What is the reason and mechanism for regioselectivity of Birch reduction?

The explanation lies in the distribution of electron density in the intermediate radical anions that appear during the Birch reduction.
TheBirch reductionis an organic reaction which is particularly useful insynthetic organic chemistry. The reaction was reported in 1944 by the Australian chemistArthur Birch(1915–1995) working in theDyson Perrins Laboratoryin theUniversity of Oxford,building on earlier work by Wooster and Godfrey in 1937.It convertsaromatic compoundshaving abenzenoid ringinto a product,1,4-cyclohexadienes, in which two hydrogen atoms have been attached on opposite ends of the molecule. It is theorganic reductionofaromatic ringsin liquidammoniawithsodium,lithiumorpotassiumand analcohol, such asethanolandtert-butanol. This reaction is quite unlikecatalytichydrogenation, which usually reduces the aromatic ring all the way to acyclohexane.

The original reaction reported byArthur Birchin 1944 utilizedsodiumandethanol. Subsequently A. L. Wilds noted that better yields result with lithium.Also the use of t-butyl alcohol has become common. The reaction is one of the main organic reactions utilized in all types of syntheses.

Electron-withdrawing groups stabilize electron density at theipsoandparapositions through conjugation and so the negative charge will mainly be found in these positions; subsequent protonation occurspara.

Electron-donating groups destabilize a negative charge (when compared to simple benzene) through conjugation and this effect would be strongest at theipsoandparapositions.

Thus the negative charge will mainly be found at theorthoandmetaposition.