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Chemistry Letters Vol.35, No.9 (2006)
Regioselective Synthesis of Lipophilic Dithiocarbamate Derivatives
of Cellulose to Prepare a Novel Solvent Extraction Reagent
for Selective Extraction of PGM Ions and Cu(II)
Rabindra Prasad Dhakal,ꢀ1 Katsutoshi Inoue,2 Keisuke Ohto,2 and Yoshinari Baba1
1Department of Applied Chemistry, University of Miyazaki, 1-1 Gakuen Kibandai Nishi, Miyazaki 889-2192
2Department of Applied Chemistry, Saga University, Honjo-1, Saga 840-8502
(Received June 29, 2006; CL-060734; E-mail: dhakalrabi@cc.miyazaki-u.ac.jp)
Regenerated cellulose from cellulose triacetate was regiose-
to regenerate the cellulose by mild alkaline hydrolysis for 18 days
which is presumed to be turned into amorphous cellulose 2. By
doing so, the degradative hydrolysis (peeling effect of polymer
chain) was avoided while using strong alkaline solution, and at
the same time, complete regeneration of hydroxy group was
achieved. It was then protected regioselectively by trityl group
to keep the primary hydroxy group intact for further chemical
modification after refluxing dehydration with pyridine repeatedly
at elevated temperature.3,4 We have observed that the tritylation
degree was as high as 1.002 with trityl chloride equivalent to a
primary hydroxy group where as 1.007 on using monomethoxy
trityl chloride as tritylating reagent, as latter is the better protect-
ing group. It was evaluated by elemental analysis (C, 74.42
(obs.); 74.25 (cal.); H, 6.00 (obs.); 5.94 (calc.)), 1H NMR
(270 MHZ, CDCl3): ꢀ 7.45–7.24 (m, br), and IR: C–H stretching
at 3032 cmꢁ1 and bending at 1468 cmꢁ1 representing for trityl
group that is also on agreement with conventional trends to pro-
tect only primary hydroxy group. It was then treated with
n-bromodecane in presence of dimsyl anion solution5,6 to get 3
substituting two equivalent of hydroxy groups as revealed by
1H NMR, particularly, ꢀ 1.55–1.26 (m, br), 3.18 (br) for methyl-
ene moieties and 0.88 (t) for methyl moieties of decyl groups.
Moreover, lacks of O–H stretching band in IR spectroscopy
strongly supports the successful conversion. The subsequent
deprotection was done by HCl in 1,4-dioxane7 as other solvents
appeared inappropriate to regenerate hydroxy group which was
evidenced by O–H stretching band at 3498 cmꢁ1 with slight blue
shift compared to previous non alkylated products suggesting less
hydrogen bonding than before. Moreover, there is no more ab-
sorbance for aromatic moieties on 1H NMR suggesting complete
regeneration of primary hydroxy group previously occupied by
lectively converted into its lipophilic amino and dithiocarbamate
derivatives to prepare novel solvent extraction reagents with
series of substitutions and deoxy substitution reaction. The 6-de-
oxydithiocarbamate-2,3-didecyl cellulose prepared in this work
significantly demonstrated the high lipophilicity coupled with
high selectivity for PGM ions and also to the Cu(II) over Ni(II)
and Zn(II) in liquid–liquid extraction.
Precious metal ions are tremendously used in many industrial
processes that obviously draw due attention for its proper
recovery from its waste, particularly from spent catalyst and elec-
trolytes as well as electronic and jwellery scraps. On the
other hand, cellulose, the most abundant biopolymer in nature
that possesses two kinds of hydroxy groups in its pyranose ring,
was utilized to increase its selectivities for specific metal groups
like PGM (Platinum Group Metal) ions and some others. Howev-
er, several chemical modifications are needed to prepare a solvent
extraction reagent from such a biopolymer as evidenced and in-
spired by previous works on chitosan.1 On contrast, the novelty
of this study is the cost effective use of available tremendous
resources, innovative and regioselective synthetic approach,
and preferable stripping process by incineration, among others.
Hence, the lipophilic moieties were regioselectively introduced
into the secondary positions of the cellulose whereas reactive
functional moiety to the primary position as shown in Scheme 1
tends to create stereo electronically free environment for reactive
functional groups. The microcrystalline structures of cellulose2,3
usually pose set back for complete conversion of such polymer,
hence, cellulose triacetate (1), was taken as a starting material
OH
H
OAc
OTr
H
H
aq. NH (28%),
3
18 days
1. Trityl chloride, pyridine
H
O
H
H
O
O
2. DMSO, NaH, C
H Br
O
10 21
O
HO
O
H
AcO
O
H
O
OAc
C
O
O
10 21
H
H
OH
H
H
n
OC
H
10 21
H
H
(1)
H
2
n
H
n
3
1. Conc. HCl, 1,4-dioxane
2. TsCl, Pyridine
3. NaI, 4-methyl-2-pentanone
S
NH
2
H
C
SH/NH
4
HN
H
H
I
O
H
O
2M NH in MeOH,
H
CS , NH OH
EtOH
3
2
4
O
H
C
H O
10 21
O
O
pyridine, K CO
2
O
3
H
OC
H
O
10 21
C
H O
10 21
O
H
C
H O
10 21
O
H
H
OC
H
n
10 21
H
OC
H
H
10 21
H
H
n
H
n
6
5
4
Scheme 1. Synthetic route to prepare lipophilic cellulose and its dithiocarbamate derivative.
Copyright Ó 2006 The Chemical Society of Japan