Selective reductions of the carbonyl compounds and aryl halides with Ni-Al alloy in aqueous alkali medium

Article abstract of DOI:10.2174/157017811799304115

The carbonyl compounds and the aryl halides are efficiently reduced by Ni-Al alloy powder, in aqueous alkali medium, with or without organic solvents. The relative reactivities in a series of structurally selected reducible compounds showed a sequence of reactivity in agreement with the mechanism involving a direct electron transfer from aluminum to the organic phase. In some cases an important change in reactivity with respect to conversion of the reducing reagent was notified. Most probably this complicated variability is the result of the changes in composition and morphology of the reacting Ni-Al alloy.

Full text of DOI:10.2174/157017811799304115

690
Letters in Organic Chemistry, 2011, 8, 690-695
Selective Reductions of the Carbonyl Compounds and Aryl Halides with
Ni-Al Alloy in Aqueous Alkali Medium
Mirela Suceveanu*,a,b, Matei Raicopolb, Raluca Enacheb, Adriana Fînarua and Sorin I. Roꢀcab
a”Vasile Alecsandri” University of Bacau, Faculty of Engineering, Calea Marasesti 157, 600115 Bacau, Romania
b”Politehnica” University of Bucharest, Department of Organic Chemistry, Polizu 1-7, 011061, Bucharest, Romania
Received April 18, 2011: Revised September 06, 2011: Accepted September 15, 2011
Abstract: The carbonyl compounds and the aryl halides are efficiently reduced by Ni-Al alloy powder, in aqueous alkali
medium, with or without organic solvents. The relative reactivities in a series of structurally selected reducible
compounds showed a sequence of reactivity in agreement with the mechanism involving a direct electron transfer from
aluminum to the organic phase. In some cases an important change in reactivity with respect to conversion of the reducing
reagent was notified. Most probably this complicated variability is the result of the changes in composition and
morphology of the reacting Ni-Al alloy.
Keywords: Aryl halides, ketones, Ni-Al alloy, reduction, relative reactivity.
INTRODUCTION
Despite numerous preparative applications of the
reduction with Ni-Al alloy of various classes of organic
substrates, mechanistic aspects of above reaction remain
obscure. Little information is available even for important
aspects such as selectivities and relative reactivities within
classes of reactive substrates, thus limiting the scope of this
important preparation tool.
Although known for a long time, Raney type catalysts
continue to pay intensive attention in modern organic
synthesis because of the increasing uses. The main and
relevant research directions in the recent literature on Raney
type catalysts follow: (a) improvement of the activity of
these catalysts by addition of promoters such as Mo, Cr, Fe,
Cu or Co from 1.5 up to 6.5% [1]; (b) possibility to use
Raney type catalysts in enantioselective syntheses by the
adsorption of modifiers – chiral molecules, e.g. (R,R)-tartaric
acid and co-modifiers, such as NaBr, sodium pivalate [2]; (c)
investigation of structure and morphology of Raney type
catalysts by techniques XRD (X-Ray Diffraction), SEM
(Scanning Electron Microscopy), TEM (Transmission
Electron Microscopy) et alii [3]; (d) use of nickel
nanoparticles in hydrogen transfer reactions [4]; (e) nickel-
promoted reduction of various classes of organic
compounds, e.g. carbonyl compounds [5] and organic
halides [6].
The present paper (representing first report of a series)
aims to collect quantitative data concerning reactivity in Ni-
Al reduction of two classes of substrates (carbonyl
compounds and aryl halides) in order to identify factors
influencing the course of reaction.
RESULTS AND DISCUSSIONS
Preliminary experiments consisting in the reduction of
representative ketones with 50 wt%-50 wt% Ni-Al alloy in
20% aqueous sodium hydroxide, without organic solvent,
were performed. Selected substrates included cyclic aliphatic
ketone 1, acyclic aliphatic ketone exhibiting major steric
hindrance 2, aryl ketone with benzene- 3 or naphthalene ring
4.
