Iron(III) chloride-catalysed aerobic reduction of olefins using aqueous hydrazine at ambient temperature

Article abstract of DOI:10.1002/adsc.201200110

A chemoselective reduction of olefins and acetylenes is demonstrated by employing catalytic amounts of ferric chloride hexahydrate (FeCl ₃·6 H₂O) and aqueous hydrazine (NH ₂NH₂·H₂O) as hydrogen source at room temperature. The reduction is chemoselective and tolerates a variety of reducible functional groups. Unlike other metal-catalysed reduction methods, the present method employs a minimum amount of aqueous hydrazine (1.5-2 equiv.). Also, the scope of this method is demonstrated in the synthesis of ibuprofen in aqueous medium. Copyright

Full text of DOI:10.1002/adsc.201200110

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DOI: 10.1002/adsc.201200110
Iron(III) Chloride-Catalysed Aerobic Reduction of Olefins using
G
Aqueous Hydrazine at Ambient Temperature
Manjunath Lamani,a Guralamata S. Ravikumara,a
and Kandikere Ramaiah Prabhua,*
a
Department of Organic Chemistry, Indian Institute of Science, Bangalore 560012, Karnataka, India
E-mail: prabhu@orgchem.iisc.ernet.in
Received: November 21, 2011; Revised: February 9, 2012; Published online: May 15, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200110.
Abstract: A chemoselective reduction of olefins
and acetylenes is demonstrated by employing cata-
lytic amounts of ferric chloride hexahydrate
(FeCl3·6H2O)
and
aqueous
hydrazine
(NH2NH2·H2O) as hydrogen source at room tem-
perature. The reduction is chemoselective and toler-
ates a variety of reducible functional groups. Unlike
other metal-catalysed reduction methods, the pres-
ent method employs a minimum amount of aqueous
hydrazine (1.5–2 equiv.). Also, the scope of this
method is demonstrated in the synthesis of ibupro-
fen in aqueous medium.
Scheme 1. Decompostion of hydrazine.
Keywords: air; aqueous hydrazine; diimide; ferric
chloride hexahydrate; olefins; reduction; selectivity
Scheme 2. Reduction of olefins through the diimide route.
and Naota[7] reported an efficient reduction of olefins
catalysed by organocatalysts such as flavin and its de-
rivatives employing a lesser amount of aqueous hy-
Transition metal-catalysed reductions are one of the drazine. However, flavin-based catalysts are prepared
prominent research areas in organic chemistry.[1,2] The in multi-step reactions and a few of them require stor-
reduction of olefins and acetylenes has been reported age at low temperature.[7b]
by employing homogeneous and heterogeneous tran-
Designing a reduction method using less than
sition metal catalysts.[3] There is always a demand for 2 equiv. of aqueous hydrazine is a challenging task.
a more selective, milder, user friendly and safer meth- Several reduction strategies reported require forcing
ods of reduction to accompolish complex synthetic conditions like heating the reaction mixture to
challenges.[4] In this context, aqueous hydrazine (for- 1008C.[10] Alternative strategies involving hydrazine
merly known as hydrazine hydrate)[5] is one of the re- derivatives such as sulfonylhydrazide[7c,g] have also
agents that has been a subject of repeated investiga- been employed to promote the reduction efficiently.
tion as it caters to the above needs.
Recently, nanocrystals such as Rh-Fe3O4, and Fe3O4
Generally, aqueous hydrazine is an efficient and have been used to reduce olefins,[10] which employ 6–
safe precursor for the generation of molecular hydro- 12 equiv. of hydrazine at higher temperatures. Among
gen (Scheme 1).[6,7] It is well known that hydrazine is all these attempts, the reduction of olefins catalysed
oxidised to diimide (Scheme 2),[8] which is a good re- by cationic flavin derivatives and isoalloxazines are
ducing agent. However, disproportionation of diimide some of the successful attempts that did not require
to molecular nitrogen and hydrazine is one of the un- a large excess (1.2–2 equiv.) of aqueous hydrazine.[7]
wanted side reactions, which affects the efficiency of However, in these cases one has to specially design
the reduction. As a consequence, generally a large the flavin derivatives using a multistep reaction se-
excess of aqueous hydrazine (10–400 equiv.) has to be quence, and in many cases they are not stable[7b]. It is
employed for the reductions.[9] More recently, Imada evident from the mechanism of reduction of olefins
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reduction of olefins,[10] to the best of our knowledge,
there is no report on the utility of FeCl3 for the re-
duction of olefins. The initial reduction reaction with
anhydrous FeCl3 revealed that 5 mol% of catalyst is
required along with 1.5 equiv. of aqueous hydrazine
(entries 4–6) to reduce styrene to ethylbenzene.
