Exploiting ligand effects in development of new methods and total synthesis with Pd-catalyzed C(spᶟ)-H activation

Owing to the everlasting pursuit for the economical and sustainable development of molecular complexity, methods that enable the selective functionalization of C(sp3)–H bonds are highly desirable. In this context, transition metal catalyzed C(sp3)–H activation has emerged as a promising tool for enabling and simplifying synthetic routes towards valuable natural products, building blocks and other molecules of interest. Motivated by previous developments in the field of Pd0-catalyzed C(sp3)–H activations, we sought to implement lessons learned on C–H arylation in an intermolecular fashion. We have managed to successfully identify conditions for a selectivity switch in arylation of heteroaromatic ketones with a competent coordinating ability from a classic α-selectivity to β-selectivity via the simple introduction of a pyridone ligand. These compounds are important blocks for the pharmaceutical industry and therefore, the development of a method for their expedient functionalization would simplify potential SAR studies. During our investigation, the origins this selectivity switch were probed, and it was found that they operate under different mechanistic manifolds, with α-arylation operating under a classical PdII–PdIV mechanism while β-arylation proceeded via a more exotic Pd-mediated desaturation-1,4-addition mechanism. At the same time, the problem of enantioselective intramolecular methylene C(sp3)–H activation – a previously unsolved challenge – was found to be possible by mediation of IBiox-type NHC ligands. Following on an initial reporting of this method, we sought to expand potential applications towards total synthesis of Indidene natural products. This was achieved via Pd0-mediated methylene C(sp3)–H activation as a key step, with a further divergent functionalization. Installation of a ketone side chain from a common diol precursor lead towards synthesis of Indidene A, while installation of the biphenyl system led towards Indidene C. The latter natural product was also synthesized in an enantioselective fashion with a high degree of enantioselectivity. Current efforts center around elucidation of the proposed structure.