Birch Reduction Regioselective

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:

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.

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 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.

Applications

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

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