Kinetics and mechanism of the aminolysis of S-aryl O-ethyl dithiocarbonates in acetonitrile

Article abstract of DOI:10.1135/cccc20042174

The aminolysis of S-aryl O-ethyl dithiocarbonates with benzylamines are studied in acetonitrile at -25.0°C. The β_X (β_nuc) values are in the range 0.67-0.77 with a negative cross-interaction constant, ρ_XZ = -0.24, which are interpreted to indicate a concerted mechanism. The kinetic isotope effects involving deuterated benzylamine nucleophiles (XC₆H₄CH ₂ND₂) are large, k_H/k_D = 1.41-1.97, suggesting that the N-H(D) bond is partially broken in the transition state by forming a hydrogen-bonded four-center cyclic structure. The concerted mechanism is enforced by the strong push provided by the EtO group which enhances the nucleofugalities of both benzylamine and arenethiolate from the putative zwitterionic tetrahedral intermediate.

Full text of DOI:10.1135/cccc20042174

2174
Oh H. K., Oh J. Y., Sung, Lee:
KINETICS AND MECHANISM OF THE AMINOLYSIS OF
S-ARYL O-ETHYL DITHIOCARBONATES IN ACETONITRILE
b1
b,c,
Hyuck Keun OHa1, Ji Young OHa2, Dae Dong SUNG and Ikchoon LEE
*
a
Department of Chemistry, Chonbuk National University, Chonju 560-756, Korea;
1
2
e-mail: ohkeun@chonbuk.ac.kr, jyoh@chonbuk.ac.kr
b
c
Department of Chemistry, Dong-A University, Busan 604-714, Korea;
1
e-mail: ddsung@mail.donga.ac.kr
Department of Chemistry, Inha University, Inchon 402-751, Korea; e-mail: ilee@inha.ac.kr
Received Jun e 11, 2004
Accepted July 29, 2004
Dedicated to Professor Otto Exner on the occasion of his 80th birthday in recognition of his
outstanding contributions to physical organic chemistry.
Th e am in olysis of S-aryl O-eth yl dith iocarbon ates with ben zylam in es are studied in aceton i-
trile at –25.0 °C. Th e βX (βn uc) values are in th e ran ge 0.67–0.77 with a n egative cross-
in teraction con stan t, ρXZ = –0.24, wh ich are in terpreted to in dicate a con certed m ech an ism .
Th e kin etic isotope effects in volvin g deuterated ben zylam in e n ucleoph iles (XC6H4CH2ND2)
are large, kH/kD = 1.41–1.97, suggestin g th at th e N–H(D) bon d is partially broken in th e tran -
sition state by form in g a h ydrogen -bon ded four-cen ter cyclic structure. Th e con certed m ech -
an ism is en forced by th e stron g push provided by th e EtO group wh ich en h an ces th e
n ucleofugalities of both ben zylam in e an d aren eth iolate from th e putative zwitterion ic tetra-
h edral in term ediate.
Keyw ord s: S-Aryl O-eth yl dith iocarbon ates; Am in olysis; Con certed m ech an ism ; Cross-
in teraction con stan ts; Kin etic isotope effects; Reaction m ech an ism s.
Th e m ech an ism of am in olysis of esters an d carbon ates h as been exten sively
in vestigated because of its relevan ce to th e en zym atic catalysis of carbon yl
group tran sfer reaction s. In m an y of th ese n ucleoph ilic substitution reac-
tion s curved Brøn sted-type plots h ave been foun d, wh ich h ave been attrib-
uted to a ch an ge in th e rate-lim itin g step from breakdown (βn uc ≈ 0.8–1.0)
to form ation (βn uc ≈ 0.1–0.3) of a tetrah edral zwitterion ic in term ediate, T±,
in th e reaction path as th e basicity of th e am in e n ucleoph ile in creases1.
Quite in terestin gly, h owever, con certed n ucleoph ilic displacem en ts are
foun d on ly in th e reaction s S-aryl O-eth yl th iocarbon ates (structure 2 with
R = EtO) with good leavin g groups2 (a Ar = 2,4,6-(NO2)3C6H2 or b Ar =
2,4-(NO2)2C6H3) an d alicyclic secon dary am in es (βn uc ≈ 0.4–0.6)3.
Collect. Czech. Chem. Commun. (Vol. 69) (2004)
doi:10.1135/cccc20042174

