Modified hydroborate agent: (2,2′-bipyridyl)(tetrahydroborato)zinc complex, [Zn(BH4)2(bpy)], as a new, stable, efficient ligand-metal hydroborate and chemoselective reducing agent

Article abstract of DOI:10.1246/bcsj.76.317

(2,2′-Bipyridyl)(tetrahydroborato)zinc complex, [Zn(BH₄)₂(bpy)], is a new white stable compound which has been used for efficient reduction of variety of carbonyl compounds such as aldehydes, ketones, acyloins, α-diketones and α, β-unsaturated carbonyl compounds (1,2-reduction) to their corresponding alcohols in acetonitrile at room temperature. Excellent chemoselectivity was also observed for the reduction of aldehydes over ketones with this reducing agent.

Full text of DOI:10.1246/bcsj.76.317

c. Jpn.
© 2003 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 76, 317–326 (2003)
317
e Chemical
ciety of Ja-
n
e Chemical
ciety of Ja-
n
Modified Hydroborate Agent: (2,2ꢀ-Bipyridyl)(tetrahydroborato)zinc
Complex, [Zn(BH4)2(bpy)], as a New, Stable, Efficient Ligand-Metal
Hydroborate and Chemoselective Reducing Agent
Modified Hydroborate Agent: [Zn(BH4)2(bpy)]
B. Zeynizadeh
03
7
Behzad Zeynizadeh
6
Chemistry Department, Faculty of Sciences, Urmia University, Urmia 57159, Iran
(Received April 23, 2002)
SJA8
09-2673
105
(2,2ꢀ-Bipyridyl)(tetrahydroborato)zinc complex, [Zn(BH4)2(bpy)], is a new white stable compound which has been
used for efficient reduction of variety of carbonyl compounds such as aldehydes, ketones, acyloins, α-diketones and α,β-
unsaturated carbonyl compounds (1,2-reduction) to their corresponding alcohols in acetonitrile at room temperature. Ex-
cellent chemoselectivity was also observed for the reduction of aldehydes over ketones with this reducing agent.
02
02
The absence of a selective reduction of the carbonyl func-
tion of aldehydes, ketones, acids, esters and amides in the pres-
ence of carbon double bonds by catalytic hydrogenation, has
led to the wide-spread use of certain complex metal hydrides
for the reduction of carbonyl groups. In spite of the great con-
venience of lithium aluminum hydride and sodium tetrahy-
droborate in laboratory for organic synthesis, they suffer from
certain limitations. LiAlH4 as a powerful reducing agent
shows poor selectivity for the reduction of multifunctional
molecules. On the other hand, Na[BH4] is a mild reducing
agent and is used for the reduction of few organic functional
groups.1 Thus developing means for controlling the reducing
power of such reagents is desirable; such control could be
achieved either by decreasing the reducing power of LiAlH4 or
by increasing that of Na[BH4].1,2 In fact, this has been
achieved by different types of modifications. Methods can be
summarized as: a) Substitution of the hydride(s) with other
substituents which may exert marked steric and electronic in-
fluences upon the reactivity of the substituted complex ion,
e.g., Na[BH(OAc)3],3,4 Na[BH3(OAc)],4,5 Na[BH(OMe)3],3,6
sodium (dimethylamino) and (t-butylamino) trihydroborates,3,7
Na[BHEt3],8 Na[BH(O2CCF3)3],3 Na[BH3(CN)]9 and Na[BH2-
(S3)];10 b) Variation of alkali-metal cation and metal cation in
