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DOI: 10.1055/s-0029-1219167
Selectivity in the Grignard Reaction with Silanes
Publication History
Publication Date:
04 January 2010 (online)
Abstract
Selectivity problems in preparation of silanes by Grignard reaction were discussed. A quantitative approach in terms of LFE analysis was proposed. It appeared that the inductive effect controls the rate of replacement more considerably than steric requirements in the transition state. Rates of subsequent substitution reactions at silicon decrease stepwise. Only with methylmagnesium halides are the subsequent steps faster.
Key words
Grignard reactions - selectivity - silicon - steric hindrance - substituent effects
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References and Notes
See ref. 3, p. 387.
19The rate constants were determined
under pseudo-first-order conditions with a great excess of Grignard
reagent. The rate measurements were carried out with 0.5 M solutions
of Grignard reagents. The rate constants will be published elsewhere
together with a discussion of mechanistic issues.
Method A
Method A is a GLC method.
The reaction vessel equipped with a magnetic stirrer and containing
40 mL of the Grignard reagent was thermostatted. According to the
concentration of the Grignard reagent, 0.1-1 mL of silane
(providing a 9- to 20-fold excess of the Grignard reagent) was added
into the flask to start the reaction. At appropriate times 2 mL
aliquots were taken from the reaction mixture and quenched with
ice cold water. The organic layer was instantly separated, dried, and
analyzed for the silane using GLC.
Method
B
Method B is a thermographic method. Fast reactions
were investigated in a thermostatic flask equipped with a stirrer and
a thermistor. The thermistor was connected through a bridge circuit
to a recording potentiometer. The reaction vessel was purged thoroughly
with pure argon. All parts of the equipment and the reagents were
thermostatted. After the thermal equilibrium was set, 0.05 mL of
silane was added to 15 mL of the Grignard reagent (providing a 20-40-fold excess
of the Grignard reagent), and the temperature change of the reaction
solution (less than 1 ˚C) was recorded as a plot of temperature
vs. time.
Values of ES(Si) for methyl, methoxy, ethoxy, ethyl, n-butyl, and isopropyl groups are: 0, -0.12, -0.14, -0.15, -0.23, and -0.56, respectively.8
21Scales of parameters in Equation [³] are not normalized, therefore the intercept has a formal meaning. Nevertheless, Equation [³] can be used for calculation of relative effects.