Feasibility of room temperature reduction of aromatic carbonyl and nitro compounds by Zn/dil. HCl-Et2O system: An experimental and DFT study

Article abstract of DOI:10.14233/ajchem.2017.20623

This experimental study is about the feasibility of the room temperature reduction of the ether soluble aromatic carbonyl/nitro compounds by atomic hydrogen. The atomic hydrogen is produced by the novel Zn/dil. HCl-Et2O reducing system (wherein dil. HCl is slowly added to the ethereal solution of Zn and substrate).This study resulted in single or mixture of anticipated reduced products in different yields. The DFT study with B3LYP/6.311g ++ (d,p) basis set revealed that the stability of the first formed free radical (energy factor) and the homo nuclear nature of the carbonyl and nitro group (charge factor) decide the yield. It is also found that the presence of –M group at o- or p- position to the carbonyl/nitro group results in the favourable modification of above-mentioned factors. The above-mentioned factors also explain the preferential reduction of nitro group when it is present along with carbonyl group. The free radical mechanism was confirmed by the formation of pinacol coupled product in one instance. In one of the reduction reactions, an unreported compound viz. the dimer of o-amino benzaldehyde was obtained in good yield.

Full text of DOI:10.14233/ajchem.2017.20623

Asian Journal of Chemistry; Vol. 29, No. 8 (2017), 1761-1766
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2017.20623
Feasibility of Room Temperature Reduction of Aromatic Carbonyl and Nitro
Compounds by Zn/dil. HCl-Et
2
O System: An Experimental and DFT Study
*
S. RAJAMATHE and G. SELVARAJ
Department of Chemistry, Kanchi Mamunivar Centre for Post Graduate Studies, Pondicherry-605 008, India
*
Corresponding author: E-mail: rajamathi@gmail.com
Received: 3 March 2017; Accepted: 10 April 2017;
Published online: 12 June 2017;
AJC-18438
This experimental study is about the feasibility of the room temperature reduction of the ether soluble aromatic carbonyl/nitro compounds
by atomic hydrogen. The atomic hydrogen is produced by the novel Zn/dil. HCl-Et O reducing system (wherein dil. HCl is slowly added
2
to the ethereal solution of Zn and substrate).This study resulted in single or mixture of anticipated reduced products in different yields.
The DFT study with B3LYP/6.311g ++ (d,p) basis set revealed that the stability of the first formed free radical (energy factor) and the
homo nuclear nature of the carbonyl and nitro group (charge factor) decide the yield. It is also found that the presence of –M group at o-
or p- position to the carbonyl/nitro group results in the favourable modification of above-mentioned factors. The above-mentioned factors
also explain the preferential reduction of nitro group when it is present along with carbonyl group. The free radical mechanism was
confirmed by the formation of pinacol coupled product in one instance. In one of the reduction reactions, an unreported compound viz. the
dimer of o-amino benzaldehyde was obtained in good yield.
Keywords: Charge factor, Energy Factor, Homonuclear, Pinacol coupling, Atomic hydrogen.
the velocity of the liberated hydrogen gas can be controlled,
by controlling the rate of addition of the dil. HCl to the Zn
suspension in the organic phase.
The aims of the investigation, are to study the feasibility
of room temperature reduction of the ether soluble aromatic
INTRODUCTION
Reduction is one of the frequently used reactions in organic
synthesis and a vast variety of reducing agents have been
introduced for this achievement [1]. The reduction of nitro
compounds is of paramount importance for the preparation of
amino derivatives in the organic synthesis. The synthesis and
biological evaluation of aromatic amines are also active and
most important areas of research and their chemistry by deriva-
tive formation is widely studied [2,3]. Likewise, the conversion
of carbonyl compound to alcohol or alkane is important in
pharmaceutical industries as the transformation changes the
hydrophilicity of the compound.A plethora of reducing agents
is available for the reduction of aromatic carbonyl/nitro
compounds, some reagents claim to be chemo selective [4-10].
