Current Green Chemistry - Volume 6, Issue 3, 2019

Some transformations are not possible with ground-state reactions even in the presence of a catalyst, hence they are performed under photochemical conditions. Electron transfer occurs even with the photochemical excitement of one molecule where redox reaction is not possible at the ground state. The side products are obtained from ground-state reactions. For C-C bond formation during photochemical reactions, there is no requirement of any chemical activation of the substrates. Therefore, these reactions are presented here for the synthesis of fused five-membered O-heterocycles in the context of sustainable processes from 1964 to 2019.

In the last few decades, considerable research has led to the introduction of selective and efficient green as well as sustainable synthetic methods of functionalization of carbon-hydrogen bonds to form new carbon-carbon and carbon-heteroatom bonds. In this emerging field, significant development has been attained under various environmentally benign conditions including aqueous medium. In this review, we have summarized the current development of C-H functionalization carried out in an aqueous medium and its synthetic applications according to carbon-carbon and carbon-heteroatom bond formations under green conditions.

Five membered nitrogen heterocycles are historically known to exhibit diverse therapeutic properties. In particular, the pyrazole analogues have shown a wide range of pharmacological profiles and a number of drugs containing this scaffold approved for the treatment of various ailments and disorders. Hence, a lot of efforts have been made to develop efficient synthetic strategies for the preparation of a variety of pyrazoles in the past several decades. However, this review describes the environmentally benign protocols for the synthesis of substituted and fused pyrazole molecules.

Spirooxindoles, particularly 3,3’-spirooxindoles constitute a privileged structural scaffold owing to the intensive biological activities which they possess. Because of this over the last twenty years, a large number of methods were devised for their synthesis and some of these molecules have entered pre-clinical trials. Of late, methods for spirooxindole synthesis using green protocols have developed rapidly. Reactions based on multicomponent strategies using non-catalytic / biocatalytic pathways and those done in aqueous media have been largely employed for the synthesis of 3,3’- spirooxindoles. This review focusses on the synthesis of 3,3’-spirooxindoles via green protocols and covers the literature from 2016 onwards (2016 - mid 2019); a review on the same topic has appeared in 2016. The green methods discussed here include reactions done in aqueous media, multicomponent strategies, alternate solvents and photocatalysis.

Aziridines are highly versatile intermediates in organic synthesis due to their easy access and their susceptibility to ring-opening by facile C-N bond cleavage. They are synthetically very important as they are valuable precursors of amino sugars, β-lactam antibiotics and alkaloids or present in various natural products that exhibit potent biological activities. The synthesis of this moiety from the easily available chemicals is desirable due to its tremendous use in the various branches of chemistry. Here, a short review has been reported on the synthesis of this scaffold employing different strategies under different greener conditions. Various methods have been developed in the presence of green catalysts and solvents.

Hydrolysis of Cinnamaldehyde (CNM) is one of the important processes for the production of industrially essential natural benzaldehyde. Benzaldehyde is a vital precursor in the production of perfumes, cosmetics, food, beverages, and pharmaceutical intermediates. As homogeneous base catalysts are polluting and difficult to separate, heterogeneous catalysts should be used. Hydrolysis of cinnamaldehyde to benzaldehyde was studied over modified hydrotalcite (HT) base catalysts wherein HT was activated with either zinc or titanium, by combustion synthesis using glycine or glycerol as fuel. Both the catalyst composition and combustion fuel affect the activity of modified HT catalysts. SEM, EDXS, BET surface area and porosimetry were used to characterize all catalysts. Zinc modified hydrotalcite using glycine as fuel (Zn-HT-Glycine) was the most active, selective, and reusable catalyst under mild reaction conditions, and it was used to study the influence of different process parameters on the reaction rate, conversion and selectivity. Reaction mechanism and kinetics were established. The reaction follows pseudo-first-order kinetics. At 1:92 mole ratio of cinnamaldehyde to water and 0.005 g/cm3 catalyst loading, the reaction gives 75.8 % conversion of cinnamaldehyde and 100 % selectivity to benzaldehyde at 130oC in 4 h. The apparent activation energy was 19.15 kcal/mol. The overall process is green and the catalyst reusable.