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Synlett 2010(2): 291-293  
DOI: 10.1055/s-0029-1219167
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© Georg Thieme Verlag Stuttgart ˙ New York

Selectivity in the Grignard Reaction with Silanes

Ants Tuulmets*, Anu Ploom, Dmitri Panov, Jaak Järv
Institute of Chemistry, University of Tartu, 2 Jakobi St., Tartu 51014, Estonia
Fax: +372(7)3795264; e-Mail: ants.tuulmets@ut.ee;
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Publication History

Received 10 September 2009
Publication Date:
04 January 2010 (online)

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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

    References and Notes

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4

See ref. 3, p. 387.

19

The 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.

20

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

21

Scales 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.