Unlike conventional catalytic hydrogenation using Raney
nickel and hydrogen, the literature revealed a simple and
convenient reduction of various organic substrates using
Raney type alloys (Ni-Al, Co-Al, Fe-Al and Cu-Al) in
alkaline aqueous solutions [7].
Reaction products identified by GC-MS method
consisted in corresponding alcohols as unique or major
reduction product. In the case of reduction of the
acetophenone 3 apart from major reductive compound 1-
phenylethanol, 3a, another reduction product, ethylbenzene,
3b, and a pinacol type coupling product, 3c, were also
identified (Scheme 1). It is noteworthy that in the case of 2-
acetylnaphthalene 4, the reduction product contained a small
amount of a compound with a hydrogenated benzene ring
4b, a typical example of hydrogenation being reported in the
literature [11].
In these reactions is considered that hydrogen is
generated in situ [8] and the reducing system (e.g. Ni-Al/
NaOH/H2O) was tested on a wide range of substrates (see,
for example, a comprehensive review [9] and the citated
literature herein).
The possibility to carry out reduction in aqueous
medium, usually without any organic solvent, is a largely
promoted method as “green chemistry” tool [10].
Preliminary experiments were also performed in Ni-Al
reduction of aryl halides: according to GC-MS analyses
reaction of all mono halogenobenzenes (5-8) in 20% wt.
aqueous NaOH and 1,4-dioxane showed a total conversion in
*Address correspondence to this author at the Faculty of Engineering,
”Vasile Alecsandri” University of Bacau, Calea Marasesti 157, 600115
Bacau, Romania; Tel: 0040234542411; Fax: 0040234545753;
E-mail: mirela_scv@yahoo.com
1875-6255/11 $58.00+.00
© 2011 Bentham Science Publishers

Selective Reductions of the Carbonyl Compounds
Letters in Organic Chemistry, 2011, Vol. 8, No. 10
691
O
H
OH
Ni-Al alloy
20% aq. NaOH
2-4 h, 20 °C
1
1a
60-80%
CH3
C
CH3
C
Ni-Al alloy
H3C
C
O
CH3
H3C CH
CH3
20% aq. NaOH
4 h, 20 °C
CH3
OH CH3
2a
2
85%
H3C
OH
CH3
OH
OH CH CH3
CH2CH3
COCH3
Ni-Al alloy
+
+
20% aq. NaOH
4 h, 50 °C
3
3a
3b
3c
45%
35%
20%
OH
CH
O
OH
CH
Ni-Al alloy
CH3
CH3
CH3
+
20% aq. NaOH
4 h, 20 °C
4
4a
25%
4b
5%
Scheme 1.
dehalogenation process (Scheme 2). The starting materials 5-
8 were converted exclusively into benzene detected by GC-
MS method.
From this data relative reactivity of acetophenone 3 with
respect to cyclohexanone 1 results from the increase of
relative molar concentration of 2.53 : 1 for initial mixture to
3.43 : 1 (30 minutes reaction time) and 4.74 : 1 (120 minutes
reaction time). This is standing for reactivity sequence 1 > 3.
Following the same rationalization for substrates 9 with
respect to 3 and 10 to respect to 9, the complete sequence of
reactivities for series A is 1 > 3 > 9 ꢀ 10.
X
H
Ni-Al alloy
20% aq. NaOH, dioxane
2 h, 20 °C
Above sequence of reactivity is explicable in terms of
stereoelectronic effects of structure on reactivity of carbonyl
compounds essentially consisting of negative charge density
located on oxygen atom of the carbonyl group. According to
such influence aryl ketones, benefiting of aryl-carbonyl
electron delocalization, have a higher negative charge
density on oxygen atom and consequently exhibit a lower
reactivity as compared to aliphatic ketones.
5-8
~100%
X=F, Cl, Br, I
Scheme 2.
Next objective was to evaluate relative reactivities of the
investigated ketones. In this respect two mixtures of three-
four structurally related ketones were subjected to reduction
under previously established reaction conditions by
competing for a limited amount of reducing reagent and then
analyzed by GC-MS to determine the total conversion of
each substrate.
