Ligand design and mechanism in hydroformylation
In this work the synthesis of new potentially trans-chelating biphosphine ligands is described and their value in rhodium catalysed hydroformylation evaluated. The reactions of biphosphine diolefin rhodium complexes with hydrogen in methanol, monitored by 1H- and 31P-NMR spectroscopy were used to determine trans-chelating ability. Complexes of 1,5-bis(diphenylphosphino)-3-oxapentane and 1,3-bis- (4-diphenylphosphinobenzyl)benzene formed rhodium dihydrides solely whereas the more flexible 1,7-bis(diphenylphosphino)-4-oxaheptane gave isomeric rhodium dihydrides and a solvate complex. The reaction of diolefin complexes with hydrogen and carbon monoxide in dichloromethane was also investigated. The 3-oxapentane ligand, readily synthesized from 3-oxapentane-1,5-diol, as its rhodium complex gave a n—/iso aldehyde ratio of 9:1 for 1-octene hydroformylation (100°, 80 psi, 1:1 hydrogen/carbon monoxide). Secondly, reactive intermediates relevant to hydroformylation were identified using 13C- and 2H-labelling and 1H, 13C and 31P-NMR spectroscopy. Hydridocarbonylbis(triphenylphosphine)rhodium(I), the probable catalytic cycle initiator, was conclusively shown to be the initial product of hydridocarbonyltris(triphenylphosphine)rhodium(I) under hydroformylation conditions. The kinetics of interconversion of these latter two complexes were examined by saturation transfer 31P-NMR. On reaction of the dicarbonyl complex with styrene no alky1-rhodium complexes were observed, but an iso-acyl intermediate which isomerizes rapidly at ambient temperature was identified and a structure proposed. A similar n-acyl complex, from 1-octene, shows dynamic NMR behaviour explained in terms of triphenylphosphine isomerization at lower temperature and acyl-alkyl interconversion at high temperature.