Design of New Hemilabile (P,S) Ligands for the Highly Diastereoselective Coordination to Alkyne Dicobalt Complexes:  Application to the Asymmetric Intermolecular Pauson−Khand Reaction

Here we synthesized a new generation of hemilabile P,S ligands designed for diastereoselective coordination to terminal alkyne-hexacarbonyldicobalt complexes. The camphor-derived CamPHOS (2) and MeCamPHOS (12) ligands were obtained in 68% and 59% yield in their borane-protected form starting from readily available 1,3-oxathiane 4. Thermally induced coordination to dicobalt-hexacarbonyl-alkyne (HCCR, R = Ph, But, SiMe3, C(CH3)2OH) complexes was studied. MeCamPHOS (12) provided an unprecedented diastereoselectivity on coordination to terminal complexes (up to 90% de). The solid state structure of a coordinated CamPHOS ligand with [Co2(μ-ButSO2C)2(CO)6] was determined and confirms the preference to coordinate the bimetallic cluster in a bridged manner. The resulting adducts were submitted to the intermolecular Pauson−Khand cyclization. Remarkably, CamPHOS and MeCamPHOS provided Pauson−Khand products of opposite absolute configurations. This behavior could be explained on the basis that 2 and 11 ligands lead to pseudo-enantiomeric tetracarbonyl complexes, as confirmed by circular dichroism analysis. Introduction of a CH3− group on the carbon bridge between P and S increased CamPHOS selectivity dramatically from 33% to 90% de of the opposed sign with an overall destabilizing effect of 2.15 kcal/mol for 7b.