Enantioselective 1,4-addition of arylboronic acids to α,β-unsaturated carbonyl compounds catalyzed by rhodium(I)-chiral phosphoramidite complexes

Article abstract of DOI:10.1016/j.jorganchem.2006.04.042

A chiral bidentate phosphoramidite (5a) was synthesized from Shibasaki's linked-(R)-BINOL and P(NMe₂)₃ as a new ligand for rhodium(I)-catalyzed asymmetric 1,4-addition of arylboronic acids to α,β-unsaturated carbonyl compounds. The effects of 5a and Feringa's monodentate phosphoramidite (4, R¹, R² = Et) on the yields and enantioselectivities were fully investigated. The reaction was significantly accelerated in the presence of a base such as KOH and Et₃N, allowing the reaction to be completed at the lower temperatures than 50 °C. The addition to cyclic enones such as 2-cyclopentenone, 2-cyclohexenone and 2-cycloheptenone at 50 °C in the presence of an [Rh(coe)₂Cl]₂-4 (R¹, R² = Et) complex resulted in enantioselectivities up to 98%, though it was less effective for acyclic enones (0-70% ee). On the other hand, a complex between [Rh(nbd)₂]BF₄ and 5a completed the addition to cyclic enones within 2 h at room temperature in the presence of Et₃N with 86-99% yields and 96-99.8% ee. This catalyst was also effective for acyclic enones, resulting in 62-98% yields and 66-94% ee. The 1,4-additions of arylboronic acids to unsaturated lactones and acyclic esters with rhodium(I)-phosphoramidites complexes were also investigated.