Olefin ligands

Entry Ligand Olefin Reaction Time (h) Yield (%) ee(%)... 

Ligand variation has resulted in considerable improvement of the reaction s efficiency (Scheme 38). With 9-0-(9 -phenanthryl) (PHN) ethers and 9-0-(4 -methyl-2 -quinolyl) (MEQ) ethers of dihydroquini-dine and dihydroquinine as ligands, olefins of various substitution patterns, mono-, gem-di-, trans-di-, and certain trisubstituted substrates can be dihydroxylated with high enantioselectivity (89). Some examples of these reactions are given in Scheme 38. The reaction occurs at either ambient or ice-bath temperature with the solid, nonvolatile Os(IV) salt, K20s02(0H)4, instead of 0s04. The substrate/catalyst ratio for this reaction is 2000 for terminal olefins. 

Different types of migratory insertions are exemplified by reactions 10, 11, and 12 in the Table II. An evaluation scheme is given in Matrix 6. In the present study we consider this reaction according to its general mechanism, shown in Eq. (6), where L is a neutral ligand (olefin, carbon monoxide, carbene, etc.) which becomes X (anionic c-donor ligand). X is also an anionic a-donor ligand. 

The Kharasch addition reactions promoted by [RuCl2(PPh3)3] are believed to proceed through a redox chain mechanism (Eqs. 1-3) [ 16]. Their kinetics show a first-order dependence both on the ruthenium complex and on CC14. Whereas no clear-cut evidence for alkene coordination to the metal was found with catalyst precursor 1 (which readily loses one phosphine ligand), olefin coordination cannot be excluded because there is a saturation kinetic rate dependence on the alkene. This observation led to the proposal of a reversible step involving olefin coordination to the metal center [ 16,19,20]. Recent work with other ruthenium-based catalysts further supports olefin coordination (see later). 

Chiefly through the work of Jonas and co-workers ( ) mixed-metal organometallic complexes also are known that involve interactions of lithium atoms with unsaturated r-bonded hydrocarbon ligands (olefin, cyclopentadienyl, arene, etc.). While reviews already are available 88, S9), we include examples for comparison purposes. The lithium complexes in this section show increasing complexity and diversity both in the geometries around the lithium atoms and in the degree and type of interactions involved. The common feature in these compounds is the interaction of a lithium atom with a hydrocarbon ligand which is n... 

Thus, for a highly active, single-component catalyst for the addition polymerization of norbornenes it seems that a necessary requirement is accessibility to the metal by the norbornene substrate. We refer to this family of catalysts as being naked since all of their ligands (olefins) are readily displaced by the norbornene monomer to afford a highly active cationic metal nucleus nickel catalysts being preferred. The envisaged active site is illustrated in Fig. 4.5. 

D. Reactivity of Five-Coordinate [M (Ai-ligand)(olefin)] Complexes (M = Rh, Ir)... 

In olefin and acetylene complexes of transition metals, in contrast to those of silver(I) and possibly copper(I), the 7r-component of the co-ordinate bond is considered to predominate over the o-component. This has been inferred from the large shifts in nc=c on co-ordination of the unsaturated ligands. Olefins on co-ordination to platinum and palladium show a decrease in vc=c of 140 cm-1 and 120 cm-1 respectively acetylenes, on the other hand, show decrease of 250 cm-1 and 400-575 cm-1 on co-ordination to platinum(II) and platinum (0) respectively ls2h... 

Transition-Metal-Promoted Reactions of Unsaturated Hydrocarbons. IV. Reactions of Norbornenyl Complexes of Palladium(II) with Group V Donor Ligands, Olefins, 1,3-Dienes and 1,2-Dienes, E. Ban, R. P. Hughes, and J. Powell, J. Organometal. Chem., 69, 455 (1974). 

The water-soluble pyrazolato complex (Rh(p-Pz)(CO)(TPPTS)]2 was used as precursor for olefin hydroformylation in an aqueous heptane solvent system 9]. Without additional ligand, olefin isomerization dominated hydroformylation at 7 bar CO/H2 (1 1) giving large amounts of 2-hexene. Isomerization was suppressed most effectively at 49.8 bar, leading to aldehyde chemoselectivities >90% and a 2.4-3.S njiso ratio. It was demonstrated that aerobic recycling of the aqueous catalyst phase by... 

Decarbonylation reactions and removal of other supporting ligands (olefins, acetylene, aienes, dinitn en)... 

Ligand Olefin Rh-cat. L/Rh S/C" Solvent Temp/ - Time (°C/h) Product Alcohol ... 

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