Excited- and ground-state versions of the tri-π-methane rearrangement:: Mechanistic and exploratory organic photochemistry

The di-pi -methane rearrangement with two pi -groups bonded to a single carbon leading to pi -substituted cyclopropanes is now well established. The present research had as its goal the exploration of molecular systems having three pi -moieties attached to an sp(3)-hybridized atom in a search for a tri-pi -methane rearrangement. Indeed, it was found that such systems do rearrange photochemically to afford cyclopentenes. However, it was also established that vinylcyclopropanes ring-expand to cyclopentenes on direct irradiation. Since both three-ring and five-ring photoproducts often are found to be produced, it was important to establish that the observed photochemistry was really the result of a true single-step tri-pi -methane rearrangement and not the consequence of two sequential rearrangements, first to: form a vinyl cyclopropane which subsequently ring expanded to the cyclopentene. The general situation has three species-A, B, and C-corresponding to tri-pi -methane reactant A, vinylcyclopropane photoproduct B,and cyclopentene photoproduct C. Three rate constants are involved, k(1) for A --> B, k(2) for A --> C, and k(3) for B --> C. The kinetics were applied to two examples with provision to avoid differential light absorption; this utilized singlet sensitization. It was determined that direct formation of the cyclopentene photoproduct proceeds more rapidly than the ring-expansion route. In contrast to the di-pi -methane rearrangement, the tri-pi -methane reaction was found to be preferred by the singlet, while in these sterically congested systems, the triplet led to di-pi -methane reactivity. Finally, a ground-state counterpart of the reaction was obtained.