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The metal can also be Lithium or Potassium and the hydrogen atoms are supplied by an Alcohol such as Ethanol or Tert-butanol . Sodium in liquid ammonia gives an intense blue color due to a Solvated Electron . Several reviews have been published.Birch, A. J.; Smith, H. ''Quart. Rev.'' 1958, ''12'', 17. (Review) Caine, D. ''Org. React.'' '''1976''', ''23'', 1-258. (Review) Rabideau, P. W.; Marcinow, Z. ''Org. React.'' '''1992''', ''42'', 1-334. (Review) Mander, L. N. ''Comp. Org. Syn.'' '''1991''', ''8'', 489-521. (Review) REACTION MECHANISM The first step of a Birch reduction is a One-electron Reduction of the aromatic ring to a Radical Anion . Sodium is oxidized to the sodium ion Na+. This intermediate is able to dimerize to the dianion. In the presence of an alcohol the second intermediate is a Free Radical which takes up another electron to form the Carbanion . This carbanion abstracts another Proton from the alcohol to form the cyclohexadiene. In the presence of an Alkyl Halide the Carbanion can also engage in Nucleophilic Substitution with Carbon-carbon Bond formation. In substituted aromatics an Electron-withdrawing Substituent , such as a Carboxylic Acid 1, stabilizes a Carbanion and the least-substituted Olefin is generated. With an Electron-donating Substituent the opposite effect is obtained.2 The reaction produces more of the less thermodynamically stable non-conjugated 1,4-addition product than the more stable Conjugated 1,3-diene because the largest orbital coefficient of the HOMO of the conjugated pentadienyl anion intermediate is on the central carbon atom. Once formed, the resulting 1,4-cyclohexadiene is unable equilibrate to the thermodynamically more stable product; therefore, the observed kinetic product is produced. Experimental alkali metal alternatives that are safer to handle, such as the M-SG Reducing Agent , also exist. BIRCH ALKYLATION In Birch alkylation the Anion formed in the Birch reduction is trapped by a suitable Electrophile such as a Haloalkane 3 for example in the reaction depicted below ''Formation of Benzo-Fused Carbocycles by Formal Radical Cyclization onto an Aromatic Ring'' Derrick L. J. Clive and Rajesh Sunasee Org. Lett. ; '''2007'''; 9(14) pp 2677 - 2680; (Letter) : : REFERENCES SEE ALSO |
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