Phosphine ligands, commercially available

HR are catalyzed by Pd(0) complexes of phosphines. Mainly commercially available Pd(PPh3)4, Pd2(dba>3 and Pd(OAc>2 are used as precursors of Pd(0) catalysts with or without phosphines. When overligated Pd(PPh3)4 is used, reactions of congested molecules may be slow due to the presence of too many ligands, which inhibit coordination of reactants. Pd(OAc)2, Pd(dba>2 and even Pd on carbon are used with phosphines. 

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. 

Reetz et al. (16) functionalized commercially available DAB-dendrimers with diphenylphosphine groups at the periphery (1) via a double phosphination of the amines with diphenylphosphine and formaldehyde. The transition metal complexes la-le have been prepared in which the dendrimer-N-(CH2PPh2)2 groups act as bidentate ligands. 

Mowery and DeShong used the commercially available hypervalent silicate complex TBAT as a phenylating agent for the cross-coupling reaction with allylic esters. They later reported on the use of the same organosilane for the coupling with aryl iodides and triflates and electron-deficient aryl bromides. The reactions were catalyzed by either Pd(dba)2 or [Pd(allyl)Cl]2 without the need of added phosphine ligands. 

The procedure described below can be applied to several derivatives of the parent tris(imidazolyl)phosphine ligand. The details are provided for preparing tris(l,4-diisopropylimidazolyl)phosphine, but the same route can be used to synthesize tris(l-isopropyl-4-tm-butylimidazolyl)phosphine, tris(l-iso-propyl-4-phenylimidazolyl)phosphine, and tris( 1 -isopropyl-4-methylmidaz-olyl)phosphine, starting with commercially available 1-bromopinacolone, 2-bromoacetophenone, and 1-chloroacetone, respectively. 

All reagents were commercially available and used without further purification. The phosphine ligands were obtained commercially and recrystallized or distilled. Solvents were deoxygenated by vacuum-nitrogen technique. 

The following catalytic cycle is postulated.14 The (commercially available) initiating catalyst 33 is transformed into the propagating catalyst 41 running the following mechanism one time. 41 then enters the cycle by loss of one phosphine ligand. Henceforth 42 catalyzes the transformation of 10 to 11. 

Almost simultaneously, several groups developed efficient procedures for Heck reactions of deactivated chloroarenes 22 involving sterically crowded monodentate phosphines as activating ligand on the palladium (Scheme 11) [31]. Littke and Fu employed commercially available P(t-Bu)3, Hartwig P(t-Bu)3 or bis-t-butyl-ferroce-nylphosphine, and Beller di(l-adamantyl)-n-butylphosphine. The use of biscyclo-hexylmethylamine as the base instead of alkali metal carbonates or phosphates significantly extends the scope of the Fu procedure in respect of the olefin partner. 

The relatively high optical purities obtained with the Rh-NMDPP system are particularly interesting from a practical viewpoint since the NMDPP ligand is prepared from an inexpensive, commercially available, chiral precursor, (-)-menthol (17). Tertiary phosphines chiral at phosphorus, on the other hand, are much less accessible and require a classic resolution step (see later discussion for details Section II. B). 

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