a(E)-1-methoxy-1,3-butadiene and 1,1-dimethoxy-1,3-butadiene in (4+2) cycloadditions. A mechanistic comparison

The reactions of (E)-1-methoxy-1,3-butadiene (1) and 1,1-dimethoxy-1,3-butadiene (2) with a series of dienophiles of increasing electrophilicity are described Stereochemical studies reveal that the cycloadditions of 1 are concerted processes, even for the most electron-deficient olefins dimethyl dicyanofumarate and dimethyl dicyanomaleate. 1,1-Dimethoxy-1,3-butadiene reacts under our conditions (dilute solutions and temperatures less than or equal to 60 degrees C) only with those dienophiles which can give zwitterions out of the antiperiplanar conformation of the diene. Zwitterionic intermediates can be trapped by methanol. In the case of tetracyanoethene the kinetics of decay of an intermediate, interpreted as the zwitterion, can be followed by stopped flow techniques: E(a) = 14.8 +/- 0.2 kcal mol(-1), log A = 11.9 +/- 0.1, Delta H double dagger = 10.8 +/- 0.1 kcal mol(-1), Delta S double dagger = -6.2 +/- 0.1 cal mol(-1) K-1, and Delta G double dagger = 11.40 +/- 0.03 kcal mol(-1).