There are many reducing agents reported in the literature
for the reduction of carbonyl/nitro group, but a non-catalytic
reagent, which can perform the task at room temperature with
simple work up technique and with commonly available non
toxic chemicals, have not been reported. The solvent chosen
was ether, as it is easily recoverable and suitable for room temp-
erature reaction and dissolves most of the organic compounds.
carbonyl and nitro compounds by Zn/dil. HCl-Et O system,
2
to explore the theoretical reasoning for the experimental results
by performing DFT study of the energies of the geometry
optimized structures of the substrates and the possible free
radical intermediates and to suggest a plausible mechanism
for the reduction of aromatic carbonyl and nitro compounds
2
by Zn/dil. HCl-Et O system.
EXPERIMENTAL
The general schemes for the reduction of aromatic
carbonyl and nitro groups by the Zn/dil. HCl-Et O system are
2
given in Schemes I and II respectively and the details of the
various products formed are given in Table-1.
All the reagents and substrates were purchased from com-
mercial sources with the best quality and were used without
further purification. SiO was used in the form of silica gel 60
2
Zn/dil. HCl-Et
2
O system is a commonly available reducing
(70-230 mesh ASTM) and was purchased from Merck
company. IR and H NMR spectra were recorded on Thermo
1
system, capable of slow in situ generation of hydrogen. The
excess of zinc can be filtered and reused.At room temperature
Nicolet Nexus 670 FT-IR and Bruker Avance DPX-400 MHz

1
762 Rajamathe et al.
Asian J. Chem.
R2
13
O
and at 15.2 and 65.9 ppm in C NMR. The spectral data of the
X
unreported compound 22 are given below:
Zn
2HCl
ZnCl
2
2
-(4-Nitrobenzylideneamino)benzaldehyde (22): Pale
Et O
2
1
3h , r.t
yellow low melting solid, mp 25 °C; H NMR (400 MHz,
R1
R1 = H; R2 = H
;
R1
1
CDCl
proton), 8.05-6.95 (m, 8H, aromatic protons) ppm; C NMR
(100 MHz, CDCl ) δ 194.0, 188.2, 156.0, 154.6, 134.1, 133.7,
3
) δ 10.35 (s, H, aldehydic proton), 9.10 (s, 1H, alkenic
R1 = H; X = CH
OH
2
13
6;
1
7; R1 = OCH
; X= CH
R1 = H; X= C(CH
)(OH)C(OH)(CH
)C(CH )C
)(OH)C(OH)(C
= H; X = CH(OH)COC H
R1 = OCH ; R = H
2
3
3
3
2
3
4
;
3
8;
9;
)(C
H
)
3
3
6
5
;
;
R1 = H; R2 = CH
R1 = H; R2 = C H
3
131.3, 130.9, 129.6, 129.4, 124.37, 119.7, 124.43, 114.9 ppm.
To compensate the loss of the atomic hydrogen atoms
generated as hydrogen molecules (bubbles) without effecting
reduction, the stoichiometric ratios of the zinc and acid was
scaled up for the optimum yield. The scaled up ratio of the
acid was lesser than that of Zn, to avoid residual acid medium
and loss of acid sensitive products to the aqueous phase. The
acid of the required concentration was prepared by using brine
solution to minimize the loss of water soluble products to the
aqueous phase. By this technique, it was found that the amount
of product in the organic phase was increased by two folds
R1 = H; X = C(CH
R1 = H; X = C(C
; R
2
2
6 5
H
6
5
5;
R1 = H; R2 = COC H5
6
10;
6
H
5
6 5 2
H )
11
1
6
5
Scheme-I: Reduction of aromatic carbonyl compounds 1-5
NO2
X
R2
R2
6HCl
3ZnCl
2
2
2H O
3Zn
Et
2
O
3h,r.t
R1
2; R1 = H; R = H
R1
2
1
1
2
1
8; R = H; R = H; X = NH(OH)
9; R1 = H; R = H; X = NH
; R2 = H; X = NH
21; R1 = CHO; R2 = H; X = N=CH-polymer
1
1
3; R1 = CH
3
; R2 = H
2
1
2
(
mixture NMR evidence) without the regular workup.
The technique of adding dil. HCl in drops by using addition
4; R1 = NH
2
1
2
; R = H
20;
R
= CH
3
2
1
2
1
5; R = H; R = NH2
funnel/burette to organic phase containing zinc and substrate
dispersed in ether, at the rate of 50 mL/h provides a steady
velocity of hydrogen generation and more contact time for
atomic hydrogen to effect reduction on the substrate. The rate
of addition was optimized after several trial runs, by running
TLC with hexanes/ethyl acetate (8:2) as eluent.
