Theoretical Studies on the Gas-Phase Pyrolysis of Esters The effect of ￥á- and ￥a-methylation of Ethyl Formates

摘要

The gas-phase thermolysis reactions of メ- and モ-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 Cメ or Cモ is shown to elevate the ヰ-HOMO of the donor fragment (Y = C) and depress the ヲ*-LUMO of the acceptor fragment (Cモ-H), increasing the nucleophilicity of Y toward モ-hydrogen which in turn increases the reactivity. The two bond breaking processes of the Cメ-X and Cモ-H bonds are concerted but not synchronous so that the reaction takes place in two stages as Taylor suggested. The initial cleavage of Cメ-X is of little importance but the subsequent scission of Cモ-H occurs in a rate determining stage.