New method for the reduction of benzophenones with Raney Ni-Al alloy in water

Article abstract of DOI:10.1080/00397910801929762

Raney Ni-Al alloy in a dilute alkaline aqueous solution has been shown to be a powerful reducing agent, which is highly effective in the reduction of benzophenones to the corresponding hydrocarbon derivatives, in the absence of any organic solvents. Copyright Taylor & Francis Group, LLC.

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New Method for the Reduction of
Benzophenones with Raney Ni‐Al Alloy in
Water
Guo‐Bin Liu a , Hong‐Yun Zhao a , Jia‐Da Zhu a , Hong‐Jie He a , Hong‐Jie
Yang a , Thies Thiemann b , Hideki Tashiro c & Masashi Tashiro d
a Department of Chemistry , Fudan University , Shanghai, China
b Interdisciplinary Graduate School of Engineering Sciences , Kyushu
University , Fukuoka, Japan
c Department of Bioscience Development, School of Bioresources ,
Hiroshima Prefectural University , Hiroshima, Japan
d National Institute of Advanced Industrial Science and Technology Central ,
Tsukuba, Ibaraki, Japan
Published online: 24 Apr 2008.
To cite this article: Guo‐Bin Liu , Hong‐Yun Zhao , Jia‐Da Zhu , Hong‐Jie He , Hong‐Jie Yang , Thies
Thiemann , Hideki Tashiro & Masashi Tashiro (2008) New Method for the Reduction of Benzophenones with
Raney Ni‐Al Alloy in Water, Synthetic Communications: An International Journal for Rapid Communication of
Synthetic Organic Chemistry, 38:10, 1651-1661, DOI: 10.1080/00397910801929762
To link to this article: http://dx.doi.org/10.1080/00397910801929762
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Synthetic Communicationsw, 38: 1651–1661, 2008
Copyright # Taylor & Francis Group, LLC
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910801929762
New Method for the Reduction of
Benzophenones with Raney Ni-Al Alloy
in Water
Guo-Bin Liu,1 Hong-Yun Zhao,1 Jia-Da Zhu,1 Hong-Jie He,1
Hong-Jie Yang,1 Thies Thiemann,2 Hideki Tashiro,3
and Masashi Tashiro4
1Department of Chemistry, Fudan University, Shanghai, China
2Interdisciplinary Graduate School of Engineering Sciences, Kyushu
University, Fukuoka, Japan
3Department of Bioscience Development, School of Bioresources,
Hiroshima Prefectural University, Hiroshima, Japan
4National Institute of Advanced Industrial Science and Technology
Central, Tsukuba, Ibaraki, Japan
Abstract: Raney Ni-Al alloy in a dilute alkaline aqueous solution has been shown to be
a powerful reducing agent, which is highly effective in the reduction of benzophenones
to the corresponding hydrocarbon derivatives, in the absence of any organic solvents.
Keywords: Benzophenones, cyclohexanes, cyclohexanols, Raney Ni-Al alloy, reduction
INTRODUCTION
Reduction of functional groups to saturated systems is not always straightfor-
ward, especially where more stable functional groups such as the carbonyl
function or the arene unit are involved.
The transformation of a carbonyl group to a methylene unit is of great
importanceinorganicsyntheticchemistry.Thusfar,thetransformationhasbeen
accomplished mainly by classical methods such as alkali metal-mediated
Received June 20, 2007
Address correspondence to Guo-Bin Liu, Department of Chemistry, Fudan Univer-
sity, 220 Handan Road, Shanghai 200433, China. Tel.: þ 86-2165642261; E-mail:
liuguobin@fudan.edu.cn
1651

