Differences in rates of Diels-Alder reactions as experimental indicators of synchronous or asynchronous transition states

Article abstract of DOI:10.1016/S0040-4020(99)00667-5

Assessment of the relative rates of the Diels-Alder reactions of the unsymmetrical diene 2-(trimethylsilyloxy)-1,3-cyclohexadiene (1) and its 6,6- and 5,5-dimethyl analogs 2 and 3 indicated that with symmetrical, ethylenic dienophiles (para-benzoquinone, maleic anhydride and N-phenylmaleimide) the Diels-Alder reaction is almost synchronous, but with tetracyanoethylene and with diethyl acetylenedicarboxylate the addition is sufficiently asynchronous as to lead to different rates of reaction with dimethyl-dienes 2 and 3.