Kinetics and mechanism of desulfurization reaction of 1-methyl-2- phenylquinazoline-4(1H)-thiones

Article abstract of DOI:10.1135/cccc20042212

Kinetics and mechanism of desulfurization reaction of 1-methyl-2- (substituted phenyl)-quinazoline-4(1H)-thiones in sodium methoxide solutions have been studied, giving the corresponding 1-methyl-2-(substituted phenyl)quinazolin-4(1H)-ones. The reaction proceeds in two steps. The first step involves splitting off of sulfur in the form of SH^- and is much faster than the second step, whose rate is almost independent of the concentration of water in methanol. At lowest concentrations of methoxide, the rate of the first step increases linearly, but at higher concentrations a gradual decrease in the rate takes place. The rate of the second step, i.e. the transformation of the intermediate formed In into 1-methyl-2-(substituted phenyl)quinazolin-4(1H)-one (2a-2e), is independent of the methoxide concentration but increases with increasing concentration of water in methanol. On the basis of the kinetic dependences, the mechanism for both steps of desulfurization and the structure of intermediate In were proposed.

Full text of DOI:10.1135/cccc20042212

2212
Hanusek, Sedlák, Keder, Štěrba:
KINETICS AND MECHANISM OF DESULFURIZATION REACTION OF
1-METHYL-2-PHENYLQUINAZOLINE-4(1H)-THIONES
1,
2
3
4
Jiří HANUSEK *, Miloš SEDLÁK , Roman KEDER and Vojeslav ŠTĚRBA
Department of Organic Chemistry, Faculty of Chemical Technology, University of Pardubice,
1
Nám. Čs. legií 565, 532 10 Pardubice, Czech Republic; e-mail: jiri.hanusek@upce.cz,
2
3
4
milos.sedlak@upce.cz, roman.keder@upce.cz, vojeslav.sterba@upce.cz
Received Jun e 29, 2004
Accepted Septem ber 9, 2004
Dedicated to Professor Otto Exner on the occasion of his 80th birthday.
Kin etics an d m ech an ism of desulfurization reaction of 1-m eth yl-2-(substituted ph en yl)-
quin azolin e-4(1H)-th ion es in sodium m eth oxide solution s h ave been studied, givin g th e cor-
respon din g 1-m eth yl-2-(substituted ph en yl)quin azolin -4(1H)-on es. Th e reaction proceeds in
two steps. Th e first step in volves splittin g off of sulfur in th e form of SH– an d is m uch faster
th an th e secon d step, wh ose rate is alm ost in depen den t of th e con cen tration of water in
m eth an ol. At lowest con cen tration s of m eth oxide, th e rate of th e first step in creases lin -
early, but at h igh er con cen tration s a gradual decrease in th e rate takes place. Th e rate of th e
secon d step, i.e. th e tran sform ation of th e in term ediate form ed In in to 1-m eth yl-2-(substi-
tuted ph en yl)quin azolin -4(1H)-on e (2a–2e), is in depen den t of th e m eth oxide con cen tration
but in creases with in creasin g con cen tration of water in m eth an ol. On th e basis of th e ki-
n etic depen den ces, th e m ech an ism for both steps of desulfurization an d th e structure of in -
term ediate In were proposed.
Keyw ord s: Kin etics; Mech an ism ; Desulfurization ; Quin azolin es; Quin azolin e-4(1H)-th ion es;
Meth an olysis; Hydrolysis.
In an alogy to th e tran sform ation of carbon yl group in to th iocarbon yl
group, th ere exist a large n um ber of reaction s in volvin g th e opposite pro-
cess, i.e. replacem en t of sulfur by oxygen . Th is problem was dealt with in
an exten sive review article1 in 1998, wh ich also m en tion s desulfurization s
of h eterocyclic com poun ds. Most often , desulfurization is ach ieved by th e
action of oxidizin g agen ts an d by h ydrolytic processes com bin ed with treat-
m en t with tran sition m etal salts. Besides th e review article m en tion ed,
th ere exist a couple of papers particularly dealin g with th e desulfurization
of quin azolin e-4-th ion e derivatives.
