Crossover experiments

There is still one tiny doubt. Supposing the reaction is not intramolecular at all, but intermolecular. The CO₂H group might be lost from one molecule as protonated CO₂ and be picked up by another molecule of alkene. No migration would be involved at all.

This mechanism can be checked by using a 50:50 mixture of doubly labelled and unlabelled starting material. The molecule of alkene that captures the roving protonated labelled CO₂ might happen to be labelled too but equally well it might be unlabelled. If this last mechanism is correct, we should get a mixture of unlabelled, singly labelled, and doubly labelled product in the ratio 1:2:1 as there are two types of singly labelled product. The two singly labelled compounds are called the crossover products and the experiment is called a crossover experiment as it discovers whether any parts of one molecule cross over to another.

In fact, no singly labelled compounds were found: NMR analysis showed that the product consisted entirely of unlabelled or doubly labelled molecules. The CO₂H group remains attached to the same molecule (though not to the same atom) and the fi rst mechanism is correct. Crossover experiments demand some sort of double labelling, which does not have to be isotopic. An example where crossover products are observed is the light-initiated isomerization of allylic sulfides.

This is formally a [1,3] sigmatropic shift of sulfur but that is an unlikely mechanism (and you should be able to suggest why). A crossover experiment was carried out in which the two molecules had either two phenyl groups or two para-tolyl groups. The mixture was allowed to rearrange in daylight and the products were examined by mass spectroscopy. There was a roughly 1:2:1 mixture of products having two phenyl groups, one phenyl and one para-tolyl group, and two para-tolyl groups. The diagram shows the starting materials and the two crossover products only.

Clearly, the ArS group had become separated from the rest of the molecule and the most likely explanation was a radical chain reaction with the light producing a small amount of ArS^• to initiate the chain. The para-methyl group acts as a label. The whole system is in equilibrium and the more highly substituted alkene is the product.

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مواضيع ذات صلة

The Hammett relationship

Systematic structural variation

Labelling experiments reveal the fate of individual atoms

Be sure of the structure of the product

Variation in the structure of the aldehyde

formation of an ester intermediate

formation of an intermediate dimeric adduct

Be sure of the structure of the product

Variation in the structure of the aldehyde

Proposed mechanism C: formation of an ester intermediate

Proposed mechanism B: formation of an intermediate dimeric adduct

Cross metathesis