“See ref 4 for experimental proceduresand references to the nano-second flash kinetic instrumentation. 6 All measurements were carried out in deoxygenated butyronitrile solvent. In earlier work, 4 we found that therates were insensitive to solvent, the values in acetonitrile and toluene agreeing to within a factor of 2.“Excitation with a 337-nm laser pulse. dExcitationwith a 355-nm laser pulse, except for the ma-leic anhydride data, which were derived from 337-nm experiments. The yield of transient per pulse (and hence the signal-to-noise ratio) was substantially greater from the 355-nm irradiations, which fall in a stronger absorption region of the diazene precursors of 1 and 2. of dienophiles with cyclopentadiene and with 9, 10-dimethylanthracene follow AAH*, not-TAAS*. In fact, AS*, is nearly invariant in both of these series, having the value-36±2 cal K" 1 mol" 1 5for the dienophiles maleic anhydride, fumaronitrile, dimethyl fumarate, and acrylonitrile, the extremes of the range contributing only about 1 kcal/mol (as-TAAS*) to AAG*. The major cause of the> 5000-fold range of reactivities of this group of dienophiles is the variationof about 5-6kcal/mol in AH*} A strong correlation exists between these relativedienophilic reactivities and the relative diylophilic reactivities of the same alkenes toward the singlet biradicals 3, 4-dimethylenefuran 1 and 3, 4-dimethylenethiophene 2 (generated from the corresponding diazenes.) 4, 5 The orders and magnitudes of the relative rates are essentially the same, despite the factor of 10m in absolute rates favoring the biradical reactions. We now find that, incontrast to the diene reactions, the relative rates of the diyl reactions are under activation entropy control and are but little influenced by variations in activation enthalpy. Nevertheless, we suggest that although they express themselves here in entropic guise, the causes of the rate order of the biradical cycloadditions are the same as those of the Diels-Alder reactions. Table I shows that, for all the alkenes, the rates of the biradical cycloadditions are virtually independent of temperature. The acrylonitrile/1 data cover four temperatures over a 95 C range and can be expressed in terms of the Arrhenius parameters£ a= 1.3 kcal/mol and A= 4 X 106 s “!. Despite the small (in the case of dimethyl fumarate, apparently even slightly negative) activation energies, the rates themselves span a range of about 2200 and 30000for 1 and 2, respectively. The reactivities therefore are largely controlled by the entropies of activation. Why does the same reactivity orderappear in the diene and diyl cycloadditions, even though one series is enthalpy controlled