Collect. Czech. Chem. Commun. (Vol. 69) (2004)
doi:10.1135/cccc20042212

Kinetics and Mechanism of Desulfurization
2213
Most preparative desulfurization s in volve S-alkylation of quin azolin e-
4-th ion e with m eth yl iodide2a,2b, diazom eth an e2c, 1-brom o-(2-dieth yl-
am in o)eth an e2d, or 1-(ch loroacetyl)piperidin e2d, an d subsequen t h ydrolysis
of th e 4-(alkylsulfan yl)quin azolin e or 4-(alkylsulfan yl)quin azolin ium iodide
form ed, th e h ydrolysis bein g realized in acid or base m edia, possibly in th e
presen ce of h ydrogen peroxide. Oth er desulfurization m eth ods are based on
th e application of ben zon itrile oxide an d trieth ylam in e in ben zen e2b or
m ercury(II) acetate in acetic acid2e. However, we are n ot aware of an y refer-
en ce of desulfurization m ediated by alkoxides.
EXPERIMENTAL
Measurem en ts
Kin etic m easurem en ts were carried out on an HP UV/VIS 8453 diode array apparatus. A
1-cm quartz cell was ch arged with 2 m l of dried m eth an olic sodium m eth oxide (water con -
ten t 200 ppm ) at 25 °C, th en 50 µl of a m eth an olic solution of substrate 1a–1e was in jected
an d th e absorban ce at 388 n m was m easured. Th e resultin g con cen tration of substrate c ≈
1 × 10–4 m ol l–1. After com pletin g th e first step, th e reaction m ixture was treated with aque-
ous sodium h ydroxide of th e sam e con cen tration , an d th e spectra correspon din g to th e sec-
on d step were recorded (see Table III). Th e values of both observed rate con stan t (kobs) were
calculated from th e spectral records. Th e m easurem en ts of th e effects of water on th e first
step were carried out in m ixtures of sodium m eth oxide an d sodium h ydroxide in m eth an ol–
water (see Table II).
Th e m ass spectra were m easured with a m ass spectrom eter VG Platform II with ch em ical
ion ization at atm osph eric pressure (APCI) an d quadrupole an alyzer (0–3000 Da). Before th e
m ass spectrom eter, th ere was a separation un it in volvin g a h igh -pressure pum p Waters 616,
an autosam pler Waters 717, UV detector Waters 996, an d HPLC colum n filled with Separon
SGX C18 (150 × 3 m m , particle size 7 µm ). Th e m obile ph ase con sisted of a m ixture of 70%
aceton itrile an d 30% redistilled water. Th e outlet of th e liquid ch rom atograph was con -
n ected directly to th e quadrupole m ass spectrom eter with APCI for recordin g positive ion s.
Th e data were obtain ed in th e ran ge of m/z 15–600. Th e ion source tem perature was 100 °C,
an d th at of th e APCI probe was 500 °C. Th e voltage im posed on th e con ical in put electrode
was 10 V.
