Phosphines JOSIPHOS

The josiphos ligands arguably constitute the most versatile and successful ferro-cenyl ligand family. Because the two phosphine groups are introduced in consecutive steps with very high yields (as shown in Scheme 25.1), a variety of ligands is readily available with widely differing steric and electronic properties. 

Thus, while the behaviour of the mono-rhodium tetraphosphine complexes in homogeneous hydrogenation may be expected to correspond to that previously observed for Josiphos or Mandyphos if the additional phosphine act purely in a spectator mode, new behaviour may be observed if either of the two new cross-... 

The introduction of chirafity into NHCs will therefore follow different strategies than those that have proved to be successful in phosphine-based asymmetric catalysis. For example, N-heterocydic carbene units will not create an edge-to-face arrangement of their aryl substituents, a structural feature common to many chiral diphosphines, such as the derivatives of Diop, Binap, Josiphos, Chiraphos and others. Results obtained in asymmetric catalysis, using chiral phosphine ligands, are therefore not directly transferable to the respective NHC-analogues. 

Scheme 53 Pd(Josiphos)-catalyzed asymmetric allylic phosphination...

Enantioselective allylic phosphination was catalyzed by Pd(Josiphos) complexes. As with related nucleophiles, these reactions presumably occur by nucleophilic attack of a secondary phosphine on a Pd-aUyl intermediate (Scheme 53) [86]. 

Heterocyclic aryl chlorides can be coupled with aryl magnesium chlorides using a Pd(dba)2-dppf catalyst system. Even unactivated aryl tosylates have been successfully coupled with aryl Grignard reagents in the presence of as little as 0.1% of a catalyst prepared from Pd(dba)2 and chelating phosphines of the Josiphos-type. Syrmnetrical biaryls can be prepared from the direct homocoupling of aryl iodides and bromides using a combination of phosphine-free Pd(dba)2 and TBAF in DMF. ... 

Ferrocene hgands represent an important group of chiral phosphines, among which the most interesting are discussed in Josiphos et al. [143]. Most of these hgands incorporate both carbon-centered chirahty and planar chirality, and they have been proven to be effective in numerous asynunetric reactions. Catalysts based upon ferrocene hgands are efficient in various asymmetric reactions, and have attracted considerable interest over the last few years [144-149]. For example, Chen et al. [145] have described a method for the incorporation of P-chirogenic... 

Consequently, a significant research effort has focused on the aminations of heterocyclic halides. Most prominent here are the pyridyl hahdes, for which several catalyhcally active systems based on ligands, such as biphenyl monophosphines, carbenes, TAPs or Josiphos-type phosphines, have been reported [52, 98, 62, 67,... 

Several other palladium complexes Pd(L )(Ph)(I)] and Pd(L )(trans-stil-bene)] with different chiral diphosphine ligands L were also employed as catalyst precursors. The yield and enantioselectivity of the product are strongly dependent on the diphosphine. It was found that the DuPhos series (Me/Et/f-Pr-DuPhos) produces results similar to those listed in Table 6.6. In contrast, changing to the more flexible (R,R)-Me-BPE gives a very active catalytic system but with a disappointing 18% ee. With DuXantphos a very slow system results, with the formation of by-products. Ferrocenyl phosphines (Me-FerroLANE, Et- FerroTANE, Josiphos, BoPhoz) were also employed but were found to be inferior to DuPhos. Finally, with several Ni(Me-DuPhos) catalyst precursors, reactions do not reach completion and formation of by-products is observed. 

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