Phosphonium salts formation

Loss of metal by ligand degradation. The oxidation of phosphorus ligands by peroxide impurities in the feed is an example. Purification of the feed is an obvious remedy. It is much more difficult to find a solution when ligand degradation is inherent to the catalytic reaction mechanism (e.g., phosphonium salt formation). 

A problem in these couplings is the identity of the end groups of the formed polymers. Pd-catalyzed dehalogenation and/or phosphonium salt formation are side reactions that are difficult to avoid. A concern for the structural integrity of the backbone is the formation of butadiyne defects. While there is no direct measure to determine the amount of butadiyne defects in PAEs, the numbers are estimated to range from 1 to 10% of all repeat units. 

Firstly, it was necessary to develop the reaction sequence of the Wittig reaction — synthesis of the phosphonium salt, formation of the ylide and reaction with a carbonyl compound to give the olefin — into an industrial process, under the stringent criteria of safety and cost-efficiency. 

When the methyl complex Pd(PPh3)2(I)(Me) is treated with Ag in CeDe at 85 °C, the only product is PPh3Me+. No benzene alkylation takes place. Heating the cationic Pd(PPh3)2(CD3CN)(Me)+ also produces only PPh3Me+. The relative instability of methyl radical probably accounts for the different reaction paths of the benzyl and methyl cationic complexes. When phosphonium salt formation is not possible, for example, in the neopentyl complex... 

Figure 7. High-pressure phosphonium salt formation.

The direct attack on the halogen of a-halocarbonyl compounds by a tertiary phosphine is implicated by substrate reactivity indices (22). The change of venue from the Perkov and Arbuzov pathways concurs with the HSAB rationale as the phosphine P is softer than the phosphite P. Studies on enol phosphonium salt formation (23) support this contention. 

While high molecular weight dialkyl-PPEs 13 are obtained by ADIMET, Pd-catalysis can also be used to make dialkyl-PPEs (Charts 8.6-8.8). However, in most cases the formed PPEs 16 display defect structures that are due to the presence of diyne units. These defect structures are difficult to control, but seem to arise from oxidative dimerization and Pd-catalyzed dehalogenation with or without concomitant phosphonium salt formation. 

Benyei, A. Stafford, J.N.W. Katho, A. Darensbourg, D.J. Joo, F. (1993) The effect of phosphonium salt formation on the kinetics of homogeneous hydrogenations in water utilizing a rhodium mera-sulfonatophenyl-diphenylphosphine complex, J. Mol Catal, M, 157-63. 

Tri-p-tolylphosphine refluxed several hrs. with 10%-excess monodiloromethyl ether in benzene, and the resulting dhloride treated with Na-fluoroborate soln. in water (methoxymethyl)tris-(p-tolyl)phosphonium fluoroborate (Y 91%) added to nitryl fluoroborate and sulfolane, heated 6 hrs. at 110, pyridine hydrochloride added, and stirred 2 hrs. at 115° tris-(3-nitro-4-methylphenyl)phosphine (Y 51%). - Phosphonium salt formation protects the phosphorus from oxidation. F. e. s. G. P. Schiemenz and K. Rohlk, B. 104, 1219 (1971). 

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