Phosphine type ligands

Non-phosphine type ligands are studied time by time with the aim to obtain water-soluble transition metal complexes with catalytic properties. However, with the exception of a few specific reaction types (e.g. oxidations) these catalysts cannot cope with tertiary phosphines - with the ligands on Figure 20 this has been found once again. 

Scheme 1. Chemistry typical of d8, d20 Pt metal systems M = Rh(I), Jr(/), Ni(0), Pd<0), Pt<0)9 with ancillary ligands L, typically a tertiary phosphine type ligand, CO, etc. (4, 33, 34)...

L = tertiary phosphine type ligand, phen, bipy S = solvent) (t)... 

For an analysis of the stereochemical outcome using phosphine-type ligands, see [20]. 

It must be emphasized that the success of palladium-catalyzed cross-couplings is crucially restricted to the use of phosphine-type ligands [55], some of which are shown in Fig. 2. 

JohnPhos(LS) DPEphos(L6) Xantphos(L7) Fig. 2 Typical phosphine-type ligands in CTOss-couplings... 

The major drawback of the latter protocols relies on the fact that the success of the process was limited to the use of enol triflates or tosylates bearing an aryl substituent or an electron-withdrawing group in the jS-position. This substrate limitation was shortly after overcome by Willis, who introduced alternative palladium catalysts involving other phosphine-type ligands that also assisted the... 

In this case, the success of the process relied on the use of the phosphine-type ligand Johnphos in combination with Pd2(dba)3. Furthermore, the proper choice of base played a determinant role in circumventing the formation of the UUmann-type homocoupling compound as a side product. Under the optimized conditions, a wide range of vinyl halides (jS-bromostyrene, 1-bromodienes, 4-bromoenynes, and even less-reactive 1-chlorodienes) smoothly underwent the coupling process to furnish the target N-alkenylhydrazines. 

Second generation BINAPHOS-type ligands have been developed recently. Placing 3-methoxy substituents on the aryl phosphine unit furnishes a catalyst which allows for an enantioselective hydroformylation of vinylfurans (Scheme 19) [68]. 

The linkers may be nitrogen and carbon, as in BoPhoz - that is, a ferrocene-type ligand, as has already been mentioned. An example of a phosphine-phos-phoramidite is provided by QuinaPhos (48) [292]. 

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