Selective reduction of alkenes, α,β-unsaturated carbonyl compounds, nitroarenes, nitroso compounds, N,N-hydrogenolysis of azo and hydrazo functions as well as simultaneous hydrodehalogenation and reduction of substituted aryl halides over PdMCM-41 catalyst under transfer hydrogen conditions

Article abstract of DOI:10.1016/j.tetlet.2004.02.098

Chemoselective reductions of alkenes, α,β-unsaturated carbonyl compounds, nitro and nitroso compounds, N,N-hydrogenolysis of azo and hydrazo functions as well as simultaneous reduction and hydrodehalogenation of substituted aryl halides, including bulkier substrates, were achieved by catalytic transfer hydrogenation (CTH) using mesoporous PdMCM-41 catalyst. The yields were practically unaffected upon recycling of the catalyst. Further, the CTH process is accomplished without affecting the reduction of any other reducible functional group.

Full text of DOI:10.1016/j.tetlet.2004.02.098

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 3071–3075
Selective reduction of alkenes, a,b-unsaturated carbonyl
compounds, nitroarenes, nitroso compounds, N,N-hydrogenolysis
of azo and hydrazo functions as well as simultaneous
hydrodehalogenation and reduction of substituted aryl halides
over PdMCM-41catalyst under transfer hydrogen conditions
Parasuraman Selvam,a,* Sachin U. Sonavane,b Susanta K. Mohapatraa
and Radha V. Jayaramb
aDepartment of Chemistry, Indian Institute of Technology-Bombay, Powai, Mumbai 400 076, India
bApplied Chemistry Division, University of Mumbai, Institute of Chemical Technology, Matunga, Mumbai 400 019, India
Received 31 December 2003; revised 13 February 2004; accepted 19 February 2004
Abstract—Chemoselective reductions of alkenes, a,b-unsaturated carbonyl compounds, nitro and nitroso compounds, N,N-
hydrogenolysis of azo and hydrazo functions as well as simultaneous reduction and hydrodehalogenation of substituted aryl halides,
including bulkier substrates, were achieved by catalytic transfer hydrogenation (CTH) using mesoporous PdMCM-41 catalyst. The
yields were practically unaffected upon recycling of the catalyst. Further, the CTH process is accomplished without affecting the
reduction of any other reducible functional group.
Ó 2004 Elsevier Ltd. All rights reserved.
The selective reduction of alkenes, a,b-unsaturated
carbonyl compounds, nitro and nitroso compounds as
well as simultaneous reduction and hydrodehalogen-
ation of substituted aryl halides is an important step in
the industrial synthesis of dyes and biologically active
compounds.1 Hence, attempts have been devoted to
developing suitable synthetic methods for such trans-
formations.2 Among the various available processes,
catalytic transfer hydrogenation (CTH)3 is emerging as
a viable alternative to the commonly used reduction
processes, which involve hazardous molecular hydrogen
or a metal hydride donor. Such reactions have also been
performed over homogeneous catalysts.4 However, in
view of the drawbacks of the homogeneous catalyzed
processes as well as environmental concerns associated
with these methods, in recent years the scientific com-
munity has switched over to heterogeneous catalysts for
such transformations,5 which offer several advantages
over homogeneous counterparts with respect to han-
dling, easy recovery and recycling of catalysts as well as
minimization of undesired toxic wastes. Furthermore,
the process is simple, reaction conditions are mild, and
reduction of bulky molecules can be carried out without
the use of expensive reagents. In this regard, palladium
supported6 or functionalized7 catalysts are, in general,
considered good catalysts.
We recently reported several transition metal-based
mesoporous silicate and aluminophosphate molecular
sieves, which show promise for a variety of industrially
important organic reactions and the use of which may
represent a possible alternative to standard syntheses of
a wide variety of precursors and intermediates.8 Fur-
thermore, they were also found to be very efficient,
highly selective, and rapidly synthesized. More impor-
tantly, these materials catalyzed some of the following
reactions, for example, oxidation of alkyl aromatics,8a
phenols,8b cyclohexane,8c–e and cyclohexene,8f reduction
of aromatic nitro and carbonyl functions,8g–i more effi-
ciently than the corresponding microporous analogues
or supported metal oxide systems. In continuation of
Keywords: Alkenes; a,b-Unsaturated carbonyl compounds; Nitro-
arenes; Nitroso compounds; Azo and hydrazo functions; Aryl halides;
PdMCM-41 catalyst; Hydrogen transfer.
* Corresponding author. Tel.: +91-22-2576-7155; fax: +91-22-2572-
3480; e-mail: selvam@iitb.ac.in
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.02.098

