Using light to make Z alkenes from E alkenes
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص680-681
2025-07-05
592
Light allows the interconversion of the two isomers of an alkene by promoting a π electron into the π* orbital and transiently breaking the π bond, but the way light favours formation of the Z isomer is rather subtle. One difference between cis and trans alkenes is that the trans alkenes usually absorb light better than the cis alkenes—they absorb light of a higher wave length and they absorb more of it, particularly when conjugated with carbonyl groups. Steric hindrance forces the cis alkene to twist about the σ bond joining the alkene to the carbonyl group and conjugation is then less efficient. In a mixture of E and Z alkenes, the E alkene is more prone to isomerization by light, so the Z isomer builds up in the mixture. Here is an example. Aldol condensation of cyclohexanone and benzaldehyde gives pure E alkene for the reasons explained above. Irradiation with longer-wavelength UV light equilibrates this to the Z alkene in excellent yield.
It is not possible for the benzene ring and the enone system to be planar in the Z-enone and so they twist, making conjugation not as good as in the E-enone. Longer-wavelength light is absorbed only by the E-enone, which is continually equilibrated back to the excited state. Eventually, all the E-enone is converted to the Z-enone, which is not as efficiently excited by the light. The final mixture of E- and Z-enone is known as a ‘photostationary state’.
The chemistry of vision
The human eye uses a cis alkene, 11-cis-retinal, to detect light, and a cis–trans isomerism reaction is at the heart of the chemical mechanism by which we see. The light-sensitive pigment in the cells of the retina is an imine, formed by reaction of 11-cis-retinal with a lysine residue of a protein, opsin. Absorption of light by the opsin–retinal compound, known as rhodopsin, promotes one of the electrons in the conjugated polyene system to an antibonding orbital. Free rotation in this excited state allows the cis double bond to isomerize to trans, and the conformational changes in the protein molecule that result trigger a cascade of reactions that ultimately leads to a nerve signal being sent to the brain.
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