Ligands furyl phosphines

The unsaturated c.vo-enol lactone 17 is obtained by the coupling of propargylic acetate with 4-pentynoic acid in the presence of KBr using tri(2-furyl)-phosphine (TFP) as a ligand. The reaction is explained by the oxypalladation of the triple bond of 4-pentynoic acid with the ailenyipailadium and the carbox-ylate as shown by 16, followed by reductive elimination to afford the lactone 17. The ( -alkene bond is formed because the oxypalladation is tnins addition[8]. 

Many procedures use Pb(OAc)2 or other Pd(II) salts as catalysts with the catalytic-ally active Pd(0) being generated in situ. The reactions are usually carried out in the presence of a phosphine ligand, with Lris-o-tolylphosphine being preferred in many cases. 7ra-(2-furyl)phosphine (tfp) is also used frequently. Several chelating diphosphines, shown below with their common abbreviations, are also effective. Phosphites are also good ligands.130... 

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>. 

A variety of palladium(O) or palladium(ii)/phosphine systems have been used as catalyst precursors (Figure 9). Triphenylphosphine was usually the ligand of choice until Farina showed in 1991 that the use tri-(2-furyl)phosphine enhanced reaction rates. The positive effects of additives such as copper salts and diethylamine ... 

An example that used this protocol the substrate of which contains a sensitive functionality is depicted in Scheme 45 [202]. 1,3-Diene monoepoxide is easily attacked by a palladium(O) complex to form the corresponding 7r-allylpalladium species. Thus such a process could be banished from the desired selective transformation as depicted in Scheme 46 by the employment of the alkynyltin reagent with an aid of triphenylarsine ligand [203]. Organotin protocol is also convenient for introduction of a small alkynyl moiety such as C2 or C3 or preparation of symmetrical diarylethynes (Scheme 47) [204]. Shirakawa et al. reported recently that iminophosphine 4 is much more effective ligand for palladium than tris(2-furyl)phosphine in this reaction (Scheme 48) [32,205]. 

The transmetallation step (iii) is certainly the most enigmatic part of the catalytic cycle. Generally, it is assumed to be rate limiting, and several mechanisms are proposed depending on the solvent. An open transition state with inversion of the stereochemistry would arise with polar solvents which are able to stabilize the transient partial charges , whereas a cyclic transition state with retention of the stereochemistry would arise in less polar solvents. It should be noted that the nature of the ligands on the palladium may influence dramatically the kinetics of the transmetallation step. A 1000-fold rate enhancement was observed when replacing triphenylphosphine by tri(2-furyl)phosphine . However, the dissociative or associative nature of the substitution on the palladium is stiU under discussion . ... 

In 1991 Farina carried out a detailed study on the effect of ligands on the reaction between vinyltributyltin and several halides and triflates [77], His conclusion was that the best ligand for this type of reaction was tris(2-furyl)phosphine. Halides (particularly iodides) and triflates are indeed the most frequently used leaving groups, although Roth and Sapino showed that fluorosulfonates can also be used [78] their catalyst was palladium(II) acetate. A look at the catalyst (or to be exact precatalyst) and cocatalyst combinations, together with solvent variations, which have been used in the papers cited above will make it clear that there is in fact no ideal system, but that each reaction will basically require optimization. 

Ether, thioether, and thiophene-modified ligands, e.g., (77)-(79) (Scheme 5) are known and are prepared by standard procedures, as illustrated in Equation (24) for (78).17 174 The tri-2-furyl-phosphine (80) has a rich and varied chemistry and is widely used as a ligand in various metal-catalyzed reactions.175... 

Some of the most significant improvements in catalyst development were the discoveries of alternative palladium ligands made by Farina in the early 1990s. In particular, tri(2-furyl) phosphine (15) and triphenylarsine proved to be far superior to triphenylphosphine when used as ligands in palladium-catalyzed Kosugi-Stille reactions. Perhaps... 

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