Ethylene catalysts, iridium complexes

Iridium complexes with triphenylphosphine modified with poly(ethylene oxide)s 14 at one of the phosphine groups were active in two-phase hydrogenation of allylbenzene, although the catalyst activity and selectivity with respect to the hydrogenation product, propylbenzene, was substantially lower than in the case of its low molecular analogue and one of the main products was propenylbenzene. Similar results were obtained for an iridium complex with polyethylene oxide modified with pyridyl groups [53],... 

A wide range of carbon, nitrogen, and oxygen nucleophiles react with allylic esters in the presence of iridium catalysts to form branched allylic substitution products. The bulk of the recent literature on iridium-catalyzed allylic substitution has focused on catalysts derived from [Ir(COD)Cl]2 and phosphoramidite ligands. These complexes catalyze the formation of enantiomerically enriched allylic amines, allylic ethers, and (3-branched y-8 unsaturated carbonyl compounds. The latest generation and most commonly used of these catalysts (Scheme 1) consists of a cyclometalated iridium-phosphoramidite core chelated by 1,5-cyclooctadiene. A fifth coordination site is occupied in catalyst precursors by an additional -phosphoramidite or ethylene. The phosphoramidite that is used to generate the metalacyclic core typically contains one BlNOLate and one bis-arylethylamino group on phosphorus. 

The use of ethylene adduct lb is particularly important when the species added to activate catalyst la is incompatible with one of the reaction components. Iridium-catalyzed monoallylation of ammonia requires high concentrations of ammonia, but these conditions are not compatible with the additive [Ir(COD)Cl]2 because this complex reacts with ammonia [102]. Thus, a reaction between ammonia and ethyl ciimamyl carbonate catalyzed by ethylene adduct lb produces the monoallylation product in higher yield than the same reaction catalyzed by la and [Ir(COD)Cl]2 (Scheme 27). Ammonia reacts with a range of allylic carbonates in the presence of lb to form branched primary allylic amines in good yield and high enantioselectivity (Scheme 28). Quenching these reactions with acyl chlorides or anhydrides leads to a one-pot synthesis of branched allylic amides that are not yet directly accessible by metal-catalyzed allylation of amides. 

Detailed mechanistic studies with respect to the application of Speier s catalyst on the hydrosilylation of ethylene showed that the process proceeds according to the Chalk-Harrod mechanism and the rate-determining step is the isomerization of Pt(silyl)(alkyl) complex formed by the ethylene insertion into the Pt—H bond.613 In contrast to the platinum-catalyzed hydrosilylation, the complexes of the iron and cobalt triads (iron, ruthenium, osmium and cobalt, rhodium, iridium, respectively) catalyze dehydrogenative silylation competitively with hydrosilylation. Dehydrogenative silylation occurs via the formation of a complex with cr-alkyl and a-silylalkyl ligands ... 

The activity of complex [lT2(CH3CN)(H)3(p-H)(P Pr3)2(p-Pz)2] as a catalyst for the hydrogenation of diphenylacetylene and ethylene contrasts with its inactivity when employed in the hydrogenation of A -benzylideneaniline. However, when transformed into its protonated derivative, for example, [lr2(CH3CN)(H)2(H2) ( 4-H)(P Pr3)2(p-Pz)2]BF4 by reaction with HBF4, the new complex becomes a very active catalyst for C=N hydrogenation [111]. The catalytic cycle involves fast elementary steps of hydride and proton transfer according to an ionic outer sphere mechanism that takes place at one of the iridium centers of the binuclear complex (Scheme 27). 

---