Recent Synthetic Applications of Catalyst-Free Photochemistry

Abstract

Catalyst-free photochemistry provides numerous opportunities toward sustainable synthesis because catalyst separation can usually be avoided, which is consistent with green chemistry principles. Complementary to the well-reviewed photoredox chemistry, this review specifically summarizes the synthetic applications of photochemistry without external catalysts reported since 2000. The selected examples include both natural product synthesis and new methodology development. This review is arranged based on the type of chromophore. It is our hope that this review will inspire more synthetic chemists to embrace photochemistry into their research plans.1 Introduction2 Photochemistry of Olefins2.1 [2+2] Cycloaddition of Enones and Olefins2.2 Cycloaddition of Olefins without Carbonyl Groups2.3 Z/E Isomerization2.4 Cyclization2.5 Others3 Photochemistry of C=O3.1 The Paternò–Büchi Reaction3.2 The Yang Photoenolization3.3 The Norrish Type I Reaction3.4 The Norrish Type II Reaction3.5 Others4 Photochemistry of Nitrogen-Containing Functional Groups5 Photochemistry of Halogen-Containing Compounds6 Conclusion and Outlook

Preparation of Cyclobutene Acetals and Tricyclic Oxetanes through Photochemical Tandem and Cascade Reactions

“…Such processes are attractive in the context of sustainable development, since the explicit reagent is a simple photon. Significant recent advances have been collated in the areas of catalyst‐free photochemical synthesis, visible‐light photocatalysis, enantioselective catalysis, photochirogenesis and the use of flow processes, testifying to the potential of organic photochemistry for selective synthesis.…”

Metal‐Free Photoinduced Transformation of Aryl Halides and Diketones into Aryl Ketones

“…Photo‐induced organic reactions have opened a new era for the preparation of diverse important organic molecules in novel and meaningful ways Via light as the energy input, photoreactions can be carried out without catalysts . More significantly, photoreactions could realize transformations challenging to achieve by traditional methodologies, thus complementing conventional reactions, especially the metal‐catalyzed ones.…”

“…Recently, we have uncovered a series of metal‐free light‐enabled chemical transformations that traditionally require transition‐metal catalysts or organometallic reagents. [2b], [2c], [2d], [3a], [3c], We thus considered the possibility of directly coupling aryl halides with carbonyl compounds to obtain aryl ketones facilitated by light under metal‐free conditions (Scheme c). Our hypothesis consists of two steps.…”

Diastereoselective Access to Tetracyclic Eight‐Membered Lactams through a Dearomative Heck Reaction and an Alkylative Ring‐Opening Driven by Photoexcited Spiroindolines

“…Based on the experimental and computational results, we proposed a mechanism for the alkylative ring‐opening reaction involving dual roles of the spiroindoline (Scheme 2 c). Under UV light irradiation, the excited state of spiroindoline I is generated, which behaves as a single‐electron reductant [26] . A single electron transfer (SET) from I to perfluorobutyl iodide delivers radical cation II and the perfluorocarbon‐centered radical.…”