1
2
1
6; R1 = CHO; R2 = H
6 4
22; R = H; R =CHO; X= N=CH-C H -NO
2
1
2
1
7; R = H; R =CHO
Scheme-II: Reduction of aromatic nitro compounds 12-17
spectrometers, respectively. The products were characterized
by a comparison with authentic samples (melting or boiling
points) and their H NMR or C NMR or IR spectra. TLC was
applied for the purity determination of substrates, products
and the reaction mixture by using silica gel 60 F254 aluminum
sheet.
As the starting materials for the investigation are known
compounds, the experiments are performed in a higher scale
for the ease of qualitative identification of the products. 0.2
mol of zinc dust was added to a solution 0.05 mol aromatic
carbonyl compound in 150-200 mL of diethyl ether, taken in
a 500 mL 2 necked round bottom flask. To the stirred zinc
suspension of the ethereal solution of the substrate, 150 mL
of very dilute HCl of the concentration 0.3 mol per 150 mL is
added very slowly through the addition funnel at the rate of
1
13
The present investigation is aimed only at finding the
feasibility of the reduction of aromatic carbonyl/nitro group
and the extent of reduction possible, under given the set of
experimental condition, so the technique used to calculate the
yield was to cull the ratio of unreacted substrate and the products
1
formed (percentage of conversion), from the H mixture NMR
of the crude product. It was done on the basis of the peak area
integration of the aromatic protons or the characteristic protons
of the substrate/products.
The principle for spectral assignment for the aromatic
protons is that, when an carbonyl group is converted to alcohol/
alkane, or when a nitro group is converted to hydroxyl amine
or amine, the aromatic protons of the products appear slightly
up field in comparison to the aromatic protons of the substrate
5
0 mL/h which is the optimum rate of addition for the optimum
(
starting material) as most of the product formed are known
velocity of the generation of hydrogen for the maximum yield,
tested by several trial runs and monitored byTLC (8:2) hexanes
and ethyl acetate system as eluant. To compensate the loss of
solvent due to evaporation, 20 mL of ether was added at end
of each hour.
compounds. The products aromatic peaks are identified by
comparing them with the authentic spectra available in NMR
data base [11].
RESULTS AND DISCUSSION
After the completion of the reaction, the zinc was filtered
off and usual work up resulted in the crude. The mixture NMR
of the crude was obtained to determine the product to reactant
ratio (conversion percentage) and in the case of formation new
compound, isolated yield was determined. In the case of the
reduction of nitro compound the amount of zinc and HCl was
scaled up by 3 times to match the stochiometric requirement.
As most of the substrates and products involved in our
study are known compounds, the NMR data of the mixture
are compared with the NMR of the authentic samples available
with SDBS [11] and with the NMR of the starting materials
available. In some mixture NMR, the residual solvent (diethyl
The results of this investigation of reduction of aromatic
carbonyl and nitro compounds by Zn/dil. HCl-Et
are presented in the Table-1.
2
O system
Based on studies of Turro and others [12], we suggest a
plausible free radical mechanism (Schemes III and IV) for
the room temperature reduction of aromatic carbonyl and nitro
com-pounds by Zn/dil. HCl-diethyl ether system.
The mechanism for the carbonyl reduction (Scheme-III)
is suggested to account for the three types of products (normal,
coupled and clemmenson) formed. The first step is the slow
generation atomic hydrogen (free radicals) or the ‘nascent
hydrogen’. The Zn metal suspended in the organic phase,
1
ether) signal occur at 1.16-1.19 and 3.42-3.47 ppm, in H NMR

Vol. 29, No. 8 (2017)
Reduction of Aromatic Carbonyl and Nitro Compounds by Zn/dil. HCl-Et
2
O System 1763
TABLE-1
DETAILS OF THE REDUCED PRODUCTS OF THE AROMATIC CARBONYL (1-5) AND NITRO COMPOUNDS 12-17
Entry
Substrates
Products
Name
Yield* (%)
1
2
3
4
5
6
7
8
9
1
2
3
4
5
12
13
14
15
16
17
6
Phenylmethanol
1-Methoxy-4-methylbenzene
2,3-Diphenylbutane-2,3-diol & buta-1,3-diene-2,3-diyldibenzene
1,1,2,2-Tetraphenylethane-1,2-diol
2-Hydroxy-1,2-diphenylethanone
N-Phenylhydroxylamine & aniline
p-Toluidine
–
42
4
7
8 & 9
10
11
18 & 19
20
–
–
21
22
13
42
25
33
5
0
0
90**
90**
–
1
1
0
1
Iminobenzylidene polymer
2-(4-Nitrobenzylideneamino)benzaldehyde
1
*
Culled from H mixture NMR spectra; **Calculated based on the isolated yield.
