Oxygen heterocycles
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص794-795
2025-07-16
597
Oxygen heterocycles
Ring-opening chemistry is characteristic of oxygen heterocycles too, and there is no need for us to revisit epoxide opening here. Epoxides are particularly reactive because ring-opening releases ring strain, driving the reaction forward. In general, though, oxygen heterocycles, as cyclic ethers, are relatively unreactive: ethers are the least reactive of all the common functional groups. This is one of the main reasons why THF and dioxane are such important sol vents. A second reason is that they solvate organometallics by donating a lone pair to stabilize an electron-deficient metal cation (Li, for example). Cyclic ethers are better donors (more nucleophilic) than acyclic ones for the same reason that cyclic amines are more nucleophilic than acyclic ones. This interaction of the lone pair with a Lewis acid can be exploited to make ethers more reactive. BF3 is commonly used to activate cyclic ethers towards nucleophilic attack, and even with epoxides it increases the rate and yield of the reaction when organometallic reagents are used as nucleophiles. BuLi does not react with oxetane unless a Lewis acid, such as BF3, is added, when it opens the four-membered ring to give a quantitative yield of n-heptanol. Without ring strain to help the reaction along, THF, by contrast, gives only a low yield of the product even with a Lewis acid.
A more common (if often unwanted) reaction between BuLi and THF is not nucleophilic attack, but deprotonation. You will have noticed that reactions involving BuLi in THF are invariably carried out at temperatures of 0 °C or below—usually –78 °C. This is because, at temperatures above 0 °C, deprotonation of THF begins to take place. The deprotonated THF is unstable, and it undergoes a reaction we call a reverse [2 + 3] cycloaddition (see Chapter 34). Here is the mechanism (we have represented the organolithium as an anion to help with the arrows). The products are: (1) the (much less basic) enolate of acetaldehyde and (2) ethylene. The fi rst tends to polymerize, and the second usually (but see the box below!) evaporates from the reaction mixture.
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