Solvent-free Multicomponent Synthesis of Biologically-active Fused-imidazo Heterocycles Catalyzed by Reusable Yb(OTf)3 Under Microwave Irradiation

A rapid, efficient and solvent-free-green-protocol for Groebke-Bienayme'-Blackburn reaction (G-B-B reaction) for the synthesis of fused-imidazo heterocycles has been developed. The methodology reported here involves multi-component reaction (MCR) catalyzed by reusable Yb(OTf)(3) (a mild and water-compatible Lewis acid) under microwave irradiation which allows fast and efficient preparation of the title compounds in excellent yield. The salient features of our protocol are solventfree, low catalyst loading (2.5-0.1 mol%) with good turnover number (TON: 890) and turnover frequency (TOF: 178/min), less reaction time (5 min), no dependency over specialized purifica-tion (by either column chromatography or recrystallization) and very high isolated yield (95-99%) with excellent green chemistry metrics (E-factor: 0.071 and Mass Intensity: 1.071). The water compatibility of the catalyst Yb(OTf)(3) has been exploited for its efficient recovery through water washings. In addition, the other exciting milestones of the protocol are catalyst and workup solvent recycling, excellent conversion with notorious substrates such as enolizable aldehyde or isonitrile bearing reactive substituent, very efficient at higher scale (50 mmol) and easy to couple with other methods (one-pot two-step cyclization: G-B-B reaction and Ullmann-type coupling).