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Bulletin of the Korean Chemical Society (BKCS)

ISSN 0253-2964(Print)
ISSN 1229-5949(Online)
Volume 11, Number 1
BKCSDE 11(1)
January 20, 1990 

Theoretical Studies on the Gas-Phase Pyrolysis of Esters The effect of α- and β-methylation of Ethyl Formates
Ikchoon Lee*, Ok Ja Cha, Bon-Su Lee
The gas-phase thermolysis reactions of α- and β-methylated ethyl formates, Y = CH-X-CHR1CH2R2 where X = Y = O or S and R1 = R2 = H or CH3, are investigated theoretically using the AM1 method. The experimental reactivity order is reproduced correctly by AM1 in all cases. The thermolysis proceeds through a six-membered cyclic transition state conforming to a retro-ene reaction, which can be conveniently interpreted using the frontier orbital theory of three-species interactions. The methyl group substituted at Cα or Cβ is shown to elevate the π-HOMO of the donor fragment (Y = C) and depress the σ*-LUMO of the acceptor fragment (Cβ-H), increasing the nucleophilicity of Y toward β-hydrogen which in turn increases the reactivity. The two bond breaking processes of the Cα-X and Cβ-H bonds are concerted but not synchronous so that the reaction takes place in two stages as Taylor suggested. The initial cleavage of Cα-X is of little importance but the subsequent scission of Cβ-H occurs in a rate determining stage.
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