Triarylphosphine-Derived Catalysts

All approaches to the design of enantiomerically pure rhodium(II) catalysts had depended on the attachment of enantiomerically pure ligands to the rhodium core. In collaboration with Professor Pascual Lahuerta of the University of Valencia, Spain, we undertook a complementary strategy, the preparation of rhodium(I I)-dimers (P)-56 and its enantiomer (M)-56, having backbone chirality [23]. Using the approach outlined above, we calculated that the transition state 52 should be favored over the transition state 53 by 4.2 kcal moU.  

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Dehalogenation of monochlorotoluenes can be readily effected with hydrogen and noble metal catalysts (34). Conversion of -chlorotoluene to Ncyanotoluene is accompHshed by reaction with tetraethyl ammonium cyanide and zero-valent Group (VIII) metal complexes, such as those of nickel or palladium (35). The reaction proceeds by initial oxidative addition of the aryl haHde to the zerovalent metal complex, followed by attack of cyanide ion on the metal and reductive elimination of the aryl cyanide. Methylstyrene is prepared from -chlorotoluene by a vinylation reaction using ethylene as the reagent and a catalyst derived from zinc, a triarylphosphine, and a nickel salt (36). 

Investigation of the derivatized triarylphosphine in PP2 demonstrated that this catalyst system was much more stable under the same reaction conditions compared to that derived from the phosphite. The rhodium leaching level was dramatically reduced (0.05 % in one case). Omission of toluene from this system allows development of a process which is nearing the rigorous retention that would be required for commercial application, whilst retaining a high rate and good selectivity to the linear aldehyde product. The refined system also compares well with commercial processes. 

Cluster or bimetallic reactions have also been proposed in addition to monometallic oxidative addition reactions. The reactions do not basically change. Reactions involving breaking of C-H bonds have been proposed. For palladium catalysed decomposition of triarylphosphines this is not the case [32], Likewise, Rh, Co, and Ru hydroformylation catalysts give aryl derivatives not involving C-H activation [33], Several rhodium complexes catalyse the exchange of aryl substituents at triarylphosphines [34] ... 

Triarylphosphines, which are often employed with palladium in catalysts, also can transfer aryl groups to the palladium and cause vinyl substitution reactions with alkenes.49,30 Fortunately, this reaction is usually slower than other methods for generating arylpalladium derivatives so that it usually is not a problem, but there are exceptions (equation 16). 

Water-soluble derivatives of alkylidenes 8 and 9 were prepared via phosphine ligand substitution reactions. Exchange of the phosphines in 8a for PhP(p-C6H4S03-Na)2 afforded a water-soluble vinyl alkylidene [20]. This alkylidene was soluble in water, but the triarylphosphine ligands were too small and insufficiently electron-donating to produce an active catalyst [48], Analogous substitution of the phosphines in 9 a for more sterically demanding, electron-rich, water-soluble phosphines yielded ruthenium alkylidenes 10 and 11 (Scheme 2), which were soluble in both water and methanol [49]. 

In spite of intensive studies of phosphine-containing catalysts for hydroformylation and their application in industry, publications concerning the stability of these catalysts under conditions of hydroformylation reactions have appeared only recently.It was found that the phosphorus-carbon bond is relatively rapidly broken. The decomposition of triarylphosphines leads to the formation of alkyldiarylphosphines as well as aromatic hydrocarbons and their derivatives such as aldehydes and alcohols. Several... 

A number of nickel salts can be used to generate the catalyst. Nickel chloride and bromide were the most active. Zinc, magnesium and manganese in combination with the nickel catalyst gave high yields of coupled products from chloroaromatic derivatives. Triarylphosphines were the best ligands. 

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