However such rationalization for Ni-Al reduction of
ketones is valid only in case of a heterolytic mechanism
involving an electron transfer directly from metal to carbonyl
group, thus disfavoring the concept of catalytic reaction with
in situ preformed hydrogen molecules (Scheme 4).
The mixtures were composed of cyclohexanone 1,
acetophenone 3, 2,4-dimethylacetophenone 9 and 2,4,6-
trimethylacetophenone 10 (Series A) and acetophenone 3, 1-
acetylnaphthalene 11 and 2-acetylnaphthalene 4 (Series B)
(Scheme 3).
Electron transfer mechanism explains the formation of
pinacol type by-product 3c and is supported by effect on
reactivity of orto, para-methyl disubstitution in compound 9
(versus 3). The reactivity of compound 10 (similar to 9)
could be the result of compensation of antagonistical factors
(inductive effect, steric inhibition of resonance and increased
steric hindrance of reduction process itself).
Initial and reacted product compositions (reaction time of
30 and 120 minutes, respectively) for mixture of series A are
showed in Table 1.

692 Letters in Organic Chemistry, 2011, Vol. 8, No. 10
Suceveanu et al.
O
CH3
O
CH3
O
CH3
CH3
O
CH3
H3C
Series A
CH3
CH3
10
1
3
9
O
CH3
O
CH3
O
CH3
Series B
3
11
4
F
Cl
Br
I
Series C
5
6
7
8
Scheme 3.
Table 1. Relative Concentrations of Compounds 1, 3, 9, 10.
Relative concentrations (CG-SM)
Ratio
Reaction time, minutes
1
3
9
10
9/3
10/3
10/9
0
1
1
1
2.53
3.43
4.74
3.44
6.42
4.10
7.17
1.35
1.87
2.14
1.62
2.09
3.01
1.19
1.11
1.41
30
120
10.13
14.26
shown in Table 2, for the initial period of reaction (30
minutes) 2-acetylnaphthalene 4 proves to be more reactive
than 1-acetylnaphthalene 11 (the ratio 11/4 changes from
1.20 to 1.41) but, on the contrary, for a longer reaction time
(from 30 to 120 minutes) the reactivity is reversed, the ratio
11/4 being changed from 1.41 to 1.13.
C
O
C
O
e
C
O
C
O
This result is explicable in terms of a complex variability
of the reaction conditions as the conversion is in progress.
Since the solid phase consisting of Ni-Al alloy was set as
limiting reagent, its drastic change in composition and
morphology during reaction is conceivable.
M
C
C
O
C
O
O
M
Above rationalization is furthermore supported by the
experiment involving reduction of the mixture representing
series C. (Scheme 5, Table 3).
H2O
OH
H2O
a(Xt) a(Dt+1)
a(It) a(Dt+1)
k X
k I
C
H
+
2M
+
2HO
C
C
krXel
=
= ln
ꢀ
ln
ꢀ
(1)
OH OH
a(Dt) a(Xt+1)
a(Dt) a(It+1)
M
= conventional representation of an electron donor metal
where:
a(Xt) - Integral value corresponding to halogenated
derivative C6H5X at time t;
Scheme 4.
A more complicated mechanistic pattern is revealed by
the reactivity of the compounds included in the series B: as

Selective Reductions of the Carbonyl Compounds
Letters in Organic Chemistry, 2011, Vol. 8, No. 10
693
Table 2. Relative Concentrations of Compounds 3, 11 and 4
Relative concentrations (CG-SM)
Reaction time, minutes
3
Ratio 11/4
11
4
0
1
1
1
3.04
1.58
1.86
2.54
1.12
1.65
1.20
1.41
1.13
30
120
F
Cl
Br
I
Ni-Al alloy
+
+
+
+
NaOH, dioxane
5
6
7
8
internal standard
Scheme 5.
