A STOPPED-FLOW KINETIC-STUDY OF THE REACTION OF TRANS-1-(DIMETHYLAMINO)-1,3-BUTADIENE WITH DIMETHYL DICYANOFUMARATE

trans-1-(Dimethylamino)-1,3-butadiene and dimethyl dicyanofumarate react in acetonitrile and dichloromethane at - 40-degrees-C with a second-order rate constant of 10(7) to 10(8) 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 (lambda(max) = 400 nm, lg-epsilon = 2.514) is recorded which decays by a first-order process. The analysis of the kinetic data (E(a) = 14.5 +/- 0.1 kcal mol-1, lgA = 14.71 +/- 0.05, DELTA-H(not-equal) = 14.1 +/- 0.1 kcal mol-1, DELTA-S(not-equal) = 7.2 +/- 0.2 cal mol-1 K-1, DELTA-G(not-equal) = 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.