Small rings are opened by electrocyclic reactions
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص928-929
2025-07-24
471
Small rings are opened by electrocyclic reactions
Ring strain is important in preventing a reaction that would otherwise change your view of a lot of the chemistry you know. Allyl cations are conjugated systems containing 2π electrons, so if you knew no other chemistry than what is in this chapter you might expect them to cyclize via disrotatory electrocyclic ring closure. The product would be a cyclopropyl cation.
In fact, it is the cyclopropyl cations that undergo this reaction (very readily indeed— cyclopropyl cations are virtually unobservable) because ring strain encourages them to undergo electrocyclic ring opening to give allyl cations. The instability of cyclopropyl cations means that, even as they start to form as intermediates, they spring open to give allyl cation derived products. Try nucleophilic substitution on a cyclopropane ring and this happens.
Electrocyclic ring opening of one type of three-membered ring tells us about the stereo chemistry of the process. Many aziridines are stable compounds, but those bearing electron withdrawing groups are unstable with respect to electrocyclic ring opening. The products are azomethine ylids and can be trapped by [3 + 2] cycloaddition reactions with dipolarophiles.
Because the cycloaddition is stereospecific (suprafacial on both components), the stereo chemistry of the products can tell us the stereochemistry of the intermediate ylid, and con fi rms that the ring opening is conrotatory (the ylid is a 4π electron system).
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