Table 3. Relative Rate of Reduction Reaction of the Aryl Halides 5-8
X
Relative rate of reaction* (
; X = F, Cl, Br)
kI
Period,
kII
(t, t+1), minutes
kIBr
kIF
kICl
0-10
10-20
20-40
2.09
1.28
1.66
0.90
0.93
1.06
1.11
0.31
0.39
1
1
1
*The relativerateofreactionwascalculated usingtheintegral values corresponding to compounds in the gas chromatograms obtained at different reaction time, according to formula 1.
a(Dt+1) - Integral value corresponding to decaline D at time
The relative reactivities in series of structurally related
reducible compounds (alkyl aryl ketones) showed a sequence
t+1.
of reactivity in agreement with stereoelectronic influence of
the substrate on a heterolytic course of reaction thus
providing an evidence for a direct electron transfer from
aluminum to organic substrate.
The data presented in Table 3 (in terms of relative first
order rate constants for reduction of phenyl fluoride, chloride
and bromide versus corresponding iodide) show, again, an
apparent irregular dependence of reactivity on the reaction
time (see for example the case of the phenyl fluoride
showing consecutive periods of decrease (10-20 minutes)
and respectively increase (20-40 minutes) of the relative
reactivity. Most probably this complicated variability is the
result of the changes in composition and morphology of the
reacting solid phase thus strongly influencing stereochemical
matching of reagents and diffusion process.
In some cases an important change in reactivity with
respect to conversion of reducing the reagent is noticeable.
This stands for a complex influence of the nanosites of the
reducing solid phase possible involving both electronic
process (influenced by concentration of reactive sites) and
steric requirements (access of reacting substrate in gradually
changed pores of the solid phase).
A morphologic analysis by AFM method for samples of
Ni-Al alloy previously subjected to a partial, rigorously
controlled dissolution of the aluminum, as well as the study
of reducing properties of these samples is now in progress in
our laboratory.
EXPERIMENTAL
NMR spectra were recorded with a Bruker Advance
DRX 400 spectrometer in CDCl3. IR spectra were recorded
with a Bruker Vector 33 spectrometer. GC-MS analyses
were performed with an Agilent model 6890N gas
chromatograph coupled with an Agilent Model 5975 mass
spectrometer.
CONCLUSIONS
Our present results reveal the possibility to find relative
reactivities in Ni-Al alloy reduction by measuring individual
total conversions of reducible substrates allowed to compete
(in reasonable mixtures of three-four components) for a
limited amount of reducing reagent. By introduction in the
reacting mixture of a suitable amount of an internal standard
(GC-MS detectable, inert to the reaction mixture, e.g.
decaline) first order reaction rate constants are deducible.
Nickel-aluminum alloy composition Ni 50 wt% and Al
50 wt% was purchased from the company Merck as powder
for the production of Raney nickel for synthesis.
Among the substances listed in Scheme 3, some
substances are commercially available. The purification was
done properly, followed by a check of purity by GC-MS

694 Letters in Organic Chemistry, 2011, Vol. 8, No. 10
Suceveanu et al.
chromatographic methods and verification of physical usual
properties for boiling point, melting point.
portions at room temperature. The mixture obtained is
poured gradually with stirring over ice water. The organic
layer is separated and is washed for several times with 5%
aq. Na2C03. The CCl4 evaporates at rotavapor and then the
residuum is distillated at low pressure (b.p. 164 °C / 13 mm
Hg). The obtained product weightning 2.59 g (61%).
For substrates that are not commercially available, we
carried out the synthesis and structural characterization, as in
the following procedures:
IR: 1672 cm-1 (ꢀC=O
1H-RMN: 2.55 ppm, s, 3H (CH3CO); 7.70 ppm, (H2);
7.25 ppm, (H3); 7.77 ppm, (H4), 7.68 ppm, (H5); 7.36 ppm,
(H6); 7.45 ppm, (H7); 8.75 ppm, (H8).