Either lowering the catalyst loading or decreasing the
amount of aqueous hydrazine resulted in a lowering
of the yield of the reduced product (entries 4 and 5).
However, reduction of 1a is very effective with
1.5 equiv. of aqueous hydrazine, anhydrous FeCl3
(5 mol%), and required 14 h reaction at ambient tem-
perature to afford the reduced product ethylbenzene
in almost quantitative yield (entry 6). Other iron cata-
Table 1. Optimisation studies.
lysts such as Fe2O3, FeSO4·7H2O, Fe
Fe(NO3)3·9H2O under the similar reaction conditions
ACHTUGNTNERN(UNG ClO4)3·xH2O, or
AHCTUNGTRENNUNG
are not as effective as anhydrous FeCl3 (entries 7–12).
Ultimately, it was found that the reduction of styrene
to ethylbenzene is very effective with 5 mol%
FeCl3·6H2O and 1.5 equiv. of aqueous hydrazine at
room temperature (14 h, entry 13, Table 1).
Having established the optimal reduction condi-
tions, we continued our investigation to evaluate the
substrate scope. Therefore, a variety of olefins was
subjected to the reduction under the optimised condi-
tions at room temperature using 1.5–2 equiv. of aque-
ous hydrazine and 5 mol% FeCl3·6H2O (Table 3). Al-
lylbenzene 1b underwent a smooth reduction at room
temperature to furnish 1-propylbenzene 2b in almost
quantitative yield (4 h, entry 1, Table 2). Similarly, a-
[a]
Yield determined by GC-MS.
Reaction carried out in an N2 atmosphere.
[b]
using hydrazine (Scheme 1) that at least one equiva- methylstyrene under the standard reaction conditions
lent of aqueous hydrazine is necessary. In this per- required 24 h to afford the corresponding reduced
spective, we planned the utility of inexpensive transi- product 2c in almost quantitative yield (99%, entry 2,
tion metal catalysts to reduce olefins using 1–2 equiv. Table 2). 1-Octyne 1d was reduced efficiently to
of aqueous hydrazine under aerobic conditions. As octane, but as expected needed 4 equiv. of aqueous
a result, herein we report our recent finding that a cat- hydrazine to furnish the completely saturated product
alytic amount of FeCl3·6H2O and aqueous hydrazine 2d in 99% yield (16 h, entry 3, Table 2). To exploit the
(1.5–2 equiv.) are sufficient to promote an efficient utility of this mild reduction, several functionalised
chemoselective reduction of olefins under aerobic olefins were subjected for the reduction. As seen in
conditions.
the examples provided in Table 2, the reduction ap-
As it is apparent from the literature, an efficient pears to be very selective. O-Allyl, sulphides, and
and controlled formation of diimide is important in nitro functional groups were unaffected during the re-
designing an efficient reduction. In this direction, the duction, while the double bonds were saturated suc-
reductions employing flavin derivatives are promising. cessfully to provide the reduced products 2e, 2f, and
Otherwise most of the methods use a large excess of 2g in excellent yields (entries 4, 5, and 6, Table 2).
aqueous hydrazine. Therefore, we started screening The entry 6 illustrates the chemoselectivity observed
a variety of metal catalysts to reduce styrene (1a). As in the reduction of olefins, as the nitro group was
we proposed to design a method to use the minimum inert under the conditions to produce the correspond-
amount of aqueous hydrazine, we used 1–1.5 equiv. of ing reduced product 2g in excellent yield. Similarly,
aqueous hydrazine during the optimisation reactions the azido functionality was also observed to be stable
(Table 1) using air as the oxygen source.[11] As seen in under the reduction conditions as 1-(allyloxy)-4-(azi-
Table 1, catalysts such as PdACHTNUTGRNEG(UN OAc)2, RuCl3·xH2O and domethyl)benzene 1h under optimal reduction condi-
RhCl3·xH2O, did not produce promising results (en- tions produced the product 2h (entry 7, Table 2). The
tries 1–3). Further optimisation reactions were carried entry 8 illustrates a multiple selectivity that is seen in
out with iron catalysts, as they are easily available the reduction of 1i as the N-benzyl and nitro groups
and iron is an inexpensive metal. Although, there are were inert under the employed conditions to produce
limited reports on the utility of iron catalysts for the the corresponding reduced product 2i in excellent
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IronACHTUNGTRENNUNG(III) Chloride-Catalysed Aerobic Reduction of Olefins using Aqueous Hydrazine
Table 2. Reduction of olefins.[a]
[a]
Reaction conditions: air, aqueous hydrazine (1.5 equiv.), FeCl3·6H2O (5 mol%),
EtOH, room temperature.
