Palladium catalysts phosphination

In Section 8.2.3.2, we discussed arylation of enolates and enolate equivalents using palladium catalysts. Related palladium-phosphine combinations are very effective catalysts for aromatic nucleophilic substitution reactions. For example, conversion of aryl iodides to nitriles can be done under mild conditions with Pd(PPh3)4 as a catalyst. 

A palladium catalyst with a less electron-rich ligand, 2,2-dipyridyl-methylamine-based palladium complexes (4.2), is effective for coupling of aryl iodides or bromides with terminal alkynes in the presence of pyrrolidine and tetrabutylammonium acetate (TBAB) at 100°C in water.37 However, the reactions were shown to be faster in NMP solvent than in water under the reaction conditions. Palladium-phosphinous acid (POPd) was also reported as an effective catalyst for the Sonogashira cross-coupling reaction of aryl alkynes with aryl iodides, bromides, or chlorides in water (Eq. 4.18).38... 

Shylesh, S., Wang, L. and Thiel, W.R. (2010) Palladium(II)-phosphine complexes supported on magnetic nanoparticles filtration-free, recyclable catalysts for Suzuki-Miyaura crosscoupling reactions. Advanced Synthesis and Catalysis, 352 (2-3), 425-432. 

Enantioselective allylations of a-nitro ketones and a-nitro esters with allyl acetates are carried out in the presence of 2 equiv of alkali metal fluorides (KF, RbF, CsF) and 1 mol% palladium catalysts prepared in situ from Pd2(dba)3-CHC13 and chiral phosphine ligands. Moderate enantio-selectivity (ca 50% ee) is reported for allylation of cx-nitroketones (Eq. 5.60). The highest selectivity (80% ee) is observed for allylation of the reaction of tert-butyl ester (Eq. 5.61).93... 

The most active palladium catalyst system developed for the asymmetric hydrosilylation of cyclopentadiene (Scheme 23) involves the use of the (/ )-MOP-phen ligand (38), which shows significant enhancement of enantioselectivity compared to (R)-MeO-MOP (80% ee from (38), 39% ee from (36a)).114 Other phosphine ligands that afford active palladium catalysts for the same transformation include the /3-7V-sulfonylaminoalkylphosphine (39) and phosphetane ligand (40) (Equation (13)).115-117 A comparison of the enantioselectivities of these ligands for the palladium-catalyzed hydrosilylation of cyclopentadiene is given in Table 8. 

Exposure of the benzo[b]thiophene derivative 44 to a palladium catalyst in the presence of tri(2-furyl)phosphine (TFP) as the ligand led to the product 45, which incorporated two heterocyclic units from the starting material. The mechanistic aspects of this transformation were also discussed, which appears to involve palladacycle intermediates <06JA722>. 

Fluorous ligands introduce an ease of purification in that the tagged phosphine ligand, the palladium catalyst complexed ligand, and the oxidized ligand can be completely removed by direct fluorous solid-phase separation (F-SPE) prior to product isolation. Similarly, an example of a fluorous palladium-catalyzed microwave-induced synthesis of aryl sulfides has been reported, whereby the product purification was aided by fluorous solid-phase extraction [91]. 

The problems associated with the in situ approaches can be avoided by using a discrete catalyst. The presumed structure of the monometallic palladium catalyst contains the sulfonated phosphine ligand chelated to the palladium and a palladium-carbon bond (polymer), most probably in cis geometry with respect to the phosphorous (Fig. 8). 

The addition of the neutral silylborane to carbon-carbon triple bonds, in which both the boron and silicon groups were retained in the products, was achieved in 1996 by using palladium catalysts.217 Although, in the preliminary report, tert-alkyl isocyanide was used as the ligand on palladium, it was later reported that conventional phosphine... 

In the asymmetric hydrosilylation of 1,3-cyclohexadiene 38 (Scheme 10, Table 4), catalyzed by chiral ferrocenylphosphines 5 and 40, the enantioselectivity is higher with phenyldifluorosilane than that with trichlorosilane or methyldichlorosilane (entries 1—4). The reaction of 38 with phenyldifluorosilane in the presence of a palladium catalyst coordinated with ferrocenylphosphine 40b gave allylsilane (A)-39c with 77% ee.58,59 The use of (j3-N-sulfonylaminoalkyl (phosphine 35a for the reaction of 38 with methyldichlorosilane exhibited the same level of asymmetric induction (entries 5-6).53 In this asymmetric hydrosilylation, combination of trichlorosilane and... 

Reduction of the cyclic alkynyl carbonate 31 afforded two different products depending on the phosphine ligands on the palladium catalyst [48], Whereas Pd(dba)2/dppe gave the allenic product 32, the homopropargyl alcohol 33 was obtained with nBu3P (Scheme 3.16). 

---