1652
G.-B. Liu et al.
reduction in liquid ammonia,[1] Lewis acid–promoted metal-hydride
reduction,[2] Clemmensen reduction,[3] and Wolf–Kishner–Huang–type
reductions.[4] Over the years, further methods have been developed for the
direct conversion of C55O to CH2, such as an H3PO2-mediated reduction,[5]
a reduction in supercritical i-PrOH at high temperatures (350–400 8C),[6a]
a
hydrogenation of aromatic ketones catalyzed by sol-gel entrapped combined
Pd-[Rh(COD)Cl]2 under high hydrogen pressure (400 psi),[6b] a hydrogen-
ation procedure in a three-phase emulsion,[6c] and a Pd-catalyzed hydrogen-
ation of ketones at high temperatures (200 8C) under a hydrogen
atmosphere (30 bar).[6d]
The complete reduction of aromatic groups is also a very important trans-
formation in the preparation of organic molecules containing cyclohexane
units, useful intermediates in the chemical and pharmaceutical industries.
Many of the known procedures for these molecules rely on a cyclization as
the crucial step of the sequence.[7] Here, it must be noted that K. Ishimoto
et al.[8] have reported that aromatic ketones and aldehydes can be reduced
to the corresponding cyclohexane derivatives with Raney Ni-Al alloy in
water, although a large amount of alloy was employed in their studies.
Many of the documented methods for both of the transformations
mentioned previously, namely for the reduction of a carbonyl to a
methylene group and for the complete hydrogenation of an arene unit to a
cyclohexane, exhibit some disadvantages based on practical and economical
considerations. Often, harsh reaction conditions, high reaction temperatures,
and/or high pressure are employed, toxic reagents are used, and a large
amount of reducing agents is required.
In the continuation of our work on the reduction of aromatic compounds
with Raney Ni-Al alloy in aqueous media,[9] we found that benzophenones
were easily reduced by Raney Ni-Al alloy to give the corresponding diphenyl-
methanes and/or aromatic-ring hydrogenated derivatives, in dilute basic
solution under mild reaction conditions, in the absence of any organic
solvents. These results are summarized here.
RESULTS AND DISCUSSION
At first, the reaction of benzophenone itself with Raney Ni-Al alloy under
various conditions was studied. The reductions were carried out by dropwise
addition of an aqueous alkaline solution to a suspension of benzophenone
and Raney Ni-Al alloy in water. No organic solvent was used. The resulting
mixture was heated at 90 8C. The reduction was monitored by gas chromato-
graphy (GC). When a 1 wt% aq. KOH solution was added to a mixture of ben-
zophenone (1a) and Raney Ni-Al alloy in water, 1a was reduced to give a
mixture of dicyclohexylmethane (2), benzylcyclohexane (3a), and diphenyl-
methane (4a) (Table 1, runs 1–9). A trend that the ratios of 2 and 3a rise
with an increase of the amount of Raney Ni-Al alloy could be noted

Table 1. Reduction of benzophenones (1)a
Ratio (%)c,d
3
Run
Substrate
Ni-Al (g)
Alkaline solutionb (ml)
Time (h) Product
2
4
1
2
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
1d
1e
1f
2
4
6
8
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
1%KOH (40)/H2O (40)
1%KOH (40)/H2O (40)
1%KOH (40)/H2O (40)
1%KOH (40)/H2O (40)
1%NAOH (40)/H2O (40)
1%CsOH (40)/H2O (40)
1%CA(OH)2 (40)/H2O (40)
1%LiOH (40)/H2O (40)
1%BA(OH)2 (40)/H2O (40)
1%KOH (40)/H2O (40)
1%KOH (40)/H2O (40)
1%KOH (40)/H2O (40)
1%KOH (40)/H2O (40)
1%KOH (40)/H2O (40)
1%KOH (40)/H2O (40)
1%KOH (40)/H2O (40)
1%KOH (40)/H2O (40)
1%KOH (40)/H2O (40)
1%KOH (40)/H2O (40)
1%KOH (40)/H2O (40)
1%KOH (40)/H2O (40)
6
5
5
5
5
5
6
7
7
5
5
6
6
7
7
9
9
9
9
9
9
2, 3a, 4a 27.4
29.5
43.1
2, 3a, 4a 62.4 (55.7)[10] 26.3 (21.6)[10]
11.3 (8.8)
10.1
16.8
3
2, 3a, 4a 63.0
2, 3a, 4a 62.6
2, 3a, 4a 60.0
2, 3a, 4a 61.8
2, 3a, 4a 39.9
2, 3a, 4a 25.5
2, 3a, 4a 19.8
2, 3a, 4a 51.8 (45.7)
2, 3a, 4a 50.0
2, 3a, 4a 48.5
2, 3a, 4a 50.4
23.9
20.6
4
5
25.1
25.9
14.9
12.3
6
7
21.3
20.5
38.8
54.0
8
9
18.8
25.5 (20.2)
61.4
10
11
12
13
14
15
16
17
18
19
20
21
22.7 (17.8)
25.9
29.3
24.1
22.2
21.2
18.0 (11.4)[11]
28.4
82.0 (73.6)[12]
100 (86.7)[12]
100 (87.6)[14]
100 (81.7)[14]
100 (84.5)[15]
100 (82.2)[16]
100 (83.5)[17]
100 (85.5)[18]
3f, 4f
4g
0
0
0
0
0
0
0
0
1g
1h
1i
0
0
0
0
0
0
0
4h
4i
1j
4j
1k
1l
4k
4l
1m
4m
(continued)

