5.06 Di-π-methane, Oxa-di-π-methane, and Aza-di-π-methane Photoisomerization

Enantioselective Photochemical Reactions Enabled by Triplet Energy Transfer

“…A hallmark of photochemistry is the fact that simple starting materials can be used to form complex polycyclic structures that would otherwise be inaccessible. The di-π-methane rearrangement and its most common variant, the oxa-di-π-methane rearrangement, are classic examples which have enjoyed great interest from the synthetic community regarding their application and selectivity. − Despite some cases in which the reaction proceeds on the singlet hypersurface, the triplet state is most frequently involved, allowing the reaction to take place by triplet energy transfer from an adequate sensitizer . In this context, β,γ-unsaturated ketones such as rac - 76 are suitable substrates for a triplet sensitized oxa-di-π-methane rearrangement.…”

Photochemical Rearrangements in Heterocyclic Chemistry

“…Complex structures in a broader scope become more easily available . Typical examples are the di‐π‐methane rearrangement and corresponding heteroatom oxa‐ and aza‐di‐π‐methane rearrangements . Often photochemical products possess an increased reactivity in particular the ground state reactions and can serve as interesting synthesis intermediates for natural product synthesis or for the preparation of new compounds in many fields such as pharmaceutical and agro‐chemistry or for material sciences.…”

Di-π-ethane Rearrangement of Cyano Groups via Energy-Transfer Catalysis

“…This newly developed synthetic approach not only achieves the CN functional group translocation under an energy-transfer mechanism but also extended the di-π-methane rearrangement (Zimmerman rearrangement) − for the first time. Following Zimmerman’s definition of di-π-methane rearrangement, − we have termed our synthetic transformation as a di-π-ethane rearrangement, in which photochemical rearrangement involves substrates wherein two π systems are separated by two sp 3 carbon atoms (ethane-like).…”

“…Di-π-methane rearrangement was originally discovered by Zimmerman and co-workers in 1967. , As shown in Scheme B, a typical photochemical di-π-methane rearrangement includes diradical generation, functional group relocation, and diradical recombination. − Compared to most established radical translocations, the development of di-π-methane rearrangement is slow. − However, di-π-methane rearrangement is quite unique, because the diradical is formed in the initial step and a three-membered ring is always formed due to the final diradical recombination step. So far, this approach has been successfully applied in the migration of aryl, alkenyl, carbonyl, and imine functional groups. − The cyano (CN) functional group is a very versatile functional group, which could be easily converted to a variety of other functional groups such as aldehydes, ketones, amines, carboxylic acids, and heterocycles. − Actually, CN functional group translocation has already been explored through an electron-transfer mechanism by Beckwith, Zhu, and Liu employing either halide or alkene , as radical precursors. C–H bonds could also be concerted to radical intermediates through either intramolecular heteroatom mediated 1,5-hydrogen atom transfer (HAT) by Kalvoda, Watt, , and Zhu or intermolecular HAA by Xu …”