Andreas Heidbreder

Radical ion cyclizations

Radical ions generated by single-electron transfer from neutral organic compounds are known as important intermediates in a variety of interesting chemical processes and reactions. Our aim in this article will be to provide a comprehensive review of the design and application of cyclization reactions that utilize radical ions as reactive intermediates. Examples from the last ten years will be reported and mechanistic aspects will be discussed critically. The cyclization reactions are classified into three categories regardless of the method of generating of the reactive species. After a brief introduction, cyclization reactions via radical cations are described. The subsequent category is devoted to radical-anion-mediated cyclization reactions. The last category includes cyclization reactions via reactive intermediates containing both radical anions and radical cations.

Photoinduced electron transfer (PET) in organic synthesis. [3 + 2]-type cycloaddition, cyclization and CC 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.

Mechanistic studies on PET-oxidative cyclization of unsaturated silyl enol ethers: dependence of the regioselectivity on alcohol addition and pressure effects

Unsaturated silyl enol ethers are irradiated in the presence of electron transfer sensitizers. The efficiency of the cyclization reaction using different sensitizers is investigated. The endo/exo regiochemistry of the ring closure reaction can either be controlled by variation of the silyl group or by addition of alcohol. Furthermore, a dependence of the regiochemistry on pressure is revealed and it seems that it can be related to acetonitrile acting as a nucleophile at 1500 bar. As key intermediates radical cations and radicals are involved.