Table 1. Continued
Ratio (%)c,d
3
Run
Substrate
Ni-Al (g)
Alkaline solutionb (ml)
Time (h) Product
2
4
22
23
24
1n
1o
1p
4
4
4
1%KOH (40)/H2O (40)
1%KOH (40)/H2O (40)
1%KOH (40)/H2O (40)
8
7
7
3n, 4n
3o, 4o
3p, 4p
0
0
0
17.1 (11.5)[11]
65.4 (55.6)[20]e
63.4 (52.1)[20]e
82.0 (75.8)[19]
34.6 (28.8)[21]
26.6 (18.8)[22]
a1 (10 mmol).
bAdded dropwise in 1.0 h.
cGC ratio.
dIsolated yields in parenthesis.
1
esyn/ant ¼ 1/3 (determined by H NMR).

Reaction of Benzophenones
1655
Scheme 1.
(Table 1, run 1 vs runs 2–4). Similarly, the use of a 1 wt% aq. solution of other
alkali and alkali earth hydroxides such as CsOH, NaOH, Ca(OH)2, LiOH, or
Ba(OH)2 gave mixtures of 2, 3a, and 4a (Table 1, runs 5–9) (Scheme 1).
Next, the reaction of substituted benzophenones was examined. The
reduction of halogenated benzophenones (1b–e) also afforded a mixture of
2, 3a, and 4a (Table 1, runs 10–13). Clearly, concommittant dehalogenation
occured, as was expected. Otherwise, the outcome of the reduction depends
very much on the substitution pattern of the benzophenone. Thus, hydroxy
and methoxy substituted benzophenones regularly produced fair amounts of
benzylcyclohexanes. It must be noted, however, that hydroxybenzophenones
yielded benzylcyclohexanols, whereas in 4-methyl and 4-methoxy substituted
benzophenone only the nonsubstituted phenyl moiety was reduced (Table 1,
runs 14 and 22–24). In the 4,40-dimethyl- and di-t-butyl-dimethylbenzophe-
nones (1g–i), only the carbonyl group was reduced, and the corresponding

1656
G.-B. Liu et al.
Scheme 2.
diphenylmethanes (4g–i) were formed (Table 1, runs 15–17). In short,
reduction of 4-methyl-, 4-methoxy-, 2-hydroxy, and 4-hydroxybenzophe-
nones (1f, n–p) with Raney Ni-Al alloy produced a mixture of benzylcyclo-
hexanes (3f, n–p) and diphenylmethanes (4f, n–p), whereas many of the
other substituted benzophenones gave diphenylmethanes. In the case of
nitro- and cyanobenzophenones (1j–m), the diphenylmethanes also were
formed, albeit with the concommittant conversion of NO2 (or CN) to NH2
(or CH2NH2) under the reaction conditions (Table 1, runs 18–21). No
aromatic-ring saturated product was detected.
To ascertain that diphenylmethane (4a) and diphenylmethanol (5a) can
function as common intermediates for the products under the reaction con-
ditions employed, 4a and 5 were subjected to the reaction with Raney Ni-Al
alloy under the same reaction conditions (Scheme 2 and Table 2). Treatment
of 5a with Raney Ni-Al alloy by addition of a 1.0 wt% aq. NaOH solution
did indeed give a mixture of 2, 3a, and 4a in a ratio of 59.2: 22.8: 18.0
(Table 2, run 1). In the case of a dropwise addition of a 1.0 wt% aq. KOH
solution, again a mixture of 2, 3a, and 4a was formed (Table 2, run 2).
Treatment of 4a with Raney Ni-Al alloy (4.0 g), upon addition of a 1 wt%
aq. NaOH or KOH solution, gave a mixture of 2 and 3 along with recovered
starting material 4a (Table 3). A similar mixture of 2, 3a, and 4a was formed
when the reaction was performed in an aq. 1 wt% KOH solution (run 2). These
results indicate that 4a and 5a can be viewed as intermediates on a pathway
leading to the products found in the reduction of 1a.
The reduction of other compounds containing benzophenone-units (1s–z)
were investigated (Table 4) as well. 1,4-Dibenzoylbenzene (1s), 1,3-diben-
zoylbenzene (1t), dibenzosuberone (1u), and dibenzosuberenone (1v) were
Table 2. Reduction of diphenylmethanol (5a)a
Ratio (%)c,d
Run
Allaline solutionb
Time (h)
2
3a
4a
1
2
1%NaOH (40)/H2O (40)
1%KOH (40)/H2O (40)
5
5
59.2 (46.2) 22.8 18.0 (12.7)
60.3 (51.6) 24.5 (19.2) 15.2 (10.4)
a5a (10 mmol), Raney Ni-Al alloy (4.0 g).
bAdded dropwised in 1.0 h.
cGC ratio.
dIsolated yields in parentheses.