Procedure
Th e startin g 1-m eth yl-2-(substituted ph en yl)quin azolin e-4(1H)-th ion es (1a–1e) an d th eir
desulfurization products 1-m eth yl-2-(substituted ph en yl)quin azolin -4(1H)-on es were pre-
pared by a procedure described in our previous paper3,4 th at also in cluded th eir ch aracteriza-
tion . Th e desulfurization reaction was carried out for th e 4-ch loro derivative on preparative
scale as follows:
A 100-m l flask was ch arged with 2-(4-ch loroph en yl)-1-m eth ylquin azolin e-4(1H)-th ion e
(1d ; 0.32 g, 1.12 m m ol) an d 0.35 M sodium m eth oxide (50 m l). Th e reaction m ixture was
refluxed un til decolorization (ca. 30 m in ) an d th en cooled an d n eutralized with acetic acid
to pH ≈ 7–8. Meth an ol was distilled off an d collected in a flask cooled with a dry ice–
Collect. Czech. Chem. Commun. (Vol. 69) (2004)

2214
Hanusek, Sedlák, Keder, Štěrba:
aceton e m ixture. Th e residue after evaporation was triturated with water an d th e un dis-
solved portion (0.2 g, 67%) m eltin g at 216–219 °C was recrystallized from a water–eth an ol
(2:1) m ixture to give wh ite crystals of 2-(4-ch loroph en yl)-1-m eth ylquin azolin -4(1H)-on e
(2d ), m .p. 218–220 °C. Th e iden tity of product was verified by 1H an d 13C NMR spectra. Th e
distillate was treated with 1-fluoro-2,4-din itroben zen e (0.42 g, 2.25 m m ol) an d sodium h y-
droxide (0.1 g, 2.5 m m ol). Th e m ixture was left to stan d at room tem perature for 12 h an d
th en an alyzed by HPLC/MS, wh ich revealed 2,4-din itroben zen eth iol MS (m/z): [M + H]+
–
200.7.
RESULTS AND DISCUSSION
In our previous paper3 we studied th e kin etics an d m ech an ism of base-
catalyzed cyclization givin g derivatives of quin azolin -4(1H)-on e an d
quin azolin e-4(1H)-th ion e. Studyin g th is reaction , we foun d out th at in th e
case of 1-m eth yl-2-(substituted ph en yl)quin azolin e-4(1H)-th ion es (1a–1e),
th e desulfurization reaction s in m eth an olic sodium m eth oxide at room
tem perature took place leadin g to th e correspon din g quin azolin -4-on e. Th e
aim of our work was a detailed in vestigation of kin etics an d m ech an ism of
th is desulfurization reaction .
Th e reaction was studied by UV/VIS spectroph otom etry in 0.1–1.6 M solu-
tion s of sodium m eth oxide. Th e desulfurization of com poun d 1d was also
carried out on preparative scale in order to verify th e reaction course. We
foun d out th at th e desulfurization proceeds kin etically as a system of two
con secutive reaction s. Th e first, m uch m ore faster step produces a de-
sulfurized in term ediate, wh ich subsequen tly decom poses to th e fin al prod-
uct (oxygen -con tain in g derivative) (Sch em e 1).
S
O
N
N
In
N
N
CH3
CH3
R1
R1
1a-1e
2a-2e
a
b
c
d
e
1, 2
R1
H
OCH3
CH3
Cl
NO2
SCHEME 1
At th e lowest con cen tration s of m eth oxide, th e rate of form ation of In
depen ds lin early upon [CH3O–] (Fig. 1). Th e secon d-order rate con stan ts k
correspon din g to th is reaction step are given by th e slopes of th e straigh t
lin es (Table I). Th e depen den ce of log k on th e σ con stan ts of substituen ts is
lin ear (Fig. 2), th e value of reaction con stan t ρ = 0.40 ± 0.01 bein g in accor-
Collect. Czech. Chem. Commun. (Vol. 69) (2004)

Kinetics and Mechanism of Desulfurization
2215
dan ce with th e fact th at th e attack of th e reaction cen ter by th e n ucleoph ile
occurs in a large distan ce from th e substituen t.