Aminolysis of S-Aryl O-Ethyl Dithiocarbonates
2175
O
O
C
S
C
R
C
OAr
R
SAr
R
SAr
1
2
3
Ar
1–3
C6H2-2,4,6-(NO2)3
C6H3-2,4-(NO2)2
C6H4-4-NO2
a
b
c
Th e con certed m ech an ism is en forced by (i) stron ger push provided by
R = EtO, wh ich en h an ces th e n ucleofugality of both th e am in e an d
arylth iolate ion from T± relative to oth er acyl groups (R = alkyl or ph en yl)
an d (ii) m uch faster expulsion of a given am in e an d ArS– from T± form ed
with 2 th an th ose from T± form ed with 1 an d 3 due to a stron ger π-bon din g
en ergy of th e carbon yl group (C=O) com pared with th iocarbon yl (C=S) cou-
pled with a greater n ucleofugality of ArS– in curred by m uch less basicity of
ArS– th an ArO–.
In th is work, we carried out kin etic studies of am in olysis of S-aryl O-eth yl
dith iocarbon ates, 3 with R = EtO an d Ar = C6H4Z (Z = 4-Me, H, 4-Cl an d
4-Br), usin g ben zylam in es (XC6H4CH2NH2; X = 4-OMe, 4-Me, H, 4-Cl an d
3-Cl) in aceton itrile at –25.0 °C. Th e prim ary purpose of th is work is to ex-
am in e th e effects of (i) am in e n ature (secon dary alicyclic am in es versus
ben zylam in es) an d (ii) substitution of C=S for C=O on th e am in olysis
m ech an ism of S-aryl O-eth yl carbon ates. In addition we are m uch in terested
4
in the m echanistic criteria based on the sign of ρXZ in Eqs (1), where X and Z
are substituents in the nucleophile and leaving group, respectively. It has been
postulated and experim entally confirm ed that in a stepwise acyl transfer
log (kXZ/kHH) = ρXσX + ρZσZ + ρXZσXσZ
ρXZ = ∂ρZ/∂σX = ∂ρX/∂σZ
(1a)
(1b)
th rough a tetrah edral in term ediate, th e sign of ρXZ is in variably positive
an d reactivity–selectivity prin ciple (RSP) h olds5. In con trast, th e sign of ρXZ
is normally negative and the RSP is violated6 in concerted acyl transfer reactions.
Collect. Czech. Chem. Commun. (Vol. 69) (2004)

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Oh H. K., Oh J. Y., Sung, Lee:
RESULTS AND DISCUSSION
Th e reaction s studied in th e presen t work followed th e rate law given by
Eqs (2) an d (3), wh ere [BA] is th e con cen tration of th e ben zylam in e n ucleo-
ph ile.
rate = kobs [substrate]
kobs = kN [BA]
(2)
(3)
Th e values of kN were determ in ed by th e slopes of plots of kobs again st
[BA], an d th e results are sum m arized in Table I. In th is work, th e selectivity
param eters obtain ed as σ an d pKa depen den ce of th e rate con stan t, kN (ρX,
βX, ρZ, βZ an d ρXZ) are also sh own . In th e determ in ation of Brøn sted coeffi-
cien ts, βX an d βZ, th e pKa values in water are used: th e pKa (CH3CN) values
for structurally sim ilar am in es are kn own to ch an ge in parallel with th e pKa
(H2O) values so th at th e βX (= βn uc) values given are reliable7. For th e βZ
(= βlg) values, a factor of 0.62 was used to all th e βZ values determ in ed usin g
th e pKa (H2O) values8.
Th e rates are extrem ely faster th an th ose of correspon din g reaction s of
aryl dith ioates in aceton itrile. For exam ple, th e kN value for th e am in olysis
of th e S-aryl O-eth yl dith iocarbon ate is 8.62 × 10–1 l m ol–1 s–1 at –25.0 °C
wh ereas th ose of aryl dith ioacetate (R = CH3 in 3) an d ben zoate (R = C6H5
in 3) are 6.99 × 10–1 l m ol–1 s–1 (at 20.0 °C)9 an d 3.82 × 10–1 l m ol–1 s–1
(at 30.0 °C)10, respectively (for X = Z = H), with ben zylam in es in aceto-
n itrile. A stron g push provided by th e EtO group is due to an efficien t
ch arge tran sfer of th e lon e pair electron s on th e oxygen atom toward th e
σ*C–S orbital, nO → σ*C–S in teraction , wh ich leads to a facile bon d cleavage
of th e C–S bon d in th e tran sition state (TS). Th is is especially stron g sin ce
(i) th e nO level is h igh due to th e electron don atin g effect of Et group an d
(ii) th e σ*C–S level is low11 relative to σ*C–O. Th e secon d-order ch arge tran s-
fer en ergy12 is given by ∆E(2)
= –2F2nσ*/∆E, wh ere ∆E = Eσ* – En an d Fnσ*
n→σ*
is th e m atrix elem en t wh ich is proportion al to th e overlap, Snσ*, between
th e two in teractin g orbitals.
An oth er factor th at is en h an cin g th e rate is th e low π*C=S level (relative to
π*C=O level)11. Sin ce th e in itial attack by th e lon e pair electron s of th e n itro-
gen atom in th e ben zylam in e n ucleoph ile occurs at th e π*C=S orbital, th e
breakin g of th e π*C=S orbital to form a tetrah edral TS or in term ediate is fa-
cilitated by a stron g nN → π*C=S in teraction wh ere nN is th e lon e pair orbital
of th e n itrogen atom in ben zylam in e. Th us th e stron g rate en h an cem en t is
Collect. Czech. Chem. Commun. (Vol. 69) (2004)

