Synthesis, Separation, and Reactivities of Multidentate Phosphine Ligands and Investigation into Dirhodium Hydroformylation and Hydrocarboxylation Catalysis
A dirhodium homogeneous hydroformylation catalyst based on a tetraphosphine ligand, rac-Et2PCH2CH2P(Ph)CH2P(Ph)CH2CH2PEt2, rac-et,ph-P4, is under investigation. The ligand synthesis produces a racemic mixture and a facile and efficient method of separation of the racemic and meso diastereomers was achieved through reaction of et,ph-P4 with two equivalents of NiCl2 in EtOH to yield an almost quantitatively isolable precipitate of meso-Ni2Cl4(et,ph-P4) and the soluble rac-Ni2Cl4(et,ph-P4). Subsequent cyanolysis of these complexes liberates the et,ph-P4 ligand, and the formation of a thermodynamically favored racemic monometallic intermediate during cyanolysis facilitates isomerization of meso to racemic ligand.
The addition of even small amounts of PPh3 to the dirhodium tetraphosphine hydroformylation catalyst synthesized from the diastereomerically pure rac-et,ph-P4 causes a dramatic drop in the aldehyde linear to branched regioselectivity (25:1 to 3:1) in acetone solvent (90 °C, 6.1 bar, 1-hexene). The results indicate extremely effective inhibition of the regioselective bimetallic hydroformylation catalyst and the formation of an inefficient monometallic catalyst system, but not fragmentation to generate free RhH(CO)(PPh3)2 catalysts.
For the dirhodium hydroformylation catalyst the addition of 30% water (by volume) to the acetone solvent gives the highest rate (73 min-1) and highest selectivity (33:1 linear:branched (L:B) aldehyde ratio, <1% isomerization or hydrogenation products) as compared to that in acetone with initial TOF of 20 min-1, 25:1 L:B, 2.5% isomerization, and 3.4% alkene hydrogenation for 1-hexene. The dramatic improvement is the result of the more polar water-acetone solvent system preventing phosphine ligand dissociation from the dirhodium catalyst and subsequent formation of inactive species. Comparisons of the catalytic results in water-acetone to those of four representative monometallic, rhodium, modified phosphine systems indicate that the dirhodium catalyst is one of the fastest and the most selective catalyst overall. The dirhodium catalyst also converts aldehydes, but more interestingly alkenes, to carboxylic acids in the presence of water and under hydrogen-depleted conditions. Alkenes are converted via a novel tandem catalysis reaction first involving hydroformylation then aldehyde-water shift catalysis.
Advisor:George G. Stanley; Andrew W. Maverick; David Spivak; Brian J. Hales; Jim H. Belanger
School Location:USA - Louisiana
Source Type:Master's Thesis
Date of Publication:06/09/2004