the hydride complex, e.g., Li[BH4],11 K[BH4],12 Ca[BH4]2,13
Cu[BH4]2,14 Zn[BH4]2,15 Ti[BH4]3,16 Zr[BH4]4 and the
others;16b c) A concurrent cation and hydride exchange, such as
K[BH(OAc)3],3,17 K[BH(i-PrO)3],18 K[BHPh3],19 Li[BH-
(OAc)]3,20 Li[BH3(CN)],3,21 Li[BHEt3] (Super Hydride),22
Li[BH(n-butyl)],23 lithium 9,9-di-N-butyl-9-borabicyclo-
[3.3.1]nonate,24 lithium isopinocampheyl-9-borabicyclo-
[3.3.1]nonyl hydride,25 lithium B-iso-2-ethylapopinocam-
pheyl-9-borabicyclo[3.3.1]nonyl hydride,26 lithium (amino)tri-
hydroborate,27 lithium trihydro(pyrrolidino)borate,28 Li-
[BH(sec-butyl)3] (L-Selectride),29 K[BH(sec-butyl)3] (K-Selec-
tetrahydroborates with metals, metal salts, Lewis acids, ad-
ditives36 and mixed solvent systems, especially those contain-
ing methanol;37 f) Changing the cation to quaternary ammoni-
um and phosphonium tetrahydroborates, such as Et4N-
[BH4],38,39 Bu4N[BH4],39,40 PhCH2(Et)3N[BH4],41 Bu4N[BH-
(OAc)3],42 Bu4N[BH3(CN)],43 4-aza-N-benzylbicyclo[2.2.2]-
octylammonium tetrahydroborate44 and Ph3PMe[BH4];45 g)
Use of the polymers or solid supports to support hydride spe-
cies: Amberlyst (A-26 or A-400), zeolite, alumina, silica gel
and polyvinylpyridine are usually used as bed supports for dif-
ferent hydride species.46
Zinc tetrahydroborate, Zn[BH4]2, as a transition metal hy-
droborate, is a potential reducing agent that has attracted much
attention in the last two decades.47 This reagent could be pre-
pared conveniently from inexpensive zinc chloride and sodium
tetrahydroborate in ether.15 The reagent must be used as its
ethereal solution and should be stored in a cold place, which
puts some restriction on its uses. This compound with high co-
ordination ability is neutral and its combination systems such
as Zn[BH4]2/TMEDA,48 Zn[BH4]2/Me3SiCl,49 and Zn[BH4]2/
TFA/DME50 have also been used for different reduction pur-
poses. Preparation and use of poly(4-vinylpyridine) supported
zinc tetrahydroborate as a first polymeric transition metal hy-
droborate have been reported.46a This reagent is very stable
and has excellent chemoselectivity for reduction of aldehydes
over ketones. The other reported modified tetrahydroborate is
poly{tetrahydro[η-pyrazine]boratozinc} complex, [Zn(BH4)2-
(pyz)]n.51 This compound is stable to heat and light, but ex-
plodes violently into flames when in contact with water. Re-
cently, we reported new tertiary amino and phosphino ligand
complexes of zinc tetrahydroborate as (1,4-diazabicyclo-
[2.2.2]octane)(tetrahydroborato)zinc, [Zn(BH4)2(dabco)]2b,45b
and mono or bistriphenylphosphinezinc tetrahydroborate,
[Zn(BH4)2(Ph3P)] or [Zn(BH4)2(Ph3P)2],52 of which we investi-
gated the full reducing abilities. Continuing our mentioned
strategies and our interest in preparation of modified ligand-
metal tetrahydroborate, now we wish to report a new coordina-
tion complex of zinc tetrahydroborate and a commercially
tride),29a,c Ca[(BH2(S3)]2 and Ba[(BH2(S3)]2;31 d) Use of the
30
ligands to alter behavior of the metal hydroborates: (Ph3P)2-
Cu[BH4],32 (Ph3P)4Cu2[BH3(CN)]2,33 (C5H5)2Zr[BH4]Cl34 and
(i-PrO)2Ti[BH4]35 are examples in this area; e) Combination of

