Stereoselective and Stereospecific Reactions of Cobalt Sandwich Complexes: Synthesis of a New Class of Single Enantiomer Bulky Planar Chiral P−N and P−P Ligands

Starting from (η 5 ‐acetylcyclopentadienyl)(η 4 ‐tetraphenylcyclobutadiene)cobalt(I), highly enantioselective (99 % ee ) ( S )‐CBS catalysed ketone reduction followed by stereospecific alcohol‐azide exchange, azide reduction and dimethyllation gave ( R )‐(η 5 ‐α‐ N , N ‐dimethylaminoethylcyclopentadienyl)(η 4 ‐tetraphenylcyclobutadiene) cobalt(I) (Arthurs’ amine). This underwent highly diastereoselective cyclopalladation to give di‐μ‐acetate‐bis‐( R )‐[(η 5 ‐( S p )‐2‐(α‐ N , N ‐dimethylaminoethyl)cyclopentadienyl, 1‐ C , N )(η 4 ‐tetraphenylcyclobutadiene)cobalt(I)]dipalladium, and highly diastereoselective lithiation to give ( R )‐(η 5 ‐( S p )‐1‐(α‐ N , N ‐dimethylaminoethyl)‐2‐(diphenylphosphino)cyclopentadienyl)(η 4 ‐tetraphenylcyclobutadiene)cobalt(I) (PPCA) following the addition as electrophile of chlorodiphenylphosphine. This PN‐ligand was converted into ( R )‐(η 5 ‐( S p )‐1‐(α‐dicyclohexylphosphinoethyl)‐2‐(diphenylphosphino)cyclopentadienyl)(η 4 ‐tetraphenylcyclobutadiene)cobalt(I), a PP‐ligand (Rossiphos), by stereospecific amine‐phosphine exchange using HPCy 2 . These air‐stable P−N and P−P complexes are the first examples of a new class of bulky planar chiral ligands for application in asymmetric catalysis.