Electron-rich and electron-deficient alkenes
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص280-281
2025-05-20
652
The same sort of thing happens with alkenes. We’ll concentrate on cyclohexene so as to make a good comparison with benzene. The six identical protons of benzene resonate at 7.27 ppm; the two identical alkene protons of cyclohexene resonate at 5.68 ppm. A conjugating and electron-withdrawing group such as a ketone removes electrons from the double bond as expected—but unequally. The proton nearer the C=O group is only slightly down field from cyclohexene but the more distant one is over 1 ppm downfi eld. The curly arrows show the electron distribution, which we can deduce from the NMR spectrum.
Oxygen as a conjugating electron donor is even more dramatic. It shifts the proton next to it downfi eld by the inductive effect but pushes the more distant proton upfield a whole 1 ppm by donating electrons. The separation between the two protons is nearly 2 ppm. For both types of substituents, the effects are more marked on the more distant (β) proton. If these shifts reflect the true electron distribution, we should be able to deduce something about the chemistry of the following three compounds. You might expect that nucleophiles will attack the electron-deficient site in the nitroalkene, while electrophiles will be attacked by the electron-rich sites in silyl enol ethers and enamines. These are all important reagents and do indeed react as we predict, as you will see in later chapters. Look at the difference— there are nearly 3 ppm between the shifts of the same proton on the nitro compound and the enamine!
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