318 Bull. Chem. Soc. Jpn., 76, No. 2 (2003)
Modified Hydroborate Agent: [Zn(BH4)2(bpy)]
cheap bidentate ligand, 2,2ꢀ-bipyridyl, as a stable bench-top
reducing agent for the reduction of a variety of carbonyl com-
pounds to their corresponding alcohols.
Results and Discussion
Scheme 1.
(2,2ꢀ-Bipyridyl)(tetrahydroborato)zinc, [Zn(BH4)2(bpy)], is
a white stable compound which is prepared by the dropwise
addition of an equimolar ethereal solution of 2,2ꢀ-bipyridyl to
an equimolar ethereal solution of Zn(BH4)2 at room tempera-
ture. The formation of the complex is very fast and the reagent
is formed quantitatively. Filtration and drying of the precipi-
tate resulted in a white fluffy powder, which could be stored
for months without losing its activity. The amount of Zn con-
tent in the complex is determined by both gravimetric and
atomic absorption techniques, which confirmed the [Zn(BH4)2-
(bpy)] formula (Fig. 1).
duced by the reagent (Scheme 1). In Table 3, we see the gener-
al trend of chemoselective reduction of various carbonyl com-
pounds. In most cases the selectivity ratio was excellent.
In order to show both advantages and limitations of the re-
ducing agent, we compared our results with those reported for
[Zn(BH4)2(dabco)],2b [Zn(BH4)2(pyz)]n,51 [Zn(BH4)2(Ph3P)],52
[Zn(BH4)2(Ph3P)2],52 Zn[BH4]2,47a–c [Zn(BH4)2-XP4],46a Ph3-
PMe[BH4],45a, c 4-aza-N-benzylbicyclo[2.2.2]octylammonium
tetrahydroborate44 and Bu4N[BH4]40 (Table 4).
α
2. Reduction of -Diketones and Acyloins.
Reduction
1. Reduction of Aldehydes and Ketones. [Zn(BH4)2(bpy)]
is a slightly soluble reagent and works efficiently in a range of
aprotic solvents such as CH2Cl2, Et2O, THF and CH3CN at
room temperature. This reagent is unstable in protic solvents
such as methanol, ethanol and water and decomposed vigor-
ously with the evolution of hydrogen gas. Because of higher
rate and efficiency of reduction in CH3CN relative to other sol-
vents, we selected it as a suitable solvent for the reduction re-
actions. The molar ratio of the reducing agent varies between
0.25–1.5 according to the nature of the carbonyl function in the
molecules. As shown in Tables 1 and 2, different types of alde-
hydes and ketones were reduced with the reducing agent in ac-
etonitrile at room temperature. Aldehydes are reduced rapidly
with 0.25–0.5 molar amounts of the reagent in short times and
85–99% yields (Table 1). Ketones can also be reduced in ace-
tonitrile in 50–99% yields (Table 2). Relative to aldehydes, re-
duction of ketones required higher molar amounts of the re-
agent (0.5–1.5 mol) and longer reaction times (Table 2).
Since aldehydes are reduced much more rapidly than ke-
tones and the reagent has a bulky nature, this reagent can act as
a good chemoselective reagent for discrimination of aldehydes
over ketones or less hindered ketones over hindered ones. In
order to show the chemoselectivity towards various carbonyl
compounds, we performed the competitive reduction of ace-
tophenone in the presence of an equimolar amount of benzal-
dehyde with 0.25 mol of the reagent at room temperature. We
observed that aldehyde with excellent selectivity ratio was re-
of α-diketones to their diols usually gives a mixture of the
diols and the corresponding acyloins. With this reagent, α-
diketones are reduced very easily to their corresponding vici-
nal diols with excellent yields (Table 5). Attempts to reduce α-
diketones to their corresponding acyloins have been unsatis-
factory and only the corresponding vicinal diols have been iso-
lated from a mixture with high yields (92–98%). Acyloins are
also reduced very easily to their vicinal diols by the reagent in
excellent yields (90–96%) (Table 5).
α β
3. Regioselective 1,2-Reduction of , -Unsaturated Car-
bonyl Compounds. A regioselective reduction of α,β-unsat-
urated aldehydes and ketones is an easy way to obtain allyl al-
cohols that are important synthetic materials in organic synthe-
sis. This achievement with Na[BH4] is highly solvent-depen-
dent and generally does not result in a useful regioselectivi-
ty.4,53 Regioselective 1,2-reduction is usually achieved using
modified hydride reagents, which are formed a) by the replace-
ment of hydride with sterically bulky substituents or electron-
withdrawing/releasing groups in order to discriminate between
the structural and electronic environments of the carbonyl
groups, b) by changing the metal cation, c) by combination
with metal salts and mixed solvents, and d) finally immobiliza-
tion on polymeric supports.4,13,14b,23,53,54 [Zn(BH4)2(bpy)] is
also regioselective in this case and α,β-unsaturated aldehydes
and ketones are efficiently reduced to their corresponding allyl
alcohols with 0.5–1 molar amount in CH3CN at room tempera-
ture (92–98%) (Table 6).
The chemo- and regioselectivity of the reagent are also dem-
onstrated by the following competitive reaction (Scheme 2).
The selectivity ratio for the 1,2-reduction of citral over β-
ionone is 6.7 (Table 3).
In this case, we also compared our results with those report-
ed for [Zn(BH4)2(dabco)],2b [Zn(BH4)2(pyz)]n,51 [Zn(BH4)2-
(Ph3P)],52 [Zn(BH4)2(Ph3P)2],52 Zn[BH4]2,47a–c [Zn(BH4)2-
Fig. 1. (2,2ꢀ-Bipyridyl)(tetrahydroborato)zinc Complex.
Scheme 2.

