.800
1.45c
0.698d
9.84
3.46
39.5
21.5
47.8
26.7
59.1c
28.9d
2.77 ^ 0.12
3.68 ^ 0.14
[1.14 ^ 0.11
[1.52 ^ 0.17
p-Cl
m-Cl
0.310
1.31
[2.24 ^ 0.11
2.19 ^ 0.10
10.2
14.1
3.99 ^ 0.22
[1.63 ^ 0.12
n
o
o e
x
[2.86 ^ 0.07
[1.53 ^ 0.09
[1.39 ^ 0.06
p o f \ 3.51 ^ 0.29
b g
x
2.80 ^ 0.04
1.49 ^ 0.10
1.35 ^ 0.08
xz
a The p values were taken from C. Hansch, A. Leo and R. W. Taft, Chem. Rev., 1991, 91, 165. Coefficients were better than 0.995 in all cases.
b The pK values were taken from J. Buckingham, Dictionary of Organic Chemistry, 5th edn., Chapman and Hall, New York, 1982. For Z \ p-Br,
a
an extrapolated value of 5.87 was used. Correlation coefficients were better than 0.996 in all case. c At [15.0 ¡C. d At [25.0 ¡C. e The source of p
is the same as that for footnote a. Correlation coefficients were better than 0.994 in all cases. f Correlation coefficient was 0.995. g The pK values
a
were taken from A. Fischer, W. J. Galloway and J. Vaughan, J. Chem. Soc., 1964, 3588. Correlation coefficients were better than 0.993 in all cases.
(For X \ p-CH O, pK \ 9.67 was used; see: H. K. Oh, J. Y. Lee and I. Lee, Bull. Korean Chem. Soc., 1998, 19, 1198.
3
a
Table 2 Comparison of rate constants k and selectivity parameters for the reactions of benzylamines (XC H CH NH ) with thiol [I;
N
6
4
2
2
RC(2O)SC H Z] and dithio [II; RC(2S)SC H Z] esters in acetonitrile
6
4
6 4
Ester
103 k /M~1 s~1a
o b
b b
o c
b c
o
Ref.
N
x
x
z
z
xz
Ia
3.93(45 ¡C)
7.32(45 ¡C)
3.76(45 ¡C)d
2.51(45 ¡C)
[1.40
[2.09
[1.50
[1.88
[0.56
[2.24
[2.21
[0.65
[1.37
[1.00
1.36
2.11
1.55
1.86
0.55
2.19
2.03
0.24
1.40
1.02
5.32
2.74
1.61
3.84
1.19
2.77
3.51
0.56
1.85
1.26
[2.21
[1.18
[1.66
[1.63
[0.50
[1.15
[1.38
[0.24
[0.81
[0.56
0.90
2.36
0.92
0.27
0.40e
3.51
2.05
0.50e
0.65
0.59
7
6
4
Ib
Ic
Id
16b
2
This work
5
16d
f
IIa
IIb
IIc
IId
IIe
IIf
699([20 ¡C)
9840([35 ¡C)
11 600([25 ¡C)
382(30 ¡C)
34.0(15 ¡C)
25.6(15 ¡C)
f
a For X \ Z \ H. b o and b values are for Z \ H. c o and b values are for X \ H. d Extrapolated value. e Rate-limiting formation of TB.
x
x
z
z
f H. K. Oh, S. Y. Woo, C. H. Shin and I. Lee, Int. J. Chem. Kinet., 1998, 30, 849.
314
New J. Chem., 2001, 25, 313È317

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TNPDTC) but they reverse to slower above the pKo than with
to a tighter structure in organic reactions.8b,17 Since a steri-
cally hindered structure in the expulsion of the LG from the
intermediate, TB, should lead to a faster rate, the fastest rate
observed with R \ CH CH supports this argument.
a
the corresponding thio (Ig) esters. In other words, in the rate-
limiting breakdown of TB (below pKo) the dithio esters react
a
faster but in the rate-limiting formation of TB (above pKo) the
a
3
2
thio esters react faster, as shown schematically in Fig. 1.