)
2,4-Dimethylacetophenone
Over a stirred mixture of 10 mL meta-xylene (8.656 g, 72
mmoles) and 7 g anhydrous AlCl3 (52 mmoles) cooled at a
temperature below 5 °C, is added a mixture of 12 mL meta-
xylene (10.424 g, 108 mmoles) and 3.2 mL acetyl chloride
(3.53 g, 45 mmoles). The obtained mixture is poured
gradually with stirring over ice water. The organic layer is
extracted with diethyl ether and then it is washed with 5%
aq. Na2CO3 and dried over anhydrous magnesium sulphate.
The ether is evaporated at normal pressure. The unreacted
meta-xylene is separated from the crude residue by
distillation at normal pressure on oil bath (b.p. 138 °C). The
residue is distilled under reduced pressure to afford 4.10 g of
2,4-dimethylacetophenone (62% with respect to acetyl
chloride).
2-Acetylnaphthalene
In three-necked flask are placed 3.20 g (25 mmoles)
naphthalene freshly sublimated and 15 mL nitrobenzene
(17.5 g; 142 mmoles). Over this solution, 2 mL (2.175 g,
27.5 mmoles) freshly distilled acetyl chloride are added and
then the mixture is cooled to -5 °C. Gradually, 3.67 g AlCl3
(27.5 mmoles) is added at 0 °C temperature below. The
obtained reaction mixture is poured over a mixture of water
and ice; then the organic layer is separated and is washed
with 5% aq. Na2CO3. The nitrobenzene is separated by steam
distillation and the reaction product is extracted with diethyl
ether. The solvent is removed by distillation at low
temperature to afford 0.80 g of 2-acetylnaphtalene (18.8%).
IR: 1678 cm-1 (ꢀC=O
)
1H-RMN: 2.34 ppm, s, 3H (CH3 posn. 2); 2.52 ppm, s,
3H (CH3 posn. 4); 2.56 ppm, s, 3H (COCH3); 7.06 ppm, d,
1H (H5); 7.63 ppm, d, 1H (H6); 6.89 ppm, s, 1H (H3).
IR: 1677 cm-1 (ꢀC=O
)
13C-RMN: 21.31 ppm (CH3 posn. 4); 21.77 ppm (CH3
posn. 2); 29.30 ppm (CH3-CO); 142.12 ppm, 134.64 ppm,
126.27 ppm, 136.90 ppm, 129.96 ppm, 132.90 ppm (C1-C6
aromatic); 200.97 ppm (CO).
1H-RMN: 2.55 ppm, s, 3H (CH3CO); 8.30 ppm, (H1);
7.92 ppm, (H3), 7.76 ppm, (H4); 7.75 ppm, (H5), 7.48 ppm,
(H6); 7.43 ppm, (H7); 7.83 ppm, (H8).
Preliminary Reactions
2,4,6-Trimethylacetophenone
The reactions of ketones 1-4 (Scheme 1) were performed
according to the protocol described below. In a two-necked
flask fitted with condenser, there are placed 6 mL 20% aq.
NaOH, then are added 5 mmoles of ketone and 1-2 mL
MeOH were introduced for its solubilization, if necessary.
270 mg Ni-Al alloy (5 mmoles aluminum) are introduced in
small portions with stirring. After the reaction time specified
in Scheme 1, the reaction mixture is diluted with water, then
is extracted with 3x10 mL diethyl ether. The extract is dried
over MgSO4 and it is evaporated at low temperature. The
characterization of reaction products was done by GC-MS
method and comparison with authentic samples.
To a cooled mixture of 10 mL 1,3,5-trimethylbenzene
(8.642 g, 72 mmoles) and 7 g AlCl3 (52 mmoles) is gradually
added a mixture of 10 mL 1,3,5-trimethylbenzene (8.642 g,
72 mmoles) and 3.2 mL acetyl chloride (3.53 g, 45 mmoles).
The stirring is continuing for 80 minutes at the 5 °C
temperature below. The obtained mixture reaction is treated
as the previous case. The unreacted mesitylene is distilled at
low pressure (b.p. 80 °C / 12 mm Hg). The reaction product,
2,4,6
- trimethylacetophenone is purified by vacuum
distillation (b.p. 120 °C / 12 mm Hg) to obtain 4.23 g of
2,4,6-trimethylacetophenone (58%).
