H-BINOL-Derived Chiral η-Benzene Ligands: New Opportunities for the Ruthenium-Catalyzed Asymmetric C-H Activation.

Given the tremendous success of (p-cymene)Ru-catalyzed C-H activation over the past 20 years, the community has long been aware that the development of chiral η-benzene (Ben) ligands should be a potent strategy for achieving the attractive but incredibly underdeveloped ruthenium(II)-catalyzed asymmetric C-H activation. However, it has rarely been achieved due to the severe difficulty in developing proper chiral Ben ligands. In particular, designing chiral Ben ligands by connecting a benzene fragment to a chiral framework including benzene rings remained an unsolved challenge until this effort. Here we present a novel class of axially chiral Ben ligands derived from readily available (S)-5,5',6,6',7,7',8,8'-octahydro-1,1'-bi-2-naphthol ((S)-H-BINOL) in 4-8 steps. Notably, when coordinated with ruthenium, such chiral Ben ligand containing three benzene rings only forms one of the three possible isomeric BenRu complexes. The related chiral BenRu catalysts could effectively catalyze the asymmetric C-H activation of N-sulfonyl ketimines with alkynes, affording a range of chiral spirocyclic sultams in up to 99 % yield with up to >99 % ee. These catalysts are expected to find broad applications in future.