Site-specific role of bifunctional graphitic carbon nitride catalyst for the sustainable synthesis of 3,3-spirocyclic oxindoles in aqueous media

Functionalized graphitic carbon nitride (Sg-C(3)N(4)) has been manufactured and used as a reusable catalyst for the one-pot production of various spiro-pyrano chromenes and spiro indole-3,1′-naphthalene tetracyclic systems in aqueous media. An ultrasound-assisted method has been used for the functionalization of g-C(3)N(4). The catalytic functionalities and the structural integrity of the catalyst were characterized via different analytical tools. The catalytic site-specific role of Sg-C(3)N(4) was confirmed via various control experiments in one-pot reaction sequences. We recognized that Sg-C(3)N(4) acts as a bifunctional acid–base catalyst for the first reaction sequence whereas it is an acidic catalyst for the second reaction sequence during the one-pot production of various spiro-pyrano chromenes. In addition, the bifunctional acid–base catalytic role of Sg-C(3)N(4) has been confirmed for the first reaction sequence whereas it has a basic catalytic role for the second reaction sequence during the one-pot production of spiro indole-3,1′-naphthalene tetracyclic systems. Diverse C–C, C–O, and C–N bonds, six-membered cycles, stereogenic centers, and spiro frameworks were formed in a single reaction, enhancing the biocidal profile and possibly resulting in the discovery of new medicinal properties. The mild reaction environment, simple workup, easy separation, low cost, heterogeneity, and recyclability of Sg-C(3)N(4) are some rewards of this approach.