| Four different sources of kinetic information were combined to study the effect of ortho substituents on the rate of Bergman cycloaromatization. All of these methods confirm that the cyclization barrier is highly sensitive to the nature of ortho-substituents. However, the measured activation energies strongly depend on the choice of experimental technique: the relative trends provided by the different methods agree with each other only in the case of acceptor substituents. Both the onset peaks and the activation energies determined by Differential Scanning Calorimetry (DSC) (either in neat enediynes or in their solutions in 10.6 M 1,4-cyclohexadiene (1,4-CHD)) strongly overestimate the reactivity of 1,2-diethynylbenzene suggesting that DSC is not a reliable indicator of enediyne reactivity. This discrepancy is likely to stem from the presence of side reactions with low activation barriers, especially important when the reaction is carried out in neat enediyne. On the other hand, kinetic measurements based on monitoring the concentrations of enediyne reactants and naphthalene products provide reliable general trends that include the parent benzannelated enediyne. These measurements confirm that both substituents in 2,3-diethynyl-1-nitrobenzene (ortho-NO 2) and 2,3-diethynyl-1-formylbenzene (ortho-CHO) substantially decrease activation energies for the Bergman cyclization supporting our earlier computational predictions. Activation energies derived from k eff, the effective rate constants, depend on the 1,4-CHD concentrations. The "true" rate constants, k1 for the cycloaromatization step and the ratio of constants for the retro-Bergman ring opening, k-1, and intermolecular H-atom abstraction, k2, were determined from the dependence of cycloaromatization kinetics of ortho- and para-NO2 substituted enediynes on the concentration of 1,4-CHD. Interestingly, intramolecular hydrogen-atom (H-atom) abstraction from the ortho-OCH3 group effectively intercepts p-benzyne intermediate in the Bergman cycloaromatization of 2,3-diethynyl-l-methoxybenzene leading to the formation of a new diradical and rendering the cyclization step essentially irreversible. Chemical and kinetic consequences of this phenomenon were investigated through the combination of computational and experimental studies.;Diaryl acetylenes, in which one of the aryl groups is either a pyridine or a pyrazine, undergo efficient triplet state photocycloaddition to 1,4-cyclohexadiene with formation of 1,5-diaryl substituted tetracyclo[3.3.0.02,8.0 4,6]octanes (homoquadricyclanes). In the case of pyrazinyl acetylenes, the primary homoquadricyclane products undergo a secondary photochemical rearangement leading to diaryl substituted tricyclo[3.2.1.04,6]oct-2-enes. (Abstract shortened by UMI.). |
