Direct synthesis of arenes and heterocycles from esters and their use as organophotocatalysts

Abstract

The development of modern methods for the synthesis of important molecular scaffolds is crucial for sustainable chemistry. This encompasses the synthesis of substituted arenes and heterocyclic compounds as they are vital motifs in pharmaceuticals, functional materials, dyes and nutritional products. Nowadays, most robust syntheses employ versatile transition metal-catalyzed cross-coupling reactions. However, the use of organic reagents and catalysts is often more environmentally benign, modular and economical, particularly for long-term processes. This thesis describes the development of a practical, one-step synthesis of acenes from carboxylic acid esters with 1,5-bifunctional organometallic reagents. The mild double nucleophilic addition of 1,5-organodimagnesium reagents, followed by a 1,4-elimination reaction gives direct access to mono-substituted anthracenes, tetracenes and highly reactive pentacenes as well as nonlinear dibenz[a,j]anthracenes. Furthermore, due to the ideal reactivity of organometallic reagents and the high variety of esters, a broad scope in yields of up to 99% was obtained. Manifold applications in e.g. bioimaging, material sciences and organic synthesis require organic fluorophores with specific dye properties. However, current fluorophore syntheses lack the necessary variability and flexibility for suitable tunability. Hence, we investigated a modular assembly to transform carboxylic acid esters with the use of heteroatom-incorporated 1,5-bifunctional organometallic reagents into heteroanthrylium dyes. With this approach, we gained access to a general synthesis of various 3,6-bis(dimethylamino)-xanthylium, -SiR and -acridinium fluorophores in up to 90% yield. Additionally, the newly formed acridinium scaffolds exhibit attenuated excited-state reduction potentials and were found to be complementary to metallophotocatalysts in various transformations such as a decarboxylative photoredox/Ni dual-catalytic C(sp2)-C(sp3) coupling. Photocatalysis has become an integral part of synthetic chemistry. Due to their favorable electrochemical and photophysical properties, the most frequently used photocatalysts today are based on earth-unabundant Ru- and Ir-polypyridyl complexes. For the further diversification of heterocyclic fluorophore structures and modulation of their photophysical and electrochemical properties, we investigated the synthesis of 1,5-bifunctional organometallic reagents via double directed ortho-metalation reactions. The ensuing transformation of carboxylic acid esters afforded 1,8-dimethoxy substituted acridinium fluorophores with unique photophysical and electrochemical properties compared to prototypical 3,6-dimethoxy-acridinium dyes. Furthermore, exceptional tunability of the organic fluorophore properties was attained by employing a combined halogen-metal exchange and directed ortho-metalation. This provided access to non-symmetric xanthylium and acridinium salts with remarkable photocatalytic properties in decarboxylative fluorination and oxidative [3+2]-cycloaddition. To additionally improve the sustainability of photocatalysis, we investigated common, inexpensive organic fluorophores in the transformation of Boc-proline and methyl iodobenzoate and found distinct reactivities compared to our newly formed acridinium catalysts under identical reaction conditions. Especially Na2-eosin Y was highly selective in forming the aryl carboxylate via triplet-triplet energy transfer, instead of the decarboxylative coupling.