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-NO2) 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 keff, the effective rate constants, depend on the 1,4-CHD concentrations. The Ħ§trueĦ¨ 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-1-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[3.3.0.02,8.04,6]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. Mechanistic and photophysical studies suggest that photocycloaddition proceeds through an electrophilic triplet excited state whereas the subsequent rearrangement to the tricyclooctenes proceeds through a singlet excited state. Chemical and quantum yields for the cycloaddition, in general, correlate with the electron acceptor character of aryl substituents but are attenuated by photophysical factors, such as the competition between the conversion of acetylene singlet excited state into the reactive triplet excited states (intersystem crossing: ISC) and/or to the radical-anion (photo-electron transfer from the diene to the excited acetylene: PET). Dramatically enhanced ISC between à-à* S1 state and Ħ§phantomĦ¨ n,à* triplet excited state is likely to be important in directing reactivity to the triplet pathway. The role of PET can be minimized by judicious choice of reaction conditions (solvent, concentration, etc.). Furthermore, the 1,5-diaryl substituted homoquadricyclanes were used as building blocks in supramolecular scaffolds with a ca. 60o angle formed by the two aromatic rings defining a hydrophobic cavity. These structural features of pyridinyl homoquadricyclanes were applied to the design of composite organic/inorganic materials with topologies depending on the ratio of ligand to metal. Crystal structures of complexes varied from polymeric material, where the metal is shared between two homoquadricyclane molecules and alternating ligand-metal units, to supramolecular rhomboids, where crystal packing of the chain of rhomboids generates cavities which are filled with disordered solvent molecules. Substituents on the polycyclic moiety of the homoquadricyclane cause restricted rotation of the pyridine rings. This observation suggests that the flexibility of such systems can be fine-tuned to create a family of supramolecular scaffolds of controlled rigidity.