Ligands phosphonium salt

The bis-phosphonium salt 56 in presence of Pd(OAc)2 leads to the formation of the neutral bis-ylide 57 which reacts with TICIO4 to give the dinuclear cationic complex 58 (Scheme 22) [89,90]. The bis-ylide part, which has potentially two carbons and one oxygen donor atoms, acts as a C,C-chelating ligand through its two soft ylidic carbons. 

Complexes containing the ylide ligand can be also obtained by reaction of the yellow solution of ylides (prepared by addition of butyllithium to tetrahydrofurane or diethylether solutions of the phosphonium salt) with [AulCfiFs) (tht)]. The weak ligand tht is replaced and the ylide complex is obtained (Equation 3.5) [70]. 

Scheme 6.2 Heck reactions of aryl chlorides involving air-stable phosphonium salts as ligand precursors.

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). 

The bis-ylide ligands [R2P(CH2)2] can easily be prepared by treatment of the phosphonium salts [R2PMe2] (R = Me, Ph) with strong deprotonating reagents such as lithium derivatives, NaNH2, or non stabilized ylides R3P=CH2. However, Au Ag, and Cu complexes (99) were prepared by reaction of ClMPMe3 (M = Ag, Au) or CuCl with excess of R3P=CH2 [202, 203] (Scheme 30). 

The Heck reaction is a C-C coupling reaction where an unsaturated hydrocarbon or arene halide/triflate/sulfonate reacts with an alkene in presence of a base and Pd(0) catalyst so as to form a substituted alkene. Kaufmann et al. showed that the Heck reaction carried out in presence of ILs such as tetra-alkyl ammonium and phosphonium salts without the phosphine ligands, resulted in high yields of product. They attributed the activity to the stabilizing effect of ammonium and phosphonium salts on Pd(0) species. Carmichael et al. used ionic liquids containing either A,A -dialkylimidazolium and A-alkylpyridinium cations with anions such as halide, hexafluorophosphate or tetrafiuoroborate to carry out reactions of aryl halide and benzoic anhydride with ethyl and butyl acrylates in presence of Pd catalyst. An example of iodobenzene reacting with ethyl acrylate to give trans-et vy cinnamate is shown in Scheme 14. 

Phenyl-substituted diphosphines, R2P(CH2) PR2, can also be cleaved by Na or Li in liquid ammonia or THF to yield MPhP(CH2) PPhM,M which are very useful starting materials for the production of unsymmetrical bidentate ligands PhRP(CH2) PRPh. However, care should be taken in these reactions as Ph—P cleavage is sometimes accompanied by P—CH2 fission. For the production of unsymmetrical bidentate phosphines, such as R2P(CH2) PPh2, stepwise formation of mono- and di-phosphonium salts provides a facile route (see Section 14.2.4). 

Ferrocenyl ligands, via zinc reagents, 9, 120 Ferrocenylmethyl phosphonium salts, with gold(I), 2, 274 Ferrocenylmonophosphine, in styrene asymmetric hydrosilylation, 10, 817 Ferrocenyl oxazolines, synthesis, 6, 202 Ferrocenylphosphines with chromium carbonyls, 5, 219 in 1,3-diene asymmetric hydrosilylation, 10, 824-826 preparation, 6, 197 various complexes, 6, 201 Ferrocenylselenolates, preparation, 6, 188 Ferrocenyl-substituted anthracenes, preparation, 6, 189 Ferrocenyl terpyridyl compounds, phenyl-spaced, preparation 6, 188 Ferrocifens... 

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