Ionic phosphine ligands

QCdmJfSbF,] Rh(cod) with ionic phosphine ligand Enamides 1 bar, 20°C activity drops after the third cycle enantioselectivity remains basically unchanged. [41]... 

Fig. 3.5-10 Examples of ionic phosphine ligands employed for the immobilization of transition metal catalyst precursors in ILs.

Also Pdotalyzed Negishi cross-coupling reactions have been described in ionic liquids. Knochel and coworkers investigated the reaction between organometalhc zinc compounds and aryl iodide in [BMMIM][Bp4] using an ionic phosphine ligand. Scheme 5.3-30 illustrates the reaction for the formation of a 3-substituted cyclohexenone from 3-iodo-cyclohex-2-en-l-one [170]. The reaction vras carried out in an ionic liquid/toluene biphasic system, which allowed easy product recovery from the catalyst by decantation. However, attempts to recycle the ionic catalyst phase resulted in significant catalyst deactivation, after only the third recycle. 

A new ionic phosphine ligand (1-19) is effective for the coupling of arylzinc 13 with aryl iodide 14 at room temperature to give 15 in a biphasic system containing toluene, THF, and an ionic liquid [bdmim][BF4], The Pd catalyst always stays in the ionic liquid and can be recycled [53]. 

While certain TSILs have been developed to pull metals into the IL phase, others have been developed to keep metals in an IL phase. The use of metal complexes dissolved in IL for catalytic reactions has been one of the most fruitful areas of IL research to date. LLowever, these systems still have a tendency to leach dissolved catalyst into the co-solvents used to extract the product of the reaction from the ionic liquid. Consequently, Wasserscheid et al. have pioneered the use of TSILs based upon the dissolution into a conventional IL of metal complexes that incorporate charged phosphine ligands in their stmctures [16-18]. These metal complex ions become an integral part of the ionic medium, and remain there when the reaction products arising from their use are extracted into a co-solvent. Certain of the charged phosphine ions that form the basis of this chemistry (e.g., P(m-C6H4S03 Na )3) are commercially available, while others may be prepared by established phosphine synthetic procedures. 

It was recently found that the modification of neutral phosphine ligands with cationic phenylguanidinium groups represents a very powerful tool with which to immobilize Rh-complexes in ionic liquids such as [BMIM][PFg] [76]. The guani-dinium-modified triphenylphosphine ligand was prepared from the corresponding iodide salt by anion-exchange with [NH4][PFg] in aqueous solution, as shown in Scheme 5.2-15. The iodide can be prepared as previously described by Stelzer et al. [73]. 

The use of ionic liquids as reaction media for the palladium-catalyzed Heck reaction was first described by Kaufmann et ak, in 1996 [85]. Treatment of bromoben-zene with butyl acrylate to provide butyl trans-cinnamate succeeded in high yield in molten tetraallcylammonium and tetraallcylphosphonium bromide salts, without addition of phosphine ligands (Scheme 5.2-16). 

When water-miscible ionic liquids are used as solvents, and when the products are partly or totally soluble in these ionic liquids, the addition of polar solvents, such as water, in a separation step after the reaction can make the ionic liquid more hydrophilic and facilitate the separation of the products from the ionic liquid/water mixture (Table 5.3-2, case e). This concept has been developed by Union Carbide for the hydroformylation of higher alkenes catalyzed by Rh-sulfonated phosphine ligand in the N-methylpyrrolidone (NMP)/water system. Thanks to the presence of NMP, the reaction is performed in one homogeneous phase. After the reaction. 

Additional work on alkyl platinum reactions has been completed which mostly supports the generality of these observed insertion reactions. Treichel and Wagner 153) have varied the choice of phosphine ligand utilizing the complexes Pt(phos)2(CH3)X (phos = PEtj, X = Br, I and phos = PPhMe2, X = I) both ionic intermediate and inserted products were obtained. 

The concept makes use of the complimentary strengths and weaknesses of the two unconventional media. While ionic liquids are known to be excellent solvents for many transition metal catalysts, the solubility of most transition metal complexes in scC02 is poor (if not modified with e. g. phosphine ligands with fluorous "ponytails" [64]). However, product isolation from scC02 is always very simple, while from an ionic catalyst solution it may become more and more complicated depending on the solubility of the product in the ionic liquid and on the product s boiling point. 

Whether of nM 7tL or n- -OL (off-axis) type, pi-backbonding interactions correspond to partial inclusion of ionic resonance structures featuring a metal-ligand double bond, as illustrated schematically in (4.79) for phosphine ligands ... 

Fig. 2 Phosphine ligands and ionic liquids used in continuous hydroformylation of propene [32]...

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