Pseudorotational Mechanism for the Inversion of 10-Si-5 Siliconates: Ligand Structure and Reactivity

Article abstract of DOI:10.1021/ja00308a030

X-ray crystal structure determinations of 8-Si-4 spirosilane 2 and 10-Si-5 phenylsiliconate 5a illustrate the changes in molecular geometry which accompany the increase in coordination state of silicon; 2 is a distorted tetrahedron while 5a is essentially trigonal bipyramidal at silicon, with oxygens occupying apical positions.Rates of ligand permutation (inversion of configuration at silicon), as measured by 19F NMR monitoring of site interchange for the geminal CF3 groups, for a series of siliconates <3; Y=n-Bu, 4-MeOC6H4, C6H5, 3-CF3C6H4, 3,5-(CF3)2C6H3, OPh, C6F5, CN, and F> are consistent with a mechanism proceeding by a nondissociative, Berry-type pseudorotation.There is an excellent linear correlation between the energy barrier ΔG* to inversion at silicon and ?* of the variable ligand Y, with electron-withdrawing ligands favoring pseudorotation.Diastereoisomeric siliconates 3g and 3h are separately observable by 19F NMR.Their thermal interconversion, directly followed by NMR, proceeds with a rate similar to that measured by magnetization transfer in 19F NMR spectroscopy for 3a (Y=n-Bu).This is also consistent with the observed correlation between ΔG* and ?*.The energy barrier to inversion at 424 K for phenylsiliconate 5b (ΔG*=26.0 kcal/mol) is significantly lower than that of the isostructural, isoelectronic phosphorane 7 (ΔG*=28.3 kcal/mol).The bidentate ligand of 1 preferentially stabilizes 10-Si-5 compounds relative to 8-Si-4 and 12-Si-6 compounds as indicated by addition of one molecule of nucleophile to 2 and by the absence of evidence for coordination of a second molecule of nucleophile to give observable 12-Si-6 compounds. (Pentafluorophenyl)siliconate 6 decomposes to give fluorosiliconate 4 by a fluoride ion chain mechanism proposed to proceed by initial coordination of fluoride ion at Si in 6 to give a high-energy 12-Si-6 species whichrapidly losesC6F5^- to give 4.Loss of fluoride ion from C6F5(1-) provides the fluoride ion required to carry on the chain.The rate of monodenta Si-C bond cleavage of arylsiliconates by electrophiles bromine and triflic acid decrease markedly as the electronegativity of the monodenta group increase.Phenylsiliconate 5 and n-butylsiliconate 9 react with bromine at comparable rates to give selective cleavage of the modentate ligands Si-C bond.