Cationic Phosphines

The synthesis of AMPHOS, the prototype of cationic phosphines, known for almost 20 years, requires intermediate protection of the phosphorus in Ph2P -(CH2)2 -NMe2 by oxidation or coordination to a transition metal before N-quaternization [99, 100], In the case of the ligands 28a, 28b, and 30, borane was used as the protecting group. 

Palladium complexes of AMPHOS (28, R( = Me3 and Me2H X = Cl) are suitable catalysts for telomerization of butadiene and isoprene in biphasic systems in the presence of CH3OH/CH3ONa. Conversion rates and selectivities are altered only very little upon catalyst recycling, the results obtained for butadiene comparing favorably with those obtained for TPPTS [109]. The cationic phosphines 28 a, b 

Rh complexes of composition [Rh(nbd)(33)2]+X containing the phosphonium phosphines 33 (PHOPHOS II, III, VI, X) have been shown to be very active catalysts for the biphasic hydrogenation of n-hexene and maleic acid in water [103, 110]. A definite chain length effect was observed, the system where n = 6 being the most active. The biphasic systems containing longer chain ligands are not well behaved as catalysts since they are prone to formation of stable emulsions which are quite difficult to break. 

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Cationic phosphine ligands containing guanidiniumphenyl moieties were originally developed in order to make use of their pronounced solubility in water [72, 73]. They were shown to form active catalytic systems in Pd-mediated C-C coupling reactions between aryl iodides and alkynes (Castro-Stephens-Sonogashira reaction) [72, 74] and Rh-catalyzed hydroformylation of olefins in aqueous two-phase systems [75]. 

At the same time, Schmidtchen et al. compared cationic phosphine ligands containing the hydrophilic guanidinium (4.3, 4.4) and the anionic phosphine ligand TPPTS for this palladium-catalyzed coupling reaction. They found that the cationic ligands were effective for the coupling reaction but less efficient than TPPTS 43... 

Green used the cationic phosphine palladium hydride complex for the reaction of methanol (44). 

Efforts to tune the reactivity of rhodium catalysts by altering structure, solvent, and other factors have been pursued.49,493 50 Although there is (justifiably) much attention given to catalysts which provide /raor-addition processes, it is probably underappreciated that appropriate rhodium complexes, especially cationic phosphine complexes, can be very good and reliable catalysts for the formation of ( )-/3-silane products from a air-addition process. The possibilities and range of substrate tolerance are demonstrated by the two examples in Scheme 9. A very bulky tertiary propargylic alcohol as well as a simple linear alkyne provide excellent access to the CE)-/3-vinylsilane products.4 a 1 In order to achieve clean air-addition, cationic complexes have provided consistent results, since vinylmetal isomerization becomes less competitive for a cationic intermediate. Thus, halide-free systems with... 

Carbonylation of Ethene in Methanol Catalysed by Cationic Phosphine Complexes of Pd(ll) from Polyketones to Monocarbonylated Products... 

Catellani M (2005) Novel Methods of Aromatic Functionalization Using Palladium and Norbornene as a Unique Catalytic System. 14 21-54 Cavinato G, Toniolo L, Vavasori A (2006) Carbonylation of Ethene in Methanol Catalysed by Cationic Phosphine Complexes of Pd(II) from Polyketones to Monocarbonylated Products. 18 125-164... 

Strongly basic cationic phosphine ligands 75, 76 containing guanidino functions were prepared either in the reaction of 3-... 

Cationic phosphine Hgands containing guanidiniumphenyl moieties were originally developed in order to make use of their pronounced solubility in water [72, 73]. 

Cationic phosphine/phosphite -allyliron complexes are more rare. Aside from the previously mentioned report of the preparation of (139a), complex (139b) has been made from the reaction of allyl halides and Fe[P(OMe)3]5. Additionally, complexes with mixed CO/phosphine ligands (139c) have been prepared by the reaction of either (135 X = Br) or [( -allyl)Fe(CO)3]2 with two equivalents of phosphine, followed by NaBPlu addition. ... 

A systematic study of the synthesis and reactions of various dithiocar-benes has been undertaken by Angelici et al. (4). The ylide complexes obtainable from certain of these carbenes, [FeCp(CO)2 CH(SMe)PR2H ] (61) (R = Ph or c-Hex), undergo pyrolysis at temperatures around 200°C to give cationic phosphine complexes, [FeCp(CO)2 PR2(CH2SMe))] (62) (79). 

Additionally, the Negishi cross-coupling of arylzinc reagents and aryl iodides using Pd(dba)2 and the cationic phosphine ligand 4 was performed in a biphasic system of an imidazolium salt 1 and toluene [34]. 

Strongly basic cationic phosphine ligands 75, 76 containing guanidino functions were prepared either in the reaction of 3-aminopropyldiphenylphosphine with lH-pyrazole-l-carboxamide under basic conditions in DMF [75] or by the addition of dimethylcyanamide to the amino groups of tertiary (3-aminophenyl)phosphines in acidic medium [70], These phosphines (as acetate or chloride salts) are highly soluble in water in some cases the solubility reaches that of TPPMS. Another noteworthy feature of these compounds that they are considerably less sensitive to air oxidation then the anionic (e.g. sulfonated) phosphines. 

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