Chemoselectivity in the reactions of dianions
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص547-548
2025-06-23
523
A similar idea is central to a useful bit of chemoselectivity that can be obtained in the react ions of dianions. 1-Propynol can be deprotonated twice by strong bases—first, at the hydroxyl group to make an alkoxide anion (the pKa of the OH group is about 16) and, secondly, at the alkyne (pKa of the order of 25) to make a dianion. When this dianion reacts with electrophiles it always reacts at the alkynyl anion and not at the alkoxide.
●Reactivity of dianions The anion that is formed last reacts first.
Vollhardt used this sort of chemoselectivity in his 1977 synthesis of the female sex hormone oestrone. He needed an alkyl iodide, which could be made by reacting an anion of a bis alkyne with ethylene oxide.
Although anions can often be formed straightforwardly next to alkynes, there are two other more acidic protons (black) in the molecule that would be removed by base before the green proton. However, treatment with three equivalents of butyl lithium removes all three, and the trianion reacts with ethylene oxide at the last-formed anionic centre to give the required compound.
الاكثر قراءة في مواضيع عامة في الكيمياء العضوية
اخر الاخبار
اخبار العتبة العباسية المقدسة
