Synthesis and Characterization of Azine‐[5]Helicene Hybrids

Abstract

Novel azine‐helicene hybrids (pyridine‐, pyrazine‐ and quinoxaline‐fused along the central ring [5]helicenes) have been prepared in good overall yields through a five‐step synthetic sequence. Commercially available 2,3‐dihaloazines were used as starting materials. To discern the effect of merging an azine moiety within a helical skeleton, the X‐ray structures, UV/Vis absorption spectra, cathodic and anodic electrochemistry of the helicene hybrids were investigated and compared to that of the parent [5]helicene.

Synthesis, crystal structures and properties of carbazole-based [6]helicenes fused with an azine ring

“…Apparently, interaction of this type is also realized in solution since the signal of the inner helix proton in the 1 H NMR spectra of azine-fused [6]helicenes appeared in the low field at δ 9.3-9.7 ppm. A similar pattern was observed by us in the structures of the azine-fused [5]helicenes [72].…”

“…Compound 8b was synthesized in 97% yield by the treatment with a 3-fold excess of ICl on monoiodide 7b. The ICl-induced cyclization of the pyridinebased starting compound 6 proceeded smoothly giving rise to product 7c in 62% ( The fourth step of the synthesis of the target helicenes, namely the Sonogashira coupling of iodides 7a and 7c with p-tolylacetylene, was carried out in the Pd(PPh 3 ) 2 Cl 2 /CuI/Et 3 N/THF catalytic system that proved itself well in the azine-fused [5]helicenes synthesis [72] (Table 4, entries 1 and 3). The corresponding compounds 9a and 9c were obtained in 84 and 88% yields, respectively.…”

“…The structure of 9c was unambiguously proved by X-ray structural analysis (see Supporting Information File 1, Figure S34). Earlier, at the final step of a similar synthesis of the azine-fused [5]helicenes, we used trifluoroacetic acid as a cyclizing agent [72]. Unfortunately, only in the case of compound 9c, heating in trifluoroacetic acid led to isomerization into the required carbazole-based [6]helicene 10c ( Table 5, entry 3).…”

“…Transition-metal-catalyzed, electrophile-induced and oxidative radical cyclizations of ortho-alkynylated biaryls are widely used for the synthesis of polynuclear aromatics [57][58][59][60][61][62][63][64][65][66][67][68][69]. Recently, we have described a versatile method for the preparation of aza [4]helicenes [70], diaza [4]helicenes [70,71] and azine-fused [5]helicenes [72] through a five-step synthetic sequence, using commercially available 2,3-dihaloazines as starting materials. Based on this approach, we synthesized carbazole-based [6]helicenes fused with an azine ring (quinoxaline, pyrazine or pyridine ones).…”

The synthesis and crystal structure of pH-sensitive fluorescent pyrene-based double aza- and diaza[4]helicenes

“…28 It can also be assumed that the protonation of the inner helix aza group will destabilize molecules 9a and 9b , destroying the C–H⋯N hydrogen bond and disrupting the planarity of their diazahelicene fragments. Indeed, this study and our previous studies 15–18 have shown that diazahelicenes are essentially planar, in contrast to monoaza analogs. It is also known that benzo[ h ]quinoline (p K a = 10.84 (MeCN) and 4.25 (H 2 O)) is a weaker base by 1.82 p K a units in MeCN and 1.37 units in water compared to isomeric acridine.…”