1. Walphos versus biferrocene-based walphos analogues in the asymmetric hydrogenation of alkenes and ketones.
Manuela A Groß, Kurt Mereiter, Yaping Wang, Afrooz Zirakzadeh, Walter Weissensteiner. Organometallics. 2014 Apr 28; 33(8): 1945-1952. DOI: 10.1021/om401074a. PMID: 24795493.
Two representative Walphos analogues with an achiral 2,2″-biferrocenediyl backbone were synthesized. These diphosphine ligands were tested in the rhodium-catalyzed asymmetric hydrogenation of several alkenes and in the ruthenium-catalyzed hydrogenation of two ketones. The results were compared with those previously obtained on using biferrocene ligands with aC2-symmetric 2,2″-biferrocenediyl backbone as well as with those obtained with Walphos ligands. The application of one newly synthesized ligand in the hydrogenation of 2-methylcinnamic acid gave (R)-2-methyl-3-phenylpropanoic acid with full conversion and with 92% ee. The same ligand was used to transform 2,4-pentanedione quantitatively and diastereoselectively into (S,S)-2,4-pentanediol with 98% ee.
2. Biferrocene-based diphosphine ligands: synthesis and application of walphos analogues in asymmetric hydrogenations.
Manuela A Groß, Kurt Mereiter, Felix Spindler, Yaping Wang, Afrooz Zirakzadeh, Walter Weissensteiner. Organometallics. 2013 Feb 25; 32(4): 1075-1084. DOI: 10.1021/om3012147. PMID: 23457421.
A total of four biferrocene-based Walphos-type ligands have been synthesized, structurally characterized, and tested in the rhodium-, ruthenium- and iridium-catalyzed hydrogenation of alkenes and ketones. Negishi coupling conditions allowed the biferrocene backbone of these diphosphine ligands to be built up diastereoselectively from the two nonidentical and nonracemic ferrocene fragments (R)-1-(N,N-dimethylamino)ethylferrocene and (SFc)-2-bromoiodoferrocene. The molecular structures of (SFc)-2-bromoiodoferrocene, the coupling product, two ligands, and the two complexes ([PdCl2(L)] and [RuCl(p-cymene)(L)]PF6) were determined by X-ray diffraction. The structural features of complexes and the catalysis results obtained with the newly synthesized biferrocene-based ligands were compared with those of the corresponding Walphos ligands.