IR: 1698 cm-1 (ꢀC=O
1H-RMN: 2.20 ppm, s, 6H (CH3 posn. 2, 6); 2.25 ppm, s,
3H (CH3 posn. 4); 2.43 ppm, s, 3H (COCH3); 6.81 ppm, s,
2H (H3, H5).
13C-RMN: 18.86 ppm (CH3 posn. 2,6); 20.77 ppm (CH3,
posn. 4); 31.84 ppm (CH3-CO); 139.73 ppm, 132.05 ppm,
136.03 ppm, 128.29 ppm, (C1, C2 and C6; C4, C3 and C5
aromatics).
)
Compound 1a: GC: RT 31.783 min; MS: m/z (%) = 100
(2) [M+], 82 (100) [M+-2H], 67 (45) [C5H8 ], 57 (40) [C4H9 ]
+
+
Compound 2a: GC: RT 4.157 min; MS: m/z (%) = 102
(5) [M+], 85 (21) [M+-CH3], 57 (100) [C4H9 ]
+
Compound 3a: MS: m/z (%) = 122 (59) [M+], 107 (100)
[M+-CH3], 77 (44) [C6H5 ], 45 (50) [(CH3CHOH)+]
+
Compound 3b: RT 19.800 min; MS: m/z (%) = 107 (2)
[M++1], 106 (27) [M+], 91 (100) [M+-CH3], 77 (12) [C6H5 ]
+
Compound 3c: MS: m/z (%)
=
121 (100)
1-Acetylnaphthalene
[C6H5(CH3)C+(OH)], 107 (11) [C6H5(OH)C+], 43 (62)
[(CH3CO)+]
To a cooled mixture of 10 mL CCl4 (15.94 g, 103
mmoles) and 4 mL of acetyl chloride (4.416 g, 56 mmoles)
is added 7 g anhydrous AlCl3 (52 mmoles). A solution of 32
g naphthalene (25 mmoles) in 10 mL CCl4 is added in small
Compound 4a: MS: m/z (%) = 173 (3) [M+1], 172 (24)
[M+], 157 (38) [M+-CH3], 129 (100) [C10H8 +1], 128 (48)
+
[C10H8 ], 43 (15) [(CH3CO)+]
+

Selective Reductions of the Carbonyl Compounds
Letters in Organic Chemistry, 2011, Vol. 8, No. 10
695
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[M+], 161 (100) [M+-CH3], 132 (30) [C10H12
]
+
Reduction Reactions in the Series A
In a 25 mL volumetric flask, there is prepared a stock
solution consisting of 0.2 g (2 mmoles) cyclohexanone, 0.6 g
(5 mmoles) acetophenone,
dimethylacetophenone, 1.3
1
g
g
(6.7 mmoles) 2,4-
(8 mmoles) 2,4,6-
[5]
trimethylacetophenone and dioxane up to the mark.
In a two-necked flask fitted with condenser, there is
added 10 mL of the stock solution of ketones, containing
approximately 5 mmoles of reducible compounds and 540
mg Ni-Al alloy (10 mmoles aluminum). 2 mL 20% aq.
NaOH are dripped over the mixture and stirred for 30
minutes and then 2 mL of reaction mixture are taken as
sample. There is added 2 mL 20% aq. NaOH over the
reaction mixture in the flask, the stirring continues for 90
minutes before the second sample has been taken. The two
samples are processed as follows: they are diluted with 50
mL water, are extracted with 3x10 mL CH2Cl2. The extract
is washed with water up to neutral pH, then it is dried over
MgSO4 and it is evaporated at low temperature. The samples
are analyzed by GC-MS.
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The reduction reactions in the series B and C are
performed by the same procedure as in series A.
performance
in
chemoselective
hydrogenation
of
2-
ethylanthraquinone. J. Catal., 2006, 237 (1), 143-151; (d) Liu, G.-
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ACKNOWLEDGEMENT
Financial support from the CNCSIS is gratefully
acknowledged.
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