Excited- and Ground-State Versions of the Tri-π-methane Rearrangement

What is it about?

The di-π-methane rearrangement with two π-groups bonded to a single carbon leading to π-substituted cyclopropanes is now well established. The present research had as its goal the exploration of molecular systems having three π-moieties attached to an sp3-hybridized atom in a search for a tri-π-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-π-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 speciesA, B, and Ccorresponding to tri-π-methane reactant A, vinylcyclopropane photoproduct B, and cyclopentene photoproduct C. Three rate constants are involved, k1 for A → B, k2 for A → C, and k3 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-π-methane rearrangement, the tri-π-methane reaction was found to be preferred by the singlet, while in these sterically congested systems, the triplet led to di-π-methane reactivity. Finally, a ground-state counterpart of the reaction was obtained.

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Why is it important?

The di-π-methane rearrangement with two π-groups bonded to a single carbon leading to π-substituted cyclopropanes is now well established. The present research had as its goal the exploration of molecular systems having three π-moieties attached to an sp3-hybridized atom in a search for a tri-π-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-π-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 speciesA, B, and Ccorresponding to tri-π-methane reactant A, vinylcyclopropane photoproduct B, and cyclopentene photoproduct C. Three rate constants are involved, k1 for A → B, k2 for A → C, and k3 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-π-methane rearrangement, the tri-π-methane reaction was found to be preferred by the singlet, while in these sterically congested systems, the triplet led to di-π-methane reactivity. Finally, a ground-state counterpart of the reaction was obtained.

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