Arkivoc 2018, ii, 241-251
Rayhan, U. et al.
Introduction
Addition of hydrogen to aromatic alkynes is one of the important transformations in synthetic organic
chemistry. Catalytic hydrogenation using hydrogen gas or hydride transfer agents is commonly employed for
this transformation. Numerous hydrogenation reactions have been reported, for which the active catalysts
commonly employed include systems based on metals such as Pd, Rh, Ru, Ni, Pt, Ir, Os, V, Fe, and Nb.1 There
is particular interest in the use of Pd given its excellent performance in the selective hydrogenation of
alkynes.2 Nowadays, isolated single atom Pd sites in intermetallic nanostructures are used for high catalytic
selectivity in the semihydrogenation of alkynes and light mediated preparation of palladium nanoparticles as
catalysts for alkyne cis-semihydrogenation.3,4 It is found that N,N-dimethylformamide (DMF) can act as a
hydride source in nickel-catalyzed asymmetric hydrogenation of α,β-unsaturated esters.5 Also a Hantzsch
amido dihydropyridine has been reported as a transfer hydrogenation reagent for α,β-unsaturated ketones
while pyridine derivatives were reduced via borane-catalyzed transfer hydrogenation reaction with ammonia-
borane.6,7 Recently, use of a copper catalyst as a monophasic catalytic system and silica-supported copper
nanoparticles have been employed for the selective semireduction of alkynes.8,9 In another study, a nonclasical
Co-H2 system was used to assess the effect of hydrogen spin on isomerization and E-selectivity in an alkyne
semihydrogenation reaction.10 Moreover, rationally tuned micropores within enantiopure metal-organic
frameworks for highly selective separation of acetylene and ethylene as well as tuning the gate opening
pressure of metal-organic frameworks (MOFs) for the selective separation of hydrocarbons have appeared in
the literature.11–12 Microporous metal-organic framework with dual functionalities for highly efficient removal
of acetylene from ethylene/acetylene mixtures have also been reported. In addition, a microporous metal-
organic framework for highly selective separation of acetylene, ethylene and ethane from methane at room
temperature has been reported.13,14
Over a century ago Paul Sabatier developed nickel-catalyzed hydrogenation as one of the most commonly
applied procedures for organic synthesis.15,16 The use of complex hydrides as reducing reagents causes many
environmental concerns. Catalytic hydrogenation is widely considered to be an environmentally benign
process while both heterogeneous and homogeneous alternatives are popular in industry.17–23 Hydrogen is an
o
explosive gas and its production through steam reforming is a highly energy-intensive (700-1000 C) process
and it also causes considerable emission of carbon dioxide, a well-known greenhouse gas.24 So the current
method of production of hydrogen cannot be considered to be environmentally-friendly. Therefore, the
overall environmental impact of catalytic hydrogenation cannot be considered negligible. Nowadays,
heterogeneous catalysts are being used for the selective reduction of condensed N-heterocycles using water
both as a solvent and a hydrogen source.25 More recently, the application of a heterogeneous catalyst in
combination with microwave irradiation has been employed as an environmentally benign tool for some
contemporary organic syntheses.26,27 Based on the application of Raney-type Ni-Al alloy in aqueous medium
as the hydrogen source as well as solvent, the selective reduction of ketones and reductive amination of
carbonyl compounds has been reported.28,29
Tashiro et al. have pioneered the use of various metal-Al alloys (Ni-Al, Co-Al, Cu-Al, Fe-Al) in water for the
reduction of aryl ketones to the corresponding alcohols or alkane derivatives. Raney Ni-Al alloy in water was
found to be a good reducing agent for the reduction of aromatic ketones and aldehydes to the corresponding
cyclohexane derivatives, but a large amount of alloy was employed in these studies.30 Thereafter, under
microwave irradiation, the reduction of acetophenones was performed using Raney Ni-Al alloy in water. Also
reported were the reductive reactions of a series of aromatic ketones with noble metal catalysts such as Rh,
Ru, Pd or Pt along with added Al powder in water in a sealed tube.31,32 Raney Ni-Al was found to be very
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