Reaction of Benzophenones
1657
Table 3. Reduction of diphenylmethane (4a)a
Ratio (%)c,d
Run
Allaline solutionb
Time (h)
2
3a
1e
2f
1%NaOH (40)/H2O (40)
1%KOH (40)/H2O (40)
5
5
55.5 (40.2)
57.3 (41.4)
44.5 (38.4)
42.7 (36.2)
a4a (10 mmol), Raney Ni-Al alloy (4.0 g).
bAdded dropwised in 1.0 h.
cGC ratio.
dIsolated yields in parentheses.
e15% of 4a was recovered.
f13% of 4a was recovered.
treated with Raney Ni-Al alloy in the way described previously to afford
the corresponding hydrocarbon derivatives 4s–y in good yields (Table 4,
runs 1–4). Moreover, anthraquinone (1w), which holds two benzophenone
units, was transformed to dihydroanthracene (4w) in 91% yield under
similar reaction conditions (Table 4, run 5) (Scheme 3).
In conclusion, we have developed a new and efficient method for the
reduction benzophenones using commercially available Raney Ni-Al alloy
in a dilute alkaline solution. Benzophenones were easily reduced to afford
dicyclohexylmethanes (2), benzylcyclohexanes (3), and/or diphenylmethanes
Table 4. Reduction of ketones (1s–w)a
Run
1
Substrate
Time
7
Product
Yield (%)b,c
95[23]
2
7
96[23]
3
4
5
8
8
8
90
92
91
a1 (10 mmol).
b1% KOH (40 ml) was added dropwise to a mixture of 1, Raney Ni-Al alloy (4.0 g),
and water (40 ml) at 90 8C in 1.0 h.
cIsolated yields.

1658
G.-B. Liu et al.
Scheme 3.
(4), respectively, under mild reaction conditions. Anthraquinone, benzoyl-
naphthalenes, benzoylanthracene, dibenzosuberenone, and dibenzosuberone,
were also successfully converted to the corresponding hydrocarbons selec-
tively in good and/or excellent yields. The advantages of the present
approach lie in terms of the ease of manipulation, quickness of the reaction,
and mildness of reaction conditions. The conditions used for the reduction
of the carbonyl group and hydrogenation of the aromatic ring, as compared
to existing methods, especially in regard to temperature and reaction
medium, should make this protocol a valuable alternative. Commercially,
Raney Ni-Al alloy is readily available and is, of course, cheaper than the
Raney Ni catalyst made from it. Further work on the reduction of carbonyl
compounds such as ketones and aldehydes and related substrates is
currently under way in our laboratory, and the results will be published in
due course.
EXPERIMENTAL
Typical Procedure
A 1 wt% aq. KOH solution (40 ml) was added to a mixture of 1a (10.0 mmol,
1.82 g), Raney Ni-Al alloy (4.0 g), and water (40 ml) at 90 8C within 1.0 h.
After the mixture was heated at 90 8C for an additional 5 h, it was cooled to
rt and filtered over Celite.w The residue was washed with ethyl acetate. There-
after, the filtrate was neutralized with hydrochloric acid and extracted with
ethyl acetate (3 ꢀ 15 ml), and the organic layer was dried over anhydrous
magnesium sulfate. After removal of the solvent, a mixture of dicyclohexyl-
methane (2), benzylcyclohexane (3a), and diphenylmethane (4a), in the
ratio of 62.4:26.3:11.3 was obtained (Table 1, run 2).
Where mixtures of products were formed, 2, 3, and 4 were separated by
column chromatography on silica gel.
Data
Benzylcyclohexylmethane (3a).[13] 1H NMR: d ¼ 0.71–0.88 (2H, m), 1.12–
1.21 (3H, m), 1.33–1.46 (1H, m), 1.66–1.80 (5H, m), 2.39 (2H, d,
J ¼ 6.2 Hz), 6.92–7.05 (2H, m), 7.20–7.26 (3H, m); GC/MS (m/z): 174
(Mþ), 92, 83, 77, 67, 55, and 41.

