Phosphine-metal complexes ruthenium

Ruthenium complexes of a novel silsesquioxane-based tridentate phosphine ligand have been prepared and characterized by Mitsudo et al The synthesis of the ligand 178 is depicted in Scheme 60. Reactions of 178 with several late transition metal complexes were examined. A typical example is the reaction with three equivalents of [RuCl2(cymene)]2, which produced the red triruthenium complex (c-C5H9)7Si709[0SiMe2CH2CH2PPh2RuCl2(cymene)]3 (179) in almost quantitative yield. 

Ruthenium complexes containing this phosphine ligand are able to reduce a variety of double bonds with enantiomeric excesses above 95%. In order to achieve high enantio-selectivity, the compound to be reduced must show a strong preference for a specific orientation when complexed with the catalyst. This ordinarily requires the presence of a fimctional group that can coordinate with the metal. The ruthenium binaphthyldiphosphine catalyst has been used successfully with unsaturated amides,11 allylic and homoallylic alcohols,12 and unsaturated carboxylic acids.13... 

It is clear from the preceding section that the field of tethered arene-metal complexes is dominated by ruthenium and by arene-phosphines as ligands. In part, this situation has arisen because of the current surge of interest in the catalytic properties of ruthenium complexes in organic synthesis.85,86 Moreover, the tethered arene complexes are usually air-stable, crystalline solids with a well-defined, half-sandwich molecular geometry that, in principle, can lock the configuration at the metal centre. These compounds should, therefore, be ideal both for the study of the stereospecificity of reactions at the metal centre and for stereospecific catalysis. 

The resulting extraordinary stability of NHC-metal complexes has been utilized in many challenging applications. However, an increasing number of publications report that the metal-carbene bond is not inert [30-38]. For example, the migratory insertion of an NHC into a ruthenium-carbon double bond [30], the reductive elimination of alkylimidazolium salts from NHC alkyl complexes [37] or the ligand substitution of NHC ligands by phosphines [36,38] was described. In addition, the formation of palladium black is frequently observed in applications of palladium NHC complexes, also pointing at decomposition pathways. 

Metal complex chemistry, homogeneous catalysis and phosphane chemistry have always been strongly connected, since phosphanes constitute one of the most important families of ligands. The catalytic addition of P(III)-H or P(IV)-H to unsaturated compounds (alkene, alkyne) offers an access to new phosphines with a good control of the regio- and stereoselectivity [98]. Hydrophosphination of terminal nonfunctional alkynes has already been reported with lanthanides [99, 100], or palladium and nickel catalysts [101]. Ruthenium catalysts have made possible the hydrophosphination of functional alkynes, thereby opening the way to the direct synthesis of bidentate ligands (Scheme 8.35) [102]. 

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