1
)
Zn
2HCl
ZnCl
2
2 H
3ZnCl
O
1
)
3Zn
O
6HCl
2
6H
R2
R
2
OH
O
OH
O
N
N
4
R
4
R
H
2
)
2)
H
R1
R
1
3
3
Free radical intermediate
R
R
First Free radical (cation) intermediate
H
R2
OH
R2
OH
O
N
O
OH
N
4
4
R
3
)
H
R
2
H O
3
)
H
R1
Normal product
R1
3
3
R
R
Nitraso derivative
R2
OH
R2
OH
O
HO
N
R2 HO
N
R1
R1
4
4
3a)
R
R
OH R2
H
R1
R1
Pinacol coupled product
4)
3
3
R
R
H
H
R2
OH
R2
H
Second Free radical intermediate
2
H O
4
)
H
H
HO
HO
H
N
N
4
R
4
R1
R1
R
H
Clemmensen product
5)
Scheme-III: Mechanism for the carbonyl reduction
3
R
3
R
+
Hydroxyl amine derivative
transfers electrons to H ions of the falling dil. HCl drops to
reduce them to atomic hydrogen. The hydrogen free radicals,
HO
H
H
H
N
N
before combine to form H molecule (gas) and bubble out,
2
6
)
4
4
R
R
they induce the homolytic fission of the carbonyl π bond with
the generation of carbon and oxygen radicals. As it is a room
temperature reaction, the πbond of the carbonyl group cannot
undergo heterolytic fission (ionic mechanism) in the absence
H O
H
2
3
3
R
R
Third Free radical intermediate
–
of a energetically favourable nucleophile like H ion etc. In
H
H
N
the second step, the hydrogen free radical combines with the
oxygen free radical (which is less stable) to form the hydroxyl
carbon free radical species as intermediate whose stability
should decide the further course of reaction.
Similar steps are involved in the reduction mechanism of
nitro compounds (Scheme-IV), It is well documented that the
N
4
R
4
R
7
)
H
3
R
3
R
Amino derivative
conversion of NO
2
group to NH group involves ‘-N=O’ and
2
Scheme-IV: Mechanism for the nitro group reduction

1
764 Rajamathe et al.
Asian J. Chem.
‘
-NHOH’intermediate. The suggested free radical mechanism
accounts for the production of hydroxyl amine. The second step
Scheme-IV), which is similar to the second step of carbonyl
TABLE-2
ENERGIES OF THE GEOMETRY MINIMIZED
SUBSTRATES (1-5) AND (12–17) AzND THEIR FREE RADICALS
(
Energy (hf) (DFT-
311g ++ (d,p))
Energy
(kJ/mol)
-907554
-909042
reduction, the π bond of the nitro group under goes homolytic
fission under the influence of the hydrogen free radical to yield
the nitrogen radical cation and oxygen free radical. The
generated hydrogen free radical attacks the oxygen free radical
of the substrate to generate a hydroxyl nitrogen radical cation
of the substrate first free radical (cation) intermediate whose
stability decides the further course of the reaction.