Aminolysis of S-Aryl O-Ethyl Dithiocarbonates
2177
th e com bin ed results of th e low σ*C–S an d π*C=S levels an d th e h igh nO an d
nN levels of th e EtO group an d am in o n itrogen in BA.
Referen ce to Table I reveals th at th e βX values are 0.67–0.77, wh ich are in
th e ran ge of values th at h ave been foun d for th e con certed m ech an ism 2a
Th e con certed am in olysis reaction s n orm ally exh ibit th e Brøn sted slopes in
th e ran ge 0.4–0.7 2. However, β values sm aller th an 0.4 13 an d larger th an
.
TABLE I
Th e secon d-order rate con stan ts, kN (in l m ol–1 s–1) for th e reaction s of S-aryl O-eth yl
dith iocarbon ates (EtO–C(=S)SC6H4Z) with X-ben zylam in es (XC6H4CH2NH2) in aceton itrile
at –25.0 °C
Z
a
b
ρZ
βZ
X
4-Me
H
4-Cl
2.52
4-Br
2.78
4-OMe 0.915
1.39
1.17 ± 0.06 –0.40
4-Me
1.17c
3.57
2.23
1.37
0.767
0.507d
1.15
2.09
1.14 ± 0.04
–0.29
H
0.581
0.416
0.862
0.602
1.52
1.05
1.64
1.11
1.10 ± 0.05
1.05 ± 0.04
–0.29
–0.27
4-Cl
3-Cl
0.497
0.331
0.217
1.43
0.483
0.813
0.852
0.512
1.01 ± 0.03
–0.26
e
f
ρX
–0.68 ± 0.01 –0.71 ± 0.02 –0.76 ± 0.02 –0.79 ± 0.02
0.67 ± 0.02 0.70 ± 0.01 0.74 ± 0.02 0.77 ± 0.01
ρXZ = –0.24 ± 0.07
g
βX
a
Th e σ values were taken from Dean J. A.: Handbook of Organic Chemistry, Table 7-1,
McGraw–Hill, New York 1987. Correlation coefficien ts were better th an 0.997 in all cases.
b
Th e pKa values were taken from Bukin gh am J. (Ed.): Dictionary of Organic Chemistry,
5th ed., Ch apm an an d Hall, New York 1982. Z = 4-Br was excluded from th e Brøn sted plot
for βZ due to an un reliable pKae values. Correlation coefficien ts were better th an 0.997 in all
c
d
cases. At –15 °C. At –35 °C. Th e σ values were taken from McDan iel D. H., Brown H. C.:
J. Org. Chem. 1958, 23, 420. Correlation coefficien ts were better th an 0.999 in all cases.
f
g
Correlation coefficien t was 0.997. Th e pKa values were taken from Fisch er A., Galloway
W. J., Vaugh an J.: J. Chem. Soc. 1964, 3588. Correlation coefficien ts were better th an 0.999
in all cases. X = 4-CH3O were excluded from th e Brøn sted plot for βX (ben zylam in e) due to
an un reliable pKa value listed.
Collect. Czech. Chem. Commun. (Vol. 69) (2004)

