Toward the advancement of tetraphosphine ligand synthesis for homogeneous bimetallic catalysis
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- Homogeneous; Catalysis; Hydroformylation
thesis / dissertation description
A tetraphosphine ligand rac-et,ph-P4 (Et2PCH2CH2(Ph)PCH2P(Ph)CH2CH2PEt2) is used for the formation of a highly active and regioselective bimetallic hydroformylation catalyst. The proposed active catalytic species in acetone, [Rh2H2(µ-CO)2(rac-et,ph- P4)]2+, is formed in situ under H2/CO pressure. This is one of the most impressive examples of 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 = Et2P(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 diastereomers led to the attempted alteration of the et,ph-P4 tetraphosphine ligand. Where the et,ph-P4-Ph ligand attempts to change the traditional ligand via altering the carbon system between internal and external phosphines, these systems attempt to replace the methylene linker between the internal phosphines with an amine linker. 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 first synthetic route involves a simple Grignard-mediated phosphorus-carbon coupling reaction between an amine bridge of the type RN(PhPCl)2 with the “small arm” phosphine moiety Et2P(o-C6H4)I. 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 of the “large arm” phosphine moiety Et2P(o-C6H4)PPhCl with a primary amine to afford the desired ligand. This route was also unsuccessful due to the inability to obtain the pure phosphorus compound due to the “large arm” reaction not consistently going to completion.