Abstract
Boomerang-shaped bipyrroles containing donor–acceptor units were obtained through a tandem palladium-mediated reaction consisting of a cyclization step, involving double C–H bond activation, and a double α-oxygenation. The latter reaction can be partly suppressed for the least reactive systems, providing access to α-unsubstituted boomerangs for the first time. These “α-free” systems are highly efficient fluorophores, with emission quantum yields exceeding 80% in toluene. Preliminary measurements show that helicene-like boomerangs may be usable as circularly polarized luminescent materials.
Highlights
Nanographenes and other polycyclic aromatics as well as their heterocyclic analogues are typically obtained by tandem cyclodehydrogenations of oligoaryl precursors [1,2,3]
Scheme 1: The previously reported family of the boomerang bipyrroles obtained by Pd-induced double C–H bond activation [32]
The present work shows that the tandem cyclization–oxygenation reaction is a general strategy for the synthesis of lowbandgap bipyrrole boomerangs and is applicable to targets with variable donor–acceptor character and increasing curvature of the bipyrrole linkage
Summary
Nanographenes and other polycyclic aromatics as well as their heterocyclic analogues are typically obtained by tandem cyclodehydrogenations of oligoaryl precursors [1,2,3]. Scheme 1: The previously reported family of the boomerang bipyrroles obtained by Pd-induced double C–H bond activation [32]. Subsequent screening revealed that the yield of cNMI2H and cNMI3H could be increased when reactions were performed in more dilute solutions (Table 1, entries 14 and 20).
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