The halogen-atom transfer (XAT) is one of the most important and applied processes for the generation of carbon radicals in synthetic chemistry. In this review, we summarize and highlight the most ...important aspects associated with XAT and the impact it has had on photochemistry and photocatalysis. The organization of the material starts with the analysis of the most important mechanistic aspects and then follows a subdivision based on the nature of the reagents used in the halogen abstraction. This review aims to provide a general overview of the fundamental concepts and main agents involved in XAT processes with the objective of offering a tool to understand and facilitate the development of new synthetic radical strategies.
Anilines are some of the most used class of substrates for application in photoinduced electron transfer. N,N‐Dialkyl‐derivatives enable radical generation α to the N‐atom by oxidation followed by ...deprotonation. This approach is however elusive to monosubstituted anilines owing to fast back‐electron transfer (BET). Here we demonstrate that BET can be minimised by using photoredox catalysis in the presence of an exogenous alkylamine. This approach synergistically aids aniline SET oxidation and then accelerates the following deprotonation. In this way, the generation of α‐anilinoalkyl radicals is now possible and these species can be used in a general sense to achieve divergent sp3 C−H functionalization.
Divergent radical sp3 C−H functionalization of N‐alkyl anilines has been achieved using a photoredox strategy. This approach enables to overcome back‐electron transfer and therefore allows direct access to previously elusive α‐anilinoalkyl radicals.
Conspectus Developing new methods that enable the synthesis of new and complex molecules with complete control of their 3-D shape is central to the advancement of synthetic chemistry with ...applications spanning from medicine to materials. Our approach consists of the iterative combination of small building blocks through the use of boron chemistry to essentially “grow” molecules. This approach, which we term assembly-line synthesis (ALS), resembles the way that nature assembles natural products (e.g., the polyketide synthase machinery) and has the advantage that many structural variations can be easily introduced and the products can be evaluated in structural or biological contexts. Chiral boronic esters have been recognized as valuable building blocks due to their unique chemical properties. They are both chemically and configurationally stable, and they can be prepared with very high levels of enantioselectivity. Additionally they undergo a broad array of transformations that lead to the stereocontrolled formation of C–C and C–X (X = heteroatom) bonds. This versatility makes boronic acids ideal building blocks for iterative molecular assembly. A powerful reaction platform for chemical diversification using chiral boronic esters is their homologation using lithium carbenoids via 1,2-metalate rearrangement. In the 1980s, Matteson described the use of boronic esters bearing a chiral diol in a two-step homologation process with dichloromethyl lithium and Grignard reagents (substrate-controlled approach). We have focused on reagent control and have found that Hoppe’s chiral lithiated carbamates can be used as carbenoid equivalents in conjunction with achiral boronic esters. This reagent-controlled process offers many advantages due to the easy access of both the chiral lithiated carbamates and stable boronic esters. The carbamates can be derived from primary or secondary alcohols, and a broad range of functionalized boronic esters and boranes can be employed. Multiple homologations can be carried out in a one-pot sequence thereby streamlining the process to a single operation. This methodology has enabled the synthesis of many molecules containing multiple contiguous stereogenic centers with exquisite 3-D control. In this Account, we trace our own studies to establish the lithiation–borylation methodology and describe selected synthetic applications.
This Minireview highlights advances in the Suzuki–Miyaura cross‐coupling of secondary boron reagents for the creation of CC bonds with control of stereochemistry. It also includes ...non‐transition‐metal coupling of secondary and tertiary boronic esters to electron‐rich aromatics.
Just couple it: In the past decade, highly efficient protocols have been developed to allow the stereospecific arylation of chiral organoborons. This Minireview documents the rapid development of this area, thus providing a clear overview of the various processes available together with mechanisms, as well as their scope and limitations.
Photoredox Imino Functionalizations of Olefins Davies, Jacob; Sheikh, Nadeem S.; Leonori, Daniele
Angewandte Chemie (International ed.),
October 16, 2017, Letnik:
56, Številka:
43
Journal Article
Recenzirano
Odprti dostop
Shown herein is that polyfunctionalized nitrogen heterocycles can be easily prepared by a visible‐light‐mediated radical cascade process. This divergent strategy features the oxidative generation of ...iminyl radicals and subsequent cyclization/radical trapping, which allows the effective construction of highly functionalized heterocycles. The reactions proceed efficiently at room temperature, utilize an organic photocatalyst, use simple and readily available materials, and generate, in a single step, valuable building blocks that would be difficult to prepare by other methods.
Light up: Polyfunctionalized nitrogen heterocycles can be easily prepared by a visible‐light‐mediated radical cascade process. This divergent strategy features the oxidative generation of iminyl radicals and subsequent cyclization/radical trapping, which allows the effective construction of highly functionalized heterocycles.
In recent years, hydroxylamines derivatives have been exploited as nitrogen‐radical precursors in visible‐light photochemistry. Their ability to serve as electrophores in redox chemistry has ...propelled the development of many novel transformations. Fundamental mechanistic aspects as well as the importance in the preparation of nitrogen‐containing molecules will be highlighted.
Visible‐light photocatalysis enables the formation of nitrogen radicals from simple hydroxylamine derivatives. This Concept article highlights the most notable developments in the field. Relevant applications in the synthesis and late‐stage modification of complex molecules are included along with the underpinning mechanistic analysis.
A photoinduced cascade strategy leading to a variety of differentially functionalised nitriles and ketones has been developed. These reactions rely on the oxidative generation of iminyl radicals from ...simple oximes. Radical transposition by C(sp3)−(sp3) and C(sp3)−H bond cleavage gives access to distal carbon radicals that undergo SH2 functionalisations. These mild, visible‐light‐mediated procedures can be used for remote fluorination, chlorination, and azidation, and were applied to the modification of bioactive and structurally complex molecules.
Radical transposition: A photoinduced cascade strategy for remote fluorination, chlorination, and azidation relies on the oxidative generation of iminyl radicals from simple oximes. Radical transposition by C(sp3)−(sp3) and C(sp3)−H bond cleavage gives access to distal carbon radicals that undergo SH2 functionalisations.
The formation and use of iminyl radicals in novel and divergent hydroimination and iminohydroxylation cyclization reactions has been accomplished through the design of a new class of reactive O‐aryl ...oximes. Owing to their low reduction potentials, the inexpensive organic dye eosin Y could be used as the photocatalyst of the organocatalytic hydroimination reaction. Furthermore, reaction conditions for a unique iminohydroxylation were identified; visible‐light‐mediated electron transfer from novel electron donor–acceptor complexes of the oximes and Et3N was proposed as a key step of this process.
Divergent SET: Visible‐light irradiation of O‐aryl oximes leads to the generation of N‐centered radicals. Depending on the reaction conditions, these radicals cyclize either in a photoredox‐catalyzed hydroimination or in an iminohydroxylation via electron donor–acceptor complexes (EY=eosin Y).
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