B. Zeynizadeh
Bull. Chem. Soc. Jpn., 76, No. 2 (2003) 319
Table 1. Reduction of Aldehydes to Their Alcohols with [Zn(BH4)2(bpy)]a)
Molar ratio
Reag./Subs.
Entry
Substrate
Product
Time/min Yield/%b)
1
2
3
4
5
6
0.25
0.25
0.25
0.35
0.35
0.35
1
5
95
98
98
99
90
94
5
10
10
8
7
8
0.25
0.25
0.25
0.4
8
5
99
92
95
91
9
11
3
10
11
0.3
5
96
12
13
14
0.3
0.3
0.3
6
2
1
97
93
94
15
0.35
2
94
16
17
0.25
0.5
3
98
93
25
18
19
0.4
4
6
95
85
0.25
a) All reactions were performed in CH3CN at room temperature.
b) Yields referred to isolated products.
XP4],46a Ph3PMe[BH4],45a,c 4-aza-N-benzylbicyclo[2.2.2]oct-
ylammonium tetrahydroborate44 and Bu4N[BH4]40 (Table 7).
A comparison shows that this reagent is more efficient and re-
active than those reported.
Conclusion
In this article we have shown that 2,2ꢀ-bipyridyl is a good
ligand for stabilizing Zn[BH4]2 and does not affect the reduc-

