Key Words: Zwitterion formation and decay, kinetics of J Cycloadditions trans-1-(Dimethylamino)-1,bbutadiene and dimethyl dicy-by a first-order process. The analysis of the kinetic data (E, = anofumarate react in acetonitrile and dichloromethane at 14.5 f 0.1 kcal mol-l, 1gA = 14.71 f 0.05, AH* = 14.1 f 0.1 -40°C with a second-order rate constant of 1 0 ' to lo8 1 mol-' kcal mol-', AS* = 7.2 f 0.2 cal mol-' K-', AG* = 11.95 f s-'. This was determined by following the decay of the UV 0.01 kcal mol-') in combination with stereochemical studies absorption of diene and dienophile with a stopped-flow spec-on the cycloaddition of the E/Z isomeric dienophiles dimethyl trofluorimeter. After disapperance of the reagents a new ab-dicyanofumarate and dicyanomaleate leads to the interpretasorption (h,,, = 400 nm, lg& = 2.514) is recorded which decays tion that the new species is a zwitterion.The systematic study of (4 + 2) cycloadditions between a diene and a dienophile as a function of substitution has shown that these reactions do not occur via one single Evidence has been presented that biradical and zwitterionic structures are the most plausible intermediate^ [^-^]. These conclusions derive from indirect evidence, most often from stereochemical studies. If a cycloaddition takes place with loss of stereochemistry in the reactants an intermediate is indicated. If further the rate of reaction is measured as a function of the solvent polarity a distinction between a biradical and zwitterionic pathway is possible, Hui~gen'~.'] has shown this in a very elegant manner for (2 + 2) cycloadditions.The measurement of activation volumes can also support the discrimination between different pathways, even allowing the recognition of concurrent concerted and two-step mechanisms[31. Direct observation of zwitterions has become possible in recent years. Hartmann and Heuschmann[*] have established the structure of a zwitterion which is formed from a tetrazine and a ketene N,N-acetal by X-ray crystallography and have discussed its significance in the cycloaddition process. We have characterized zwitterions formed from bis(l,l-dimethylamino)-1,3-butadiene and TCNEf9' or dimethyl dicyanofumaraterlol which, however, do not cyclize to (4 + 2) adducts. Quast et al. ["] have carried out similar studies on 1,3-dipolar cycloadditions of electrophilic azides to a ketene aminal. Direct kinetic measurements on zwitterions have not been reported so far. It has also not been known what the kinetics of disappearance of such species should be like. Here we present results of measurements obtained by stopped-flow techniques which suggest that we have observed for the first time a transient zwitterion and determined the activation parameters for its disappearance in the cycloaddition of trans-l-(dimethylamino)-l,3-butadiene to dimethyl dicyanofumarate.trans-I -(Dimethylamino)-l,3-butadiene (1) reacts with dimethyl dicyanofumarate (2) or dimethyl dicyanomaleate in high yield to give a (4 + 2) cycloadduct 3 the structure of which has been determined by NMR...

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