The reactions of (
E)-1-methoxy-1,3-butadiene(
1) and 1,1-dimethoxy-1,3-butadiene (
2) with aseries ofdienophiles of increasing electrophilicity are described.Stereochemical studies reveal that the cycloadditions of
1are concerted processes, even for the most electron-deficient olefinsdimethyl dicyanofumarate and dimethyldicyanomaleate. 1,1-Dimethoxy-1,3-butadiene reacts under ourconditions (dilute solutions and temperatures
![](/images/entities/le.gif)
60
![](/images/entities/deg.gif)
C) only with those dienophiles which can give zwitterions out of theantiperiplanar conformation of the diene.Zwitterionic intermediates can be trapped by methanol. In thecase of tetracyanoethene the kinetics of decay of anintermediate, interpreted as the zwitterion, can be followed by stoppedflow techniques:
Ea = 14.8 ± 0.2kcalmol
-1, log
A = 11.9 ± 0.1,
H![](/images/entities/thermod.gif)
= 10.8 ± 0.1 kcalmol
-1,
S![](/images/entities/thermod.gif)
=-6.2 ± 0.1 cal mol
-1K
-1, and
G![](/images/entities/thermod.gif)
=11.40 ±0.03 kcal mol
-1.