Regioselective Birch Reduction of aromatic rings by alkali metals (Li, Na,) in liquid ammonia with ethanol as a proton donor. The regioselectivity of the Birch reduction depends on the nature of the substituent of an aromatic ring. The basic Birch aromatic ring reduction is as follows:

Birch Reduction

This method can reduce aromatic rings, heterocycles, conjugated alkenes, α,β-unsaturated carbonyl compounds, internal alkynes. Isolated double bonds usually not affected by this method.

MECHANISM

FIRST STEP: alkali metal donates an electron to the aromatic ring forming stable 1,4 radical anion.

SECOND STEP: radical anion is very basic abstracts proton from alcohol proton donor.

THIRD STEP: radical picks another electron to give anion followed by proton abstraction from alcohol once again. Finally, the unconjugated cyclohexadiene is seen.

MECHANISM

Regioselective Birch Reduction

Electron withdrawing group as a substituent on aromatic ring reduction preferentially takes place at withdrawing group attaching position also called the IPSO position. Reason carbanion stabilized by the electron-withdrawing group.

Electron withdrawing group

Electron donating group as a substituent on aromatic ring reduction preferentially takes place at the ORTHO position of donating group. Reason carbanion destabilized by the electron-donating group, hence carbanion needs to be far from the donating group.

Electron donating group

Applications

Find the final product for the given organic functional group conversion reaction?

applications

BIRCH REDUCTION: step 1 due to electron-donating group present reduction happens at Ortho position giving unconjugated cyclohexadiene derivative.

OZONOLYSIS: step 2 using Ozone(1 eq) oxidation is regioselective at double bond with more electron-donating substituents.

References Birch, A. J. Reduction by dissolving metals. I. J. Chem. Soc. 1944, 430-436

Also read Barton Reaction

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