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P. Selvam et al. / Tetrahedron Letters 45 (2004) 3071–3075
our work on the development of novel eco-friendly
molecular sieves-based heterogeneous catalysts and
catalytic processes, as well as realizing the importance of
the development of novel synthetic methods for certain
important transformations, including bulkier molecules,
we report herein the first of a series of studies that ex-
plore a CTH approach for the reduction of aromatic
a,b-unsaturated ketones, nitroso compounds, and hyd-
rodehalogenation of substituted aryl halides using a
newly developed palladium-based mesoporous silicate
molecular sieve catalyst, designated as PdMCM-41.9
Mesoporous materials are novel molecular sieves, hav-
ing a high surface area, and large pore size and vol-
ume.10 The PdMCM-41 catalyst was hydrothermally
synthesized11 and characterized using various analytical
and spectroscopic techniques. Since, the CTH process
requires acidic sites, this catalyst is very well suited for
this purpose, as it possesses such characteristics,12 and
therefore, in this investigation mesoporous PdMCM-41
catalyst was employed for the titled reactions.13 Fur-
thermore, the mesoporous matrix gives a better disper-
sion of the active palladium ions as compared to other
supported systems in addition to the benefits from
having a low palladium content.
Table 1 summarizes the results of CTH of several alk-
enes and a,b-unsaturated carbonyl compounds over
PdMCM-41 catalyst, wherein the substrates were
reduced in excellent yields. The catalyst reduces only the
olefinic bonds, the carbonyl groups remaining unaf-
fected. The selective reduction can be explained on the
basis of the actual active species involved in the reduc-
tion process, which strongly depends on the reaction
conditions. Thus, when PdMCM-41 is used in combi-
nation with ammonium formate and methanol, selective
reduction of the olefinic bonds produces saturated car-
bonyl compounds. Interestingly, when PdMCM-41 was
used in combination with potassium hydroxide and
isopropanol, the unsaturated carbonyl compounds were
transformed into saturated alcohols (Scheme 1) since the
catalyst in combination with potassium hydroxide and
isopropanol produces metal alkoxide,8i which has an
Table 1. CTH of unsaturated carbonyl compounds over PdMCM-41
Entry
Substrate
Time (h)
Product
Yield (%)
Third run
First run
87
O
O
CH3
CH3
C
C
1
3.5
88
CH=CH
CH2 CH2
O
O
2
3
2.0
4.0
92a
91
89
88
CHO
CHO
CH2OH
CH2OH
CH=CH
CH2 CH2
4
2.0
89
88
CH=CHCHO
CH=CH2
CH2 CH2CHO
5
6
2.5
3.0
90
89
91
CH2 CH3
93a
CH=CH2
CO2H
CH2 CH3
CO2H
7
5.0
80
80
8
9
1.5
1.5
62
88
60
84
O
O
CH
CH CH
2
2
CH
CH=CH
CH
3
CH
3
a Isolated yield based on single experiment.

