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N.G. Khaligh et al. / C. R. Chimie xxx (2013) xxx–xxx
Table 2
Table 4
Stability of P(BVP)BH4 and NaBH4 in 100% ethanol at room temperature.
Reduction of benzaldehyde and acetophenone and benzophenone to their
corresponding hydroxyl compounds with NaBH4, (PVP)ꢃBH3 and
P(BVP)BH4.
Time (min)
0
50
100
150
200
Hydride loss (%)
NaBH4
Substrate
NaBH4
PVPꢃBH3
P(BVP)BH4
0.0
0.0
3.8
1.4
8.4
2.5
12.0
3.6
16.4
4.4
P(BVP)BH4
Benzaldehyde
Acetophenone
Benzophenone
99
32
18
24
32
40
92
71
68
The initial apparent pH of the used ethanol was approximately 5.1.
Reaction conditions: solvent, 5 mL ethanol; reaction time, 240 min.
Table 3
Stability of P(BVP)BH4 and NaBH4 in water (pH = 6.9) at room
temperature.
aldehyde or ketones (1 mmol) and NaBH4 (1 mmol),
PVPꢃBH3 (1 mmol) and P(BVP)BH4 (0.32–0.64) in the
presence of 5 mL of ethanol at room temperature in air.
The produced alcohols were isolated and the yields were
determined by GC–MS analysis. As shown in Table 4,
benzaldehyde was rapidly reduced to benzyl alcohol, even
by NaBH4 alone. Unlike benzaldehyde, the reduction of
ketonesusingsodiumborohydrideitself wasnotcompleted.
For example, the reduction of acetophenone provided 1-
phenylethanolin only 32% yield in240 min, withrecoveryof
the unreacted starting ketone in 68% yield. However, in the
presence of P(BVP)BH4, reduction gives the produced
alcohol in 71% yield in 240 min. This methodology was
successfully applied to the reduction of benzophenone, but
1 mmol of benzophenone required 0.64 mmol of the
reducing agent for complete reduction. It seems to be due
to the fact that the solution of NaBH4 in ethanol is rapidly
decomposed into borates, which reduce only very reactive
substrates.
Obviously, some functional groups were more resistant
to reduction than others and, therefore, higher amounts of
the reducing agent were used to obtain reasonable yields of
the products in acceptable times. The chemoselective
reduction of aldehydes in the presence of ketones is one
of the most important transformations in organic synthesis
[4]. Table 5 shows the reduction of various carbonyl
compounds with P(BVP)BH4. This reagent reduced alde-
hydes and ketones into the primary and secondary alcohols
in good to high yields, respectively (Table 5, entries 1–13).
But, ketones were reduced to their corresponding alcohols
using a higher reducing agent:substrate molar ratio and
longer reaction times (Table 5, entries 14–18).
Time (min)
0
50
100
150
200
Loss hydride (%)
NaBH4
0.0
0.0
7.2
0.4
8.1
0.6
9.5
0.6
9.8
0.6
P(BVP)BH4
The final pH values were greater (generally 8–9) than the initial one.
hydride content. The P(BVP)BH4 resin was substantially
more stable solvolytically than sodium borohydride.
3.2. Reductive activity of P(BVP)BH4
The preparation and characterization of some of the
coordination complexes of poly(4-vinylpyridine) with a
large number of metal salts have been reported in the
literature [20]. These complexes are easily formed when
alcoholic solutions of the metal salts are added to a
solution or a suspension of the polymer. n-Butyl chloride
was supported on cross-linked poly(4-vinylpyridine) by
adding n-butyl chloride to a suspension of the polymer in
sulpholane. The polymeric reagent was then obtained by
an exchange reaction between the polymer-supported
poly(n-butyl-4-vinylpyridinium) chloride and sodium
borohydride in water. This PSBR could be stored without
appreciable change in its capacity over 6 weeks. Most
importantly, no new contaminants, such as Na+ and BO2
,
–
were added to the environment, since the borate ion
remains bonded to the reagent.
It is known that solvents can play an important role in
the stability, reducing power, and selectivity of borohy-
dride reagents in reduction reactions [4,20]. It is also
known that for using a polymeric reagent in an organic
reaction, a solvent should be chosen in which it could swell
to a considerable extent [12]. Considering these facts,
solvent optimization reactions for the reduction of
carbonyl groups was performed using different solvents,
and the best solvent was found to be ethanol. This solvent
is a greener solvent than methanol. In ethanol, the
polymeric reagent was completely insoluble and boron
moieties before and after the reduction remained firmly
bound to the insoluble polymeric support. Product
isolation and purification were performed simply by
filtration of the reaction mixture, evaporation of the
solvent, and, if necessary, by further separation of the
starting material by column chromatography.
Due to the importance of synthetic precursors of allylic
alcohols, the regioselective 1,2-reduction of
a,b-unsatu-
rated carbonyl compounds without disturbing the carbon–
carbon double bond is synthetically very important. [4].
Generally, the 1,4-addition to
a,b-unsaturated carbonyl
compounds becomes equally important than the 1,2-
addition in the presence of NaBH4 itself, which leads
usually to a mixture of 1,2- and 1,4-product with 59% and
41% yields, respectively (Scheme 2) [21]. The polymeric
reagent shows a satisfying regioselectivity in the reduction
of
a,b-unsaturated aldehydes as well as of ketones (Table
5, entries 12,13 and 20–22). On the other hand, in alcoholic
media, NaBH4 reduces halides, while P(BVP)BH4 did not
affect chlorides (Table 5, entry 25).
It may be of interest to note that selective reduction of
aldehydes in the presence of ketones is ordinarily
impracticable using sodium borohydrides [22]. The chemos-
electivity of this method was checked by the competitive
reduction of benzaldehyde and acetophenone together. The
We initially examined the reduction of benzaldehyde,
acetophenone, benzophenone and cyclohexanone using
NaBH4 itself, PVPꢃBH3 and P(BVP)BH4 (Table 4). The
reductions were carried out by mixing a mixture of the
Please cite this article in press as: Khaligh NG, et al. Poly(n-butyl-4-vinylpyridinium) borohydride as a new stable and