DIASTEREOSELECTIVE ALKYNE ALKYLATION REACTIONS IN CHIRAL-AT-METAL NIOBIUM COMPLEXES

Deprotonation with n-BuLi, TMEDA at the propargylic site of the coordi nated alkyne in Tp'Nb(Cl)(X) (PhC = C-CH3) (X = Cl, 1a, OCH3, 2a; Tp' = hydridotris(3,5-dimethylpyrazolyl)borate) followed by reaction with CH3I or PhCH2Br affords the alkylated alkyne compounds Tp'Nb(Cl)(X)(Ph C = C-CH2R) (R = CH3, X = Cl, lb, OCH3, 2b; R = CH2Ph, X = Cl, lc, OCH 3, 2c) in high yield. Adding PhCH2Br to deprotonated 2b gives a 4/1 di astereomeric ratio of Tp'Nb(Cl)(OCH3)[PhC = C-CH(CH3)(CH2Ph)] (3), whe reas a 1/6 ratio is obtained under the same conditions starting from 2 c and CH3I.