Reference to Table 1 reveals that faster rates are invariably
accompanied by smaller magnitudes of selectivity parameters;
that is, the reactivity-selectivity principle (RSP)18 holds in all
cases. This is also considered to be consistent with the step-
wise mechanism proposed.19
The faster rates in the rate-limiting expulsion for the dithio
esters should be due to the larger k /k value in the observed
b
~a
rate constants, k , since the lower ability of the (ICÈS~)
N
compared to the (ICÈO~) group in TB to form a double
bond depresses the rate of amine expulsion (k ) more than
The kinetic isotope e†ects (k /k ) for the reaction of aryl
~a
that of ArS~ expulsion from TB (k ).1 Thus, the weaker p
b
H D
propanedithioates (IIb) with deuterated benzylamines
bond12 or the lower p* LUMO level of C2S compared to
(XC H CH ND ) in acetonitrile are shown in Table 3. We
6
4
2
2
C2O13 leads to a more highly stabilized intermediate, TB,
since the C2S p bond is easier to break to form TB but re-
formation of the C2S p bond from TB is more hindered than
with C2O.12,14
note that the isotope e†ects are normal (no inverse secondary
e†ects with k /k \ 1.0) with k /k \ 1.0È1.8, and the magni-
H
D
H D
tude suggests that proton transfer is involved in the TS,8b as
has often been observed for a stepwise pathway in which
partial proton transfer to the leaving group, ArS~, occurs con-
currently with rate-limiting departure of the leaving group
from the intermediate, TB, forming a four-center type TS,4h7
as shown in Scheme 2.
The higher rates observed with R \ CH CH than with
3
2
R \ CH , as well as with C H CH , for both series of esters I
3
6
5
2
and II can be explained by the electron-donating e†ect of the
ethyl group in the intermediate, where only the polar e†ect
(but no resonance e†ect) should be e†ective (TaftÏs p* values
are 0.0, [0.10 and ]0.22 for R \ CH , CH CH and
3
3
2
C H CH , respectively).15
6
5
2
It has been shown that an electron-donating R group
enhances the rate of leaving group expulsion, k , from the
b
TB.16 We can therefore conclude that the fastest aminolysis
rate with R \ CH CH among the three esters for both thio
3
2
(IaÈc) and dithio (IIaÈc) esters and also the faster rates for IIb
compared to Ib are a consequence of the stepwise mechanism
with rate-limiting expulsion of the leaving group, ArS~, from
TB. The proposed mechanism is supported by the large
o ([1.4 to [2.9), b (1.4È2.8), o (1.7È4.0), b ([0.7 to [1.6)
Scheme 2 Proposed TS structure.
X
X
Z
Z
and o (3.5) values in Table 1. We have compared these selec-
XZ
The activation parameters *HE and *SE, determined for
the present reaction series using the Eyring equation, are col-
lected in Table 4. The low *HE (8È9 kcal mol~1) and large
negative *SE([16 to [23 e.u.) values are consistent with the
TS proposed.4h7 Since rate-limiting expulsion of the leaving
group, ArS~, from TB is assisted by a concurrent proton
tivity parameters for di†erent R groups of thio (I) and dithio
(II) esters in Table 2. It should be noted that the reactions
proceeding by rate-limiting formation of TB(IIa and d) exhibit
much smaller magnitude of b and b . Examination of Table
X
Z
2 shows that for R \ CH CH the magnitude of b is the
3
2
X
largest but that of b is the smallest among the three R (aÈc)
Z
groups (except for b of IIa, which reacts by rate-limiting for-
Z
mation of TB). Moreover, the o value for IIb is exception-
XZ
ally large, suggesting a very strong interaction of nucleophile
