Three-Component Activation/Alkynylation/Cyclocondensation (AACC) Synthesis of Enhanced Emission Solvatochromic 3-Ethynylquinoxalines.

2-Substituted 3-ethynylquinoxaline chromophores can be readily synthesized by a consecutive activation-alkynylation-cyclocondensation (AACC) one-pot sequence in a three-component manner. In comparison with the previously published four-component glyoxylation starting from electron-rich π-nucleophiles, the direct activation of (hetero)aryl glyoxylic acids allows the introduction of substituents that cannot be directly accessed by glyoxylation. By introducing N,N-dimethylaniline as a strong donor in the 2-position, the emission solvatochromicity of 3-ethynylquinoxalines can be considerably enhanced to cover the spectral range from blue-green to deep red-orange with a single chromophore in a relatively narrow polarity window. The diversity-oriented nature of the synthetic multicomponent reaction concept enables comprehensive investigations of structure-property relationships by Hammett correlations and Lippert-Mataga analysis, as well as the elucidation of the electronic structure of the emission solvatochromic π-conjugated donor-acceptor systems by DFT and time-dependent DFT calculations with the PBEh1PBE functional for a better reproduction of the dominant charge-transfer character of the longest wavelength absorption band.

Four- and Five-Component Syntheses and Photophysical Properties of Emission Solvatochromic 3-Aminovinylquinoxalines.

3-Aminovinylquinoxalines are readily accessible from (hetero)aryl glyoxylic acids or heterocyclic π-nucleophiles by consecutive four- and five-component syntheses in the sense of an activation-alkynylation-cyclocondensation-addition sequence or glyoxylation-alkynylation-cyclocondensation-addition sequence in good yields. The title compounds are highly fluorescent with pronounced emission solvatochromicity and protochromic fluorescence quenching. Time-resolved fluorescence spectroscopy furnishes radiative and nonradiative fluorescence decay rates in various solvent polarities. The electronic structure is corroborated by DFT and TD-DFT calculations rationalizing the observed spectroscopic effects.

Highly Convergent Synthesis of Intensively Blue Emissive Furo[2,3-c]isoquinolines by a Palladium-Catalyzed Cyclization Cascade of Unsaturated Ugi Products.

A convergent and diversity-oriented approach to the unusual furo[2,3-c]isoquinoline scaffold is presented. This serendipity-driven approach is characterized by an Ugi multicomponent reaction, which gives the substrate for a palladium-catalyzed insertion-alkynylation-cycloisomerization cascade to provide the furo[2,3-c]isoquinolines in moderate to high yield. Upon UV excitation, all representatives are intensively blue luminescent, as observed by the naked eye, and quantitative fluorescence spectroscopy reveals a considerable effect of the substitution pattern on the quantum yields. The electronic structure is semiquantitatively rationalized by DFT and time-dependent DFT calculations.