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Henry Rzepa's Blog

Henry Rzepa's Blog
Chemistry with a twist
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Published

Alkene metathesis is part of a new generation of synthetic reaction in which a double C=C bond is formed from appropriate reactants where no bond initially exists (another example is the Wittig reaction), with the involvement of a 4-membered-ring metallacyclobutane ring 1 (again, very similar to the Wittig). I thought it might make a good addition to my collection of reaction mechanisms and so as the first step

Published

This is the follow-up to the previous post exploring a typical nucleophilic addition-elimination reaction. Here is the elimination step, which as before requires proton transfers. We again adopt a cyclic mechanism to try to avoid the build up of charge separation during those proton movements. Elimination step to form an oxime. Click for animation of reaction mode.

Published

The mechanism of forming an oxime from nucleophilic addition of a hydroxylamine to a ketone is taught early on in most courses of organic chemistry. Here I subject the first step of this reaction to form a tetrahedral intermediate to quantum mechanical scrutiny. The first decision is to decide which atom of the hydroxylamine acts as the nucleophile. Reaction 1 shows the oxygen and reaction 2 the nitrogen.

Published

Semibullvalene is a molecule which undergoes a facile [3,3] sigmatropic shift. So facile that it appears this equilibrium can be frozen out at the transition state if suitable substituents are used. This is a six-electron process, which leads to one of those homologous questions; what happens with ten electrons? A 5,5 double Möbius sigmatropic rearrangement. Click for 3D model.

Published

It was three years ago that I first blogged on the topic of the Sn2 reaction. Matthias Bickelhaupt had suggested that the Sn2 reaction involving displacement at a carbon atom was an anomaly; the true behaviour was in fact exhibited by the next element down in the series, silicon.