Water is the most precious and essential element for the sustainability of
life. It has emerged as a versatile solvent for various chemical transformations in recent
times. It is a naturally abundant, cheap, non-toxic, inexhaustible, and non-flammable
green solvent that possesses several unique physiochemical properties like hydrogen
bonding, stays in a liquid state at a high range of temperatures, high dielectric constant,
large surface tension, and heat capacity. These features make water a powerful domain
for the eco-friendly and green synthesis of heterocycles via both in-water and on-water
methods. Diversified heterocyclic moieties are formed in an aqueous phase through
various organic reactions like multi-component reactions (MCRs), pericyclic, Wittig,
Michael, Mannich, Aldol, Suzuki, Sonogashira, hydroformylation, and other organocatalyzed
reactions with high atom economy, stereo-selectivity, and sustainability. This
article gives a systematic, comprehensive, and authoritative study of a range of
reactions in which water is used as a solvent for the synthesis of heterocycles. This
article endows an impetus to explore the synthetic and mechanistic aspects of “on” and
“in” water reactions and gives insights into the divergence between on-water and inwater
synthesis.
Keywords: Atom-economy, Catalysis, Green solvent, Heterocycles, Multicomponent
reaction, Organic synthesis, Organo catalyzed reactions, Water.