Photoinduced electron transfer (PET) in organic synthesis. [3 + 2]-type cycloaddition, cyclization and CC bond cleavage reactions

Abstract

An area of growing interest, in organic photochemistry in particular, involves the use of light to induce an electron transfer from a donor (D) to an acceptor (A) molecule, designated photoinduced electron transfer (PET). Excitation of A or D leads to well-defined changes in their redox properties, i.e. A (D) becomes a stronger acceptor (donor). In general, the feasibility of producing radical ions can be predicted using the well-known Weller equation and moreover, polar solvents and salts effects support their formation. Following a brief introduction, we focus on the synthetic applications of PET, and discuss [3 + 2] cycloadditions, reductive and oxidative cyclizations and CC bond cleavage reactions. The PET ring cleavage of azirines results in the formation of 2-azaallenyl radical cations which may be trapped by dipolarophiles to give heterocyclic compounds. Using this method, we have developed a new route to heterophanes. Porphyrins can also be prepared by this procedure. Further studies aim ot reveal the scope and limitations of these new reactions. Radical cations and radical anions can attack π bonds within the same molecule. Depending on the reaction conditions chosen to generate the radical ions, five-or six-membered ring systems can be obtained. Under PET reductive conditions, radical anions arise which preferentially form five-membered ring molecules (Baldwin rule). However using PET oxidative conditions, radical cations are generated which preferentially form six-membered ring molecules due to their electrophilic character. In addition, we have developed a new reactive CC bond cleavage reaction with annular cyclopropanes and cyclobutanes and have applied this method to a new short synthesis of the hirsutene ring system.