257
evolved. Today, the ideal catalytic systems should have characteristic common
requirements like it should be highly active, selective, durable, recyclable, compatible with practical reaction conditions, broadly applicable as well as cost, safety and
environmentally benign. Conventionally, the phrase catalysis is generally divided
into two divisions, i.e. homogeneous and heterogeneous catalysis which are just
similar to a pair of twins with dissimilar features [30–32]. Among these, homogeneous catalysts, largely organometallics, possessed a substantial development and
were utilized with varied reactive reagents to generate a huge amount of fine chemicals. In these systems, the starting reactant and active centres of catalysts exist in the
identical phase and because of clear availability of active centres to the reactant,
high activity performance is obtained. Whereas, the active centres of catalysts and
the reactants are in a distinct phase in case of heterogeneous catalysis.
The active site is recognized in homogeneous catalytic systems, and much that is
appearing in catalysis is detectable and alterable. As a result of this, extensively
used classical homogeneous catalysis has become an indispensable part of modern
organic chemistry [33]. For instance, the development of C–C, C–N, C–O bonds
using homogeneous catalysts with transition metals having extensive industrial utilization which is creating interest to chemists [34, 35], due to its capability to
accomplish the reaction under gentle conditions with improved selectivity as well as
yields [36–39]. In this way, homogeneous catalytic systems have been extensively
recognized as further efficient, selective as well as effortless for analysis and prediction as compared to heterogeneous systems.
Though the huge number of homogeneous catalysts are extensively utilized in
industries, the stability, cost, and availability of the catalysts (ligands and metal) are
the key considerations restricting its utilization on the commercial scale [40, 41].
For example, rhodium catalysts have extensive application in organic chemistry
[42–44], while a recent study exposes a severe hike in the market cost of rhodium
so that it becomes the costliest transition metal [45]. Similarly, palladium has wide
application as a catalyst in numerous C−C bond formations [46], which also exists
as an endangered and expensive element. In this view, considerable recognition has
been given for the advancement of methods for approximately quantitative recycle
as well as reuse of these precious metals. Additionally, the serious resistance in the
application of homogeneous catalytic systems exhibited by distinct disadvantages
such as complications in isolation as well as the separation of the ultimate product
from reaction media and reutilization of exhaustible valuable organometallic catalysts or costly ligands. In many cases the overall product gets contaminated with a
minute quantity of the metal catalyst (at ppm or at ppb level), so the total regaining
of the metal catalysts from the reaction mixture becomes a challenging task. In the
drug as well as pharmaceutical manufacturing, it is essential to eliminate the trace
metal catalyst from ultimate pure product entirely as it can be responsible for crucial
metal adulteration problem thus degrading the human health. According to guidelines by the European Medicines Agency, the acceptable oral exposure to such metals in a medicinal constituent is supposed to be usually not more than 10 ppm per
day [47].
Insights into Sustainable C–H Bond Activation
evolved. Today, the ideal catalytic systems should have characteristic common
requirements like it should be highly active, selective, durable, recyclable, compatible with practical reaction conditions, broadly applicable as well as cost, safety and
environmentally benign. Conventionally, the phrase catalysis is generally divided
into two divisions, i.e. homogeneous and heterogeneous catalysis which are just
similar to a pair of twins with dissimilar features [30–32]. Among these, homogeneous catalysts, largely organometallics, possessed a substantial development and
were utilized with varied reactive reagents to generate a huge amount of fine chemicals. In these systems, the starting reactant and active centres of catalysts exist in the
identical phase and because of clear availability of active centres to the reactant,
high activity performance is obtained. Whereas, the active centres of catalysts and
the reactants are in a distinct phase in case of heterogeneous catalysis.
The active site is recognized in homogeneous catalytic systems, and much that is
appearing in catalysis is detectable and alterable. As a result of this, extensively
used classical homogeneous catalysis has become an indispensable part of modern
organic chemistry [33]. For instance, the development of C–C, C–N, C–O bonds
using homogeneous catalysts with transition metals having extensive industrial utilization which is creating interest to chemists [34, 35], due to its capability to
accomplish the reaction under gentle conditions with improved selectivity as well as
yields [36–39]. In this way, homogeneous catalytic systems have been extensively
recognized as further efficient, selective as well as effortless for analysis and prediction as compared to heterogeneous systems.
Though the huge number of homogeneous catalysts are extensively utilized in
industries, the stability, cost, and availability of the catalysts (ligands and metal) are
the key considerations restricting its utilization on the commercial scale [40, 41].
For example, rhodium catalysts have extensive application in organic chemistry
[42–44], while a recent study exposes a severe hike in the market cost of rhodium
so that it becomes the costliest transition metal [45]. Similarly, palladium has wide
application as a catalyst in numerous C−C bond formations [46], which also exists
as an endangered and expensive element. In this view, considerable recognition has
been given for the advancement of methods for approximately quantitative recycle
as well as reuse of these precious metals. Additionally, the serious resistance in the
application of homogeneous catalytic systems exhibited by distinct disadvantages
such as complications in isolation as well as the separation of the ultimate product
from reaction media and reutilization of exhaustible valuable organometallic catalysts or costly ligands. In many cases the overall product gets contaminated with a
minute quantity of the metal catalyst (at ppm or at ppb level), so the total regaining
of the metal catalysts from the reaction mixture becomes a challenging task. In the
drug as well as pharmaceutical manufacturing, it is essential to eliminate the trace
metal catalyst from ultimate pure product entirely as it can be responsible for crucial
metal adulteration problem thus degrading the human health. According to guidelines by the European Medicines Agency, the acceptable oral exposure to such metals in a medicinal constituent is supposed to be usually not more than 10 ppm per
day [47].
Insights into Sustainable C–H Bond Activation