Sougata Santra

A decade update on solvent and catalyst-free neat organic reactions: a step forward towards sustainability

Particular success has been achieved in the synthesis of new products and in processes since the twelve principles of “green chemistry” were formulated in the 1990s. These products and processes are more compatible with human health, society, and the environment. In this review, a collection of research reports have been documented from the viewpoint of green chemistry. The main theme of this review is neat reactions, which are solvent and catalyst-free reactions. Neat reactions in the absence of any solvent or catalyst with concise summaries of microwave, ball milling, and neat reactions have been described.

Metal nanoparticles in “on-water” organic synthesis: one-pot nano CuO catalyzed synthesis of isoindolo[2,1-a]quinazolines

An environmentally benign nano CuO catalyzed “on-water” strategy has been developed for one-pot synthesis of isoindolo[2,1-a]quinazolines by a three-component coupling of isatoic anhydride, 2-carboxybenzaldehyde and amines in high yields. Aryl, heteroaryl and alkyl amines reacted very well under the present reaction conditions. Water plays a major role to accelerate this transformation through hydrogen bond mediated ‘electrophile–nucleophile dual activation’. The catalyst can be reused several times without significant loss of catalytic activity. The present method produces no waste, and represents a green synthetic protocol.

Organocatalysis by an aprotic imidazolium zwitterion: regioselective ring-opening of aziridines and applicable to gram scale synthesis

An imidazole-based zwitterionic-salt, 4-(3-methylimidazolium)butane sulfonate (MBS), has been found to be an efficient organocatalyst for aziridine ring-opening regioselectively by various nucleophiles like indoles, pyrroles, methanol, ethanol, acetic acid and di-iso-propylamine. The reactions are highly regioselective and they always afford the products resulting from benzylic attack. The present methodology is applicable to gram scale synthesis.

Diversified Synthesis of Furans by Coupling between Enols/1,3-Dicarbonyl Compounds and Nitroolefins: Direct Access to Dioxa[5]helicenes.

A versatile method for the diversified synthesis of furans and arenofurans has been developed that proceeds through K2CO3-promoted cyclization between enols/1,3-dicarbonyl compounds and nitroolefins at reflux in EtOH. This facile method has been successfully employed in the synthesis of benzotrifuran derivatives, which are useful hole-transporting materials. This procedure also provides direct access to dioxa[5]helicenes. This reaction offers a broad substrate scope, uses an inexpensive base and environmentally benign solvent, and is operationally simple.

Brønsted acidic ionic liquid-catalyzed tandem reaction: an efficient approach towards regioselective synthesis of pyrano[3,2-c]coumarins under solvent-free conditions bearing lower E-factors

1-Butane sulfonic acid-3-methylimidazolium tosylate, [BSMIM]OTs, is found to be a remarkable catalyst for the tandem cyclization of 4-hydroxycoumarin with chalcones for the syntheses of pyrano[3,2-c]coumarins under solvent-free conditions. The developed protocol is applicable for the construction of biologically important pyranocoumarins from easily accessible chalcones having various substituents. This reaction possibly proceeds through Michael addition followed by cyclization. The feasibility of catalyst recycling is also demonstrated. This method produces only water as the byproduct and represents a green synthetic protocol. The catalytic reaction proceeded very smoothly under solvent-free conditions and showed high regioselectivity. Clean reaction, non-chromatographic purification technique, easily accessible reactants, and metal and solvent-free and environmentally friendly reaction conditions are the notable advantages of this procedure. In addition, this method shows lower E-factors.

Solvent-free synthesis of pillar[6]arenes

An efficient solvent-free procedure for the synthesis of pillar[6]arenes has been developed. The procedure involves the solid-state condensation of finely milled 1,4-dialkoxybenzene and paraformaldehyde by grinding in the presence of a catalytic amount of H2SO4. The use of organic solvents for the extraction of products has also been avoided. Operational simplicity, compatibility with various 1,4-dialkoxybenzenes, non-chromatographic purification technique, high yields and mild reaction conditions are the notable advantages of this procedure. A large scale reaction demonstrated the practical applicability of this methodology.

One-pot multicomponent synthesis of polyhydroquinolines under catalyst and solvent-free conditions

Abstract An efficient one-pot condensation has been developed for the synthesis of polyhydroquinolines via a four component coupling reaction of aldehyde, dimedone, ethyl acetoacetate, and ammonium acetate under catalyst and solvent-free conditions. Non-hazardous experiment procedure, operational simplicity, mild reaction conditions, and the compatibility with various functional groups represent the advantages of the present method.

Combination of NH2OH·HCl and NaIO4: an effective reagent for molecular iodine-free regioselective 1,2-difunctionalization of olefins and easy access of terminal acetals

We have demonstrated a new application of our oxidizing reagent, a combination of NH2OH·HCl and NaIO4, in the first generalized regioselective 1,2-difunctionalization of olefins. It is a general method for the preparation of β-iodo-β′-hydroxy ethers, β-iodo ethers, β-iodohydrin, and β-iodo acetoxy compounds using different reaction media. The reactions are highly regioselective, always affording Markovnikovs type addition products. The methodology is also applicable for the easy access of terminal acetals. Molecular iodine-free synthesis, room temperature reaction conditions, high yields, use of less expensive reagents, mild reaction conditions, broad applicability of nucleophiles, and applicability for gram-scale synthesis are the notable advantages of this present protocol.

Solvent-free synthesis of 5-(aryl/alkyl)amino-1,2,4-triazines and α-arylamino-2,2′-bipyridines with greener prospects

A green and highly efficient method has been developed for the synthesis of 5-(aryl/alkyl)amino-1,2,4-triazines and α-arylamino-2,2′-bipyridines according to the principles of atom economy. It has been performed by two consecutive solvent-free reaction pathways: the ipso-substitution of a cyano-group in 5-cyano-1,2,4-triazines and the aza-Diels-Alder reaction of the resulting 5-arylamino-1,2,4-triazines with 1-morpholinocyclopentene used as a dienophile. Solvent and catalyst-free conditions, operational simplicity, the compatibility with various functional groups, nonchromatographic purification technique, and high yields are the notable advantages of this procedure. The present methodology possesses a low E-factor.

Extended cavity pyrene-based iptycenes for the turn-off fluorescence detection of RDX and common nitroaromatic explosives

Extended cavity pyrene-based iptycenes have been synthesized by using the Diels–Alder reaction between in situ generated dehydropyrenes and anthracene. The photophysical properties and the interaction of these iptycenes with nitro-explosive components were studied both in solution and in the solid state by using fluorescence spectroscopy and X-ray crystallography, respectively. Due to the presence of both the large iptycene cavity and the central pyrene core, an unprecedently high fluorescence-quenching response towards non-aromatic and non-planar 1,3,5-trinitroperhydro-1,3,5-triazine (RDX) has been observed both in solution (with an apparent Stern–Volmer constant value aKSV up to 1.53 × 103 M−1) and in the vapor phase (50–75% fluorescence quenching of the PU films doped with chemosensors). In the case of nitroaromatic explosives, nitrobenzene (NB), 2,4-DNT, TNT, and 2,4,6-trinitrophenol (TNP or picric acid, PA), pyrene-based iptycenes also demonstrate a good fluorescence-quenching response both in solutions (with apparent Stern–Volmer constant values aKSV = 0.4–8.0 × 103 M−1) and in the vapor phase (up to 90% fluorescence quenching of the PU films doped with chemosensors). The “sphere of action” fluorescence quenching model was suggested.