under inert atmosphere. The bimetallic properties of these NPs also provided insight
regarding their reactivity. They noted that the BMNPs are of a smaller particle
size than the individual Ni and Co NPs, suggesting that greater surface area is
available on which the reactions take place.
Hydrazine hydrate was chosen as the hydrogen donor en route to these BMNPs
due to its ease of handling as well as the inert by-product formed after use.
Temperature played a crucial role in this transformation; when run at 60
C, poor
conversion was noted, while higher temperatures led to aggregation of the NPs
resulting in reaction inhibition. A temperature of 70
C seemed to be the optimal
compromise. Water or ethanol served as the reaction medium depending upon the
solubility of the substrates. Electron-rich as well as electron-poor nitroarenes
were smoothly converted to the corresponding amines. Halogenated anilines could
also be obtained with no dehalogenation observed. Dinitro compounds were fully
consumed to the desired diamine products upon addition of excess hydrazine
hydrate. Most importantly, the nitro groups were reduced in the presence of olefins,
alkynes, and nitrile groups indicative of the selectivity of this process. However,
formyl substituents under the reaction conditions afforded the derived primary
alcohol.
Preparation of Ni 50 Co 50 BMNPs NiCl 2
. 6H 2 O (0.5 mg), CoCl 2 (0.26 mg), and
PVP (160 mg, average molecule weight ¼ 40,000) were dissolved in ethanol
(1.5 mL) and charged into a 10 mL reactor with a magnetic stirrer. Then, a freshly
prepared ethanol solution of NaBH 4 (0.8 mg, in 0.5 mL in ethanol) was added
into the reactor quickly under vigorous stirring (1,000 rpm) at rt. (25
C) under argon.
The color of the colloidal mixture turned to black immediately which indicates
that metal salts have been reduced to metal particles. The catalysts prepared were
directly used for reactions, as overexposure of catalysts containing Ni to air will
result in significantly decreased activity due to oxidation.
Typical Procedure Hydrazine hydrate (4 equiv) was added into the reactor which
contains freshly prepared catalyst, as described above. Then, the reactor was placed
into a pre-heated oil bath with a stirring speed of 500 rpm, and the substrate (1 mmol)
dissolved in 1 mL ethanol was added dropwise under argon. The reactions were
monitored by TLC. After the reaction, the mixture was vacuum filtered through
a pad of silica on a glass-fritted funnel, and an additional 15 mL of EtOAc (5 mL
portions) was used to rinse the product from the silica. The filtrate was concentrated
in vacuo and analyzed by GC. Products were purified by column chromatography
and identified by
1 H NMR and
13 C NMR.
Although reductive amination via metal hydrides or catalytic hydrogenation
has long been reported [29], uses of hydrogen transfer to facilitate the reaction
are sparse in the literature and often employ hydrazine or borohydride as the
hydrogen source. However, Yus and co-workers reported that Ni NPs can
catalyze reductive amination of aldehydes by hydrogen transfer using a relatively
environmentally benign 2-propanol as both solvent and reductant in the absence
of base (Fig. 13) [30]. Their nanoparticles were prepared by mixing anhydrous
90
M. Cortes-Clerget et al.
regarding their reactivity. They noted that the BMNPs are of a smaller particle
size than the individual Ni and Co NPs, suggesting that greater surface area is
available on which the reactions take place.
Hydrazine hydrate was chosen as the hydrogen donor en route to these BMNPs
due to its ease of handling as well as the inert by-product formed after use.
Temperature played a crucial role in this transformation; when run at 60
C, poor
conversion was noted, while higher temperatures led to aggregation of the NPs
resulting in reaction inhibition. A temperature of 70
C seemed to be the optimal
compromise. Water or ethanol served as the reaction medium depending upon the
solubility of the substrates. Electron-rich as well as electron-poor nitroarenes
were smoothly converted to the corresponding amines. Halogenated anilines could
also be obtained with no dehalogenation observed. Dinitro compounds were fully
consumed to the desired diamine products upon addition of excess hydrazine
hydrate. Most importantly, the nitro groups were reduced in the presence of olefins,
alkynes, and nitrile groups indicative of the selectivity of this process. However,
formyl substituents under the reaction conditions afforded the derived primary
alcohol.
Preparation of Ni 50 Co 50 BMNPs NiCl 2
. 6H 2 O (0.5 mg), CoCl 2 (0.26 mg), and
PVP (160 mg, average molecule weight ¼ 40,000) were dissolved in ethanol
(1.5 mL) and charged into a 10 mL reactor with a magnetic stirrer. Then, a freshly
prepared ethanol solution of NaBH 4 (0.8 mg, in 0.5 mL in ethanol) was added
into the reactor quickly under vigorous stirring (1,000 rpm) at rt. (25
C) under argon.
The color of the colloidal mixture turned to black immediately which indicates
that metal salts have been reduced to metal particles. The catalysts prepared were
directly used for reactions, as overexposure of catalysts containing Ni to air will
result in significantly decreased activity due to oxidation.
Typical Procedure Hydrazine hydrate (4 equiv) was added into the reactor which
contains freshly prepared catalyst, as described above. Then, the reactor was placed
into a pre-heated oil bath with a stirring speed of 500 rpm, and the substrate (1 mmol)
dissolved in 1 mL ethanol was added dropwise under argon. The reactions were
monitored by TLC. After the reaction, the mixture was vacuum filtered through
a pad of silica on a glass-fritted funnel, and an additional 15 mL of EtOAc (5 mL
portions) was used to rinse the product from the silica. The filtrate was concentrated
in vacuo and analyzed by GC. Products were purified by column chromatography
and identified by
1 H NMR and
13 C NMR.
Although reductive amination via metal hydrides or catalytic hydrogenation
has long been reported [29], uses of hydrogen transfer to facilitate the reaction
are sparse in the literature and often employ hydrazine or borohydride as the
hydrogen source. However, Yus and co-workers reported that Ni NPs can
catalyze reductive amination of aldehydes by hydrogen transfer using a relatively
environmentally benign 2-propanol as both solvent and reductant in the absence
of base (Fig. 13) [30]. Their nanoparticles were prepared by mixing anhydrous
90
M. Cortes-Clerget et al.