VINYL ETHER HYDROLYSIS .19. 1-METHOXY-1,3-BUTADIENE - REACTION-MECHANISM AND IMPLICATION FOR HYDROLYSIS OF THE MUTAGEN FECAPENTAENE-12

The hydrolysis of cis- and trans-1-methoxy-1,3-butadiene in aqueous so lution occurs by hydron transfer to the delta-carbon atom with little or no beta-hydronation to give crotonaldehyde as essentially the sole aldehyde product. The reaction gives appreciable hydronium-ion isotope effects in the normal direction (cis: k(H)+/k(D)+ = 2.60, trans: k(H) +/k(D)+ = 3.52) and shows general acid catalysis; five carboxylic acid catalytic coefficients for hydrolysis of the trans isomer give a good Bronsted relation with the exponent cr = 0.59. This is taken as evide nce that these reactions occur by the conventional mechanism for vinyl ether hydrolysis involving rate-determining hydron transfer to substr ate carbon followed by rapid formation and decomposition of a hemiacet al intermediate. Comparison of the reactivity of the present dienyl et hers with that of their monoenyl analog, methyl vinyl ether, shows tha t introduction of the second double bond decreases reactivity consider ably: the hydronium-ion catalytic coefficient is reduced by a factor o f 8.3 for the trans isomer and by a factor of 160 for the cis isomer. This reduction supports a hypothesis advanced to explain the occurrenc e of reaction by a different mechanism recently discovered in the hydr olysis of the strongly mutagenic polyenyl ether, fecapentaene-12.