Electrophilic attack on conjugated dienes
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص579-581
2025-06-25
516
Another way to make allylic chlorides is by treating dienes with HCl. Electrophiles attack conjugated dienes more readily than they do isolated alkenes. establishing the main point that the terminal carbon atoms are the most nucleophilic and that the initial attack produces an allylic cation. A simple example is the addition of HCl to cyclopentadiene.
Although there is a question of regioselectivity in the initial protonation, the allylic cation is symmetrical and attack by chloride at either end produces the same product. However, if the electrophile is a halogen rather than HCl or HBr then the reaction becomes regioselective as the cationic intermediate is no longer symmetrical. What happens is this:
The alternative is direct attack on the bromonium ion intermediate, which we assume would occur at the allylic site (black arrows) and not at the other (green arrows). Although this 1,2-dibromide product is not observed, it is still possible that this reaction happens because the 1,2-product can rearrange by bromide shift to the observed 1,4-dibromide.
The final product of this reaction could in fact be either of two compounds as the two bro mine atoms may be cis or trans. Bromination in chloroform at –20 °C gives mostly a liquid cis dibromide while reaction in hydrocarbon solvents gives the crystalline trans isomer. On standing the cis isomer slowly turns into the trans.
This suggests that the cis bromide is the kinetic product and the more stable trans com pound is the thermodynamic product, formed by reversible loss of bromide and reformation of the bromonium ion.
Similar questions arise when nucleophilic substitution occurs on the dibromides. Reaction of either the cis or the trans dibromide with dimethylamine gives the trans isomer of a diamine. But look at the regioselectivity—it’s not the diamine you might expect. The only explanation is one SN2 displacement and one SN2' displacement.
But what about the stereochemistry? Starting with the cis isomer, one SN2 displacement with inversion might be followed by an intramolecular SN2' displacement and finally another SN2 displacement with inversion at the allylic centre.
The reaction with the trans isomer is almost identical: the same three-membered ring is an intermediate in both sequences so the products are bound to be the same.
If the nucleophile is different from the electrophile we can get a bit more information about the course of the reaction. When butadiene is treated with bromine in methanol as solvent, two adducts are formed in a 15:1 ratio along with some dibromide. Methanol is a weak nucleophile and adds to the bromonium ion mainly at the allylic position (black arrow below); only a small amount of product is formed by attack at the far end of the allylic system. Note that no attack occurs at the other end of the bromonium ion (green dotted arrow).
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