Synthesis of triazene-substituted homoconjugated push-pull chromophores by formal [2 + 2] cycloadditions

3-Alkynylindoles as Building Blocks for the Synthesis of Electronically Tunable Indole-Based Push–Pull Chromophores

“…Following the regioselective 3-iodination 1 was obtained in 74% yield. At the same time, alkynes substituted with electron-rich and electron-poor phenyl groups 2a – e , PAHs 2f – i , required for the synthesis of N -alkyl indole-based substrates 3a – i have also been synthesized using literature procedures. − With iodo-indole 1 and terminal alkynes 2a – i in hand, the Sonogashira cross-coupling step has been performed. While preparing disubstituted alkynes 3a , 3b , 3c , and 3e , cross-coupling reactions occurred at room temperature.…”

“…Initially, 3a was tested as a substrate in CA-RE with electron-deficient TCNE 4 , and target push–pull chromophore 5a was isolated in 76% yields. At this point, it was still unclear whether the group that activates the alkyne for the reaction was N -alkyl indole or the triazene, ,, which is known to be an efficient electron donor in the literature. To confirm the electron donor role of N -alkyl indoles, 3c and 3e containing electron-withdrawing groups (phenyl and nitrophenyl) were treated with TCNE.…”

“…Commercially available chemicals were purchased, and no additional purification has been performed. Compounds 2a , 73 2b , 75 2d , 76 2e , 77 2f , 74 2g , 74 2h , 74 and 2i ( 78 ) were prepared according to literature procedures. Solvents (dichloromethane, hexanes, and ethyl acetate) used for extraction or column chromatography procedures were distilled.…”

“… 46 Similarly, long-term storage limitations and instability reduce the diversity of donor-substituted alkynes used in CA-RE. Metal acetylides, 19 , 47 dialkylaniline, 20 ferrocene, 23 thiophene, 48 p-methoxybenzene, 48 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY), 49 cyclopenta[ b ]furan-2-one, 50 metalloporphyrins, 51 carbazole-substituted alkynes, 52 ynamides, 53 tetrathiafulvalene, 17 azulene, 54 phenothiazine, 55 triazene, 56 , 57 and ureas 33 are donor substrates displaying sufficient reactivity in CA-RE reactions with TCNE. Surprisingly, the donor substrate scope (dialkylaniline, 42 ferrocene, 58 cyclopenta[ b ]furan-2-one, 59 carbazole, 60 metal acetylides, 61 azulene, 62 phenothiazine, 63 thiophene 64 ) is much more limited in CA-RE reactions with TCNQ ( Figure 1 ).…”

Click-Type Synthesis of Homoconjugated Push–Pull Chromophores: Computational Investigation of Optical and Nonlinear Optical (NLO) Properties

“…17,18 In 2019, our group successfully demonstrated that triazenes can also be used as donor groups in formal [2+2] cycloaddition. 19 Following this work, our attempts to broaden the scope of substrates that could be used in [2+2] cycloadditions with double triazene-substituted alkynes unfortunately did not work. To circumvent this issue, we have recently designed unsymmetrical dialkylaniline/triazenesubstituted unsymmetrical alkynes that undergo successful [2+2] cycloaddition-retroelectrocyclizations to form πconjugated non-planar push-pull NLOphores.…”