At h igh er m eth oxide con cen tration s, th e observed rate con stan ts kobs
becom e in depen den t of th e m eth oxide con cen tration , an d at [CH3O–] >
0.8 m ol l–1 th ey decrease (Fig. 1). Th e presen ce of water h as on ly a sligh t
effect on th e value of kobs; th e rate of form ation of in term ediate m ildly de-
creases with in creasin g water con cen tration (Table II). In aqueous solution s
of sodium h ydroxide, th e rate of desulfurization is very low, about 10 tim es
lower th an th at in m eth an ol. Th e rate of tran sform ation of In in to th e
product is alm ost in depen den t of th e m eth oxide con cen tration , but it
rapidly in creases in a lin ear way with in creasin g am oun t of added water
(Table III). If th e MeOH used h ad been dried with Mg (water con ten t
200 ppm ), th en th e decom position of In was very slow, an d th e form ation
TABLE I
Secon d-order rate con stan ts for 1a–1e → In
Com poun d
1a
1b
1c
1d
1e
k, l m ol s–1
2.15 ± 0.05
2.40 ± 0.05
2.89 ± 0.07
3.43 ± 0.11
5.82 ± 0.33
1.5
1.0
0.5
0.0
0.0
0.5
1.0
c (CH ONa), mol l
1.5
–1
3
FIG. 1
Depen den ce of th e observed rate con stan t (kobs
) for desulfurization of 1-m eth yl-2-
(4-substituted ph en yl)quin azolin e-4(1H)-th ion es 1a (᭿), 1b (᭹), 1c (᭺), 1d (ٗ) an d 1e (ꢀ) on
sodium m eth oxide con cen tration c at 25 °C. Th e curves depict th e ch aracter of th e
depen den ces an d are given for better guidan ce
Collect. Czech. Chem. Commun. (Vol. 69) (2004)

2216
Hanusek, Sedlák, Keder, Štěrba:
of com poun ds 2a–2e practically ceased: th e solution con tain ed on ly th e
un reacted in term ediate. Several attem pts to isolate th e in term ediate In
failed. After separation from th e reaction m ixture, we always obtain ed on ly
th e startin g substan ce or th e desulfurization product.
Th e low effect of water con cen tration on th e rate of form ation of In
(Table II) an d, in particular, its slow decom position in to products, in dicate
th at th e desulfurization itself takes place durin g th e reaction of th e sub-
TABLE II
Observed rate con stan ts for reaction 1d → In in 0.5 M sodium m eth oxide with addition of
water
Vol.% H2O
0
5
10
20
kobs × 103, s–1
1.40
1.27
1.13
0.92
TABLE III
Depen den ce of th e observed rate con stan ts for reaction In → 2d in 0.5 M sodium m eth oxide
at various con cen tration s of water. (Note: th e m easurem en ts can n ot be carried out at water
con ten ts above 5%, because th e product crystallizes from th e reaction m ixture.)
Vol.% H2O
1
2
3
4
5
kobs × 104, s–1
1.68
3.42
5.45
8.31
10.2
0.8
0.7
0.6
0.5
0.4
0.3
–0.0
0.0
0.2
0.4
0.6
0.8
σ
p
FIG. 2
Ham m ett correlation of secon d-order rate con stan ts
Collect. Czech. Chem. Commun. (Vol. 69) (2004)

Kinetics and Mechanism of Desulfurization
2217
strate with m eth oxide in th e first reaction step. Th is is also con firm ed by a
low value of λ (235 n m ), wh ich is typical (Fig. 3) of form ation of th e tetra-
h edral in term ediate In . For th e reaction of th e substrate with OH– in water,
it is possible to presum e th e form ation of th e followin g in term ediates lead-
in g to th e products (Sch em e 2).
O
S
S
OH
N
k1
N
N
+ OH
R1
k–1
N
N
N
CH3
CH3
CH3
R1
R1
–SH
–SH
HS
O
HS OH
N
N
N
N
CH3
CH3
R1
R1
SCHEME 2
Th e lifetim e of in term ediates given in Sch em e 2 is substan tially lower
th an 1 s in th e given m edium , wh ich m ean s th at th e tran sform ation of sub-
strate in to product by reaction with OH– does n ot reveal an y spectral m an i-
festation of in term ediate, as th is is th e case in th e reaction of carbon yl
com poun ds5. Sin ce from th e above-m en tion ed fin din gs follows th at th e
2
1
1
3
2
0
250
350
450
λ, nm
FIG. 3
Spectra of startin g substan ce (1), in term ediate (2) an d product (3). Th e spectrum after reaction
is iden tical with th at of th e product prepared by in depen den t syn th esis in th e sam e m edium
Collect. Czech. Chem. Commun. (Vol. 69) (2004)

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Hanusek, Sedlák, Keder, Štěrba:
desulfurization of substrate is exclusively caused by th e action of m eth -
oxide, In is very stable at th e given con dition s (zero rate of tran sform ation
in to product in th e absen ce of water), it can justifiably be presum ed th at
In h as th e structure of a dim eth oxy adduct. Its form ation in cludin g split-
tin g off of sulfur in th e form of SH– are presen ted in Sch em e 3.
