Abstract
When aiming to synthesize molecules with elevated molecular complexity starting from relatively simple starting materials, photochemical transformations represent an open avenue to circumvent analogous multistep procedures. Specifically, light‐mediated cycloadditions remain as powerful tools to generate new bonds begotten from non‐very intuitive disconnections, that alternative thermal protocols would not offer. In response to the current trend in both industrial and academic research pointing towards green and sustainable processes, several strategies that meet these requirements are currently available in the literature. This Minireview summarizes [2+2] and [4+2] photocycloadditions that do not require the use of metal photocatalysts by means of alternative strategies. It is segmented according to the cycloaddition type in order to give the reader a friendly approach and we primarily focus on the most recent developments in the field carried out using visible light, a general overview of the mechanism in each case is offered as well.
Highlights
It is well known that photoinduced reactions offer efficient strategies when crafting organic frameworks that might be difficult to access by summoning thermal procedures.[1]
The use of UV light as energy source for promoting these type of transformations lies in the literature since the beginning of the 19th century, when Hermann Trommsdorff discovered in 1834 that natural product santonin 1 reacted upon sunlight exposure through a puzzling series of cycloadditions (Scheme 1).[3]
This review aims to recontextualize the state-of-the-art related to photocycloadditions, by presenting interesting strategies which elude the use of metal-based photocatalysts under visible-light conditions
Summary
It is well known that photoinduced reactions offer efficient strategies when crafting organic frameworks that might be difficult to access by summoning thermal procedures.[1] This well-deserved attractiveness of photochemical transformations derives from the opportunity to convert simple feedstocks into highly functionalized molecules in a relatively reduced number of steps In this sense, cycloaddition reactions shine as a minimalistic tool, by offering the possibility to create multiple bonds and even stereocenters in one single step with predictable stereochemistry.[2] The use of UV light as energy source for promoting these type of transformations lies in the literature since the beginning of the 19th century, when Hermann Trommsdorff discovered in 1834 that natural product santonin 1 reacted upon sunlight exposure through a puzzling series of cycloadditions (Scheme 1).[3] Since a great variety of fascinating protocols highlighting the power of UV light for the assembly of polycyclic carbon scaffolds has aroused. This review aims to recontextualize the state-of-the-art related to photocycloadditions, by presenting interesting strategies which elude the use of metal-based photocatalysts under visible-light conditions
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