Monodentate ligands alcohols

The chemistry of complexes having achiral ligands is based solely on the geometrical arrangement on titanium. Optically active alcohols are the most favored monodentate ligands. Cyclopentadienyl is also well suited for chiral modification of titanium complexes. 

The MOP series of ligands59 (see Section 9.5.4.2) in conjunction with standard palladium precursors has been reported to catalyze the addition of HBcat to 1,3-enynes. With 1 mol.% catalyst produced by combination of Pd2(dba)3 and the monodentate ligand (Y)-MeO-MOP (22), axially chiral allenyl-boranes are formed (Equation (3)). Subsequent oxidation affords the corresponding alcohols with moderate ee values.60... 

However, it is possible that the heterocatalyst becomes the dominant one, either if it is more stable and thus formed in large excess, or if it is a more active, kinetically dominant catalyst. Recently, both Reetz et al. and Feringa/Min-naard/de Vries et al. have shown that this approach can be beneficial. Earlier attempts by Chen and Xiao using mixtures of monodentate phosphites based on bisphenol and a chiral alcohol were not successful [39]. In our experience, the majority of catalysts based on mixtures of monodentate ligands show a poorer performance than the individual homo-catalysts. However, in a few instances there is a positive effect. 

The substitution of [Tc02(py)4]" in DMF or DMF/MeOH mixtures with monodentate ligands such as cyanide, imidazole, or multidentate chelators such as cyclam and en has been studied. For all the ligands, complete replacement of the pyridines occurs. A dependence on the added alcohol is observed, whereas there is little or no dependence on the nature of the incoming ligand. The... 

Ali and Alper later developed a method for y-lactone formation under neutral conditions from allylic alcohols according to Scheme 23, involving Pd(0) and dppb. The same protocol was also applied to synthesis of a,/3-butenolides from propargylic alcohols. Other bidentate or monodentate ligands give inferior results, and this correlates well with bite angle effects on the rate of the CO insertion reaction. Also, the allylic alcohol needs to be a-alkyl substituted to facilitate ring closure, in accordance with other studies.t t This process was postulated by the authors to be of the carbopalladative type. 

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