Simple Enols. 5. (Z)- and (E)-1-Hydroxy-1,3-butadiene

Article abstract of DOI:10.1021/ja00223a038

(Z)- and (E)-1-hydroxybutadiene have been generated from their trimethylsilyl derivatives in slightly acidic aqueous acetonitrile and CH3OH-DMSO mixtures and characterized by NMR spectroscopy.The kinetics and products of ketonization of both dienols have been investigated.In aqueous solution they yield mixtures of (E)-2-butenal and 3-butenal whose proportions depend on pH.Analysis of the variation of product composition with pH indicates that (E)-1-hydroxybutadiene yields 90.1, 28.0, and 44.7percent of (E)-2-butenal in the H3O⁺, H2O, and HO^--catalyzed reactions, whereas (Z)-1-hydroxybutadiene yields 69.8, 17.2, and 19.6percent.It is suggested that transmission of positive charge to the oxygen in the transition state for protonation at the 4-position is more efficient with the E than with the Z isomer, possibly because it is easier for the former to attain a planar conformation.No evidence was found for a mechanism that involves a cyclic transition state for the spontaneous ketonization of (Z)-butadienol.