We consider the formation of benzo pinacol by the pinacol
coupling of the free radicals of the substrate 4, in large yield
serves as the evidence for the suggested mechanism for
carbonyl reduction. The evidence for the suggested free radical
mechanism for the nitro group reduction is the formation of
the hydroxylamine intermediate for the substrate 12, usually
reduction by using metal does not stop at intermediate stage,
but the use of mild reagent, such as the one used, can produce
mixture of intermediate products also. Moreover, we have used
the same reducing system for reducing carbonyl and nitro
groups. Liu et al. [13] have also reported the N-arylhydroxyl
Substance
Benzaldehyde
Benzaldehyde-FR
Anisole
6
-345.66920379
-346.23573214
-460.22823165
-460.79130200
-385.00178859
-385.56579451
-576.77159850
-577.35344980
-690.12893287
-690.71256956
-436.87472610
-437.46035854
-476.20367999
-476.78823310
-492.25620264
-492.83758052
-492.25642811
-492.83793181
-550.22867204
-550.80362843
-1208329
-1209808
-1010822
-1012303
-1514314
-1515841
-1811934
-1813466
-1147015
-1148552
-1250273
-1251808
-1292419
-1293945
-1292419
-1293946
-1444625
-1446135
Anisole FR
Acetophenone
Acetophenone-FR
Benzophenone
Benzophenone-FR
Benzil
Benzil-FR
Nitrobenzene
Nitrobenzene-FR
4-Nitrotoluene
4
4
4
2
2
4
-Nitrotoluene-FR
-Nitroaniline
-Nitroaniline-FR
-Nitroaniline
-Nitroaniline-FR
-Nitrobenzaldehyde
amines by the use of Zn-CO
The success of the carbonyl/nitro group reduction by our
reducing system depends as three factors:
2
/H
2
O system.
4-Nitrobenzaldehyde-
aldehyde-C-FR
4-Nitrobenzaldehyde-
-550.81890792
-1446175
nitrogen-N-FR
•
The homonuclear nature of the carbonyl group and nitro
2
2
-Nitrobenzaldehyde
-Nitrobenzaldehyde-
-550.21882762
-550.80688319
-1444600
-1446143
group, the electro negativity of the carbonyl carbon and oxygen
and nitro group nitrogen and oxygen in the given substrate.
aldehyde-C-FR
•
The stability of the free radical intermediate formed in
the second step of both mechanisms.
The concentration and rate of addition of the dil. HCl.
2-Nitrobenzaldehyde-
nitrogen-N-FR
-550.81204534
-1446157
•
The faster addition or the use of concentrated acid will
generate large concentration of the hydrogen free radicals
indicate much lower is the energy of the free radical when
compared to the substrate (i.e. Esubstrate – EFree radical) from which
it is produced. As the substrates involved are different, the
energy factor is normalized for comparison. The normalization
was done by dividing the energy factor of substrate by the
number atoms of the substrate and is called ‘effective’energy
factor. The charge factor indicates the homo nuclear nature of
the carbonyl/nitro group, It is arrived by adding the magnitudes
of ‘unlike’ charges or by subtracting the magnitudes of ‘like’
charges on the C and O atoms in the case of carbonyl group
on the N and O group on the nitro group of the substrates, the
charge factor is the approximate measure of the electro nega-
tivity difference of the atoms at reaction site of the substrate,
the lesser the difference, more is the homo nuclear nature of
the (π) bond present between the atoms and hence more likely
is the homolytic fission of (π) bond in second step of the
suggested mechanism (Schemes III and IV). The energy and
charge factors obtained from DFT studies are given in Table-3.
For handy comparison 1/charge factor called α and ‘effective’
energy factor called β are also given in Table-3 along with
yield and remarks. A scrutiny of data given in the Table-3
reveals that for the yield of the reduction products to be high,
the value of either one of the two factors (α and β) or both of
them should be high. If both the factors are less, the yield will
be less and the pattern of change of α and β is summarized in
the remarks column of the Table-3. The anomalous behaviour
of the substrates 5, 16 and 17 are due to the occurrence of
consecutive elimination (5) or polymerization reactions (16
which will recombine to form H gas and will bubble out
2
without effecting reduction. Of the three factors, the third factor
can be controlled experimentally, but for the knowledge of
first two factors, one has to resort to the theoretical study. As
DFT study is popular and more accurate, we have decided to
perform DFT calculation. We have optimized the geometries
of substrates (1-5 and 12-17) and the their first formed free
radical intermediates mentioned in Schemes III and IV by
using Gaussian 03 program. The optimization was done by
B3LYP (Becke, 3-parameter, Lee-Yang-Parr) method with
6
.311g ++ (d,p) basis set. The energies of the geometry opti-
mized structures are summarized in Table-2. The difference
in the energies of the substrate and free radical (energy factor),
gives in sight to the stability of the radicals and hence taking
care of the second factor. From the Geometry optimization
of the substrates, the partial charges on each the atom of the
substrates (Mullikan charges) are obtained. The difference in
the Mullikan charges for carbon and oxygen of the carbonyl
group and nitrogen and oxygen of the nitro group (charge
factor) gives insight into the homo nuclear nature of the groups.
