Radical ion cyclizations

Dual Catalysis Strategies in Photochemical Synthesis

The interaction between an electronically excited photocatalyst and an organic molecule can result in the genertion of a diverse array of reactive intermediates that can be manipulated in a variety of ways to result in synthetically useful bond constructions. This Review summarizes dual-catalyst strategies that have been applied to synthetic photochemistry. Mechanistically distinct modes of photocatalysis are discussed, including photoinduced electron transfer, hydrogen atom transfer, and energy transfer. We focus upon the cooperative interactions of photocatalysts with redox mediators, Lewis and Brønsted acids, organocatalysts, enzymes, and transition metal complexes.

Tandem polar/radical crossover sequences for the formation of fused and bridged bicyclic nitrogen heterocycles involving radical ionic chain reactions, and alkene radical cation intermediates, performed under reducing conditions: scope and limitations

It is demonstrated that phosphorylated forms of beta-nitro alcohols provide an excellent means of entry into beta-(phosphatoxy)alkyl radicals on exposure to tributyltin hydride and AIBN in benzene at reflux. These radicals then undergo heterolytic cleavage of the phosphate group to yield alkene radical cation/phosphate anion contact ion pairs which are trapped intramolecularly in a tandem polar/radical crossover sequence involving radical ionic chain reactions by allylic and propargylic amines. The substitution pattern of the alkene radical cation dictates the cyclization mode, and this may be engineered to form fused ring systems by an initial exo-mode nucleophilic cyclization or bridged bicyclic systems when the nucleophilic attack takes place in the endo-mode.

Diastereoselectivity in the cyclization of alkene radical cations generated under non-oxidizing conditions: contact ion pairs and memory effects

A series of highly diastereomerically enriched 1,5-dimethyl-, 2,5-dimethyl-, and 3,5-dimethyl-N-benzyl-5-nitro-4-(diphenylphosphatoxy)hexylamines were exposed to tributyltin hydride and AIBN in benzene at reflux. The ensuing reactions, interpreted in terms of radical denitration, radical ionic fragmentation, and nucleophilic substitution, lead to the formation of pyrrolidines with moderate to high diastereoselectivity. In five out of the six cases, the diastereoselectivity is best interpreted by backside attack by the amine on the initial contact ion pair generated by radical ionic fragmentation. In the exception that proves the rule, this mode of attack is disfavored by 1,3A strain in the initial contact ion pair, resulting in equilibration and subsequent attack on the opposite face.

Generation and reaction of alkene radical cations under nonoxidizing conditions: synthesis of the pyrrolizidine nucleus

[see reaction]. Stable beta-phosphatoxy nitroalkanes, readily assembled by the Henry reaction and subsequent phosphorylation, serve as good precursors to alkene radical cations on treatment with triphenyltin or tributyl hydride and AIBN in benzene at reflux. When the beta-phosphatoxy nitroalkane is suitably functionalized with nucleophilic groups, substitutions can be achieved with the formation of heterocyclic rings. When the nucleophile is an allylamine, tandem processes occur giving pyrrolizidines.

Photoinduced Electron Transfer Reactions of alpha-Cyclopropyl- and alpha-Epoxy Ketones. Tandem Fragmentation-Cyclization to Bi-, Tri-, and Spirocyclic Ketones

Reductive photoinduced electron transfer (PET) reactions have been performed with various bicyclic alpha-cyclopropyl-substituted ketones and tertiary amines. The reaction resulted in a regioselective cleavage of one cyclopropyl bond under formation of an exocyclic radical with an endocyclic enolate unit. In the case of bicyclic ketones with an unsaturated side chain, various bicyclic, spirocyclic, and tricyclic products are accessible via radical cyclization, depending on the position of the alkenyl substituent. In addition to triethylamine, N-silylated amines have also been used as electron donors, leading to a variety of compounds, among them are silylated fragmentation products, indicating that a proton is transferred from not only the amine radical cation but also the cationic silyl group. The intramolecular Paternó-Büchi reaction has also been studied for cyclopropane derivatives of the jasmone type leading to tetracyclic oxetanes. Finally, alpha-epoxy-substituted ketones have been investigated under PET conditions, yielding ring-opened products.