Rapid reduction of carbonyls with nickel boride at ambient utemperature

Article abstract of DOI:10.1081/SCC-100106209

Carbonyl compounds have been reported to undergo rapid reduction with nickel boride generated in situ from anhyd. nickel chloride and sodium borohydride in THF at ambient temperature to the corresponding alcohols in high yields.

Full text of DOI:10.1081/SCC-100106209

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RAPID REDUCTION OF CARBONYLS WITH NICKEL
BORIDE AT AMBIENT TEMPERATURE
Jitender M. Khurana a & Sushma Chauhan a
a Department of Chemistry , University of Delhi , Delhi, 110 007, India
Published online: 22 Aug 2006.
To cite this article: Jitender M. Khurana & Sushma Chauhan (2001) RAPID REDUCTION OF CARBONYLS WITH NICKEL BORIDE
AT AMBIENT TEMPERATURE , Synthetic Communications: An International Journal for Rapid Communication of Synthetic
Organic Chemistry, 31:22, 3485-3489, DOI: 10.1081/SCC-100106209
To link to this article: http://dx.doi.org/10.1081/SCC-100106209
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SYNTHETIC COMMUNICATIONS, 31(22), 3485–3489 (2001)
RAPID REDUCTION OF CARBONYLS
WITH NICKEL BORIDE AT AMBIENT
TEMPERATURE1
Jitender M. Khurana* and Sushma Chauhan
Department of Chemistry, University of Delhi,
Delhi – 110 007, India
ABSTRACT
Carbonyl compounds have been reported to undergo rapid
reduction with nickel boride generated in situ from anhyd.
nickel chloride and sodium borohydride in THF at ambient
temperature to the corresponding alcohols in high yields.
Reduction of carbonyl groups to the corresponding alcohols is one of
the important functional group interconversions in organic synthesis.
Various reagents have been employed to achieve this transformation
namely sodium in ethanol,2 borane,3 isopropanol-aluminium triisopropo-
xide,4 hydride transfer reagents e.g., lithium aluminium hydride and sodium
borohydride,5 hydrogenation in presence of a suitable catalyst6 etc. Nickel
boride has been reported as a useful catalyst in a number of hydrogenation
reactions and offers certain advantages over conventional catalysts.7 It has
also been reported as a catalyst in the hydrogenation of aldehydes to alco-
hols.8 Recently it has been reported that nickel boride acts as a reducing
*Corresponding author.
3485
Copyright & 2001 by Marcel Dekker, Inc.
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KHURANA AND CHAUHAN
agent by itself in the absence of hydrogen too.9 Borbaruah et al.10 have
reported that aldehydes can be reduced with nickel boride only in presence
of chlorotrimethylsilane. Since nickel boride behaves differently under
different reaction conditions, we believed that it may be possible to reduce
the carbonyl compounds rapidly with nickel boride under appropriate
conditions.
We report herein a simple, rapid and convenient procedure for the
reduction of a variety of aldehydes and ketones with nickel boride in tetra-
hydrofuran at ambient temperature to the corresponding alcohols (eq. 1).
RCHOHR0
Anhyd:NiCl2, NaBH4
RCOR0 ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ!
Dry THF, r:t:, ꢀ5À15 min
High yields
ð1Þ
R ¼ Aryl, Alkyl, R0 ¼ H, Alkyl; R, R0 ¼ Cyclic
The nickel boride was generated in situ from anhyd. nickel chloride and
sodium borohydride. The reductions were complete in 5–15 min using
1 : 3 : 9 molar ratio of substrate to nickel chloride to sodium borohydride.
Longer reaction times were required for complete reduction if lower molar
ratios of substrate to NiCl2 to NaBH4 were used, e.g., reduction of benzo-
phenone was complete after 15, 75 and 150 min using 1 : 3 : 9, 1 : 2 : 6 and
1 : 1 : 3 molar ratios respectively (runs 1–3). The alcohols were obtained in
nearly quantitative yields by a simple workup. Reductions were also
attempted in DMF, CH3CN and dioxane but THF has been found to be
the solvent of choice. The observed reductions are due to the involvement of
nickel boride formed in situ since benzophenone, veratraldehyde, aceto-
phenone, fluorenone and 4-heptanone did not undergo complete reduction
with sodium borohydride only in THF even after 10, 2, 8, 8 and 48 h respect-
ively (runs 4, 6, 8, 10, 27).
The reductions are chemoselective as bromo, chloro, carboethoxy,
nitro, isolated double bond and methoxy groups were unaffected under
these conditions. The detailed results are presented in Table 1. Nickel
boride loses its activity to reduce carbonyl compounds with time. Thus,
benzophenone showed complete reduction when added to preformed
nickel boride after 15 min but the reduction was incomplete even after 6 h
when benzophenone was added to preformed nickel boride after 72 h. The
reduction of N, N-dimethylaminobenzaldehyde and p-aminoacetophenone
was very sluggish and showed a mixture of products probably because basic
nitrogen has been reported to poison the reagent.11 Reduction of 9-anthrone
and xanthone also was not complete even after 240 min and yielded anthra-
cene and xanthene, respectively as the predominant products in these reac-
tions. Anthracene and xanthene are believed to be obtained from the
reaction of the initially formed alcohols. This has been confirmed by an

