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| - A Green Chemistry Approach to a More Efficient AsymmetricCatalyst: Solvent-Free and Highly Concentrated AlkylAdditions to Ketones
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| - There is a great demand for development of catalyst systems that are not only efficient andhighly enantioselective but are also environmentally benign. Herein we report investigations into the catalyticasymmetric addition of alkyl and functionalized alkyl groups to ketones under highly concentrated andsolvent-free conditions. In comparison with standard reaction conditions employing toluene and hexanes,the solvent-free and highly concentrated conditions permit reduction in catalyst loading by a factor of 2- to40-fold. These new conditions are general and applicable to a variety of ketones and dialkylzinc reagentsto provide diverse tertiary alcohols with high enantioselectivities. Using cyclic conjugated enones, we haveperformed a tandem asymmetric addition/diastereoselective epoxidation using the solvent-free additionconditions followed by introduction of a 5.5 M decane solution of tert-butyl hydroperoxide (TBHP) to generateepoxy alcohols. This one-pot procedure allows access to syn epoxy alcohols with three contiguousstereocenters with excellent enantio- and diastereoselectivities and high yields. Both the solvent-freeasymmetric additions and asymmetric addition/diastereoselective epoxidation reactions have been conductedon larger scale (5 g substrate) with 0.5 mol % catalyst loadings. In these procedures, enantioselectivitiesequal to or better than 92% were obtained with isolated yields of 90%. The solvent-free and highlyconcentrated conditions are a significant improvement over previous solvent-based protocols. Further, thischemistry represents a rare example of a catalytic asymmetric reaction that is highly enantioselective undermore environmentally friendly solvent-free conditions.
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