Are Charge‐Transfer Complexes Intermediates in Diels‐Alder Reactions? A Case Study of the Reaction of 1,2‐Dimethylenecyclopentane with Tetracyanoethylene

The Diels‐Alder reaction of 1,2‐dimethylenecyclopentane ( 1 ) and tetracyanoethylene (TCNE) is kinetically studied by conventional and stopped‐flow UV/Vis absorption‐spectroscopic techniques. A transient charge‐transfer (CT) absorption band with λm max =490 nm (half‐life at 20°C ca. 3.4 × 10 ‐2 s) is detected in dichloromethane solution. For the molar extinction coefficient a value of e 490 =310 M ‐1 cm ‐1 has been determined from measurements of the optical densitity at zero time as a function of excess diene concentration. With the aid of computer simulation the combined kinetic analysis of the disappearance of reactants and CT complex reveals that the data obtained from experiments at equimolar concentration and at a single temperature (20°C) can be interpreted equally well by both the model where the CT complex is an intermediate on the reaction path and where it is in a nonreactive side equilibrium with the reactants. Activation parameters have been determined for the disappearance of the starting compounds and of the CT complex. Contrary to what is expected for a CT complex being an intermediate in a concerted Diels‐Alder reaction a more negative activation entropy for the decay of the CT complex than for the disappearance of the reactants is found. This suggests that the CT complex is unlikely to exhibit a transition‐state‐like structure as is required for a concerted cycloaddition. Experiments under pseudo‐first‐order conditions have led to an extension of the reaction scheme by a second‐order reaction of the CT complex with the excess diene, the rate constant of which is virtually identical to that for the direct bimolecular cycloaddition. The conclusion is drawn for our case that there are more arguments against than arguments for the CT complex being a preorientation complex in the [4 + 2] cycloaddition.