Regioselective Syntheses and Functionalizations of Polycyclic Aromatic Hydrocarbons: Directed Metalation and C─H Activation
Keywords:kjemi, polycyclic aromatic hydrocarbons (PAH), directed ortho metalation (DoM), directed remote metalation, cross-coupling, C─H bond functionalization, regioselective synthesis
Polycyclic aromatic hydrocarbons (PAHs) are well-known as pollutants and carcinogenic compounds. Lately, considering their opto-electronic and photophysical properties, PAHs are being developed as materials to be used in electronics, non-linear optics (NLOs) and light-emitting diodes (LEDs). Surprisingly, their binding affinity towards DNA has evolved into a study of their potential usage as anti-cancer and anti-malarial agents. Within the realm of these possibilities, the syntheses and functionalizations of PAHs has become an important area of research. The classical method of oxidative photocyclization is used to prepare gramscale phenanthrene and chrysene derivatives required as starting materials for all the experiments. The study investigates directed ortho metalation (DoM) and non-directed C─H activation as methods to functionalize chrysene derivatives. DoM proved to be an efficient strategy in the presence of directing metalation group (DMG) affording di-substituted chrysene derivatives in 27% to quant yields. However, C─H activation needs further experiments to develop the catalyst system suitable for activating C(sp2)─H bonds in PAH derivatives. This thesis is also focussed on approaches to synthesize smaller to larger PAHs. In this context, cross-coupling, and directed remote metalation (DreM) are studied. The Suzuki-Miyaura cross-coupling protocol is optimized using a simple commercial catalyst to cross-couple ortho-substituted bulky substrates such as chrysenyl carboxamides and methylnaphthalenyl boronic esters. The importance of electronic and steric factors is discussed when sterically demanding cross-coupling partners are involved. Finally, the cross-coupled products are cyclized following a DreM strategy to achieve the planned larger 6− and 7− ring fluorescent PAHs. The UV-visible and fluorescence spectra of all the synthesized PAHs are presented. The experiments are also aimed to understand the mechanism involved in attaining the products regioselectively.
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