S
S
OCH3
N
HS OCH3
N
k1
CH3OH
Kad
N
+ CH3O
R1
CH3O
k–1
N
N
N
CH3
CH3
CH3
R1
R1
R1
R1
(K1)
Ad
Ad
k2
–SH
OCH3
OCH3
N
OCH3
N
N
N
N
N
CH3
CH3
CH3
R1
R1
O CH3
N
CH3O OCH3
N
+ CH3O
R1
N
N
CH3
CH3
In
SCHEME 3
Th e relation sh ip (1) for kobs can be defin ed on th e basis of Sch em e 3 on
th e assum ption th at th e reactin g species is th e adduct Ad , wh ich irrevers-
ibly splits off th e SH group. As th e adduct Ad is h igh ly un stable an d its con -
cen tration is n egligibly low com pared to th ose of th e reactan ts, it is possi-
ble to apply th e Boden stein approxim ation . Th e con cen tration of adduct
Ad – can be expressed by th e equilibrium con stan t Kad; so th e fin al form of
Eq. (1) reads as follows:
Kad k1 k2 [CH 3ONa]
kobs
=
.
(1)
k–1 [CH 3ONa] + k2
Sin ce th e SH– con cen tration in solution is very low, th e reverse reaction
practically does n ot occur. Th is m ean s th at th e form ation of In is, in fact, a
con secutive two-step reaction con sistin g of form ation of carbocation an d
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Kinetics and Mechanism of Desulfurization
2219
its reaction with CH3O– to give In . As th e rate con stan t of form ation of In
from carbocation is h igh , its value can n ot be determ in ed. Th e gradual de-
crease in th e slope of th e depen den ce kobs vs [CH3O–] at h igh er m eth oxide
con cen tration s (Fig. 1) can be due eith er to th e fact th at practically all th e
substrate h as reacted with m eth oxide to give n on -reactive n egatively
ch arged species or to a ch an ge in th e rate-lim itin g step. Sin ce th e n egatively
ch arged adduct of substrate with m eth oxide Ad – is an an ion of th ioacetal,
an d th e startin g substrate 1a–1e is stabilized by delocalization of th e elec-
tron pair of N-1 n itrogen , its reverse decom position is very fast, m uch faster
th an th e addition of m eth oxide to th e substrate. Th is was con firm ed by th e
fact th at th e absorption (at λm ax = 388 n m ) of th e substrate several secon ds
after th e addition of substrate to 0.5 M m eth oxide was th e sam e as th at of
th e substrate alon e (in m eth an ol). Th is m ean s th at th e observed decrease in
th e slope of th e depen den ce kobs vs [CH3O–] is due to a ch an ge in th e rate-
lim itin g step.
Th e on ly altern ative th at can be taken in to accoun t as th e rate-lim itin g
step of th e reaction in th e region of lin ear in crease is th e reaction of n ega-
tively ch arged adduct Ad – with m eth oxide, wh ich is stron gly en doth erm ic
an d very slow. Th us th e form ation of Ad – is preceeded by a fast establish ed
equilibrium directly proportion al to CH3O– con cen tration , with sm all equi-
librium con stan t Kad.