Thus, there are two factors envisaged: 1) Energy factor 2)
Charge factor. The charge factor is important for the homolytic
cleavage of the carbonyl and nitro group. The energy factor is
important for the subsequent formation of free radicals needed
for the progress of the reaction. Higher the energy factor more
stable is the free radical intermediate, because energy factor

Vol. 29, No. 8 (2017)
Reduction of Aromatic Carbonyl and Nitro Compounds by Zn/dil. HCl-Et
2
O System 1765
TABLE-3
ENERGY DIFFERENCE OF THE GEOMETRY OPTIMIZED SUBSTRATES (1-5) AND (12-17) AND THEIR
FREE RADICALS (ENERGY FACTOR) WITH CHARGE FACTOR OF THE SUBSTRATES ALONG WITH YIELD
Energy difference
k.J/mol (energy
factor)
Charge
difference
(charge factor)
Energy factor
1/Charge
factor β
Compound name
Benzaldehyde
Yield (%)
Influencing factor (s)
(effective) α
1487
1478
1481
1528
1532
1538
1535
1526
1527
1550
1557
0.094
0.191
0.018
0.005
0.302
0.118
0.140
0.144
0.248
0.147
0.138
106
92
87
64
59
110
90
85
85
97
97
10.6
5.2
55.6
200.0
3.3
8.5
7.1
6.9
4.0
42
4
α & β more
Anisole
α less, β very less
α very less, β moderate
α very less, β very high
α & β very less
α & β more
Acetophenone
Benzophenone
Benzil
13
42
25*
33
5
0
0
90*
90*
Nitrobenzene
4
4
2
4
2
*
-Nitrotoluene
-Nitroaniline
-Nitroaniline
-Nitrobenzaldehyde C-FR
-Nitrobenzaldehyde C-FR
α & β less
α & β very less
α & β very less
α & β more
6.8
7.2
α & β more
With consecutive elimination/polymerization reaction.
1
00
and 17) of the reduction products respectively. The values in
the Table-3 are presented in the form of graphs Figs. 1-3 to
depict the possible linear correlation between the yield and
stability of the free radical; yield and homo nuclear nature of
carbonyl and nitro group.
Apart from the theoretical reasoning of the yield (α and β
factors), It was observed that the free radical (intermediate)
which can be delocalized more (more canonical structures)
90 90
90
80
70
60
50
42
42
40
33
25
30
20
13
4
5
10
0
0
1
20
110
0
106
110
97
97
1
00
92
90
87
85
85
9
8
7
6
5
4
0
0
0
0
0
0
64
59
Fig. 3. Relationship between yield percentages of the substrates
are more stable, resulting in high yield of reduction product.
This fact is reflected in the yields of substrate 4, 16 and 17.
Very less yield was obtained in the case of substrate 7 and no
yields were observed in substrates 14 and 15 with electron
donating group. In the case substrates 16 and 17, the DFT
study shows that the aldehyde free radicals are of high energy
than the nitro group free radical, so in those cases, it is the
nitro group which gets reduced and not the CHO group, leading
to chemo selective reduction. So the reducing agent system is
chemo selective, with the free radical mechanism.
Fig. 1. Relationship between effective energy factors of the substrates
000
1
200
Conclusions
55.6
100
The investigation of the feasibility of the room temperature
reduction of the aromatic carbonyl and nitro group by our
novel Zn/dil. HCl-diethyl ethereal system highlights the
following factors:
10.6
8.5
7.1 6.9
6.8 7.2
1
0
4.0
5.2
3.3
• The reduction is surprisingly by hydrogen free radical
1
(
contrary to Clemmenson ionic reduction mechanism) and is
possible under the given set of experimental conditions. The
DFT study reasons out that the yield of the reduction depends
on both the stability of the free radical generated (in the second
step of the suggested mechanisms), denoted as effective energy
factor α and the charge factor (Mullikan charges) β which
makes the ‘π’ bond of both the nitro and carbonyl group
Fig. 2. Relationship between 1/charge factors of the substrates

1
766 Rajamathe et al.
Asian J. Chem.
amenable for homolytic fission which is a precondition for
the free radical reactions.
