Chiral Cobaltabis(dicarbollide) Catalysts Prepared Using a Silica Helical Nanoplatform for the Enantioselective Photooxidation of Aromatic Secondary Alcohols.

Cobaltabis(dicarbollide) anion ([Co(CBH)]) named COSAN is a sandwich-type metallacarborane that has been widely explored in a variety of fields, due to its interesting physical and chemical properties. While COSAN and its derivatives have recently been investigated in catalysis, their chiral counterparts have, to our knowledge, never been investigated in asymmetric catalysis. This may be due to the difficulty associated with preparing and isolating enantiopure COSAN derivatives. Inspired by our previous work on the chiral induction of nanoscale silica helices on achiral objects, we prepared and characterized enantiopure COSAN-based left-and right-handed hybrid silica nanohelices (NANOCOSANs). In these structures, achiral COSAN anions are encapsulated within left-and right-handed organic-inorganic hybrid nanohelices, which serve as a chiral platform for COSAN in enantioselective transformations. The morphology and structural composition of these NANOCOSAN hybrids as well as the presence of COSAN species were confirmed using UV/Vis, Raman spectroscopy, transmission electron microscopy (TEM), electron energy loss spectroscopy (EELS), and solid nuclear magnetic resonance (NMR) spectroscopy. Their optical activity was demonstrated by circular dichroism (CD) spectroscopy and Raman optical activity (ROA), highlighting the induction of chirality from chiral hybrid nanohelices to the achiral COSAN anions. To the best of our knowledge, these NANOCOSAN hybrids represent the first examples of COSAN-based chiral systems displaying chiroptical effects. In addition, they show excellent catalytic activity and enantioselectivity in the photooxidation of secondary aromatic alcohols in water. Specifically, the left-handed NANOCOSAN that selectively oxidizes the (S)-alcohol, while the right-handed oxidizes the (R)-alcohol, achieving up to 85% enantiomeric excess (ee). This proof-of-principle catalytic experiment demonstrates the transfer of chiroptical properties from the hybrid nanohelices to the catalytically active and originally achiral COSAN anion, enabling supramolecular asymmetric catalysis. Remarkably, the catalyst was recovered and reused at least three times without any loss of catalytic activity or enantioselectivity. This study reports the first example of enantiopure COSAN-containing nanohelices, which can promote photooxidation of organic substrates in water with high enantioselectivity.