Using experimental and computational approaches to probe an unusual carbon-carbon bond cleavage observed in the synthesis of benzimidazole N-oxides

Computational, Mechanistic, and Experimental Insights into Regioselective Catalytic C-C Bond Activation in Linear 1-Aza-[3]triphenylene

C-C bond activation by transition metal complexes in ring-strained compounds followed by annulation with unsaturated compounds is an efficient approach to generate structurally more complex compounds. However, the site of catalytic C-C bond activation is difficult to predict in unsymmetrically substituted polycyclic systems. Here, we report a study on the (regio)selective catalytic cleavage of selected C-C bonds in 1-aza-[3]triphenylene, followed by annulation with alkynes, forming products with extended π-conjugated frameworks. Based on density functional theory (DFT) calculations, we established the stability of possible transition metal intermediates formed by oxidative addition to the C-C bond and thus identified the likely site of C-C bond activation. The computationally predicted selectivity was confirmed by the following experimental tests for the corresponding Ir-catalyzed C-C cleavage reaction followed by an alkyne insertion that yielded mixtures of two mono-insertion products isolated with yields of 34-36%, due to the close reactivity of two bonds during the first C-C bond activation. Similar results were obtained for twofold Ir- or Rh-catalyzed insertion reactions, with higher yields of 72-77%. In a broader context, by combining DFT calculations, which provided insights into the relative reactivity of individual C-C bonds, with experimental results, our approach allows us to synthesize previously unknown pentacyclic azaaromatic compounds.

2,2′-(1,4-Phenylene)bis(7-nitro-1H-benzimidazole 3-oxide)

Bis(benzimidazol-2-yl-3-oxide)benzene derivatives have potential applications as energetic or photoactive materials. By using a two-step one-pot approach employing microwave heating as a tool, 2,2′-(1,4-phenylene)bis(7-nitro-1H-benzimidazole 3-oxide) (1) has been prepared in 94% yield. In the first step an SNAr reaction is performed using p-xylylenediamine as the central building block. Without isolating the intermediate, a base-mediated cyclization reaction follows in the second step. The product was isolated in analytically pure form by means of a pH-controlled precipitation.