meta-Selective C–H Bond Alkylation with Secondary Alkyl Halides

Ruthenium catalysts enabled C-H bond functionalizations on arenes with challenging secondary alkyl halides. Particularly, ruthenium(II) biscarboxylate complexes proved to be the key to success for direct alkylations with excellent levels of unusual meta-selectivity. The direct alkylations occurred under mild reaction conditions with ample scope and tolerated valuable functional groups. Detailed mechanistic studies were performed, including various competition experiments as well as reactions with isotopically labeled substrates. These studies provided strong support for an initial reversible cyclometalation. The cycloruthenation thereby activates the arene for a subsequent remote electrophilic-type substitution with the secondary alkyl halides. Independently prepared cycloruthenated complexes were found to be catalytically active provided that a carboxylate ligand was present, thereby highlighting the key importance of carboxylate assistance for effective meta-selective C-H bond alkylations.