P. Selvam et al. / Tetrahedron Letters 45 (2004) 3071–3075
3073
Table 3. CTH of nitroarenes over PdMCM-41
CH2 CH2CH2OH
PdMCM-41
CH2 CH2CHO
CH=CHCHO
PdMCM-41
NO2
NH2
PdMCM-41
(CH3)2CHOH
KOH
HCOONH4
MeOH
HCOONH4
MeOH
R
R
R
R
R
[R = H, OCH3, N(CH3)2]
[R = H; 4.5 h; 75%
= OCH3; 5 h; 71%
= N(CH3)2; 5 h; 69%]
[R = H; 2.5 h; 90%
= OCH3; 3 h; 85%
= N(CH3)2; 3.5 h; 88%]
Entry
R
Time (min)
Yield (%)
1
H
30
40
45
40
30
40
40
30
45
60
50
45
35
45
40
60
99
92
90
93
85
88
80
82
88
86
85
85
87
85
89
83
2
2-CH3
3-CH3
4-CH3
3-CO2H
4-CO2H
4-COCH3
3-CHO
4-OCH3
2-OH
Scheme 1.
3
4
5
affinity for carbonyl groups and therefore produces
saturated alcohols.
6
7
8
Table 2 lists the results of transfer hydrogenation of
some aromatic and heterocyclic nitroso compounds over
the PdMCM-41 catalyst. It can be seen from the table
that the catalyst successfully reduces aromatic nitroso
compounds to aromatic amines with good yields (entries
1 and 2), while denitrosofication takes place due to –N–N–
bond cleavage, in the case of aliphatic and aromatic
nitrosoamines (entries 3–5). The catalyst was also used
for the chemoselective reduction of nitroarenes and it
was found to be more efficient (Table 3) than other
mesoporous-based catalysts.7;12 It is also noteworthy
here that the PdMCM-41 catalyst can successfully
reduce a relatively hindered aromatic nitro compound,
2-nitronaphthalene (Scheme 2), with good yields in a
much shorter time than that reported in literature.7;14
However, one of the most important characteristics of
the PdMCM-41 catalyst is the simultaneous cleavage of
both –N@N– and –C–X (X ¼ a halogen) bonds. Thus,
the reduction of nitroso, nitro, azo, and hydrazo com-
pounds yields aniline (Scheme 3).
9
10
11
12
13
14
15
16
4-OH
3-CH3CHOH
2-NH2
3-NH2
4-NH2
4-CH2CN
NO2
NH2
PdMCM-41
HCOONH4
MeOH
[35 min, 88%]
Scheme 2.
yields. Functional groups such as –CHO were tolerated.
However, it is important to note that both reduction (of
carbonyl groups) and hydrodehalogenations (of halides)
takes place concurrently when isopropanol and KOH
were used for the reaction (Scheme 4). Indeed, with time,
Table 4 summarizes the results of hydrodehalogenation
of aryl halides over PdMCM-41. The hydrodehalogen-
ations of –Cl and –Br functions were achieved in good
Table 2. CTH of nitroso compounds over PdMCM-41
Entry
Substrate
Time (min)
Product
Yield (%)
Third run
First run
91
NO
NH2
1
2
30
50
89
81
NH2
NO
OH
OH
86
80
NO
N
H
N
3
120
77
O
H
N
O
NO
N
4
5
150
180
71
62
74
63
N
N
CH3
CH3
NO
N
H
N

