Promotion by Phosphine Ligands and Derivatives

Moreover, Hoveyda and coworkers have demonstrated that Mannich reactions between silyl enol ethers and aldimines are promoted by the chiral complex that was generated from AgOAc and an wo-Leu-derived phosphine ligand.26 When the reaction was conducted with trimethylsilyl enol ether and aldimine in the presence of AgOAc, /.so-Leu-derived phosphine ligand and 2-PrOH, the (3-aminoketone was obtained with high enantioselectivity (Schemes 9.15 and 9.16). 

Ligand screening experiments were conducted on the alkenes 1-pentene and pent-4-en-l-ol, because such substrates were inert to 3a-3c (15). Pentene lacks any polar or protic group and pentenol contains the alkene and OH separated by 3 carbons. The preliminary studies involved phosphines with both imidazol-2-yl and pyrid-2-yl substituents on P as well as t-Bu, i-Pr, Ph, and Me groups (16). From the screening, complex 1 derived from the phosphine ligand 4 (17) was identified as the most capable (in terms of both reaction rate and final yield) of promoting isomerization of both 1-pentene and pent-4-en-l-ol. 

Nb and Ta derivatives are hard acids and then-complexes with P- or As-donors are limited. Tertiary phosphines, especially PMes, have been widely used to stabilize low-valent derivatives. C-H activation reactions, promoted by the formation of thermodynamically stable Ta-H, Ta-C, and Ta=C bonds have resulted in metallacycles based on unusual anionic phosphorus donors. Nucleophilic Ta phosphinidene complexes could be stabilized by a tripodal tetradentate [NN3] amido ligand. The terminal PR ligand reacts smoothly with aldehydes, providing a general synthesis of phosphaalkenes RP=C(H)R and act thus as a phospha-Wittig reactant see Phosphorus Organophosphorus Chemistry). 

The anionic nature of 5 results in excellent solubility in ether solvents compared with that of the neutral, silver complex 3. Although we have isolated several silver tellurium eomplexes 3 with different phosphine ligands (PEt3, PEtaPh, PPr"3), the same cannot be said for 5 for which, to date, only the PEt3 derivative has been isolated and characterized via the condensation reaction outlined in Scheme 4. One distinct advantage this route does offer is, however, the possibility of synthesizing ternary and quaternary phase nanoclusters by promoting condensation of different cluster molecules. 

One of the most important transformations catalysed by palladium is the Heck reaction. Oxidative addition of palladium(O) into an unsaturated halide (or tri-flate), followed by reaction with an alkene, leads to overall substitution of a vinylic (or allylic) hydrogen atom with the unsaturated group. For example, formation of cinnamic acid derivatives from aromatic halides and acrylic acid or acrylate esters is possible (1.209). Unsaturated iodides react faster than the corresponding bromides and do not require a phosphine ligand. With an aryl bromide, the ligand tri-o-tolylphosphine is effective (1.210). The addition of a metal halide or tetra-alkylammonium halide can promote the Heck reaction. Acceleration of the coupling can also be achieved in the presence of silver(I) or thallium(I) salts, or by using electron-rich phosphines such as tri-tert-butylphosphine. ... 

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