- The γ-silicon effect. II. The substituent effect on the solvolysis of 3-(aryldimethylsilyl)-2,2-dimethylpropyl p-bromobenzenesulfonates
-
Solvolysis rates of 3-(aryldimethylsilyl)-2,2-dimethylpropyl p- bromobenzenesulfonates were determined in 60% (v/v) aqueous ethanol (60E) and 97% (w/w) aqueous 2,2,2-trifluoroethanol (97Tw) at 50°C. The effects of aryl substituents at the silyl atom on these solvolyses were correlated with unexalted σ°parameters, giving the ρ values of -0.87 in 60E and -1.08 in 97Tw. This indicates that there exists a certain extent of positive charge on the γ-silicon atom reflecting the delocalization of incipient carbocationic charge by participation of the Si-Cγ, bond. The size of ρ values can be regarded as the effect of aryl ring on γ-Si via percaudal interaction in the rate-determining step.
- Nakashima, Tohru,Fujiyama, Ryoji,Fujio, Mizue,Tsuno, Yuho
-
-
Read Online
- The γ-silicon effect. I. Solvent effects on the solvolyses of 2,2- dimethyl-3-(trimethylsilyl)propyl and 3-(aryldimethylsilyl)-2,2- dimethylpropyl p-toluenesulfonates
-
The solvolysis rates of 2,2-dimethyl-3-(trimethylsilyl)propyl and 3- (aryldimethylsilyl)-2,2-dimethylpropyl p-toluenesulfonates were measured in a wide variety of solvents at 45 °C. The solvent effects were analyzed by using the Winstein-Grunwald equation. The solvent effects observed did not give simple linear correlations with the 2-adamantyl Y(OTs) parameter, but showed dispersion behavior in a series of binary solvents. The m values of 0.59-.67 were remarkably lower than unity for the limiting k(c) solvolysis of 2-adamantyl p-toluenesulfonate. The deviation patterns could not be interpreted in terms of nucleophilic assistance by the solvent. The dispersion behavior with reduced m values was found to be more significant for the 3-(aryldimethylsilyl) than for the 3-(trimethylsilyl) derivatives and was compatible with the delocalization of the incipient cationic charge by participation of the Si-Cγ bond in the rate-determining step. An extended dual-parameter treatment, log (k/k(80E)) = m(c)Y(OTs) + m(Δ)Y(Δ), successfully correlated such γ-silyl assisted solvolyses. The M(Δ) values of 0.24-0.49 so obtained, where M(Δ) = 0.51 m(Δ)/(m(c) +0.51 m(Δ)), are a measure of the extent of charge delocalization, suggesting that the γ-silyl group in the percaudal interaction is more effective in delocalizing the cationic charge than the alkyl group in C-C σ-participation, but less so than σ-assisted interaction by the β-aryl group.
- Nakashima, Tohru,Fujiyama, Ryoji,Fujio, Mizue,Tsuno, Yuho
-
p. 741 - 750
(2007/10/03)
-
- The Regiochemistry and Stereochemistry of the Hydroboration of Allylsilanes
-
The hydroboration of a wide range of allylsilanes 3 and 5-21 is found to be generally regioselective for attachment of the boron to C-3 and hydrogen to C-2 of the allyl unit, and to be generally stereoselective in the sense 1, with attachment of the boron
- Fleming, Ian,Lawrence, Nicholas J.
-
p. 3309 - 3326
(2007/10/02)
-
- REGIOCHEMISTRY IN THE HYDROBORATION OF ALLYLSILANES
-
The hydroboration of most types of allylsilane is highly selective for the formation of the product having the silicon and boron in a 1,3 relationship when the hydroborating agent is 9-BBN.There is only a low degree of correlation between the difference in the chemical shifts of the trigonal carbon atoms on an alkene and the regioselectivity in hydroboration with either borane:THF or 9-BBN.
- Fleming, Ian,Lawrence, Nicholas, J.
-
p. 2073 - 2076
(2007/10/02)
-