Fluxional Processes in Asymmetric Hydroformylation Catalysts [HRhL⌒L(CO)2] Containing C 2-Symmetric Diphosphite Ligands

• Published in: Organometallics 1997-12, Vol.16 (26), p.5681-5687
• Main Authors: Buisman, Godfried J. H, van der Veen, Lars A, Kamer, Paul C. J, van Leeuwen, Piet W. N. M
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om970599k

Solution structures of hydridorhodium diphosphite dicarbonyl complexes [HRhL⌒L1 - 9(CO)2] have been studied. Diphosphites L⌒L1 to L⌒L9 are based on C 2 symmetric (2R,3R)-butane-2,3-diol, (2R,3R)-diethyl tartrate, (2R,4R)-pentane-2,4-diol, and (2S,5S)-hexane-2,5-diol backbones substituted with 1,1‘-biphenyl-2,2-diyl- or (S)-(−)-1,1‘-binaphthyl-2,2‘-diylphosphoroxy derivatives. Variable-temperature (293−163 K) 31P and 1H NMR spectroscopy revealed fluxional behavior in the trigonal bipyramidal HRhL⌒L(CO)2 complexes of L⌒L1 - 9 which could be frozen out at low temperatures. Depending on the length of the bridge between the two phosphorus atoms in the diphosphite ligands, equatorial−axial or bis-equatorial coordination takes place. Enthalpies of activation (ΔH ⧧) varying between 36.7 and 62.5 kJ mol-1 have been calculated for the phosphorus exchange observed in these hydridorhodium diphosphite dicarbonyl complexes. The enthalpies of activation increase with larger steric bulkiness of the coordinated diphosphite ligands.