Pd-Induced Double C–H Bond Activation in Annulative Syntheses of Bipyrrole Boomerangs: Mechanistic Insights from NMR Spectroscopy and Computation

Abstract

1, n-Dipyrrolylalkanes can be efficiently converted into extensively π-conjugated bipyrroles by Pd-mediated annulative double C-H activation, and this approach might be further developed into tandem processes involving further cyclization of substituents or oxygenation of pyrrolic α-positions. Herein, the mechanism of these transformations is explored using NMR spectroscopy and DFT calculations. The kinetics of the annulation are found to depend on the conjugation extent and donor-acceptor character of the pyrroles, as well as on substitution and the linker length. Combined experimental and theoretical evidence indicates that a change of the rate-determining step occurs for the most electron-deficient substrates. The unprecedented double α-oxygenation of bipyrroles is found to be a stepwise process, involving α-acetoxylated intermediates.

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

“…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].…”

“…Subsequent screening revealed that the yield of cNMI2 H and cNMI3 H could be increased when reactions were performed in more dilute solutions (Table 1, entries 14 and 20). Following our previous experimental and computational findings [32,33], we also checked whether the yields of dilactam products might be improved by increasing the concentration of acetate anions in the reaction mixture. Indeed, annulations of NDAn H and…”

“…The scope of such Pd(II)-induced couplings was further developed into tandem processes involving consecutive cyclization of substituents (dcTT EE ) and oxygenation of pyrrolic α-positions to form lactams cNDA1 O and cNMI1 O . The mechanism of those transformations was subsequently explored using NMR spectroscopy and DFT calculations [33]. In particular, the unprecedented double α-oxygenation of bipyrroles was shown to occur through stepwise acetoxylation, which we found to compete with α-α oligomerization.…”

Naphthalimide‐Fused Dipyrrins: Tunable Halochromic Switches and Photothermal NIR‐II Dyes

“…[38][39][40][41][42] Here we report on the halochromism and electronic structure properties of a family of dipyrrins bearing fused naphthal- imide (NMI) moieties (Figure 1). NMI pyrroles [43] have been previously shown to be versatile building blocks for the synthesis of electron-deficient porphyrins, [43][44][45] azacoronenes, [46][47][48] bipyrroles, [49][50][51] and polymers. [52] The family comprises two series of dipyrrins 1a-1f and 2a-2e, bearing respectively phenyl, and p-(dimethylamino)phenyl groups at the pyrrolic 𝛼 (or 1,9) positions (G1), and aryl substituents with variable donor/acceptor strength at the meso bridge (G2).…”

Synthesis of π-Extended Imidazoles by 1,3-Dipolar Cycloaddition of Polycyclic Aromatic Azomethine Ylides with Nitriles

“…Since the early 20th century, PAH molecules fused with pyridine, a nitrogen-containing six-membered arene, have played a central role in PAH chemistry due to their ready availability and high stability . Pyrrole, a nitrogen-containing five-membered arene, is also a popular framework for forming a variety of pyrrole-fused PAH molecules. ,− On the contrary, fewer examples of imidazole-fused PAH molecules have been reported, − despite the fact that the introduction of imidazole derivatives can be an efficient way to make PAH molecules more electron-deficient, suggesting a potential application as n-type semiconductors. To increase the variety of nitrogen-containing PAH molecules, the development of novel polycyclic aromatic imidazole derivatives is highly desirable.…”