Electron‐Deficient Bipyrrole Boomerangs: Bright Fluorophores Obtained via Double C−H Bond Activation

Abstract

Pd -mediated annulative double C-H activation is shown to efficiently convert 1,n-dipyrrolylalkanes into extensively π-conjugated bipyrroles not accessible by conventional oxidative coupling protocols. This approach is applicable to both electron-rich and electron-deficient systems, and has been further developed into tandem processes involving further cyclization of substituents or oxygenation of pyrrolic α-positions. The new bipyrrole intermediates show enhanced fluorescence as well as tunable optical properties controlled by the alignment of chromophore subunits. Photophysical data, including femtosecond transient absorptions, reveal solvent-induced intramolecular charge transfer in their excited states, dependent on the polarity of the medium.

Bipyrrole boomerangs via Pd-mediated tandem cyclization–oxygenation. Controlling reaction selectivity and electronic properties

“…Analogous α-unsubstituted intermediates (cNDA2 H and cNDA3 H ) were not isolated in reactions involving NDA2 H and NDA3 H . The latter behavior is consistent with the selectivity pattern observed previously for α-unsubstituted dipyrrolylmethanes NDA1 H and NMI1 H , for which we were only able to isolate the correspond-ing dilactams cNDA1 O and cNMI1 O , respectively [32]. Transient formation of cNDA1 H could, however, be observed in situ for reactions involving NDA1 H [33].…”

“…Consequently, incomplete ring fusion and various side reactions, e.g., chlorination [7], or unexpected rearrangements, are frequently observed [8]. The use of oxidative couplings is further limited for the synthesis of strained [9,10], electron-deficient, or sterically Scheme 1: The previously reported family of the boomerang bipyrroles obtained by Pd-induced double C-H bond activation [32].…”

“…As a part of our ongoing research on π-extended electron-deficient oligopyrroles [13,[28][29][30][31], we have recently reported that Pd(II)-mediated double C-H activation can be a useful tool for conversion of 1,n-dipyrrolylalkanes into boomerang-shaped N,N'-bridged α,α'-bipyrroles that are not accessible by means of conventional oxidative coupling methods (Scheme 1) [32]. Our approach is applicable to electron-deficient and sterically encumbered systems, notably those based on pyrrole derivatives fused with naphthalenediamide (NDA) and naphthalenemonoimide (NMI) moieties.…”

Stereocontrolled Synthesis of Chiral Heteroaromatic Propellers with Small Optical Bandgaps

“…The final cyclodehydrogenation was, however, challenging: the FeCl 3 oxidation, previously used to make 3 , failed with the hindered HPBs 6 a – c , yielding only mixtures of ill‐defined, possibly polymeric products, instead of the expected 4 a – c . Conversely, an attempted coupling of 6 a – c with palladium(II) acetate under double C−H bond activation conditions resulted in a nearly complete recovery of the starting materials. These setbacks indicated the potential role of steric hindrance and internal strain as limiting factors in the cyclodehydrogenation step.…”

Porphyrin‐Ryleneimide Hybrids: Low‐Bandgap Acceptors in Energy‐Transfer Cassettes

“…[11,13] Here we demonstrate the utility of ryleneimide-metalloporphyrin hybrids as acceptor units in energy transfer cassettes ( Figure 1). The parent chromophore, [14] containing four naphthalenemonoimide (NMI) units fused to a central porphyrin moiety, belongs to a recently developed family of donoracceptor (DA) oligopyrroles, [15][16][17][18][19] characterized by small electronic bandgaps, and extended redox activity. Related DA porphyrins have recently been explored as agents for antimicrobial photodynamic inactivation of Gram-positive bacteria.…”