Preparation and Reactions of Enantiomerically Pure α-Functionalized Grignard Reagents

A strategy for the generation of enantiomerically pure alpha-functionalized chiral Grignard reagents is presented. The approach involves the synthesis of alpha-alkoxy and alpha-amino sulfoxides in >= 99:1 dr and >= 99:1 er via asymmetric deprotonation (s-BuLi/chiral diamine) and trapping with Andersen's sulfinate (menthol derived). Subsequent sulfoxide -> Mg exchange (room temperature, 1 mm) and electrophilic trapping delivers a range of enantiomerically pure alpha-alkoxy and alpha-amino substituted products Using this approach, either enantiomer of products can be accessed in 99:1 er from asymmetric deprotonation protocols without the use of (-)-sparteine as the chiral ligand. Two additional discoveries are noteworthy: (i) for the deprotonation and trapping with Andersen's sulfinate, there is a lack of stereospecificity at sulfur due to attack of a lithiated intermediate onto the alpha-alkoxy and alpha-amino sulfoxides as they form, and (ii) the alpha-alkoxy-substituted Grignard reagent is configurationally stable at room temperature for 30 min.