Palladium Compounds, Complexes, and Ligands Widely Used in Organic Synthesis

2 Palladium Compounds, Complexes, and Ligands Widely Used in Organic Synthesis 

In organic synthesis, two kinds of Pd compounds, namely Pd(II) salts and Pd(0) comple.xes, are used. Pd(II) compounds are used either as stoichiometric reagents or as catalysts and Pd(0) complexes as catalysts. Pd(Il) compounds such as PdCh and Pd(OAc)2 are commercially available and widely used as 

Attention should be paid to the fact that the ratio of Pd and phosphine ligand in active catalysts is crucial for determining the reaction paths. It is believed that dba is displaced completely with phosphines when Pd2(dba)3 is mixed with phosphines in solution. However the displacement is not eom-plcte[16]. Also, it should be considered that dba itself is a monodentate alkene ligand, and it may inhibit the coordination of a sterically hindered olefinic bond in substrates. In such a case, no reaction takes place, and it is recommended to prepare Pd(0) catalysts by the reaction of Pd(OAc)2 with a definite amount of phosphinesflO]. In this way a coordinatively unsaturated Pd(0) catalyst can be generated. Preparation of Pd3(tbaa)3 tbaa == tribenzylidene-acetylacetone) was reported[17], but the complex actually obtained was Pd(dba)2[l8], 

Highly reactive Pd(0) powder is prepared by the reduction of Pd(II) salts with Li or K and used for catalytic reactions] 19,20]. Pd on carbon in the presence of PI13P is used as an active catalyst similar to PdfPh, ),] ]. 

Phosphites, such as triisopropyl and triphenyl phosphite, are weaker electron donors than the corresponding phosphines, but they are used in some reactions because of their greater rr-accepting ability. The cyclic phosphite trimethylol-propane phosphite (TMPP) or 4-ethyl-2,6,7-trioxa-l-phosphabicyclo[2.2.2]oc-tane (8), which has a small cone angle and small steric hindrance, shows high catalytic activity in some reactions It is not commercially available, but can be prepared easily[27]. 

Palladium Reagents and Catalysts—New Perspectives for the 21st Century J. Tsuji 2004 John Wiley Sons, Ltd ISBNs 0-470-85032-9 (HB) 0-470-85033-7 (PB) 

Commercially available Pd(OAc 2, PdCl2(PPh3)2, Pd(PPh3)4, Pd2(dba)3-CHCl3 and ( ] -allyl-PdCl)2 are generally used as precursors of Pd(0) catalysts with or without addition of phosphine ligands. However, it should be mentioned that catalytic activities of Pd(0) catalysts generated in situ from these Pd eompounds are not always the same, and it is advisable to test all of them in order to achieve efficient catalytic reactions. 

Pd on carbon in the presence of PPh3 may be used for reactive substrates as an active catalyst similar to Pd(PPh3) . 

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