22719-63-7Relevant academic research and scientific papers
Regioselectivity of the tri-π-methane rearrangement: Mechanistic and exploratory organic photochemistry
Zimmerman, Howard E.,Novak, Tibor
, p. 5056 - 5066 (2007/10/03)
The di-π-methane rearrangement with two π-groups attached to the central "methane carbon" of the reactant and which leads to a π-substituted cyclopropane has been studied intensively. Our present research had the goal of elucidating the regioselectivity of the tri-π-methane counterpart. The reactants with three π groups attached to the central carbon mechanistically are capable of affording both di-π-methane and tri-π-methane photoproducts. In common with the di-π-methane system, bridging of two of the π-systems affords a cyclopropyldicarbinyl diradical intermediate that opens to an allyl carbinyl diradical. This diradical has the option of closing to a three-membered or a five-membered ring. It was found that the regioselectivity of the initial π-π-bridging step and the three-ring opening of the cyclopropyldicarbinyl diradical exhibit regioselectivity parallel to that of the di-π counterpart. Both three-ring and five-ring photoproducts were formed with the ratio varying with conversion. Since the three-ring (i.e. di-π-methane) photoproducts were found to ring expand to the five-ring (i.e. tri-π-methane) products, kinetics were employed to determine to what extent the reaction proceeds in a two-step versus direct formation of the five-ring product. It was found that the direct route was the major one.
Excited- and ground-state versions of the tri-π-methane rearrangement: Mechanistic and exploratory organic photochemistry
Zimmerman,Cirkva
, p. 1839 - 1851 (2007/10/03)
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 species - A, B, and C - corresponding 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.
