(R)- and (S)-2-diphenylphosphinoyl-2′-hydroxy-1,1′-binaphthalene: versatile chiral bidentate ligands

Publisher Website
Google Scholar
Abstract
The homochiral compounds (R)- and (S)-2-diphenylphosphinoyl-2′-hydroxy-1,1′-binaphthalene, (R)- and (S)-Hbinappo, react with TiCl4 or ZrCl4 to produce complexes of bidentate binappo–, [MCl2(binappo)2](M = TiIV or ZrIV). VCl3, FeCl3 and MoO2Cl2 also react with Hbinappo, but require the presence of a base to produce [M(binappo)3](M = VIII or FeIII) or [MoO2(binappo)2]. The complex [MoO2(acac)2](acac = acetylacetonate) reacts with Hbinappo to produce [MoO2(acac)(binappo)]. The vanadium(III) tris(chelate) complexes are readily oxidised in air to [VO(binappo)2]. All of these complexes have been characterised by microanalytical, infrared, and, where appropriate, NMR, electronic and CD spectroscopic techniques. (S)-Hbinappo, [ZrCl2{(S)-binappo}2], [V{(S)-binappo}3] and [VO{(S)-binappo}2] have also been characterised by single-crystal X-ray techniques. In all of the complexes the ligands are co-ordinated through their phenolate and phosphinoyl oxygen atoms forming eight-membered chelate rings. The tris-chelate complexes form stereospecifically, (S)-binappo giving the Λ isomer exclusively, and the (R)-form producing Δ-[M(binappo)3]. Although each of the [M(binappo)3] species are necessarily sterically crowded, little M–O bond elongation is observed in the single-crystal X-ray structure of [V{(S)binappo}3]. Initial attempts at achieving asymmetric induction in TiIV-and VOIV-based oxidations of prochiral sulfides in the presence of these ligands are also presented, but observed enantiomeric excesses are generally < 10%.