Metal-free photoinduced alkylative [3 + 2] annulation of terminal alkynes with N-alkyl isoquinolin-2-iums by catalytic isoquinoline-based electron donor–acceptor complex†

A new, metal-free electron donor–acceptor (EDA) complex photocatalysis strategy for alkylative [3 + 2] annulation of N-alkyl isoquinolin-2-ium salts with common alkynes in ethyl actate/H₂O to assemble highly functionalized pyrrolo[2,1-a]isoquinolines is presented. Upon photoinducing reactivity using the catalytic isoquinoline-based EDA complex, this method enables incorporation of alkynes as two-carbon units to achieve alkylative [3 + 2] annulation of N-alkyl isoquinolin-2-ium salts, and is notable for its excellent site selectivity, tolerance of a wide range of functional groups, the use of aqueous media and avoidance of the need for expensive external photocatalysts and basic or acidic promotors. Mechanistic study indicates that the EDA complex is generated from association of a catalytic isoquinoline with N-alkyl isoquinolin-2-ium salts to photoinduce reactivity.