Table 3 The kinetic isotope e†ects for the reactions of aryl propa-
nedithioates with deuterated benzylamines in acetonitrile at [35.0 ¡C
(XC H CH NH ) and leaving group (ZC H S~) in the TS.8
6
5
2
2
6 4
Although the largest magnitude of (tightest bond
b
X
X
Z
k /M~1 s~1
k /M~1 s~1
k /k
formation) and smallest b (least bond cleavage) suggest a
H
D
H
D
Z
tightest TS8ahc for R \ CH CH as implied by the largest
p-OMe p-Me
p-OMe
p-OMe p-Cl
p-OMe p-Br
p-Cl
p-Cl
p-Cl
p-Cl
23.2 ^ 0.2a
42.9 ^ 0.8
111 ^ 2
19.7 ^ 0.2a
37.9 ^ 0.5
104 ^ 1
1.18 ^ 0.02a
1.13 ^ 0.03
1.07 ^ 0.02
1.03 ^ 0.03
3
2
o
, the reasons for it are not apparent. The steric e†ect of the
H
XZ
extra methylene group in b in the TS may provide a clue for
this odd behavior, so we are exploring this possibility by con-
ducting ab initio geometry optimizations of the ground state
and transition state structures involved in the aminolysis of Ib
and IIb. It has been shown that a sterically hindered TS leads
116 ^ 2
113 ^ 2
p-Me
H
p-Cl
p-Br
0.800 ^ 0.007
3.46 ^ 0.06
21.5 ^ 0.2
26.7 ^ 0.3
0.442 ^ 0.003 1.81 ^ 0.02
2.25 ^ 0.05
16.5 ^ 0.2
21.7 ^ 0.2
1.54 ^ 0.04
1.30 ^ 0.02
1.23 ^ 0.02
a Standard deviations.
Table 4 Activation parametersa for the reactions of aryl prop-
anedithioates with benzylamines in acetonitrile
X
Z
*HE/kcal mol~1
[*SE/kcal mol~1 K~1
H
H
m-Cl
m-Cl
p-Me
p-Br
p-Me
p-Br
8.5
8.9
8.3
8.3
20
23
16
18
a Calculated by the Eyring equation. The maximum errors calculated
(K. B. Wiberg, Physical Organic Chemistry, Wiley, New York, 1964,
p. 378) are ]0.6 kcal mol~1 and ]3 e.u. for *HE and *SE, respec-
tively.
Fig. 1 Schematic plots of log k vs. pK for the pyridinolysis of
N
a
dinitro and trinitro thiol (DNPTC and TNPTC)10 and dithio
(DNPDTC and TNPDTC)11 carbonates in aqueous solution.
New J. Chem., 2001, 25, 313È317
315

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transfer, low energy is required to break the CÈS bond in the
rate-determining step and the strained cyclic structure leads to
large negative activation entropies.
reduced pressure. Separation by column chromatography
gave the products, for which the following analytical data
were obtained.
p-Tolyl propanedithioate. Liquid; IR (KBr): 2975 (CÈH,
CH ), 2933 (CÈH, CH ), 1499, 1487 (C2C, aromatic), 1212
Conclusion
2
3
The aminolysis of aryl propanedithioates (IIb) with benzyl-
amines proceeds by a stepwise mechanism with rate-limiting
expulsion of the leaving group, ArS~, from a tetrahedral inter-
mediate, TB. This conclusion is based on faster rates for dithio
(C2S) cm~1; 1H NMR (400 MHz, CDCl ): 1.41 (3H, t, CH ),
3
3
2.41 (3H, s, CH ), 3.08 (2H, q, CH ) 7.01È7.51 (4H, m, aro-
3
2
matic ring); 13C NMR (100.4 MHz, CDCl ): 224.5 (C2S),
134.5, 132.6, 130.3, 129.3, 56.7, 44.4, 15.4; MS: m/z 196 (M`).
3
(II) than for thio (I) esters, the large selectivity parameters, b ,
Anal. calc. for C
H, 6.22%.
H
S : C, 61.2; H, 6.21%. Found: C, 61.4;
X
10 12 2
b
and o , and adherence to the reactivity-selectivity prin-
Z
XZ
ciple (RSP). The kinetic isotope e†ects involving deuterated
benzylamine nucleophiles (XC H CH ND ), giving k /k ^
Phenyl propanedithioate. Liquid; IR (KBr): 2978 (CÈH,
CH ), 2937 (CÈH, CH ), 1501, 1488 (C2C, aromatic), 1218
6
4
2
2
H D
1.0È1.8, as well as the small *HE (D8 kcal mol~1) and large
negative *SB values ([16 to [23 e.u.) suggest that proton
transfer occurs concurrently with leaving group departure in a
four-center type transition state.