In th e region wh ere th e kobs con stan t is in depen den t of m eth oxide con -
cen tration , th e rate-lim itin g step con sists in splittin g off of SH– from adduct
Ad with con com itan t form ation of th e reson an ce stabilized carbocation .
Th e ch an ge in rate-lim itin g step depen ds on th e ratio of th e reaction rates
of th e reverse an d con secutive reaction s of Ad (Sch em e 3). Wh en th e rate of
reverse reaction is lower th an th at of th e tran sform ation in to carbocation
(at low [CH3O–]), th e rate-lim itin g step is th e reaction of Ad – with m eth a-
n ol; in th e opposite case it is th e rate of splittin g off of SH– from Ad to give
th e carbocation . Wh en k–1[CH3O–] = k2, Ad is tran sform ed back to Ad – an d
forward to th e carbocation as well, an d kobs sh ould h ave a h alf value of th at
attain ed at th e plateau (zero slope of th e depen den ce kobs vs [CH3O–]). In re-
ality, th is value is partially distorted by th e retardation of reaction due to
th e ch an ge in m edium an d, to an in creasin g exten t, due to decreasin g val-
ues of σ con stan ts of th e substituen ts (see Fig. 1).
On th e basis of pKa = 9.43 for ph en ylm eth an eth iol6, th e equation for cal-
culation of pKa of tertiary alcoh ols an d th iols in water7 an d m eth an ol8, an d
th e σI con stan ts of th e substituen ts, we h ave assessed th e differen ce be-
tween pKa of m eth an ol an d adduct Ad in m eth an ol. Th e differen ce is 7–8
orders of m agn itude, wh ich m ean s th at th e reaction is stron gly exoth erm ic
Collect. Czech. Chem. Commun. (Vol. 69) (2004)

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Hanusek, Sedlák, Keder, Štěrba:
an d th e rate con stan t of reaction of Ad – with m eth an ol approach es th at of
diffusion -con trolled processes (k–1 is about 108 s–1). As th e rough calculation
of m eth oxide con cen tration s in th e equation k–1[CH3O–] = k2 gave th e val-
ues from 0.12 m ol l–1 (4-NO2) to 0.4 m ol l–1 (4-OCH3), th e rate con stan t k2
of splittin g off of SH– sh ould ran ge from ca. 0.12 × 107 s–1 (4-NO2) to 0.4 ×
107 s–1 (4-OMe).
Alth ough it is presum ed th at acetals an d ketals an d th eir th io an alogs are
stable in n eutral an d basic m edia, a very slow but m easurable spon tan eous
h ydrolysis h as been foun d9,10 with som e acetals con tain in g good leavin g
groups.
A fast spon tan eous h ydrolysis takes place in th e case of 4-H2N-C6H4-
CH(OEt)SC6H5 (kobs = 40 s–1); a m ech an ism with ion pair separation rate-
lim itin g step was suggested11. Th e drivin g force is (like with Ad ) stabiliza-
tion of carbocation by delocalization of th e electron pair from n itrogen to
carbon . Th e delocalization of electron pair is accom pan ied by in creased
arom atization of th e six-m em bered rin g.
Gen erally, th e decrease in reaction rates at m eth oxide con cen tration s
above 0.8 m ol l–1 can be due to an oth er ch an ge in th e rate-lim itin g step,
but in th e presen t case such a varian t is groun dless.
Th e drop in kobs at sodium m eth oxide con cen tration s above 0.8 m ol l–1 is
due to th e effect of a ch an ge in activity coefficien ts of th e in dividual reac-
tan ts an d in th e tran sition state of th e rate-lim itin g step. For in stan ce, th e
activity coefficien t of m eth an ol in 1.6 M sodium m eth oxide12 γ = 0.8.
At m eth oxide con cen tration s above 0.8 m ol l–1, th e rate-lim itin g step is
splittin g off of SH– from adduct Ad for all th e derivatives. In th is case, th e
reaction m ech an ism can be described by sim plified Sch em e 4.
