Enantioselective synthesis of alpha-phenyl- and alpha-(dimethylphenylsilyl) alkylboronic esters by ligand mediated stereoinductive reagent-controlled homologation using configurationally labile carbenoids

Chain extension of boronic esters by the action of configurationally labile racemic lithium carbenoids in the presence of scalemic bisoxazoline ligands was explored for the enantioselective synthesis of the two title product classes. Enantioenriched 2 degrees carbinols generated by oxidative work-up (NaOOH) of initial alpha-phenylalkylboronate products were obtained in 35-83% yield and 70-96% ee by reaction of B-alkyl and B-aryl neopentyl glycol boronates with a combination of O-(alpha-lithiobenzyl)-N, N-diisopropylcarbamate and ligand 3,3-bis[(4S)-4,5-dihydro-4-isopropyloxazol-2-yl] pentane in toluene solvent (-78 degrees C to rt) with MgBr2 center dot OEt2 additive. Enantioenriched alpha-(dimethylsilylphenylsilyl)alkylboronates were obtained in 35-69% yield and 9-57% ee by reaction of B-alkyl pinacol boronates with a combination of lithio (dimethylphenylsilyl) methyl 2,4,6-triisopropylbenzoate and ligand 2,2-bis[(4S)-4,5-dihydro-4-isopropyl-oxazol-2-yl] propane in cumene solvent (-45 degrees C to -95 degrees C to rt). The stereochemical outcome of the second type of reaction depended on the temperature history of the organolithium ligand complex indicating that the stereoinduction mechanism in this case involves some aspect of dynamic thermodynamic resolution.