Posts Tagged ‘Leaving group’

An alternative mechanism for nucleophilic substitution at silicon using a tetra-alkyl ammonium fluoride.

Friday, May 27th, 2016
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In the previous post, I explored the mechanism for nucleophilic substitution at a silicon centre proceeding via retention of configuration involving a Berry-like pseudorotation. Here I probe an alternative route involving inversion of configuration at the Si centre. Both stereochemical modes are known to occur, depending on the leaving group, solvent and other factors.[1],[2],[3]

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References

  1. L. Wozniak, M. Cypryk, J. Chojnowski, and G. Lanneau, "Optically active silyl esters of phosphorus. II. Stereochemistry of reactions with nucleophiles", Tetrahedron, vol. 45, pp. 4403-4414, 1989. http://dx.doi.org/10.1016/S0040-4020(01)89077-3
  2. L.H. Sommer, and H. Fujimoto, "Stereochemistry of asymmetric silicon. X. Solvent and reagent effects on stereochemistry crossover in alkoxy-alkoxy exchange reactions at silicon centers", Journal of the American Chemical Society, vol. 90, pp. 982-987, 1968. http://dx.doi.org/10.1021/ja01006a024
  3. D.N. Roark, and L.H. Sommer, "Dramatic stereochemistry crossover to retention of configuration with angle-strained asymmetric silicon", Journal of the American Chemical Society, vol. 95, pp. 969-971, 1973. http://dx.doi.org/10.1021/ja00784a081

The mechanism of silylether deprotection using a tetra-alkyl ammonium fluoride.

Wednesday, May 25th, 2016
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The substitution of a nucleofuge (a good leaving group) by a nucleophile at a carbon centre occurs with inversion of configuration at the carbon, the mechanism being known by the term SN2 (a story I have also told in this post). Such displacement at silicon famously proceeds by a quite different mechanism, which I here quantify with some calculations.

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