The reduction of aromatic compounds to 1.4-cyclohexadiene compounds in presence of alkali metal liquid ammonia and an alcohol is called Birch reduction.A variety of aromatic compounds containing electron donating or electronwithdrawing groups could be readily converted to the corresponding 1,4cyclohexadiene derivatives.
The regioselectivity towards the products in Birch reduction of substituted aromatic compound depends on the nature of the substituent. For example, the electron donating substituent such as alkyl or alkoxy group remains on the unreduced carbon where as the electron withdrawing groups such as carboxylic acid or primary amide remains on reduced carbon almost exclusively.
The aldehydes and esters groups are in the electron donation side because these are reduced to the corresponding alcohols in Birch condition before the reduction of aromatic ring . The addition of one or two equivalent of water or t-BuOH to the NH, before the addition of metal can preserve the ester group in Birch reduction.
Mechanism
The solvated electron accepts an electron and generates the radical anion which then takes a proton from the alcohol and forms a radical intermediate . The radical intermediate then takes another electron and converts to the carbanion which on protonation gives the desired 1,4-cyclohexadiene derivatives.The role of alcohol is to supply the proton because the NH3, is not sufficient acidic to supply the proton to all the intermediate anion.
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