Alcohols monodentate

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. 

Introduction of substituents to the cyclopentadienyl ring of 7, or the use of complexes derived from other monodentate chiral alcohols, cause decreased enantioselectivity11,1 la,35a 36. 

CU4S4] species in which the TADDOL-derived ligand was an unexpected sul-fur-monodentate and not a S/X-bidentate ligand. This hypothesis was confirmed by NOE NMR spectroscopy that suggested a different structure for the Cu complex of the alcohol ligand relative to the methylether and the dimethylamino ligands, which could explain the observed stereochemical inversion. 

Ito and co-workers observed the formation of zinc bound alkyl carbonates on reaction of carbon dioxide with tetraaza macrocycle zinc complexes in alcohol solvents.456 This reversible reaction was studied by NMR and IR, and proceeds by initial attack of a metal-bound alkoxide species. The metal-bound alkyl carbonate species can be converted into dialkyl carbonate. Spectroscopic studies suggested that some complexes showed monodentate alkyl carbonates, and varying the macrocycle gave a bidentate or bridging carbonate. Darensbourg isolated arylcarbonate compounds from zinc alkoxides as a by-product from work on polycarbonate formation catalysis.343... 

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... 

The nature of the catalysts, especially those formed in situ from chlo-rorhodium(I) precursors, deserves some comment. The catalysts have been often written as Rh(P )2Cl(solvent), where P and (P )2 represent monodentate and bidentate chiral phosphines, respectively (10), but this almost certainly pertains for nonpolar media only. In polar media, including the mixed hydrocarbon/alcohol solvents usually employed,... 

As for some of the monodentate phosphine-based catalysts, ds-[Ru(6,6 -Cl2bpy)2(0H2)2][CF3S03]2 was found to require water for the best catalytic activity in the reduction of aldehydes and ketones [57]. Aldehydes and ketones were found to be hydrogenated, with reasonable yields. Unsaturated aldehydes were reduced with selectivity towards the unsaturated alcohol, whereas unsaturated ketones showed selectivity towards the saturated ketones. 

The chiral monodentate phosphites presented in Scheme 28.6 are easily prepared from a diol, phosphoms trichloride, and an alcohol. Usually, the diol is converted into the corresponding phosphoro chloridite, followed by reaction... 

Reetz and Goossen et al. reported recently the asymmetric hydrogenation of a series of enol esters using monodentate phosphite ligands 17 and 24 based on a combination of BINOL and carbohydrates or simple alcohols the results of these studies are shown in Table 28.6. 

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. 

Catalytic asymmetric hydrosilylation of prochiral olefins has become an interesting area in synthetic organic chemistry since the first successful conversion of alkyl-substituted terminal olefins to optically active secondary alcohols (>94% ee) by palladium-catalyzed asymmetric hydrosilylation in the presence of chiral monodentate phosphine ligand (MOP, 20). The introduced silyl group can be converted to alcohol via oxidative cleavage of the carbon-silicon bond (Scheme 8-8).27... 

An asymmetric version of the Pd-catalyzed hydroboration of the enynes was reported in 1993(118]. The monodentate phosphine (S)-MeO-MOP was used as a chiral ligand for the palladium catalyst. Enantioselectivity of the asymmetric hydroboration was estimated from the enantiopurity of homopropargyl alcohols, which were obtained from the axially chiral allenylboranes and benzaldehyde via an SE pathway (Scheme 3.78). 

Catechol allenylboranes have also been used to synthesize homopropargylic alcohols [25], These reagents are prepared by hydroboration of an enyne with catechol-borane in the presence of a Pd(0) catalyst possessing monodentate phosphine ligands. Dienylboranes were formed as minor products. Optimum results were obtained by treatment of the catecholborane with molar equivalents of triphenylpho-sphine and the palladium catalyst. Although several allenylboranes were prepared, only the dimethyl reagent was further examined. Treatment of that borane with benzaldehyde afforded the homopropargylic alcohol in 62% yield (Eq. 9.21). 

The last possibility for ester formation (20, Figure 12.15) comprises the reductive elimination of esters from acyl-alkoxy-palladium complexes 17, formed by deprotonation of the alcohol adducts 16. Clearly, it requires cis coordination of the alkoxide and acyl fragment. Since monodentates have a preference for ester formation, it was thought that this mechanism was very unlikely. 

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