Synthesis of chiral-at-metal rhodium complexes from achiral tripodal tetradentate ligands: resolution and application to enantioselective Diels–Alder and 1,3-dipolar cycloadditions

Abstract

An improved synthesis of the racemic rhodium compound [RhCl₂(κ⁴C,N,N′,P-L1)] (1) containing an achiral tripodal tetradentate ligand is reported. Their derived solvate complexes [Rh(κ⁴C,N,N′,P-L1)(Solv)₂][SbF₆]₂ (Solv = NCMe, 2; H₂O, 3) are resolved into their two enantiomers. Complexes 2 and 3 catalyze the Diels–Alder (DA) reaction between methacrolein and cyclopentadiene and the 1,3-dipolar cycloaddition reaction between methacrolein and the nitrone N-benzylidenphenylamine-N-oxide. When enantiopure (A_Rh,R_N)-2 was employed as the catalyst, enantiomeric ratios >99/1, in the R at C2 adduct, and up to 94/6, in the 3,5-endo isomer, were achieved in the DA reaction and in the 1,3-dipolar cycloaddition reaction, respectively. A plausible catalytic cycle that accounts for the origin of the observed enantioselectivity is proposed.