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The hydrogenation of nitrogen-nitrogen bonds was zines, azoxy compounds, and N-oxides, under very
also achieved in short reaction times and good yields. mild reaction conditions (room temperature and at-
Thus, phenylhydrazine (5 a) and 1-methyl-1-phenyl- mospheric pressure). In general, this catalyst showed
hydrazine (5 b) were reduced to aniline and N- a reactivity superior to other nickel catalysts and,
methylaniline, respectively (Table 1, entries 12 and compared to Raney nickel, its behaviour was similar
13). Finally, this methodology found application in in the reduction of alkenes, alkynes, and hydrazines,
deoxygenation reactions of azoxy compounds and N- but better results were obtained in the reduction of
oxides. Thus, starting from azoxybenzene (6 a) and aromatic compounds, alkyl chlorides, azoxy com-
4,4'-dimethoxyazoxybenzene (6 b) the corresponding pounds and N-oxides. Further work on the chemical
azo compounds were obtained (Table 1, entries 14 nature of the catalyst will be carried out in the near
and 15, respectively), and the N-oxides isoquinoline future.
N-oxide (7 a) and 4-phenylpyridine N-oxide (7 b)
were transformed into the expected heteroaromatic
compounds (Table 1, entries 16 and 17).
Other substrates such as ketones, imines and azo
Experimental Section
compounds remained unaltered or very low conver-
sion was observed under the above described condi-
tions.
General Methods
For general information see Ref. [11d] All starting materials
(except compounds 6 a, 6 b, and 7 a) were commercially
available (Aldrich, Fluka) of the best grade and were used
without further purification. For the preparation of com-
pounds 6 a, 6 b, and 7 a, see Ref. [11f] All products gave satis-
factory physical, chromatographic, and spectroscopic data
by comparison with authentic samples which were commer-
cially available or previously prepared by us following other
methodologies.[11]
The nickel(0) prepared in the mentioned manner
was very finally divided, standing as a suspension
even after several days, and proved to be highly reac-
tive as any attempt to support it failed, resulting in de-
activation. In general, the reactivity of this catalyst is
comparable to that of Raney nickel as regards the hy-
drogenation of alkenes,[12] alkynes,[12] and hydra-
zines[13] which is also accomplished under soft reac-
tion conditions. However, the reduction of aromatic
compounds,[12] alkyl chlorides,[14] azoxy com-
pounds,[15] and N-oxides[16] by Raney nickel does not
take place or harsh reaction conditions are required.
Our catalyst also proved to be superior to the nickel
boride obtained with the NiCl2/NaBH4/DMF combi-
nation in the reduction of alkyl chlorides,[7b] and sof-
ter reaction conditions were required in the hydroge-
nation of aromatic and heteroaromatic compounds in
comparison with other nickel catalysts (H2/Ni/EtOH,
NiCl2/Zn/MeOH).[17] Better behavior of our catalyst
was observed in the reduction of carbon-carbon dou-
ble and triple bonds in comparison with the nickel-
aluminum alloy, the latter being used in large excess
in a basic medium and which only affects these func-
tions when they are benzylic or conjugated with a car-
bonyl group.[6a] Finally, and in order to establish a
comparison of our catalyst with the widely used palla-
dium on charcoal, the hydrogenation of anthracene to
furnish 1,2,3,4-tetrahydroanthracene was carried out
in 2 h at room temperature with our nickel catalyst
(Table 1, entry 10), whereas hydrogenation on Pd/C
required the use of tetralin as solvent in a sealed tube
at 340 °C.[18]
Preparation of the Catalytic Mixture
The catalytic mixture was composed of lithium powder
(175 mg, 25 mmol), anhydrous nickel(II) chloride (650 mg,
5.0 mmol), and ground naphthalene (32 mg, 0.25 mmol),
which was always kept under nitrogen.
Hydrogenation of Compounds 1±7. General
Procedure
The catalytic mixture (75 mg, 40 mol % Ni) was introduced
into a Schlenck tube together with THF (3 mL) and the start-
ing material (1 mmol) under a hydrogen atmosphere (1 atm)
at room temperature. The reaction mixture, which was initi-
ally green, changed to black indicating the formation of
Ni(0). The course of the reaction was followed by GLC and/
or TLC and after the reaction time specified in Table 1, the
resulting black suspension was diluted with diethyl ether
(20 mL) and filtered through a pad containing silica gel and
celite (ca. 3 : 1). After drying the filtrate over anhydrous
Na2SO4, it was evaporated (15 torr) and the resulting residue
analyzed without any purification or purified by column
chromatography (silica gel, hexane or hexane/diethyl ether)
(see footnotes in Table 1).
In this paper we have described the catalytic hydro-
genation of a series of organic compounds by using a
system composed of NiCl2-Li-naphthalene (cat.). This
combination, which generates a very reactive nickel,
has been applied to the catalytic hydrogenation of a
series of organic compounds such as alkenes, al-
kynes, organic halides, aromatic compounds, hydra-
Acknowledgements
This work was generously supported by the Direccio n Gener-
al de EnsenÄ anza Superior (DGES) of the Spanish Ministerio
de EducacioÂn y Cultura (MEC; grant no. PB97±0133). We
are very grateful to Dr. G. Radivoy for providing samples of
the starting materials 6 a, 6 b, and 7 a.
190
Adv. Synth. Catal. 2001, 343, 188±191
Alonso, Francisco
