Electronic effects and the stereochemistries in rearrangement-displacement reactions of triaryl(halomethyl)silanes with fluoride and with alkoxide ions.
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 degrees C are p-CF(3)-Ph, 2.72 > p-Cl-Ph, 1.67 > Ph, 1.00 > p-CH(3)-Ph, 0.91 > p-CH(3)O-Ph, 0.58 > p-(CH(3))(2)N-Ph, 0.55. For reactions of 14 with sodium methoxide in dioxane, the migratory aptitudes at 23 degrees C are p-CF(3)-Ph, 2.53 > p-Cl-Ph, 1.64 > Ph, 1.00 > p-CH(3)O-Ph, 0.84 > p-CH(5)-Ph, 0.79 > p-(CH(3))(2)N-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 sigma and/or sigma-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 degrees C, 14 (X = Br) at 23, 0, and -20 degrees C, and 15 (X = I) at 23 degrees C are not linear. (+)-(Bromomethyl)methyl-1-naphthylphenylsilane (23, +8.29 degrees, cyclohexane) reacts with CsF and with TBAF in THF to give benzylfluoromethyl-1-naphthylsilane (51, = 0.00 degrees, cyclohexane) and fluoromethyl-(1-naphthylmethyl)phenylsilane (52, impure) in 10.4:1 ratio along with unchanged 23 ( 8.29 degrees, cyclohexane). Sodium methoxide and (+)-23 in dioxane at 25 degrees C and at 0 degrees C yield (+)-benzylmethoxymethyl-1-naphthylsilane (64) and (+)-methoxymethyl(1-naphthylmethyl)phenylsilane (65) in approximately 9:1 ratio. The conversions of (+)-23 to (+)-64 occur with >93% inversion about silicon. Reaction of (+)-23 with sodium methoxide at 25 degrees C to give (+)-65 also occurs with inversion. Further, sodium ethoxide and sodium 2-propoxide react with (+)-23 at 20-25 degrees 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.
Allen
John M
JM
Department of Chemistry, The Ohio State University, Columbus, OH 43210, USA.
Aprahamian
Steve L
SL
Sans
Esther A
EA
Shechter
Harold
H