Selective Monoalkylation or Dialkylation of Indenes with Alcohols by Bis-<i>N</i>-Heterocyclic Carbene Manganese

Selective direct alkylation of indenes is important for the synthesis and decoration of indene derivatives. In contrast to traditional alkylation methods, using unactivated alcohol as alkylation reagents via borrowing hydrogen or hydrogen autotransfer is highly attractive. However, the nature of the high-lying lowest unoccupied molecular orbital of the condensation intermediate raises a big challenge for the development of the BH system for the alkylation of indenes using alcohols. Through tuning the stability and the reactivity of the metal-hydride species by theoretical prediction, herein, we succeeded in developing a non-noble Mn-catalyzed selective direct monoalkylation or dialkylation of indenes under mild conditions, producing the corresponding versatile alkylated indenes in satisfactory yields. A broad scope of substrates including primary benzylic, aliphatic, amino, hydroxyl, and second alcohols as well as various indenes are tolerated in the system. Additionally, this method could be used for the synthesis of potential precatalyst complexes and bioactive molecules. Mechanistic studies including control experiments, kinetic investigations, kinetic isotope effect, deuterium labeling experiments, and density functional theory calculations, reveal the proper metal-hydride for a successful BH process.