REFERENCES
1
2
3
4
.
.
.
.
S.D. Burke and R.L. Danheiser, Handbook of Reagents for Organic Synthesis,
Oxidizing and Reducing Agents, Wiley-VCH: New York (1999).
F. Yuste, M. Saldana and F. Walls, Tetrahedron Lett., 23, 147 (1982);
https://doi.org/10.1016/S0040-4039(00)86770-2.
R.E. Lyle and J.L. LaMattina, Synthesis, 726 (1974);
https://doi.org/10.1055/s-1974-23423.
•
For high yield, both the energy factor α and the charge
factor β must be more, for moderate yield any one of the two
factors must be more. If both the factors are low then the yield
is observed to be low.
•
Apart from energy factor and charge factor, the presence
R.S. Dhillon, Hydroboration and Organic Synthesis: 9-Borabicyclo
of electron withdrawing (-M) group in the ‘para’ or ‘ortho’
position to the carbonyl/nitro group, also facilitates reduction,
as it indirectly affects the energy and charge factor.An electrons
withdrawing group pulls the electron away from the more
electronegative oxygen of the carbonyl/nitro group and making
the charge factor favourable. It also helps in the delocalization
of the free radical formed in second step of the suggested
mechanism and thereby making energy factor also favourable.
[
3.3.1]nonan (9-BBN), Springer: Germany (2007).
5. S.H. Lee, M.H. Nam, M.Y. Cho, B.W. Yoo, H.J. Rhee and C.M. Yoon,
Synth. Commun., 36, 2469 (2006);
https://doi.org/10.1080/00397910600781224.
A.Z. Halimjani and M.R. Saidi, Synth. Commun., 35, 2271 (2005);
https://doi.org/10.1080/00397910500186177.
6
.
7. B. Zeynizadeh and S.Yahyaei, Bull. Korean Chem. Soc., 24, 1664 (2003);
https://doi.org/10.5012/bkcs.2003.24.11.1664.
B. Uysal and B.S. Oksal, J. Chem. Sci., 123, 681 (2011);
https://doi.org/10.1007/s12039-011-0116-1.
S. Chandrasekhar, S.J. Prakash and C.L. Rao, J. Org. Chem., 71, 2196 (2006);
https://doi.org/10.1021/jo052604x.
8
9
1
1
.
.
•
In the course of our investigation an unreported dimer
formed by the iminobenzyledene coupling of the 2-amino
benzaldehyde in good yield and we have also reported the new
method for the room temperature preparation of the imino-
benzylidene polymer formed by self condensation of 4-amino-
benzaldehyde.
It is reported based on the theoretical and experimental
evidences that the present investigation involves free radicals.
So in the future probe, reduction of more substrates can be
studied and it can performed by changing the experimental
conditions suitable for free radical reactions, such as use of
ordinary/UV light, radical initiator, etc. to increase the yield
of the reduction products.
0. D. Lee, D. Kim and J. Yun, Angew. Chem. Int. Ed., 45, 2785 (2006);
https://doi.org/10.1002/anie.200600184.
1. SDBSWeb, http://riodb01.ibase.aist.go.jp/sdbs/
National Institute of Advanced Industrial Science and Technology.
12. (a) N.J. Turro, Modern Molecular Photochemistry, Benjamin/Cummings
Publishing Co. Menlo Park (1978).
(b) J.N.J.Pitts Jr., R.L. Letsinger, R.P. Taylor, J.M. Patterson, G. Recktenwald
and R.B. Martin, J. Am. Chem. Soc., 81, 1068 (1959);
https://doi.org/10.1021/ja01514a014.
(c) A. Demeter, B. László and T. Bérces, Ber. Bunsenges. Phys. Chem, 92,
1
478 (1988);
https://doi.org/10.1002/bbpc.198800355.
1
3. S. Liu,Y.Wang, X.Yang and J. Jiang, Res. Chem. Intermed., 38, 2471 (2012);
https://doi.org/10.1007/s11164-012-0562-5.