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P. Selvam et al. / Tetrahedron Letters 45 (2004) 3071–3075
CH(OH)CH3
COCH3
N=N
N=N
NO
NO
NO
NH
2
PdMCM-41
(CH ) CHOH
3 2
2
2
KOH
X
X
[4 h, 71%]
Cl
N
H
N
Scheme 4.
X
H
[PdMCM-41, MeOH, HCOONH4]
[X = a halogen; 0.5-6 h; 70-95%]
NH2
NO2
Scheme 3.
PdMCM-41
HCOONH4
MeOH
X
both the reduction and hydrodehalogenation can easily
be followed throughout the reaction with nitro aryl
halides (Scheme 5). Furthermore, the PdMCM-41 cat-
alyst is more effective than several other catalytic sys-
tems, which require much longer reaction times,
typically 1–2 days, to achieve similar yields.5d;e
[X = 4-F, 30 min, 90%; X = 2-Cl, 45 min, 93%]
[X = 3-Cl, 35 min, 91%; X = 4-Cl, 40 min, 95%]
Scheme 5.
Acknowledgements
In summary, PdMCM-41 was found to be a promising
heterogeneous catalyst under mild transfer hydrogena-
tion conditions15 for the selective reduction of alkenes,
a,b-unsaturated carbonyl compounds, aromatic nitro
and nitroso compounds, N,N-hydrogenolysis of azo and
hydrazo functions as well as simultaneous reduction and
hydrodehalogenation of substituted aryl halides,
including hindered substrates. The conversion was much
faster and cleaner than conventional methods and
moreover, in the present case, the reaction occurs under
mild reaction conditions. Thus, the CTH method
employing PdMCM-41 catalyst is a highly selective
route for environmentally benign organic reductions.
The authors thank Sophisticated Analytical Instrumen-
tation Facility (SAIF), IIT-Bombay for ICP-AES data.
References and notes
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Entry
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Product
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91
O
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Br
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C
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70
NH2
NH2
Cl
COCH3
COCH3
Cl

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(1.5 g) of the total fumed silica (SiO2; Aldrich; an
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ÔSolution-AÕ. In the next step, ÔSolution-BÕ was prepared
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(CTAB; Aldrich) and sodium hydroxide (NaOH; 0.742 g
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remaining fumed silica (3.0 g), which resulted in the
formation of a gel. The resulting gel was then stirred for
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palladium acetate solution (0.127 g in 5 mL water; Loba)
was added dropwise to the gel and stirred for 15 min. A
final volume of water (29 mL) was then added to the gel,
which was then stirred for a further 1 h so that a
homogeneous gel resulted. The pH was maintained at
11.4. The final gel had the following (molar) composition:
1SiO2:0.27CTAB:0.26NaOH:0.26TMAOH:60H2O:0.01PdO
(Si/Pd ¼ 100). The gel was then transferred into a Teflon-
lined stainless steel autoclave and kept in an air oven for
crystallization at 423 K for 72 h. The solid product, that is,
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with respect to the starting amount of fumed silica.
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NH3–TPDmeasurements (peak in the range 833–913 K).
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solved in methanol (10 mL) to which ammonium formate
(30 mmol) was added as a hydrogen donor along with the
catalyst (PdMCM-41; 50 mg). The reaction mixture was
then refluxed at 343 K for several minutes to a few hours
depending on the nature of the substrates. The progress of
the reaction was monitored by TLC, and the products
were analyzed using GC (Eshika) fitted with an OV-17
column. After completion of the reaction, the catalyst was
recovered for the recycling studies by simple filtration,
washed several times with acetone followed by distilled
water, then activated at 373 K for 6 h.
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15. The PdMCM-41 catalyst was also tested for its reusability,
and interestingly, the yield was practically unaffected for
up to three cycles for these substrates. The catalyst was
also used for the reduction of cinnamaldehyde (up to six
cycles) without significant change in the yield of cinnamyl
alcohol (first run ¼ 90%; sixth run ¼ 88%). The XRD
pattern of the recycled PdMCM-41 catalyst (after the sixth
run) and BET surface area measurement showed that the
structural integrity remained unaltered after reuse. The
mesoporous nature of the reused catalyst was confirmed
by N2 adsorption measurements (BET surface area ¼
985 m2 gꢀ1, pore volume ¼ 0.85 cm3 gꢀ1, and pore size
9. X-ray diffractograms (Rigaku-miniflex, Cu-Ka) of both
synthesized and calcined PdMCM-41 showed patterns
typical of hexagonal mesoporous systems.7 Further, N2
adsorption measurements (BET surface area; 970 m2 gꢀ1
,
pore volume; 0.78 cm3 gꢀ1, and pore size; 31 A) support the
mesoporous nature of the sample. ICP-AES analysis
shows 2.8 wt% Pd loading in the catalyst.
31 A). Overall there was no major detrimental effect
ꢁ
ꢁ
observed from the reaction conditions over the structural
properties of PdMCM-41.