320 Bull. Chem. Soc. Jpn., 76, No. 2 (2003)
Modified Hydroborate Agent: [Zn(BH4)2(bpy)]
Table 2. Reduction of Ketones to Their Alcohols with [Zn(BH4)2(bpy)]a)
Molar ratio
Reag./Subs.
Entry
Substrate
Product
Time/h Yield/%b)
1
2
3
1
1
1
0.75
1.3
99
93
96
1.75
4
5
1
1
1.5
94
93
0.15
6
7
0.5
0.5
1
0.25
0.15
0.25
2.15
1
98
88
89
97
98
8
9
0.5
1
10
11
12
1
1
3.5
1
97
95
13
14
2
2
1.5
93
89
0.25
15
1.5
2
3.5
1.5
97
92
16c)
17
1
4.25
94
18
19
0.5
1
0.25
2
87
50
a) All reactions were performed in CH3CN at room temperature. b) Yields referred to
isolated roducts. c) Under reflux condition.

B. Zeynizadeh
Bull. Chem. Soc. Jpn., 76, No. 2 (2003) 321
Table 3. Conpetitive Reduction of Aldehydes and Ketones to Their Alcohols with [Zn(BH4)2(bpy)]a)
Molar ratio
Reag./Subs 1/Subs 2
Entry
Substrate 1
Substrate 2
Time/min Conv. 1/%b) Conv. 2/%b)
1
2
0.25:1:1
0.5:1:1
1
1
100
100
0
0
3
4
5
0.5:1:1
0.5:1:1
0.25:1:1
76
15
1
100
100
100
15
0
< 5
6
1:1:1
3.5 h
100
< 1
a) All reactions were performed in CH3CN at room temperature.
b) Conversions referred to TLC monitoring and isolated products.
ing ability of the reagent. [Zn(BH4)2(bpy)] is very suitable re-
agent for easy reduction of a variety of carbonyl compounds,
such as aldehydes, ketones, α-diketones and acyloins, to their
corresponding alcohols at room temperature. This reagent also
reduces effectively α,β-unsaturated carbonyl compounds to
their corresponding allyl alcohols. Reduction reactions need
low molar ratios and short times. Comparison of obtained re-
sults with those reported for Zn[BH4]2 and other reagents
shows that the reduction reactions with [Zn(BH4)2(bpy)] was
very efficient and selective. The stability, ease of preparation
from commercially available materials, high regio- and
chemoselectivity of the reagent and also easy work-up of the
reaction mixture can be make this new modified hydroborate
agent as an attractive practical bench-top reducing agent in or-
ganic synthesis.
flask (15 mL), equipped with a magnetic stirrer, a solution of
4-chlorobenzaldehyde (0.14 g, l mmol) in CH3CN (8 mL) was
prepared. The reducing agent (0.062 g, 0.25 mmol) was added
and the mixture was stirred magnetically at room temperature
for 5 min. TLC monitored the progress of the reaction (eluent;
CCl4/Et2O: 5/1). After completion of the reaction, a solution
of 10% FeCl3•6H2O (7 mL) was added and the mixture was
magnetically stirred for 20 min. Then, the mixture was ex-
tracted with CH2Cl2 (3 × 15 mL) and dried over anhydrous
sodium sulfate. Evaporation of the solvent and thick-layer
chromatography of the resulting crude material over silica gel
by eluent of CCl4/Et2O: 5/2 afforded the pure crystals of 4-
chlorobenzyl alcohol (0.14 g, 98% yield, Table 1).
A Typical Procedure for Reduction of Ketones to Their
Alcohols with [Zn(BH4)2(bpy)]. In a round-bottomed flask
(15 mL) equipped with a magnetic stirrer, a solution of ben-
zophenone (0.182 g, l mmol) in CH3CN (8 mL) was prepared.
The reducing agent (0.251 g, 1 mmol) was added and the mix-
ture was magnetically stirred at room temperature for 45 min.
TLC monitored the progress of the reaction (eluent; CCl4/
Et2O: 5/1). After completion of the reaction, a solution of 10%
FeCl3•6H2O (7 mL) was added and the mixture was magneti-
cally stirred for 20 min. Then, the mixture was extracted with
CH2Cl2 (3 × 15 mL) and dried over anhydrous sodium sulfate.
Evaporation of the solvent and thick-layer chromatography of
the resulting crude material over silica gel by eluent of CCl4/
Et2O: 5/2 afforded pure crystals of diphenylmethanol (0.182 g,
99% yield, Table 2).
Experimental
Products were characterized by a comparison with those of
authentic samples (IR, 1H NMR and TLC). All yields referred
to isolated products. TLC was used for the purity determina-
tion of the substrates, products and reactions, monitoring over
silica gel PolyGram SILG/UV 254 plates.
Preparation of (2,2ꢀ-Bipyridyl)(tetrahydroborato)zinc;
[Zn(BH4)2(bpy)]. A solution of Zn[BH4]2 (0.08 M, 250 mL)
was prepared in dry ether from ZnCl2 (2.725 g, 0.02 mol) and
Na[BH4] (1.59 g, 0.042 mol) according to procedure available
in the literature.15 Then, 2,2ꢀ-bipyridyl (3.123 g, 0.02 mol) in
ether (50 mL) was added dropwise to solution of Zn[BH4]2 and
stirred for 30 min. The resulting precipitate was filtered and
dried in a vacuum to afford a fluffy white compound in a quan-
titative yield (4.92 g, 98%). This compound decomposed at
235 °C. Found: C, 47.42; H, 6.51; N, 11.06; Zn, 25.98%.
Calcd for C10B2H16N2Zn: C, 47.80; H, 6.42; N, 11.15; Zn,
26.02%.
A Typical Procedure for Regioselective 1,2-Reduction of
α β
, -Unsaturated Aldehydes to Their Alcohols with [Zn-
(BH4)2(bpy)]. In a round-bottomed flask (15 mL) equipped
with a magnetic stirrer, a solution of cinnamaldehyde (0.132 g,
1 mmol) in CH3CN (8 mL) was prepared. The reducing agent
(0.126 g, 0.5 mmol) was added and the mixture was stirred
magnetically at room temperature for 2 min. TLC monitored
the progress of the reaction (eluent; CCl4/Et2O: 5/2). After
A Typical Procedure for Reduction of Aldehydes to
Their Alcohols with [Zn(BH4)2(bpy)]. In a round-bottomed

