Chiroptical, structural and catalytic properties of S-α-methyl-[1-(substituted phenyl)-2-(2′-pyrido)-1-ethylidene] benzylamines and their Rh(I) and Cu(I) complexes

Abstract

S-α-Methyl-[1-(substituted-phenyl)-2-(2′-pyrido)-1-ethylidene]benzylamines 15–21 and their Rh(I) complexes 22–28 are prepared and their chiroptical and conformational properties are studied. Free ligands are present as enamines in the solution and in the solid state, but are bound to Rh(I) in the imine form. The CD spectra confirm that complexation of 15–21 induces both structural change and strong conformational perturbations. The molecular structures in the crystal are reported for the chiral 1,5-bisnitrogen ligand 18, and its Rh[(norbornadiene) 2] perchlorate complex 25. The absolute conformation of the chromophore in 18 inverts on binding to Rh(I) in 25. The value of the torsional angle about C10-C9-C16-C21 bond (−69.7°) in 18, which defines the twisted stilbene-like chromophore, turns for 25 into 75.0°. Chiral S-(−)-methylbenzyl subunit in 18 has a C1-N1-C9-C10 torsional angle of 175.2°, whereas on binding to Rh(I) in 25 this angle changes to −178.4°. The absolute conformation around the styrene-like arrangement of the bonds in 15−21 can be deduced from the strong positive Cotton effect at ca. 350 nm. Cyclopropanation of styrene with ethyl diazoacetate, in the presence of in situ generated Cu(I) complexes of chiral 1,5-bidentate ligands 15–21, yielded cis/trans 2-phenylcyclopropan-1-carboxylic acid ethylesters with 5–21% e.e. Though generally low, the enantioselectivity was somewhat higher for ortho-( 16–18) than for para-( 19–21) substituted phenyl derivatives.