Asymmetric addition of phenylboronic acid to cycloalkanones mediated by a Rh/phosphoramidite complex: a comparison between tropos versus nontropos behavior

Herein we report the asymmetric addition of phenylboronic acid to different cycloalkenones, mediated by a Rh(I) complex with the tropos phosphoramidite (S)-L₁ or the nontropos phosphoramidite (S)-L₂ ligand. Different values of enantiomeric purity of the 3-phenylcycloalkanone products have been obtained, mainly depending on the ring size of the substrates, with (S)-L₁ affording higher ee values than (S)-L₂. These results could be explained by reasoning that for the tropos phosphoramidite, a ‘chiral pocket’ is formed when the ligand links to the metal atom. By fitting in this pocket, the substrate can stereoselectively coordinate to the Rh(I) atom, undergoing attack of the phenyl ring from the phenylboronic acid to a preferred enantioface. This causes the high (up to 96%) values of enantioselectivity observed with some of the substrates. A ‘chiral pocket’ cannot be formed with the nontropos ligand (S)-L₂ and, as a consequence, the stereoselectivity is greatly reduced.