322 Bull. Chem. Soc. Jpn., 76, No. 2 (2003)
Modified Hydroborate Agent: [Zn(BH4)2(bpy)]

B. Zeynizadeh
Bull. Chem. Soc. Jpn., 76, No. 2 (2003) 323
Table 5. Reduction of Acyloins and α-Diketones to Their Alcohols with [Zn(BH4)2(bpy)]a)
Entry
1
Substrate
Product
Molar ratio (Reag./Subs.) Yield/%b)
1
94
92
95
97
98
91
95
90
95
96
2
3
1
1
4
1
5
1
6
0.5
0.5
0.5
0.5
0.5
7
8
9
10
11
12
1
94
92
0.5
a) All reactions were performed in CH3CN at room temperature for 3–5 minutes.
b) Yields referred to isolated products.
Table 6. Reduction of α,β-Unsaturated Carbonyl Compounds to Their Alcohols with [Zn(BH4)2(bpy)]a)
Entry
1
Substrate
Product
Molar ratio (Reag./Subs.)
0.5
Time/h
2 min
Yield/%b)
92
2
1
1.3
97
3
4
1
3
98
90
0.5
0.5
5
6
0.5
0.5
0.2
1
89
95
7
1
1.6
96
a) All reactions were performed in CH3CN at room temperature.
b) Yields referred to isolated products.

324 Bull. Chem. Soc. Jpn., 76, No. 2 (2003)
Modified Hydroborate Agent: [Zn(BH4)2(bpy)]
completion of the reaction, a solution of 10% FeCl3•6H2O
(7 mL) was added and the mixture was magnetically stirred for
20 min. Then, the mixture was extracted with CH2Cl2 (3 × 15
mL) and dried over anhydrous sodium sulfate. Evaporation of
the solvent and thick-layer chromatography of the resulting
crude material over silica gel by eluent of CCl4/Et2O: 5/2
afforded pure liquid cinnamyl alcohol (0.l23 g, 92% yield,
Table 6).
α β
A Typical Procedure for Selective 1,2-Reduction of , -
Unsaturated Ketones to Their Alcohols with [Zn(BH4)2-
(bpy)]. In a round-bottomed flask (15 mL) equipped with a
magnetic stirrer, a solution of benzylideneacetone (0.146 g, 1
mmol) in CH3CN (8 mL) was prepared. The reducing agent
(0.251 g, 1 mmol) was added and the mixture was stirred mag-
netically at room temperature for 1.3 h. TLC monitored the
progress of the reaction (eluent; CCl4/Et2O: 5/2). After com-
pletion of the reaction, a solution of 10% FeCl3•6H2O (7 mL)
was added and the mixture was magnetically stirred for 20
min. Then, the mixture was extracted with CH2Cl2 (3 × 15
mL) and dried over anhydrous sodium sulfate. Evaporation of
the solvent and thick-layer chromatography of the resulting
crude material over silica gel by eluent of CCl4/Et2O: 5/2
afforded pure 4-phenyl-3-buten-2-ol as a product (0.144 g,
97% yield, Table 6).
A Typical Procedure for the Competitive Reduction of
Aldehydes and Ketones with [Zn(BH4)2(bpy)]. In a round-
bottomed flask (15 mL) equipped with a magnetic stirrer, a so-
lution of benzaldehyde (0.106 g, l mmol) and acetophenone
(0.12 g, 1 mmol) in CH3CN (8 mL) was prepared. The reduc-
ing agent (0.062 g, 0.25 mmol) was added and the mixture was
magnetically stirred at room temperature for 1 min. TLC mon-
itored the progress of the reaction. Then, the mixture is
quenched with a solution of 10% FeCl3•6H2O (7 mL) and this
is magnetically stirred for 20 min. Then, the mixture was ex-
tracted with CH2Cl2 (3 × 15 mL) and dried over anhydrous
sodium sulfate. Evaporation of the solvent and thick-layer
chromatography of the resulting crude material over silica gel
by eluent of CCl4/Et2O: 5/2 afforded the pure liquid benzyl
alcohol as a sole product of reduction and acetophenone as an
intact material (Table 3).
The author is grateful for the support of this work by the
Research Council of Urmia University (RCUU).
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α
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Bull. Chem. Soc. Jpn., 76, No. 2 (2003) 325
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