Isolated yields.
Yield by GC-MS.
4 equiv. of aqueous hydrazine were used.
2 equiv. of aqueous hydrazine were used.
Reaction carried out at 508C.
[b]
[c]
[d]
[e]
[f]
yield. Similarly, as can be seen in the entry 9, the O- bonds (Table 3). As reported in the literature, the re-
benzyl group was inert, whereas the allyl group was duction of internal double bonds works well with dii-
successfully reduced in 4 h to give an almost quantita- mide, but needs a large excess of hydrazine and also
tive yield of 2j (entry 9 Table 2). The excellent selec- requires heating conditions.[7,10] However, in the pres-
tivity exhibited by the present method is illustrated in ent study it was observed that the reduction of cin-
the reduction of substrate 1k, which contains a CBZ namyl alcohol (1l) proceeded well at room tempera-
protecting group. As seen in the product 2k the ture with 2 equiv. of aqueous hydrazine to furnish the
double bond was successfully saturated, whereas the corresponding alcohol 2l in almost quantitative yield
CBZ group remained intact under the reaction condi- (entry 1, Table 3). Furthermore, an excellent chemo-
tions (entry, 10, Table 2).
selectivty in the reduction of internal double bonds in
Having successfully reduced terminal double bonds, the presence of ester as well as amide functionalities
we turned our attention to reduce internal double is illustrated in entries 2 and 3. Hence, ethyl cinna-
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Table 3. Reduction of internal olefins.[a]
[a]
Reaction conditions: air, aquous hydrazine (2 equiv.), FeCl3·6H2O (5 mol%), EtOH,
room temperature.
Isolated yields.
Reaction carried out at 808C.
Reaction carried out at 508C.
[b]
[c]
[d]
[e]
Reaction carried out with corresponding sodium salt using aqueous hydrazine (2 equiv.)
in water at 1008C.
[f]
Aqueous hydrazine (4 equiv.).
Yield determined by 1H NMR with respect to starting material.
[g]
mate 1m and N,N-diethylcinnamamide 1n underwent an alcohol functionality does not affect the reduction.
a smooth reduction to produce the products 2m and Hence 1,3-diphenylprop-2-en-1-ol 1o underwent
2n in good yields. As seen in entry 4, the presence of a smooth reduction to furnish the reduced product
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IronACHTUNGTRENNUNG(III) Chloride-Catalysed Aerobic Reduction of Olefins using Aqueous Hydrazine
Table 4. Recycling experiments.[a]
Scheme 3. A tentative mechanism.
[a]
1,3-diphenylpropan-1-ol 2o in excellent yield. The suc-
cess of the present method is demonstrated in the re-
duction of 2-(4-isobutylphenyl)acrylic acid 1p to fur-
nish ibuprofen, a non-steroidal anti-inflammatory
drug, in excellent yield. In this example, it is impor-
tant to recognise that the reaction proceeds well and
the acid functionality is well tolerated under the re-
Reaction conditions: air, styrene (5 mmol), aquous hydra-
zine (1.5 equiv.), FeCl2·ACHTNUGTREN(NUNG NH2NH2)2 (5 mol%), EtOH,
room temperature.
Yield determined by GC-MS.