2178
Oh H. K., Oh J. Y., Sung, Lee:
0.7 14 h ave also been foun d for con certed processes. Th e ran ge of βZ values
(–0.26–0.40) is also con sisten t with th e con certed am in olysis reaction 1b
.
A stron g support for th e proposed m ech an ism com es from th e n egative
ρXZ value (–0.24) obtain ed in th is work. Th e sign of ρXZ h as been foun d to
be in variably positive (ρXZ > 0) for th e stepwise am in olysis reaction s5. Th e
results of am in olysis of various S-aryl O-eth yl carbon ates in H2O or in
MeCN are collected in Table II. In th is table, th e reaction s in th e upper/
righ t region proceed by th e con certed m ech an ism , wh ile th ose in th e
lower/left zon e proceed by th e stepwise m ech an ism . Th is is because th e
am in es are arran ged in th e down ward order of in creasin g kin etic stability
ren dered to th e tetrah edral in term ediate2,3b, an d th e carbon ates are ar-
TABLE II
Results of am in olysis reaction s of various S-aryl O-eth yl carbon ates in H2O or in MeCN
O
C
O
C
S
C
Am in es
EtO
OAr
EtO
SAr
EtO
SAr
Ben zylam in es
MeCN: Stepwise5
βX = 1.66–2.38
ρXZ = +1.38
MeCN: Con certed6
βX = 0.6–0.8
ρXZ = –0.47
H2O: con certed2
βX: a 0.47
MeCN: Con certed15
βX = 0.67–0.77
ρXZ = –0.24
Quin uclidin es
b 0.54
c 0.56
Secon dary alicyclic
am in es
H2O: con certed3
H2O: stepwise16
βX: a 0.48
b 0.56
βX: a 0.8
b 0.8
An ilin es
H2O:19
MeCN: Con certed6
βX = 0.50–0.70
ρXZ = –0.56
H2O: stepwise18
βX: a 0.9
βX: a 0.9
b 0.54
H2O: stepwise17
βX: a 0.80
b 0.90
Pyridin es
b 1.0
c 0.80
a Ar = 2,4,6-(NO2)3C6H2; b Ar = 2,4-(NO2)2C6H3; c Ar = 4-NO2C6H4
Collect. Czech. Chem. Commun. (Vol. 69) (2004)

Aminolysis of S-Aryl O-Ethyl Dithiocarbonates
2179
ran ged in th e order of in creasin g facility (to th e righ t) of th e n ucleoph ilic
attack of th e substrate11. We n ote th at th e sign of ρXZ is positive for th e
stepwise am in olysis but is n egative for th e con certed am in olysis reaction s.
Th e m ech an ism is stron gly in fluen ced by (i) facility of th e in itial attack,
(ii) leavin g ability of th e am in e from T±, (iii) th e n ucleofugality of th e ArS–
(an d ArO–) group an d (iv) effect of solven t. On e exam ple is am in olysis
of S-aryl O-eth yl carbon ates (2 with R = EtO) with quin uclidin es2. Th is
reaction proceeds con certedly for th e substrate with better leavin g groups
(a 2,4,6-(NO2)3C6H2 an d b 2,4-(NO2)2C6H3) but th e m ech an ism ch an ges to
a stepwise reaction with rate-lim itin g expulsion of th e ArS– group from T±
for th e substrate with low n ucleofugality (c Z = 4-NO2).
Th e secon dary alicyclic am in es an d an ilin es are in th e in term ediate ran ge
as to th e leavin g ability from T± so th at th e m ech an ism does n ot follow a
sim ple order, e.g., th e secon dary alicyclic am in es react with 3 (R = EtO) in
water by th e stepwise m ech an ism 16 but con certedly with 2 (R = EtO)3,
wh ereas th e reaction s of an ilin e with 3 (R = EtO) proceed con certedly6.
It is to be n oted th at th e faster rates are accom pan ied by larger selectivity
param eters (Table I) so th at th e RSP is violated. Th is also provides support
for th e con certed m ech an ism 20. We th erefore con clude th at th e reaction s
studied in th e presen t work proceed via a con certed m ech an ism , Eq. (4).
EtO–C(=S)–SAr + XC6H4CH2NH2 → 4 →
–
→ EtO–C(=S)·NHCH2C6H4X + H+ + SAr
(4)
Th e kin etic isotope effects, kH/kD, in volvin g deuterated ben zylam in e
n ucleoph iles21 (XC6H4CH2ND2) are sh own in Table III. Th e kH/kD values are
relatively large, 1.41–1.97, wh ich is an in dication th at partial N–H(D) bon d
cleavage is in volved in th e TS 21. Th us we propose th e TS, 4, in wh ich th e
N–H(D) h ydrogen is h ydrogen bon ded to th e leavin g group with four
m em bered cyclic structure.
S
EtO
C
SC6H4Z
H
N
CH2
C6H4X
4
Collect. Czech. Chem. Commun. (Vol. 69) (2004)

