Preparation of chiral α-oxy-[2H1]methyllithiums of 99% ee and determination of their configurational stability

(Tributylstannyl)methyl 2,2,6,6-tetramethylpiperidine-1-carboxylate was metalated with t-BuLi/TMEDA at -78 degrees C and borylated with the mixed borate derived from (R,R)-1,2-dicyclohexylethane-1,2-diol and t-butanol to give diastereomeric boronates 31/32 in equal amounts. Boronates 31 and 32 were reduced with LiBEt3D and then oxidized with basic H2O2 to give (S)- and (R)-tributylstannyl-[1-H-2(1)]methanol of 99% ee, respectively. Treatment of their respective phosphates with n-BuLi at -78 and 0 degrees C gave microscopically configurationally stable phosphinyloxy-substituted [H-2(1)]methyllithiums, which rearranged to hydroxy-[1-H-2(1)]methylphosphonates of ee > 98% (phosphate-phosphonate rearrangement). The N,N-diisopropylcarbamates of the enantiomeric tributylstannyl-[1-H-2(1)]methanols were transmetalated to give carbamoyloxy-substituted chiral [H-2(1)]methyllithiums, which were macroscopically configurationally stable for prolonged periods of time (up to 3 h, ee still 99%) at -78 degrees C, deduced from trapping experiments with benzaldehyde. The chemical stability of these methyllithiums ended at -50 degrees C. The stereochemistry of the monoprotected and monodeuterated 1-phenylethane-1,2-diols obtained was secured by spectroscopic comparison of their Mosher esters with that of all four stereoisomeric 1-phenyl-[1-H-2(1)]ethane-1,2-diols synthesized independently. Furthermore, the configurations of the boronates and the chiral methyllithiums derived from them were deduced from a single-crystal X-ray structure analysis of a carbamate in which the tributylstannyl group had been replaced by the [(1R)-menthyl]dimethylstannyl group.