Phosphine ligands hydrogenation

Scheme 1. First use of P-chirogenic phosphine ligands in asymmetric hydrogenation reaction...

Crego-Calama M, Reinhoudt DN, ten Cate MGJ (2005) Templation in Noncovalent Synthesis of Hydrogen-Bonded Rosettes. 249 in press Crepy KVL, Imamoto T (2003) New P-Chirogenic Phosphine Ligands and Their Use in Catalytic Asymmetric Reactions. 229 1-40 Cristau H-J, see Taillefer M (2003) 229 41-73... 

In 2004, a series of other chiral thioether-phosphine ligands based on a cyclopropane backbone were evaluated in the rhodium-catalysed hydrogenation of a dehydroamino acid by Molander el al As shown in Scheme 8.2, even if these ligands were generally active, only moderate enantioselectivities of up to 47% ee were obtained. 

Scheme 8.1 Hydrogenation of a-enamide ester with thioether-phosphine ligands.

In some cases an alternative sequence involving addition of hydrogen at rhodium prior to complexation of the alkene may operate.11 The phosphine ligands serve both to provide a stable soluble complex and to adjust the reactivity at the metal center. The a-bonded intermediates have been observed for Wilkinson s catalyst12 and for several other related catalysts.13 For example, a partially hydrogenated structure has been isolated from methyl a-acetamidocinnamate.14... 

The fundamental concepts of enantioselective hydrogenation were introduced in Section 2.5.1 of Part A, and examples of reactions of acrylic acids and the important case of a-acetamido acrylate esters were discussed. The chirality of enantioselective hydrogenation catalysts is usually derived from phosphine ligands. A number of chiral phosphines have been explored in the development of enantioselective hydrogenation catalysts,21 and it has been found that some of the most successful catalysts are derived from chiral 1, l -binaphthyldiphosphines, such as BINAP.22... 

Tocopherol can be produced as the pure 2R,4 R,8 R stereoisomer from natural vegetable oils. This is the most biologically active of the stereoisomers. The correct side-chain stereochemistry can be obtained using a process that involves two successive enantioselective hydrogenations.28 The optimum catalyst contains a 6, 6 -dimethoxybiphenyl phosphine ligand. This reaction has not yet been applied to the enantioselective synthesis of a-tocopherol because the cyclization step with the phenol is not enantiospecific. 

In an effort to apply the cooperative principles of metalloenzyme reactivity, involving a combination of metal-ligand and hydrogen bonding, we have reported a ruthenium catalyst incorporating imidazolyl phosphine ligands that efficiently and selectively hydrates terminal alkynes (5). We subsequently found that application of pyridyl phosphines to the reaction resulted in a >10-fold rate enhancement and complete anti-Markovnikov selectivity, even in the... 

To our knowledge, the first published report of a photocatal-ytic reaction at elevated pressure was W. Strohmeyer1s hydrogenation of 1,3-cyclohexadiene under hydrogen at 10 atm /22/. On photolysis, the iridium complex 8 formed a very active catalyst, probably by dissociation of a phosphine ligand (Equation 17). At 70 C, with hydrogen at 10 atm, and a catalyst/substrate ratio of 1/100,000, the activity was 196 per minute and the turnover number was 96,000 mol of product/mol catalyst. 

Another important use for Wilkinson s catalyst is in the production of materials that are optically active (by what is known as enantioselective hydrogenation). When the phosphine ligand is a chiral molecule and the alkene is one that can complex to the metal to form a structure that has R or S chirality, the two possible complexes will represent two different energy states. One will be more reactive than the other, so hydrogenation will lead to a product that contains predominantly only one of the diastereomers. 

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