Ligand-assisted nucleophilic addition reaction

A striking exaple of the influence of the Schlenk equilibrium on the progress of reactions of organomagnesium compounds was observed in, what was named, a ligand-assisted nucleophilic addition reaction (LANA), in which the substrate was a lithium quinol alkoxide [29]. In a typical LANA reaction (Scheme 14) the lithium quinol alkoxide is first prepared in situ, after which follows the sequential addition of l,3-dimethyl-3,4,5,6-tetrahydro-2(l//)-pyrimidone (DPMU) and a Grignard reagent. 

The highest enantiomeric excess in the palladium(0)-assisted enantiosclcctive alkylation of racemic 3-cycloalkenyl acetate has so far been observed with the chiral ligands 2, 3a and 3b60-61 (Table 17). The addition of tetrahexylammonium bromide dramatically increases enantioselec-tivity in the case of ligand 2, as does changing the solvent from tetrahydrofuran to dichloro-methane. which is believed to enhance the formation of dimeric ionic salts of the nucleophile. In contrast, additives such as tetraalkylammonium salts or crown ethers diminish the enantiomeric excess in reactions catalyzed by the phosphinoaryl oxazoline ligands 3a and 3b. bearing a chiral phosphorus on the aryl moiety. 

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