2
3
(C2S) cm~1; 1H NMR (400 MHz, CDCl ): 1.42 (3H, t, CH ),
3
3
3.10 (3H, q, CH ), 7.21È7.46 (5H, m, aromatic ring); 13C NMR
2
(100.4 MHz, CDCl ): 230.6 (C2S), 136.1, 134.9, 131.5, 129.8,
55.4, 44.5; MS: m/z 182 (M`). Anal. calc. for C H S : C,
59.3; H, 5.50%. Found: C, 59.4; H, 5.52%.
3
9
10 2
Experimental
Materials
p-Chlorophenyl propanedithioate. Liquid; IR (KBr): 2982
(CÈH, CH ), 2941 (CÈH, CH ), 1502, 1491 (C2C, aromatic),
Merck GR acetonitrile was used after three distillations.
Aldrich benzylamines and Tokyo Kasei thiophenols and
phenylacetyl chloride were used without further puriÐcation.
2
3
1211 (C2S) cm~1; 1H NMR (400 MHz, CDCl ): 1.42 (3H, t,
3
CH ), 3.10 (2H, q, CH ), 7.14È7.47 (4H, m, aromatic ring); 13C
3
2
NMR (100.4 MHz, CDCl ): 227.6 (C2S), 134.6, 133.0, 130.5,
130.3, 56.7, 44.5; MS: m/z 217 (M`). Anal. calc. for
Preparation of S-aryl propanethioates
3
Thiophenol derivatives and propionyl chloride were dissolved
in anhydrous ether and KOH added, carefully keeping the
temperature at 0È5 ¡C. Ice was then added to the reaction
mixture and the ether layer was separated, dried over MgSO
and distilled under reduced pressure to remove the solvent.
The melting point, IR (Nicolet 5BX FT-IR), 1H and 13C
NMR (JEOL 400 MHz) data are as follows: A Hewlett-
Packard quadrupole mass spectrometer with electron impact
ionization mode was used to measure MS data.
C H ClS : C, 49.9; H, 4.21%. Found: C, 49.8; H, 4.22%.
9
9
2
p-Bromophenyl propanedithioate. Liquid; IR (KBr): 2981
(CÈH, CH ), 2939 (CÈH, CH ), 1502, 1490 (C2C, aromatic),
4
2
3
1212 (C2S) cm~; 1H NMR (400 MHz, CDCl ): 1.42 (3H, t,
3
CH ), 3.09 (2H, q, CH ), 7.20È7.49 (4H, m, aromatic ring); 13C
NMR (100.4 MHz, CDCl ): 224.8 (C2S), 134.8, 132.6, 130.2,
129.3, 57.8, 44.5; MS: m/z 261 (M`). Anal. calc. for
3
2
3
C H BrS : C, 41.4; H, 3.50%. Found: C, 41.6; H, 3.49%.
9
9
2
S-p-Tolyl propanethioate. Liquid; IR (KBr): 2979 (CÈH,
CH ), 2938 (CÈH, CH ), 1494, 1459 (C2C, aromatic), 1710
Kinetic measurements
2
3
Rates were measured conductometrically in acetonitrile. The
conductivity bridge used in this work was a homemade com-
puter controlled automatic A/D converter conductivity
(C2O) cm~1; 1H NMR (400 MHz, CDCl ): 1.61 (3H, t, CH ,
3
3
J \ 6.35), 2.31 (3H, s, CH ), 2.60 (2H, q, CH , J \ 6.35 Hz),
3
2
7.16È7.26 (4H, m, aromatic ring); 13C NMR (100.4 MHz,
bridge. Pseudo-Ðrst-order rate constants, k , were deter-
CDCl ): 198.4 (C2O), 139.4, 134.4, 129.9, 124.3, 36.9, 21.2,
obs
3
mined by the Guggenheim method21 with a large excess of
9.57.
benzylamine. The k values were reproducible to within
N
^5%.
