Configurationally rigid pentaorganosilicates

The intramolecular substituent interchange in recently reported pentaorganosilicates is investigated by B3LYP calculations, which show excellent agreement with the experimental thermochemical data. Two types of ligand permutation are discerned (A and B), which both lead to racemization of the helical, spirocyclic anions. IRC calculations show that stereomutation A bifurcates into two enantiomeric reaction paths, which are inhibited by ortho substitution of the bidentate ligands. The other pathway (B) proceeds through a trigonal bipyramidal transition state with one bisequatorial bidentate ligand and is disfavored by increasing the π-electron density of the ligand. A more electronegative fifth, monodentate substituent increases the barrier of pathway A and lowers that of pathway B, as in bis(biphenyl-2,2'-diyl)fluorosilicate, which is the first tetraorganofluorosilicate to be isolated and fully characterized. These concepts enabled us to design and synthesize methyl- and ethylbis([2]naphthylpyrrol-2,1'-diyl)silicate as Si-chiral pentaorganosilicates that are configurationally rigid at room temperature.