An Unexpectedly Stable Chiral Hydrido‐Solvent Complex of Ru II : A mechanistic link in the enantioselective hydrogenation of pyrones. Preliminary communication

The ligand (6,6′‐dimethoxybiphenyl‐2,2′‐diyl)bis[3,5‐di( tert ‐butyl)phenylphosphine] ( 1 ) forms an unexpectedly stable hydrido‐bis‐solvento complex of composition [RuH(isopropanol) 2 ( 1 )]BF 4 , ( 2 ) under the conditions used in the enantioselective hydrogenation of pyrones. The structure of 2 , determined by X‐ray diffraction, represents the first well‐characterized chiral five‐coordinate bis‐phosphine ruthenium‐hydride complex stable as a solvento complex, and provides a structural link in the enantioselective pyrone hydrogenation cycle catalyzed by [Ru(OAc) 2 ( 1 )]. Using the arene complex [RuH( p ‐cymene)( 1 )]BF 4 ( 3 ), the chiral pocket of coordinated 1 is shown to be relatively rigid, via NMR spectroscopy. This is reflected in restricted rotation about one of the four P –[3,5‐di( tert ‐butyl)phenyl] PC ipso bonds at room temperature .