Electronic Effects and the Stereochemistries in Rearrangement−Displacement Reactions of Triaryl(halomethyl)silanes with Fluoride and with Alkoxide Ions
journal contributionposted on 04.05.2002, 00:00 by John M. Allen, Steve L. Aprahamian, Esther A. Sans, Harold Shechter
Tetrabutylammonium fluoride (TBAF) reacts with (halomethyl)diphenyl(para-substituted-phenyl)silanes (13, X = Cl), 14 (X = Br), and 15 (X = I) in ether solvents to give fluorodiphenyl(para-substituted-phenylmethyl)silanes (17a) and fluorophenyl(phenylmethyl)(para-substituted-phenyl)silanes (20a) by attack on silicon and migrations of the phenyl or the para-substituted-phenyl groups to C-1 with displacement of chloride ion. Sodium methoxide in dioxane effects rearrangement−displacements of 14 (X = Br) to yield methoxydiphenyl(para-substituted-phenylmethyl)silanes (17b) and methoxyphenyl(phenylmethyl)(para-substituted-phenyl)silanes (20b). The migratory aptitudes of the varied phenyl groups in rearrangement−displacements of 13 with F- at 25 °C are p-CF3-Ph, 2.72 > p-Cl-Ph, 1.67 > Ph, 1.00 > p-CH3-Ph, 0.91 > p-CH3O-Ph, 0.58 > p-(CH3)2N-Ph, 0.55. For reactions of 14 with sodium methoxide in dioxane, the migratory aptitudes at 23 °C are p-CF3-Ph, 2.53 > p-Cl-Ph, 1.64 > Ph, 1.00 > p-CH3O-Ph, 0.84 > p-CH5-Ph, 0.79 > p-(CH3)2N-Ph, 0.68. The migratory aptitudes in the above rearrangement−displacements are increased by electron-withdrawing substituents, and logarithms of the migratory aptitudes give satisfactory linear correlations with σ and/or σ-zero values of the phenyl substituents. Hammett correlations however of the migratory aptitudes from reactions of F- with 13 (X = Cl) at 0 and −20 °C, 14 (X = Br) at 23, 0, and −20 °C, and 15 (X = I) at 23 °C are not linear. (+)-(Bromomethyl)methyl-1-naphthylphenylsilane (23, +8.29°, cyclohexane) reacts with CsF and with TBAF in THF to give benzylfluoromethyl-1-naphthylsilane (51, = 0.00°, cyclohexane) and fluoromethyl-(1-naphthylmethyl)phenylsilane (52, impure) in 10.4:1 ratio along with unchanged 23 ( 8.29°, cyclohexane). Sodium methoxide and (+)-23 in dioxane at 25 °C and at 0 °C yield (+)-benzylmethoxymethyl-1-naphthylsilane (64) and (+)-methoxymethyl(1-naphthylmethyl)phenylsilane (65) in ∼9:1 ratio. The conversions of (+)-23 to (+)-64 occur with >93% inversion about silicon. Reaction of (+)-23 with sodium methoxide at 25 °C to give (+)-65 also occurs with inversion. Further, sodium ethoxide and sodium 2-propoxide react with (+)-23 at 20−25 °C by rearrangement−displacements on silicon with phenyl migrations to yield (+)-benzylethoxymethyl-1-naphthylsilane (69) and (+)-benzylmethyl-1-naphthyl-2-propoxysilane (70), respectively, each with >95% inversion about silicon. The mechanisms of rearrangement−displacements of 13−15 and (+)-23 by fluoride and by alkoxide ions are discussed.