Cycloadditions are among the most efficient chemical processes, combining atom economy, stereospecificity, and the ability to generate molecular complexity in a single step. Aromatic rings would in principle be ideal reaction partners, as they contain, at least from the topological point of view, both olefinic and diene subunits; however, the stability of the conjugated aromatic system would be broken by cycloaddition reactions, which are therefore rarely applied, because kinetics and thermodynamics hinder the process. From that aspect, photochemical activation opens interesting perspectives, as one can selectively provide excess energy to one of the reactants but not to the product, thus preventing thermal back reaction. Indeed, aromatic rings show a rich photochemistry, ranging from isomerizations, substitutions, and additions to cycloadditions. In this review, we will focus on cycloadditions, covering literature from early observations up to the present.

Arene–Alkene Cycloaddition

Electronic excitation significantly changes the reactivity of chemical compounds. Compared to ground state reactions, photochemical reactions considerably enlarge the application spectrum of a particular functional group in organic synthesis. Multistep syntheses may be simplified and perspectives for target oriented synthesis (TOS) and diversity oriented synthesis (DOS) are developed. New compound families become available or may be obtained more easily. In contrast to common chemical reagents, photons don't generate side products resulting from the transformation of a chemical reagent. Therefore, they are considered as a traceless reagent. Consequently, photochemical reactions play a central role in the methodology of sustainable chemistry. This aspect has been recognized since the beginning of the 20th century. As with many other photochemical transformations, photochemical reactions of aromatic, benzene-like compounds illustrate well the advantages in this context. Photochemical cycloadditions of aromatic compounds have been investigated for a long time. Currently, they are applied in various fields of organic synthesis. They are also studied in supramolecular structures. The phenomena of reactivity and stereoselectivity are investigated. During recent years, photochemical electron transfer mediated reactions are particularly focused. Such transformations have likewise been performed with aromatic compounds. Reactivity and selectivity as well as application to organic synthesis are studied.

Photochemical reactions of aromatic compounds and the concept of the photon as a traceless reagent

In the presence of an alkene, three different modes of photocycloaddition with benzene derivatives can occur; the [2 + 2] or ortho, the [3 + 2] or meta, and the [4 + 2] or para photocycloaddition. This short review aims to demonstrate the synthetic power of these photocycloadditions.

https://www.beilstein-journals.org/bjoc/content/pdf/1860-5397-7-61.pdf

The arene–alkene photocycloaddition

Direct Dearomatization of Pyridines via an Energy-Transfer-Catalyzed Intramolecular [4+2] Cycloaddition

A new method was developed for the photoinduced dearomatization of arenes through an intramolecular cycloaddition with aza-o-xylylenes generated by excited-state intramolecular proton transfer (ESIPT) in the readily available photoprecursors. The [2+4] topology of this cycloaddition is unprecedented for photo-dearomatizations of benzenoid aromatic carbocycles. It provides rapid access to novel heterocycles, cyclohexadieno-oxazolidino-quinolinols, as valuable synthons for a broad range of post-photochemical transformations.

Photoassisted Synthesis of Complex Molecular Architectures: Dearomatization of Benzenoid Arenes with Aza‐o‐xylylenes via an Unprecedented [2+4] Reaction Topology

N-Benzoyl-N-benzylcinnamamides 1,
N-acetyl-N-(1-benzyl)cinnamamides 3 and
N-[(1-benzyl)aminocarbonyl]-N-(1-benzyl)cinnamamides
5 have been irradiated by a high-pressure mercury lamp in the presence
of benzil in benzene at room temperature to give
3-azatricyclo[5.2.2.01,5]undeca-8,10-dien-4-ones 2, 4 and 6
with high stereoselectivity. These are the first photo[4+2]cycloadducts
of an enone and a benzene ring.

Akimoto Satoshi

Papers

Intramolecular photo[4+2]cycloaddition of an enone with a benzene ring

Intramolecular Photo(4 + 2)cycloaddition of an Enone with a Benzene Ring.