Strain-Release Photocatalysis

Abstract

The concept of strain in organic compounds is as old as modern organic chemistry and was initially introduced to justify the synthetic setbacks along the synthesis of small ring systems (pars construens of strain). In the last decades, chemists have developed an arsenal of strain-release reactions (pars destruens of strain) which can generate�with significant driving force�rigid aliphatic systems that can act as three-dimensional alternatives to (hetero)arenes. Photocatalysis added an additional dimension to strain-release processes by leveraging the energy of photons to create chemical complexity under mild conditions. This perspective presents the latest advancements in strain-release photocatalysis�with emphases on mechanisms, catalytic cycles, and current limitations�the unique chemical architectures that can be produced, and possible future directions.

Synthesis of 2‐Azanorbornanes via Strain‐Release Formal Cycloadditions Initiated by Energy Transfer

“…Very recently, Stephenson and co‐workers disclosed an intramolecular radical cyclization initiated by photoredox catalysis to prepare 2‐azanorbornanes with geminal substitution at C6 [10] . Herein, we describe a new approach toward the synthesis of these motifs by a [2π+2σ] formal cycloadditions by two different strategies (Scheme 1C) [11] . The first involves the formal cycloaddition of azahousanes with alkenes, whereas the second involves the formal cycloaddition of housanes with imines [12]…”

Energy transfer photocatalysis: exciting modes of reactivity

“…88,89 With a relatively high strain energy (B64 kcal mol À1 ), leveraging this thermodynamic driving force for cycloadditions has been a key aspect in this rapidly expanding field. [90][91][92] In this regard, our group discovered the first intermolecular [2p+2s]photocycloaddition mediated by energy transfer (Scheme 4B). 93 A series of commercially available endocyclic and aromatic double bonds, including coumarins, indoles, and flavones, were activated through energy transfer, readily undergoing cycloaddition with differently substituted bicyclo[1.1.0]butanes 29.…”

Photocatalyzed [2σ + 2σ] and [2σ + 2π] Cycloadditions for the Synthesis of Bicyclo[3.1.1]heptanes and 5- or 6-Membered Carbocycles

“…Apart from single carbon radicals, which have been intensively studied, the reactivity of 1,2 biradicals has recently been the focus of strong interest, especially in the context of the excitation of conjugated π-carbonyl systems through energy transfer catalysis (Scheme A). , In contrast, the generation of nonconjugated 1,n biradicals through the homolytic cleavage of C–C bonds in carbocycles remains challenging due to the larger bond dissociation energy of a σ C–C bond compared to a π bond. The 1,n biradicals can then be applied in further cyclizations or ring expansions to rapidly increase the complexity of small molecules …”