Modular synthesis of N-heteroaromatic cations by (2+2+2) cycloaddition

Abstract

5 Abstract N-Heteroaromatic cations represent a family of compounds with diverse application potentials including natural products, DNA probes, fluorescence dyes, herbicides, ionic liquids, organocatalysts, etc. The preparation of these species proceeds mainly through N-alkylation which is used mostly as a last step of the synthetic sequence. Only limited diversity of cationic substrates can be achieved with this manner, hence, searching for other methods leading to novel Nheteroaromatic cations is of great interest. Some examples of transition metal-catalyzed transformations on organic cations have appeared over the last two decades including Suzuki- Miyaura reaction, Sonogashira and Stille coupling, oxidative homocoupling, ring-closing metathesis, etc. Here, the successful construction of new N-heteroaromatic cations by [2+2+2] cycloaddition is presented. A methodology for the synthesis of novel pyridine-type organic monocations was developed using gaseous acetylene as a reaction partner in the cyclization step. Subsequently, the double [2+2+2] cycloaddition was utilized for the preparation of oligo-p-phenylenes with two to seven para connected phenylene units. Furthermore, the intramolecular version of the cyclization step led to two helical and four linear dications which can be considered as extended...

5 Abstract N-Heteroaromatic cations represent a family of compounds with diverse application potentials including natural products, DNA probes, fluorescence dyes, herbicides, ionic liquids, organocatalysts, etc. The preparation of these species proceeds mainly through N-alkylation which is used mostly as a last step of the synthetic sequence. Only limited diversity of cationic substrates can be achieved with this manner, hence, searching for other methods leading to novel Nheteroaromatic cations is of great interest. Some examples of transition metal-catalyzed transformations on organic cations have appeared over the last two decades including Suzuki- Miyaura reaction, Sonogashira and Stille coupling, oxidative homocoupling, ring-closing metathesis, etc. Here, the successful construction of new N-heteroaromatic cations by [2+2+2] cycloaddition is presented. A methodology for the synthesis of novel pyridine-type organic monocations was developed using gaseous acetylene as a reaction partner in the cyclization step. Subsequently, the double [2+2+2] cycloaddition was utilized for the preparation of oligo-p-phenylenes with two to seven para connected phenylene units. Furthermore, the intramolecular version of the cyclization step led to two helical and four linear dications which can be considered as extended...