| Summary: | A novel tetraphosphine ligand rac-et,ph-P4 (et,ph-P4 = PEt2CH2CH2(Ph)PCH2P(Ph)CH2CH2PEt2) is used for the formation of a highly active and regioselective hydroformylation catalyst. The active catalytic species, Rh2H2(Ý-CO)2(et,ph-P4)]2+, is formed in situ under H2/CO pressure. This is one of the most impressive examples of homobimetallic cooperativity in homogeneous catalysis. The fragmentation of this catalyst by CO has been investigated and confirmed by in situ NMR spectroscopic studies. A new tetraphosphine ligand rac-et,ph-P4-Ph (et,ph-P4-Ph = PEt2(o-C6H4)P(Ph)CH2(Ph)P(o-C6H4)PEt2) has been synthesized to combat this fragmentation problem. However, the inability to successfully separate the meso and racemic isomers of the ligand has led to more alteration of the basic structure of the tetraphosphine ligands.
The current alteration being explored in an attempt to solve this separation problem is the replacement of the central methylene bridge by a tertiary amine. Experimentation has been conducted on the basis of a retrosynthetic analysis with the possibility of two pathways for formation of these aza-bridged ligands. The new ligands have not been afforded as of yet due to the difficulty in purification of the intermediate products. A simple Grignard-mediated phosphorus-carbon coupling reaction has been attempted with an amine bridge (RN(PhPCl)2, but impurities in the starting material and decreased reactivity of the amine bridge led to results that were undesirable. The second synthetic route relies upon coupling PEt2(o-C6H4)PPhCl with a primary amine to afford the desired ligand, but the inability to obtain the pure phosphorus compound has hindered progress.
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