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CARBONYL COMPOUNDS
3487
Table 1. Reduction of Carbonyl Compounds with Nickel Boride in Dry THFa at
Ambient Temperature
Substrate
RCOR0
Substrate : NiCl2 :
NaBH4
Time
(min)
% Yield
RCHOHR0
Run no.
1
2
Benzophenone
Benzophenone
1 : 3 : 9
1 : 2 : 6
1 : 1 : 3
1 : 0 : 9
1 : 3 : 9
1 : 0 : 9
1 : 3 : 9
1 : 0 : 9
1 : 3 : 9
1 : 0 : 9
1 : 3 : 9
1 : 3 : 9
1 : 3 : 9
1 : 3 : 9
1 : 3 : 9
1 : 3 : 9
1 : 3 : 9
1 : 3 : 9
1 : 3 : 9
1 : 3 : 9
1 : 3 : 9
1 : 3 : 9
1 : 3 : 9
1 : 3 : 9
1 : 3 : 9
1 : 3 : 9
1 : 0 : 9
1 : 3 : 9
1 : 3 : 9
1 : 3 : 9
1 : 3 : 9
1 : 3 : 9
15
75
150
600c
5
88b
87b
82b
–
3
Benzophenone
Benzophenone
4
5
Veratraldehyde
Veratraldehyde
Acetophenone
87
–
6
120c
5
7
86
–
81d
–
8
Acetophenone
Fluorenone
Fluorenone
480c
5
480c
5
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
p-Chlorobenzaldehyde
2,4-Dichlorobenzaldehyde
Benzaldehyde
84
81
88
87
85
85
86
65e
83
89
85
85
69f
32g
90
92
–
5
5
m-Bromobenzaldehyde
p-Anisaldehyde
p-Tolualdehyde
Piperonal
5
5
5
5
p-Nitrobenzaldehyde
1-Naphthaldehyde
Butyrophenone
4,40-Dichlorobenzophenone
2-Amino, 5-chlorobenzophenone
Anthrone
5
15
15
60
60
240
240
5
Xanthone
Heptaldehyde
4-Heptanone
4-Heptanone
5
h
–
Cyclohexanone
Cyclopentanone
2-octanone
15
5
92
79
86
92
80
5
n-Hexadecanal
Á5-Cholesten-3-one
10
10
a25 mL of THF/g of substrate was used.
bBenzopinacol (3–4%) was also isolated by column chromatography.
cReduction was incomplete and consisted of a mixture of alcohol and the substrate.
d
ꢀ 2% of fluorene and ꢀ 2% of starting material were also recovered by column
chromatography.
eReaction mixture didn’t turn blackon addition of NaBH to mixture of reactant
4
and NiCl2 in THF unlike all other reactions and some other products were also
formed believed to be the reduction products of the nitro group.
fReaction was incomplete even after 4 h and the product isolated was anthracene.
gReaction was incomplete even after 4 h and product isolated was xanthene; rest of
the mixture consisted of xanthone and xanthydrol.
hReaction was incomplete even after 2 days.

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KHURANA AND CHAUHAN
independent reaction of xanthydrol with nickel boride which yielded
xanthene under identical reaction conditions.
EXPERIMENTAL
All the commercial carbonyl compounds were used after distillation/
recrystallization. THF was dried by reported procedure.12 Sodium borohy-
dride (E. Merck) and anhyd. Nickel chloride (Qualigens) were used in all the
reactions. Products were analyzed by m.p., mixed m.p. (wherever applic-
able), IR and NMR spectra.
In a typical experiment, carbonyl compound (1 g, n mmol), dry THF
(25 mL) and anhyd. NiCl2 (3n mmol) were placed in a 100 mL round-
bottomed flaskfitted with a reflux condenser, a CaCl guard tube and
2
mounted over a magnetic stirrer. NaBH4 (9n mmol) was added and forma-
tion of blackprecipitate of nickel boride was observed immediately. The
reaction mixture was stirred at room temperature and its progress was
monitored by TLC for disappearance of starting material. The reaction
mixture was diluted with dichloromethane (ꢀ 75 mL) and filtered through
a celite pad. The filtrate was diluted with water (ꢀ 100 mL) and extracted
with dichloromethane (3 ꢁ 10 mL). The combined dichloromethane layer
was dried (anhyd. MgSO4) and concentrated on a Buchi rotavapor to give
the alcohol which was recrystallized or purified by column chromatography
over silica gel (100–200 mesh) using petroleum ether and/or benzene as
eluent. In case of aliphatic alcohols, the combined filtrate was concentrated
on a rotary evaporator and the resulting residue was dissolved in anhyd.
diethyl ether. The ether layer was filtered and the filtrate dried over anhyd.
MgSO4. The filtrate was concentrated on a rotary evaporator and yielded
the alcohol.
ACKNOWLEDGMENT
Financial assistance by CSIR, New Delhi for the above project No. 01
(1567)/99/EMR-II, and JRF to SC is gratefully acknowledged.
REFERENCES
1. Part of the workwas presented at International Symposium on Trends in
Medicinal Chemistry and Biocatalysis, held at New Delhi, India, Jan.
2000, Hand book, pp. 62.

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CARBONYL COMPOUNDS
3489
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Received in the UK August 21, 2000

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