Humeres et al.
1051
anhydrous sodium sulfate, and distilled under vacuum: bp
160°C/8 mmHg (lit. (14) 143°C/3 mmHg); λmax (ethanol)
280 nm.
Introduction
Xanthic esters have been used in the synthesis of trithio-
carbonates (1), in Friedel–Crafts reaction (2), Chugaev’s re-
action (3), and in the synthesis of thiols (4, 5), and also as
derivatives for the isolation of carbohydrates (6). The hydro-
lysis of xanthic esters occurs as shown in eq. [1], and they
react with amines displacing the thiol and forming a thion-
carbamate ester (5). Thioncarbamates have shown high spec-
ificity as collectors in the flotation of sulfide minerals (7).
S-p-Nitrobenzyl O-ethylxanthate (EXNB)
EXK (2.2 g) dissolved in 10% aqueous EtOH, was al-
lowed to react at room temperature with a solution of 2.1 g
of p-nitrobenzyl bromide (acetone–ethanol). The reaction
was followed by the appearance of the maximum at 280 nm.
KBr was eliminated by filtration, and the solvent was evapo-
rated in a rotatory evaporator. The product was recrystallized
in ethanol: mp 61.6°C.
H2O
+
R2SH
+
COS
R1OH
[1]
R1O C(S) SR2
R3NH2
S-Phenyl O-ethylxanthate (EXP)
+
R1O C(S) NHR3 R2SH
A solution of sodium ethoxide prepared from 2.3 g of
sodium in 50 mL of dried ethanol was slowly added to a so-
lution of 12 g of thiophosgene dissolved in 100 mL of chlo-
roform at 0°C, and it was allowed to react for 3 h to obtain a
solution of ethyl chlorothionoformate that was washed with
water and dried over anhydrous magnesium sulfate. The sol-
vent was stripped off in a rotatory evaporator, and the prod-
uct was used without further purification (15a, b). The
reaction of 1 g of sodium thiophenolate in 5 mL of dried
ethanol with 1 g of ethyl chlorothionoformate produced EXP
after 15 min of reaction at room temperature. Crystals of
NaCl were filtered off, and the solution was treated with an
aqueous solution of phosphate buffer at pH 8, the product
then being extracted with diethyl ether and dried over anhy-
drous magnesium sulfate. Evaporation of diethyl ether left a
yellow oil that was identified as EXP:TLC, alumina, hex-
ane:ethyl acetate (1:1), one spot; λmax 285 nm; after reaction
with NaOH, λmax 265 nm (thiophenol); after reaction with
ethylamine, λmax 245 nm (thioncarbamate) (15c).
The study of the reactivity of xanthate ester moieties co-
valently bound to cellulose showed that the reaction of water
was much faster compared to small analogue molecules (8).
The water-catalyzed hydrolysis of p-nitrobenzyl (CelXNB)
and 2,4-ditrophenyl cellulose xanthate (CelXDNP) occurs
through two parallel processes (9, 10). The faster hydrolysis
was ascribed to the reaction of the C-2 + C-3 isomers,
whereas the slower hydrolysis corresponds to the C-6 iso-
mer. The fast hydrolysis is due to an entropic effect of a
well-oriented water molecule catalyzed by a second mole-
cule that acts as a general base, as shown by proton inven-
tory. Nucleophiles such as hydroxide ions and amines do not
discriminate between the isomers, and the kinetics are first
order (9).
In this paper, we seek to extend a previous study of the
hydrolysis and aminolysis of a series of S-substituted O-
ethylxanthate esters (11), to include sugar analogues and
2,4-dinitrophenyl cellulose xanthate, CelXDNP, a cellulose
ester with a better leaving group, in order to get an insight
into the mechanism of nucleophilic attack on cellulose
xanthate esters and small molecule analogues.
S-p-Nitrophenyl O-ethylxanthate (EXNP)
Ethyl chlorothionoformate (3.5 g), obtained as described
above, was added to 1.45 g of p-nitrothiophenol in 1.0 mL
of pyridine, and then the temperature was raised to 60°C for
1 h. The precipitate was dissolved in 30 mL of chloroform
and washed with water, subsequently being dried and then
concentrated in a rotatory evaporator. Crystals were obtained
after 8 h in the refrigerator: mp 51°C (lit. (16) 50°C).
Experimental
All reagents were of analytical grade and were used with-
out further purification except the amines that were previ-
ously distilled in a high efficiency distilling column and kept
under nitrogen atmosphere at low temperature.
The solid-state 13C CP/MAS TOSS NMR measurements
were performed using a Bruker MSL-300 spectrometer. All
S-2,4-Dinitrophenyl O-ethylxanthate (EXDNP)
1
EXK (9.2 g) dissolved in 30 mL of water was mechani-
cally shaken with a solution of 10.1 g of 1-fluoro-2,4-di-
nitrobenzene in 300 mL of methylene chloride, for 64 h at
room temperature. The methylene chloride phase was washed
with water and dried over anhydrous magnesium sulfate.
The solvent was stripped off in a rotatory evaporator, and
the residue was crystallized in methanol: mp 43.0°C; λmax
267 nm; ms, mol. wt. 288 g mol–1. Anal. (calculated values
for C9H8N2O5S2 in parentheses): C 37.5 (37.5), H 2.7 (2.8),
N 9.7 (9.7), S 21.5 (22.0).
xanthate esters were identified by H NMR and IR spectra.
S-Ethyl O-ethylxanthate (EXE)
Potassium O-ethylxanthate (EXK) was obtained using the
classical procedure (12), adding carbon disulfide to a cooled
solution of potassium hydroxide in ethanol. The product was
recrystallized twice in ethanol; λmax (ethanol) 301 nm.
S-ethyl O-ethylxanthate was obtained after refluxing for 4 h,
using an equimolar amount of EXK and ethyl bromide in
ethanol. The product was washed with water, dried over an-
hydrous sodium sulfate, and distilled under vacuum: bp
76°C/10 mmHg (1 mmHg = 133.3 Pa) (lit. (13) 78°C/18 mmHg);
λmax 283 nm.
Methyl -D-glucopyranoside 6-(S-benzyl xanthate)
(MGXB)
CS2 (2.5 mL) was added to a solution of 22.9 g of α-
methyl-D-glucopyranoside in 23.6 mL of water, with mag-
netic stirring at room temperature, and then, slowly, 6 mL of
20 M NaOH was also added. After being allowed to react
S-Benzyl O-ethylxanthate (EXB)
EXK and benzyl bromide (1:1 mol) were refluxed for 6 h
in ethanol. The product was washed with water, dried over
© 1999 NRC Canada