Leaving group effect phosphines

The 6 -phosphines dppe, dppp, and dppb were also effective for this coupling. Entry 3 is an example of use of a vinyl triflate. Entries 4 and 5 illustrate the use of perfluorobu-tanesulfonate (nonaflate) as an alternative leaving group to triflate. The organozinc... 

Iodination reagents combined with aryl phosphines and imidazole can also effect reductive conversion of diols to alkenes. One such combination is 2,4,5-triiodoimidazole, imidazole, and triphenylphosphine.215 These reagent combinations are believed to give oxyphosphonium intermediates which then serve as leaving groups, forming triphenylphosphine oxide as in the Mitsunobu reaction (see Section 3.2.4). The iodide serves as both a... 

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. 

Substituted phenolate anions react with 4-nitrophenyl diphenyl-phosphinate in a quasi-symmetrical reaction so that the effective charges can be determined for both forming and breaking bonds from the two quantities and Peq without any special experiments made on the leaving group substituents. The transition structure for the reaction, which has a concerted mechanism, has ... 

This effect is readily attributable to the very loose structure of the ion-pairs in the TS (also involving delocalization in the phenyl groups of the phosphine) and which are therefore far more polar than the initial ion-pairs in the GS. Furthermore, because tetraalkylammonium is a poor leaving group reacting under rather harsh conditions (high energy of activation), we are certainly concerned with a late TS very prone to MW effects. 

A stepwise mechanism with a rate-limiting leaving group expulsion from the intermediate was proposed for the reactions of Z-aryl dimethyl- (43a), methylphenyl-(43b), and diphenyl-phosphinates (43c) with substituted anilines in DMSO at 333 K, based on the positive cross-interaction constants, The steric effects of the two ligands (R, R ) play a role in determining the reactivity of the phosphinates, but are relatively small compared to other phosphinate systems. A dominant frontside nucleophilic attack involving a hydrogen-bonded four-centre-type transition state (44) was proposed... 

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