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

An improved synthesis of the racemic rhodium compound [RhCl 2 (κ 4 C , N , N ′, P - L1 )] ( 1 ) containing an achiral tripodal tetradentate ligand is reported. Their derived solvate complexes [Rh(κ 4 C , N , N ′, P - L1 )(Solv) 2 ][SbF 6 ] 2 (Solv = NCMe, 2 ; H 2 O, 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. A resolved chiral-at-rhodium complex catalyses asymmetric Diels-Alder and 1,3-dipolar cycloaddition reactions with enantioselectivity of up to >99/1 er.