[b]
To confirm this postulate, we have independently
duction conditions (entries 6 and 8, Table 3). Further- prepared FeCl2·ACTHUNRGTENUNG(NH2NH2)2 by reacting aqueous hy-
more, as seen in Table 3, the in situ generated sodium drazine and FeCl3·6H2O and reacted it with styrene
salts of 2-(4-isobutylphenyl)acrylic acid (1q), and 2- under the standard reaction conditions. As anticipat-
styrylbenzoic acid (1r) underwent smooth reduction ed, the reduction proceeded well to furnish the re-
in aqueous medium at elevated temperature to afford duced product. Interestingly, after the complete re-
the reduced products 2q and 2r in good yields (en- duction, the intermediate FeCl ·
2 ACHTUNGTRENN(NUG NH2NH2) was reused
tries 7 and 9, Table 3). for the reduction to find that the reduction proceeded
Furthermore, to demonstrate the selectivity in the well during the second run, whereas the third run pro-
reduction of terminal double bonds in the presence of duced a lower yield of the product (Table 4).[16]
internal double bonds, we subjected linalool (1s) to
In conclusion, as already noted, the metal-catalysed
the reduction reaction using 2 equivalents of aqueous reduction of olefins employs a large excess of aqueous
hydrazine at ambient temperature. As expected, hydrazine (10–20 equiv.). To the best of our knowl-
a good selectivity was observed as the terminal edge, the work presented here is the first and also an
double bond was selectively reduced in the presence efficient method of utilising aqueous hydrazine in low
of internal trisubstituted double bond to furnish 2s in amounts (1.5 equiv.) for the chemoselective reduction
95% yield (entry 10, Table 3). It is important to recog- of olefins and acetylenes by employing a catalytic
nise that this kind of selectivity cannot be achieved by amount of FeCl3·6H2O at room temperature. Addi-
classical hydrogenation methods using Pt, Pd or Ni.[12] tionally selective reduction of terminal double bonds
A similar selectivity was also seen in the reduction of in the presence of internal double bonds is demon-
limonene (1t) to yield 2t, in which the terminal strated. Further studies are underway in our laborato-
double bond was saturated in the presence of an in- ry to explore the scope and the mechanistic aspects.
ternal trisubstituted double bond (entry 11, Table 3).
Also, when linalool (1s) was subjected to the reduc-
tion using 4 equiv. of aqueous hydrazine at 808C, both Experimental Section
terminal and trisubstituted internal double bonds
Caution! Aqueous hydrazine is toxic reagent that is sus-
pected to induce cancer, and should be handled with care in
an efficient fume hood.
were reduced to furnish the completely reduced prod-
uct 3s in 83% yield (entry 11, Table 3). As seen earli-
er, 3,7-dimethyloct-6-en-1-ol (1u), which contains a tri-
substituted olefin also underwent a smooth reduction
to provide 2u in good yield at 808C (entry 13,
Table 3).[13]
Typical Experimental Procedure; Reduction of 1-
(Allyloxy)-4-bromobenzene (1e) to 1-Bromo-4-
propoxybenzene (2e)
It appears that aqueous hydrazine reacts with
FeCl3·6H2O to form the corresponding hydrazine
FeCl3·6H2O (6.7 mg, 5 mol%) and olefin (1e, 106 mg,
0.5 mmol) were dissolved in ethanol (4 mL), immediately
followed by addition of aqueous hydrazine (246 mL,
5 mmol). The reaction mixture was then stirred at room
temperature in air. The reaction mixture was extracted with
dichloromethane (20 mLꢁ3) and dried over anhydrous
complex of iron, FeCl2·ACTHNUTRGNEUNG
(NH2NH2)2 (I).[14] This iron
complex (a white solid), further catalyses the reduc-
tion of olefins[8t] in the presence of air to produce the
corresponding reduced product (Scheme 3).[15]
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821 cmÀ1
;
1H NMR (CDCl3, 400 MHz): d=7.34 (d, J=
8.9 Hz, 2H), 6.76 (d, J=8.9 Hz, 2H), 3.86 (t, J=6.5 Hz,
2H), 1.83–1.74 (m, 2H), 1.02 (t, J=7.3 Hz, 3H); 13C NMR
(CDCl3, 100 MHz): d=158.21, 132.14, 116.27, 112.52, 69.7,
22.46, 10.43; anal. calcd. for C9H12O; C 50.26, H 5.15;
found: C 50.61, H 5.47.
Acknowledgements
This work was supported by ARMREB, New-Delhi
(ARMREB/EMCB/2009/111), the Indian Institute of Science
and RL fine Chem. The authors thank Dr. A. R. Ramesha
and Prof. S. Chandrasekhar for useful discussion. ML thanks
CSIR, New-Delhi for a senior research fellowship.
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[15] However, formation of the intermediate II could not
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yield of the reduced product 2a (99%), whereas the
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(2a) (55%).
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