S-Phenyl propanethioate. Liquid; IR (KBr): 2993 (CÈH,
CH ), 2939 (CÈH, CH ), 1477, 1440 (C2C, aromatic), 1710
2
3
Product analysis
(C2O) cm~1; 1H NMR (400 MHz, CDCl ): 1.19 (3H, t, CH ,
3
3
J \ 6.35), 2.63 (3H, q, CH , J \ 6.44 Hz), 7.41È7.36 (5H, m,
Phenyl propanedithioate (0.05 mole) was reacted with an
excess of p-chlorobenzylamine (0.5 mole) with stirring for
more than 15 half-lives at [35.0 ¡C in acetonitrile, and the
products were isolated by evaporating the solvent under
reduced pressure. The product mixture was treated with
column chromatography (silica gel, 20% ethyl acetateÈn-
hexane). Analysis of the product gave the following results.
2
aromatic ring); 13C NMR (100.4 MHz, CDCl ): 198.4 (C2O),
3
134.5, 129.8, 129.3, 129.0, 127.4, 37.1, 9.58.
S-p-Chlorophenyl propanethioate. Liquid; IR (KBr): 2980
(CÈH, CH ), 2946 (CÈH, CH ), 1477, 1460 (C2C, aromatic),
2
3
1712 (C2O) cm~1; 1H NMR (400 MHz, CDCl ): 1.81 (3H, t,
3
CH , J \ 7.81), 2.64 (2H, q, CH , J \ 7.81 Hz), 7.28È7.35
3
2
(4H, m, aromatic ring); 13C NMR (100.4 MHz, CDCl ): 197.5
CH CH C(2S)NHCH C H Cl. Liquid; IR (KBr): 2994
3
3
2
2
2 6 4
(C2O), 135.7, 135.6, 126.3, 37.1, 9.51.
(CÈH, CH ), 2946 (CÈH, CH ), 1606 (NÈH), 1504 (CÈC,
3
aromatic), 1463 (C2C, aromatic), 1433 (CÈH, CH ), 1338
2
(CÈH, CH ), 1277 (C2S), 703 (CÈH, aromatic) cm~1; 1H NMR
3
S-p-Bromophenyl propanethioate. Liquid; IR (KBr): 2972
(CÈH, CH ), 2932 (CÈH, CH ), 1462, 1408 (C2C, aromatic),
(400 MHz, CDCl ): 2.39 (3H, s, CH ), 2.92 (1H, s, NH), 3.21
2
3
3
3
1714 (C2O) cm~1; 1H NMR (400 MHz, CDCl ): 1.20 (3H, t,
(2H, q, CH ), 7.24È7.41 (4H, m, aromatic ring); 13C NMR
(100.4 MHz, CDCl ): 225.2 (C2S), 136.6, 134.1, 133.2, 132.4,
125.6, 58.0, 47.0; MS: m/z 214 (M`). Anal. calc. for
3
2
CH , J \ 7.33), 2.67 (2H, q, CH , J \ 7.33 Hz), 7.27È7.41
3
2
3
(4H, m, aromatic ring); 13C NMR (100.4 MHz, CDCl ): 197.2
3
(C2O), 134.6, 134.2, 132.6, 130.0, 129.5, 37.2, 9.52.
C
H
ClNS: C, 56.2; H, 5.70%. Found: C, 56.4; H, 5.71%.
10 12
Preparation of aryl propanedithioates
Acknowledgement
The authors wish to acknowledge the Ðnancial support of the
Korea Research Foundation awarded in the program year of
1998.
The S-aryl propanethioates, prepared as above, were dissolved
in dry toluene and reÑuxed with LawessonÏs reagent.20 After
extraction of the reaction mixture with dichloromethane, it
was dried and the solvent removed by distillation under
316
New J. Chem., 2001, 25, 313È317

View Article Online
12 The C2S n bond is ca. 30 kcal mol~1 weaker than the C2O
bond: T. L. Cottrell, T he Strength of Chemical Bonds, Butter-
worth, London, 2nd edn., 1958, p. 275.
13 Typically, the n* levels are 0.023 and 0.079 a.u. for HC(2S)Cl and
HC(2O)Cl, respectively, calculated at the HF/6-31 ] G*//
B3LYP/6-31 ] G* level.
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New J. Chem., 2001, 25, 313È317
317