Reaction of Benzophenones
1659
Benzyl(4-methylcyclohexyl)methane (3g).[11] 1H NMR: d ¼ 0.68–0.88 (2H,
m), 1.10–1.22 (3H, m), 1.30–1.42 (1H, m), 1.62–1.78 (5H, m), 2.32 (3H, s),
2.45 (2H, d, J ¼ 6.2 Hz), 6.68 (2H, d, J ¼ 8.2 Hz), 7.34 (2H, d, J ¼ 8.2 Hz);
GC/MS (m/z): 188 (Mþ), 141, 129, 115, 97, 91, 81, and 55.
4-Methoxybenzylcyclohexylmethane (3n).[11] 1H NMR: d ¼ 0.67–0.97
(2H, m), 1.12–1.26 (3H, m), 1.32–1.46 (1H, m), 1.65–1.84 (5H, m), 2.45
(2H, d, J ¼ 6.2 Hz), 3.68 (3H, s), 6.88 (2H, d, J ¼ 8.2 Hz), 7.44 (2H, d,
J ¼ 8.2 Hz); GC/MS (m/z): 204 (Mþ), 172, 157, 144, 130, 117, 104, 81,
65, and 55.
2-Benzylcyclohexanol (3o).[20] Syn- and anti-isomer were separated by a
1
silica-gel column. syn-isomer: H NMR: d ¼ 1.12–1.20 (1H, m), 1.28–1.32
(1H, m), 1.34–1.42 (3H, m), 1.52–1.62 (2H, m), 1.60–1.70 (2H, m), 1.70–
1.75 (1H, m), 2.50 (1H, dd, J ¼ 7.4, 13.2 Hz), 2.70 (1H, dd, J ¼ 7.2,
13.0 Hz), 3.75–3.78 (1H, bs), 7.12–7.16 (3H, m), 7.24–7.28 (2H, m); anti-
1
isomer: H NMR: d ¼ 0.84–0.90 (1H, m), 1.02–1.08 (1H, m), 1.20–1.30
(2H, m), 1.45–1.60 (4H, m), 1.66–1.70 (1H, m), 1.92–1.95 (1H, m), 2.32
(1H, dd, J ¼ 9.2, 13.0 Hz), 3.16 (1H, dd, J ¼ 4.2, 13.1 Hz), 3.26 (1H, ddd,
J ¼ 4.2, 9.4, 9.4 Hz), 7.10–7.16 (3H, m), 7.22–7.26 (2H, m).
4-Benzylcyclohexanol (3p).[20] Syn- and anti-isomer were separated by a
silica-gel column. Syn-isomer: 1H NMR: d ¼ 1.32–1.60 (8H, m), 1.68–
1.74 (2H, m), 2.52 (2H, d, J ¼ 7.4 Hz), 3.92–3.98 (1H, bs), 7.10–7.18 (3H,
m), 7.25–7.28 (2H, m). anti-isomer: 1H NMR: d ¼ 0.94–1.06 (2H, m),
1.12–1.22 (2H, m), 1.40–1.60 (2H, m), 1.70–1.74 (2H, m), 1.92–1.98 (2H,
m), 2.45 (2H, d, J ¼ 7.0 Hz), 3.56 (1H, dddd, J ¼ 4.2, 4.2, 10.6, 10.6 Hz),
7.12–7.18 (3H, m), 7.24–7.28 (2H, m).
All of the compounds 2, 3, and 4 were compared with authentic samples,
1
and their structures were also assigned on the basis of IR, H NMR, and
GC-MS spectroscopic data.
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