G Model
CRAS2C-3739; No. of Pages 7
N.G. Khaligh / C. R. Chimie xxx (2013) xxx–xxx
3
ꢀ
1
completion of the reaction, the mixture was filtered and
the used reagent was washed with ethanol (2 ꢂ 5 mL). The
combined filtrates were evaporated and the pure product
was obtained in moderate to excellent yield. In a few cases,
when the reaction was not complete, the crude product
was purified on silica gel with an appropriate eluent.
distinctive vibration at 1126 cm assigned to the H–B–H
ꢀ
deformation (
d
HBH), and characteristic of the BH
BH bands are split into three peaks, around
2380, 2293 and 2227 cm respectively, which have been
assigned to the stretching of the terminal hydrogen–B. In all
spectra, a C5N band of pyridine, usually at 1600 cm , is
displaced to higher wave numbers (1650 cm ) due to
quaternisation [27]. Other vibrations of interest are at
451 cm and 1419 cm respectively; they have been
assigned to C–C/N–C.
P(BBVP)BH was subjected to elemental analysis
(Table 1). According to elemental analysis, it can be seen
that the exchange of chloride ions with BH
with 100% conversion.
4
group
[14a,b,28]. The
n
ꢀ
1
ꢀ1
ꢀ
1
4
2.5. Regeneration of cross-linked P(BBVP)BH (Scheme 1)
ꢀ1
ꢀ1
1
The utilized polymeric reagents (5.00 g) collected from
different experiments were washed successively with
hydrochloric acid (1.0 M, 2 ꢂ 10 mL), NaHCO (2.0 M,
ꢂ 10 mL), distilled water (2 ꢂ 10 mL) and ether
4
3
ꢀ
4
4
was done
(
2 ꢂ mL). The solid was dried under vacuum at 50 8C to
give the original polymer (4.87 g) as a powder precipitate,
which was stirred with sodium borohydride to provide the
initial polymeric reducing reagent.
We believe it is very unlikely that all the dihalides react
at both ends; therefore, we suppose that three types of the
reducing agents can be existing in the final polymer-
supported borohydride resin (Scheme 2).
3
. Results and discussion
It is known that solvents can play an important role in
the stability, reducing power, and selectivity of borohy-
dride reagents in reduction reactions [4,29]. It is also
known that for using a polymeric reagent in an organic
reaction, a solvent should be chosen in which it can swell
to a considerable extent [10]. The degree of swelling was as
follows: 1.0 g dried reagent was swollen in 100 mL solvent
at 25 8C for 1 h. The degree of swelling, S, was estimated as
The structure of poly(1,4-butyl-bis-vinylpyridinium)
borohydride was identified by IR [27] and elemental
analysis. Fig. 1 presents the FTIR spectra of poly(4-
vinylpyridine) (PVP) and poly(1,4-butyl-bis-vinylpyridi-
nium) borohydride P(BBVP)BH
tion bands observed in the reagent’s spectra in the range
4
. The characteristic absorp-
ꢀ1
between 2923 and 2858 cm could be ascribed to methyl
C–H) stretching vibration absorption peaks, due to the 1,4-
dichlorobetane cross-linking agent. P(BBVP)BH shows a
strong vibration frequency at about 2380–2227 cm
attributed to the B–H stretching ( BH), and another
S = (M
reagent, M
s
ꢀ M
i
)/(
r
M
i
), where M
i
is the weight of initial dry
is
(
s
is the weight of the swollen reagent, and r
4
the density of solvent.
ꢀ
1
,
4
The swelling behavior of P(BBVP)BH was revealed by
n
the degree of swelling in Table 2. The quaternarization of
Fig. 1. FTIR spectra of poly(4-vinylpyridine) (top) and poly(n-butyl-4-vinylpyridinium) borohydride (bottom).