S
HS OCH3
k2
NH
NH
K
In
+
CH3OH
N
N
R1
R1
Ad
SCHEME 4
Th e ch an ge of th e reaction m edium affects th e values of equilibrium con -
stan ts K. However, th e decreases foun d are substan tially larger with th e 4-Cl
derivative an d m uch larger with th e 4-NO2 derivative (in version of th e
depen den ces), an d th is m ean s th at th e ch an ge of th e m edium m ust h ave a
con siderable effect also on th e rate-lim itin g step, i.e. on th e k2 con stan t.
Th e spon tan eous decom position of 4-(CH3)2N-C6H4-CH(OEt)SC6H5 in -
volved th e rate-lim itin g separation of th e ion pair form ed. Alth ough th e
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Kinetics and Mechanism of Desulfurization
2221
splittin g off of SH– is by 5–6 orders of m agn itude faster, an in tern al return
of th e ion pair form ed in th is case obviously takes place, th e differen ce be-
in g in th at th e rates of in tern al return an d ion -pair separation differ sub-
stan tially less. Th e rate of in tern al return in creases with in creasin g σ con -
stan ts of substituen ts, an d th e separation of carbocation an d SH– becom es
m ore an d m ore rate-lim itin g. Th e rate of separation of SH– decreases to th e
sam e exten t as th e stabilization of SH– by form ation of H-bon ds. Th e latter
decreases with in creasin g m eth oxide con cen tration , because th e activity
coefficien t of m eth an ol is decreasin g. A decrease in stabilization of SH–
by H-bon ds by a value of 1.6 kJ m ol–1 decreases th e reaction rate alm ost to
on e h alf.
Th e tran sform ation of in term ediate In in to product is very slow; its rate
in creases on ly m oderately with th e m eth oxide con cen tration , but it in -
creases lin early with water con cen tration . In Mg-dried m eth an ol, th e tran s-
form ation proceeded at a n egligible rate. Th is is in accordan ce with th e fol-
lowin g reaction sequen ce (Sch em e 5). Th e process closely resem bles th e ex-
ten sively studied acetal h ydrolysis.
O
CH3
N
HO OCH3
N
CH3O OCH3
N
– CH3O
OH
N
N
N
CH3
CH3
CH3
R1
R1
In
R1
CH3OH CH3O
O
O
OCH3
N
– CH3O
N
N
N
CH3
CH3
R1
R1
2a-2e
SCHEME 5
CONCLUSIONS
Th e kin etic study of desulfurization of 1a–1e in sodium m eth oxide solu-
tion s sh ows th at th e reaction proceeds in two steps. At low m eth oxide con -
cen tration s, th e rate of th e first step is determ in ed by th e reaction of Ad –
with m eth an ol to give Ad ; at h igh er con cen tration s, th e splittin g off of SH–
from adduct Ad becom es rate-lim itin g. Th is beh avior is supported by th e
fin din g th at th e observed rate con stan t first in creases with th e m eth oxide
con cen tration , but th en becom es in depen den t of th is con cen tration (de-
Collect. Czech. Chem. Commun. (Vol. 69) (2004)

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Hanusek, Sedlák, Keder, Štěrba:
creasin g with water con cen tration ), wh ich excludes th e possibility of m an i-
festation of th e reaction path way given in Sch em e 2. Th e in term ediate
form ed by desulfurization probably h as th e dim eth yl acetal structure an d is
in equilibrium with th e carbocation . Relative stability of th e carbocation is
determ in ed by its pron en ess to arom atization , an d th e on ly path way of its
decom position to th e fin al product 2a–2e is th e reaction with presen t h y-
droxide ion an d/or water. Th is in terpretation is supported by th e fact th at
th e in term ediate decom position is stron gly accelerated in th e presen ce of
water an d/or h ydroxide.
The authors thank the Ministry of Education, Youth and Sports of the Czech Republic (Project
CI MSM 253 100 001) for the financial support.
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