2180
Oh H. K., Oh J. Y., Sung, Lee:
Finally the activation param eters, ∆H≠ and ∆S≠, are sum m arized in Table IV.
Th e ∆H≠ values are relatively low an d ∆S≠ values are large n egative, wh ich
are con sisten t with th e con certed m ech an ism 22
.
TABLE III
Th e secon dary kin etic isotope effects for th e reaction s of S-aryl O-eth yl dith iocarbon ates
with deuterated X-ben zylam in es (XC6H4CH2ND2) in aceton itrile at –25.0 °C
X
Z
kH, m ol l–1 s–1
kD, m ol l–1 s–1
kH/kD
4-OMe
4-OMe
4-OMe
4-OMe
4-Cl
4-Me
H
0.915 (±0.008)
1.39 (±0.01)
2.52 (±0.04)
2.78 (±0.05)
0.416 (±0.004)
0.602 (±0.007)
1.05 (±0.01)
1.11 (±0.02)
0.464 (±0.005)
0.808 (±0.006)
1.52 (±0.02)
1.97 ± 0.03a
1.72 ± 0.02
1.65 ± 0.03
1.49 ± 0.04
1.83 ± 0.02
1.68 ± 0.02
1.57 ± 0.02
1.41 ± 0.03
4-Cl
4-Br
4-Me
H
1.87 (±0.03)
0.227 (±0.002)
0.358 (±0.003)
0.669 (±0.006)
0.787 (±0.007)
4-Cl
4-Cl
4-Cl
4-Br
4-Cl
a
Stan dard deviation s.
TABLE IV
Activation param etersa for th e reaction s of S-aryl O-eth yl dith iocarbon ates with X-ben zyl-
am in es in aceton itrile
X
Z
∆H≠, kcal m ol–1
–∆S≠, cal m ol–1 K–1
4-Me
4-Me
3-Cl
3-Cl
4-Me
4-Br
4.6
5.2
4.5
5.6
40
35
41
36
4-Me
4-Br
a
Calculated usin g th e Eyrin g equation . Th e m axim um errors calculated (Wiberg K. B.: Physi-
cal Organic Chemistry, p. 378, Wiley, New York 1964.) are ±0.4 kcal m ol–1 an d ±2 e.u. for ∆H≠
an d ∆S≠, respectively.
Collect. Czech. Chem. Commun. (Vol. 69) (2004)

Aminolysis of S-Aryl O-Ethyl Dithiocarbonates
2181
EXPERIMENTAL
Materials
GR grade aceton itrile was used after th ree distillation s. Th e ben zylam in e n ucleoph iles, GR
grade, were used with out furth er purification .
Substrates
Th e dith iocarbon ates were prepared by th e kn own m eth od in th e literature23
.
Kin etic Measurem en t
Rates were m easured con ductom etrically at –25.0 ± 0.05 °C. Th e con ductivity bridge used in
th is work was a self-m ade com puter autom atic A/D con verter con ductivity bridge. Pseudo-
first-order rate con stan ts, kobs, were determ in ed by th e Guggen h eim m eth od with large ex-
cess of ben zylam in e. Secon d-order rate con stan ts, kN, were obtain ed from th e slope of a plot
of kobs vs ben zylam in e with m ore th an five con cen tration s of m ore th an th ree run s an d
were reproducible with in ±3%.
Product An alysis
Substrate (0.05 m ol) an d 4-m eth oxyben zylam in e (0.5 m ol) were added to aceton itrile an d
reacted at –25.0 °C un der th e sam e con dition s as in kin etic m easurem en ts. After m ore th an
15 h alf-lives, solven t was rem oved un der reduced pressure an d product was separated by
colum n ch rom atograph y (silica gel, 10% eth yl acetate–h exan e). An alysis of th e product gave
th e followin g results.
CH3CH2OC(=S)NHCH2C6H4-4-OCH3: liquid. IR (KBr): 3315 (N–H); 2972 (C–H, ben zyl);
2963 (C–H, CH2); 2946 (C–H, CH3); 1683 (C=O); 1549 (C=C, arom atic rin g); 1522 (N–H);
1260, 1038 (C–O). 1H NMR (400 MHz, CDCl3): 1.26 (3 H, t, CH3); 2.04 (2 H, d, J = 5.86,
CH2-N); 3.90 (3 H, s, OCH3); 4.19 (2 H, q, J = 7.08, OCH2); 5.55 (1 H, s, NH); 6.97–7.85
(4 H, m , J = 8.30, arom atic rin g). 13C NMR (100.4 MHz, CDCl3): 198.1 (C=S); 162.4, 134.2,
128.2, 113.9 (arom atic rin g); 60.7, 55.9 (OCH3); 36.3 (CH2); 14.5 (CH3). MS, m/z: 225 (M+).
For C11H15NO2S (225.3) calculated: 58.6% C, 6.71% H; foun d: 58.4% C, 6.73% H.
This work was supported by Korea Research Foundation Grant (KRF-2003-002-C00160).
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