Abstract trans‐1‐(Dimethylamino)‐1,3‐butadiene and dimethyl dicyanofumarate react in acetonitrile and dichloromethane at –40°C with a second‐order rate constant of 107 to 108 l mol‐1 s‐1. This was determined by following the decay of the UV absorption of diene and dienophile with a stopped‐flow spectrofluorimeter. After disapperance of the reagents a new absorption (λmax = 400 nm, lgϵ = 2.514) is recorded which decays by a first‐order process. The analysis of the kinetic data (Ea = 14.5 ± 0.1 kcal mol‐1, lgA = 14.71 ± 0.05, ΔH# = 14.1 ± 0.1 kcal mol‐1, δS# = 7.2 ± 0.2 cal mol‐1 K‐1, ΔG# = 11.95 ± 0.01 kcal mol‐1) in combination with stereochemical studies on the cycloaddition of the E/Z isomeric dienophiles dimethyl dicyanofumarate and dicyanomaleate leads to the interpretation that the new species is a zwitterion.
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Sustmann et al. (1992) studied this question.