Synthesis and Characterization of Pyridine‐, Pyrazine‐, and Quinoxaline‐Derived [4]Helicenes and S‐Shaped Double [4]Helicenes

Abstract

A systematic study of the synthesis and properties of 1‐aza‐, 4‐aza‐, and 1,4‐diaza[4]helicenes as well as the previously unknown S‐shaped double 1,4‐diaza[4]helicenes has been reported. The synthetic route to the aza‐ and diaza[4]helicenes involved an electrophile‐induced cyclization of 3(2)‐alkynyl‐2(3)‐(naphthalen‐1‐yl)azines (i.e., quinoxaline, pyrazine, pyridine), which were prepared in two steps from the commercially available ortho‐dihalo‐substituted azine. Trifluoroacetic acid (TFA) and ICl were used as electrophiles. The ICl‐promoted cyclization provided the 5‐iodo derivatives of the aza‐ and diaza[4]helicenes in yields of 47–71 %. The use of TFA as the electrophile afforded the helicenes along with isomeric products from an alternative cyclization and subsequent 1,2‐shift of the C–C bond (e.g., naphtho[2,1‐f]quinoxalines or naphtho[2,1‐a]phenazines, 87–89 % total yield, 1.5–1.8:1 ratio). The presence of an iodine substituent in the hetero[4]helicenes allowed for the introduction of an alkynyl group by using a Sonogashira reaction. The subsequent cycloizomerization upon treatment with TFA gave the S‐shaped double 1,4‐diaza[4]helicenes. The molecular structures and crystal packing of the synthesized hetero[4]helicenes relative to those of [4]carbohelicene have been discussed.

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

“…In accordance with the above strategy, we first synthesized ortho-halogen alkynylazines 1a,b via the Sonogashira reaction of commercially available 2,3-dichloroquinoxaline and 2,3dichloropyrazine with phenylacetylene using a known procedure [70]. Coupling of compounds 1a,b with 9-ethyl-3-(4,4,5,5tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole in the Pd(PPh 3 ) 4 /K 2 CO 3 /1,4-dioxane/H 2 O catalytic system for 24 h at 100 °C (method C) afforded the desired 3-alkynyl-2-carbazolylazines 2a,b in 82-96% yields (Table 1).…”

“…3-Alkynyl-2-carbazolylazines can also be prepared using an alternative synthetic sequence, i.e., the Suzuki-Miyaura arylation-Sonogashira reaction. It should be noted that in the case of 2,3-dibromopyridine it was the only way for us to synthesize the target [4]helicenes [70]. Unfortunately, the coupling of 2,3dichloroquinoxaline (3a) with 9-ethyl-3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole in the 5% Pd-C/PPh 3 / K 2 CO 3 /toluene/H 2 O catalytic system at 100 °C for 24 h (method A) gave the corresponding carbazolyl derivative 4a in 15% yield only ( Table 2).…”

“…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

“…This signal is typical of 1-aza[4]helicenes. 15,16 At the same time, in the spectrum of isomer 6b (see ESI, Fig. 14†), the lowest-field signal in the form of a singlet is located at 10.1 ppm and corresponds to the proton H(6).…”

“…To implement the above strategy, we first synthesized ortho -halogen alkynylazines 1 (Table 1) via the Sonogashira reaction of commercially available 2,3-dichloroquinoxaline and 2,3-dichloropyrazine with phenylacetylene using a known procedure. 16 Three catalytic systems were tested for the Suzuki coupling of chloride 1a with pyren-1-ylboronic acid (Table 1, entry 1, Methods A, B and C). The system Pd(PPh 3 ) 4 /K 2 CO 3 /1,4-dioxane, H 2 O (Method C) was the most effective giving rise to the arylation product 2a in 74% yield.…”