265
systems for biphasic reactions [78, 79]; regardless of that, rare commercially feasible instances have come to the forefront [80]. Moreover, easier and potential methodologies, e.g. employment of supercritical carbon dioxide (CO 2 ) [81, 82] and ionic
liquids [83, 84] as solvents were suggested. Despite these approaches deliver distinguished significances, among them, supercritical CO 2 operates under high pressure
with discharge of CO 2 gas. In contrast, ionic liquids are usually expensive, procedures for their preparation are arduous, and their toxic nature and ecological stress
are yet unidentified, particularly the first generation of ionic liquids are crucial with
respect to their toxic nature [85, 86]. This perhaps turns out to be an obstacle in its
evolution as a solvent for green synthesis. Also, different organic solvents are
broadly employed in organic conversions and have been a reason for a major issue
owing to their corresponding environmental threats. Thus, a visionary preference of
the reaction medium performs a crucial function in the establishment of an area of
growing viable organic transformations [87, 88] and considerable focus has been
dedicated for the progress of the effective and reusable catalytic systems for C–H
functionalizations.
In this view, conventional solvents in organic chemistry can be replaced by polyethylene glycols (PEGs). On account of the characteristics such as lower vapour
pressure, economical, thermal persistency, biodegradability, stability in both acidic
and basic conditions, easily recoverable, and less toxic, these act as a suitable solvent for ecologically favourable and harmless organic conversions [89–91].
Moreover, having certain characteristics, PEGs were served for catalyst immobilization [92], nanoparticle stabilization [93], ligand stabilization [94] and as phase
transfer catalysts [95].
In the light of the superiority shown by PEG, a green and sustainable solvent
system, it has been fruitfully employed as a medium for the metal-catalysed crosscoupling transformations like Heck reaction [96], Suzuki reaction [97, 98], the
homocoupling and cross-coupling of aryl halides [99], the straightforward arylation
of 1,2,3-triazoles [100], carbonylative Suzuki [101], and carbonylative Sonogashira
couplings [89], hydrosilylation of terminal alkynes [102] and C−H functionalization [103] with easy reusability of solvents as well as precious metals. Illustrative
examples and innovative features of the recycling approaches in C–H activation
reactions considering sustainable chemistry, cost-effectiveness, technical usefulness and toxicity viewpoint have been concisely discussed here.
In 2015, Cai and co-workers proposed recyclable [RuCl 2 (p-cymene)] 2 /Cu(OAc) 2 /
PEG-400/H 2 O catalytic system for the efficient and green synthesis of phthalide
derivatives (Scheme 11) [104]. The stated catalytic system demonstrated a higher
efficiency to achieve the cascaded intermolecular oxidative C–H bond alkenylation/
oxa-Michael addition of benzoic acids with alkenes in order to attain desired products. The proposed catalytic system potentially reused up to sixth time with negligible decrease in catalytic activity.
Encouraged by the work done by Cai group, Bhanage research group extensively
explored the recyclability of Ru(II)/PEG-400 catalytic systems for C–H functionalization reactions [103]. First, highly efficient Ru(II)/PEG-400 catalytic system has
been employed for the annulation reaction of N-methoxybenzamides, benzoic acids
Insights into Sustainable C–H Bond Activation
systems for biphasic reactions [78, 79]; regardless of that, rare commercially feasible instances have come to the forefront [80]. Moreover, easier and potential methodologies, e.g. employment of supercritical carbon dioxide (CO 2 ) [81, 82] and ionic
liquids [83, 84] as solvents were suggested. Despite these approaches deliver distinguished significances, among them, supercritical CO 2 operates under high pressure
with discharge of CO 2 gas. In contrast, ionic liquids are usually expensive, procedures for their preparation are arduous, and their toxic nature and ecological stress
are yet unidentified, particularly the first generation of ionic liquids are crucial with
respect to their toxic nature [85, 86]. This perhaps turns out to be an obstacle in its
evolution as a solvent for green synthesis. Also, different organic solvents are
broadly employed in organic conversions and have been a reason for a major issue
owing to their corresponding environmental threats. Thus, a visionary preference of
the reaction medium performs a crucial function in the establishment of an area of
growing viable organic transformations [87, 88] and considerable focus has been
dedicated for the progress of the effective and reusable catalytic systems for C–H
functionalizations.
In this view, conventional solvents in organic chemistry can be replaced by polyethylene glycols (PEGs). On account of the characteristics such as lower vapour
pressure, economical, thermal persistency, biodegradability, stability in both acidic
and basic conditions, easily recoverable, and less toxic, these act as a suitable solvent for ecologically favourable and harmless organic conversions [89–91].
Moreover, having certain characteristics, PEGs were served for catalyst immobilization [92], nanoparticle stabilization [93], ligand stabilization [94] and as phase
transfer catalysts [95].
In the light of the superiority shown by PEG, a green and sustainable solvent
system, it has been fruitfully employed as a medium for the metal-catalysed crosscoupling transformations like Heck reaction [96], Suzuki reaction [97, 98], the
homocoupling and cross-coupling of aryl halides [99], the straightforward arylation
of 1,2,3-triazoles [100], carbonylative Suzuki [101], and carbonylative Sonogashira
couplings [89], hydrosilylation of terminal alkynes [102] and C−H functionalization [103] with easy reusability of solvents as well as precious metals. Illustrative
examples and innovative features of the recycling approaches in C–H activation
reactions considering sustainable chemistry, cost-effectiveness, technical usefulness and toxicity viewpoint have been concisely discussed here.
In 2015, Cai and co-workers proposed recyclable [RuCl 2 (p-cymene)] 2 /Cu(OAc) 2 /
PEG-400/H 2 O catalytic system for the efficient and green synthesis of phthalide
derivatives (Scheme 11) [104]. The stated catalytic system demonstrated a higher
efficiency to achieve the cascaded intermolecular oxidative C–H bond alkenylation/
oxa-Michael addition of benzoic acids with alkenes in order to attain desired products. The proposed catalytic system potentially reused up to sixth time with negligible decrease in catalytic activity.
Encouraged by the work done by Cai group, Bhanage research group extensively
explored the recyclability of Ru(II)/PEG-400 catalytic systems for C–H functionalization reactions [103]. First, highly efficient Ru(II)/PEG-400 catalytic system has
been employed for the annulation reaction of N-methoxybenzamides, benzoic acids
Insights into Sustainable C–H Bond Activation