DABCO-Catalyzed β-C(sp2)-H Indolylation of 4-Arylidene Pyrazolones and 5-Arylidene Thiazolones with N-Tosyliminoindoles

A simple, efficient tactic for the direct β-C(sp²-H) functionalization reaction of various 4- or 5-arylidene pyrazolones/thiazolones using N-tosyliminoindoles catalyzed by 20 mol % DABCO in an open atmosphere is reported for the first time. This metal-free reaction represents a rare example of C3-alkenylation of 2-N-tosylaminoindoles, creating an efficient C(sp²)-C(sp²) bond selectively through a Michael-aerial oxidation process under aerobic conditions. Consequently, this method allows for the de novo access to value-added β-(2-N-tosylaminoindol-3-yl)-β-arylmethylene pyrazolones and unexpectedly, oxazolin-4(5H)-ones in satisfactory chemical yields and excellent selectivities (up to Z/E ≤ 99:1 or E/Z ≤ 99:1) with 100% carbon-atom efficiency. Moreover, incorporating a fluorine atom and alkyl peroxy group at the C4 and C3 positions on pyrazolone and indole rings, respectively, of β-(2-N-tosylaminoindol-3-yl) methylene pyrazolones was successfully achieved with remarkable selectivity through a dearomatization process, demonstrating the synthetic usefulness of the synthesized unsaturated pyrazolones.