The catalytic conversion of chemical feedstocks into products of medicinal and agricultural value is a key theme across modern synthetic chemistry. As 1,3-dienes are readily available from industrial ...cracking processes, there is great interest in the development of sustainable methods for the functionalization of these simple molecules. Although initial developments in this field have required precious-transition-metal catalysts, there has been a push toward the use of inexpensive, nontoxic, and more abundant copper catalysts to promote functionalization. This Perspective covers the many developments in the area of copper-catalyzed functionalization of 1,3-dienes, in particular hydrofunctionalization, borofunctionalization, and difunctionalization (e.g., diamination).
Sulfoxides are classical functional groups for directing the stoichiometric metalation and functionalization of C−H bonds. In recent times, sulfoxides have been given a new lease on life owing to the ...development of modern synthetic methods that have arisen because of their unique reactivity. They have recently been used in catalytic C−H activation proceeding via coordination of an internal sulfoxide to a metal or through the action of an external sulfoxide ligand. Furthermore, sulfoxides are able to capture nucleophiles and electrophiles to give sulfonium salts, which subsequently enable the formation of C−C bonds at the expense of C−H bonds. This Review summarizes a renaissance period in the application of sulfoxides arising from their versatility in directing C−H functionalization.
Classical sulfoxide directing groups have been given a new lease on life because of their unique ability to direct a variety of C−H couplings to form important C−C bonds. Sulfoxide direction operates through internal coordination to a metal (directing groups), through external coordination (ligands), or via sulfoxide capture of nucleophilic or electrophilic coupling partners (sulfonium‐directed).
The photoactivation of electron donor-acceptor complexes has emerged as a sustainable, selective and versatile strategy for the generation of radical species. However, when it comes to aryl radical ...formation, this strategy remains hamstrung by the electronic properties of the aromatic radical precursors, and electron-deficient aryl halide acceptors are required. This has prevented the implementation of a general synthetic platform for aryl radical formation. Our study introduces triarylsulfonium salts as acceptors in photoactive electron donor-acceptor complexes, used in combination with catalytic amounts of newly designed amine donors. The sulfonium salt label renders inconsequential the electronic features of the aryl radical precursor and, more importantly, it is installed regioselectively in native aromatic compounds by C-H sulfenylation. Using this general, site-selective aromatic C-H functionalization approach, we developed metal-free protocols for the alkylation and cyanation of arenes, and showcased their application in both the synthesis and the late-stage modification of pharmaceuticals and agrochemicals.
Reductive electron transfer (ET) to organic compounds is a powerful method for the activation of substrates via the formation of radicals, radical anions, anions, and dianions that can be exploited ...in bond-cleaving and bond-forming processes. Since its introduction to the synthetic community in 1977 by Kagan, SmI2 has become one of the most important reducing agents available in the laboratory. Despite its widespread application in aldehyde and ketone reduction, it was widely accepted that carboxylic acid derivatives could not be reduced by SmI2; only recently has our work led to this dogma being overturned, and the reduction of carboxylic acid derivatives using SmI2 can now take its place alongside aldehyde/ketone reduction as a powerful activation mode for synthesis. In this Account, we set out our studies of the reduction of carboxylic acid derivatives using SmI2, SmI2–H2O, and SmI2–H2O–NR3 and the exploitation of the unusual radical anions that are now accessible in unprecedented carbon–carbon bond-forming processes. The Account begins with our serendipitous discovery that SmI2 mixed with H2O is able to reduce six-membered lactones to diols, a transformation previously thought to be impossible. After the successful development of selective monoreductions of Meldrum’s acid and barbituric acid heterocyclic feedstocks, we then identified the SmI2–H2O–NR3 reagent system for the efficient reduction of a range of acyclic carboxylic acid derivatives that typically present a significant challenge for ET reductants. Mechanistic studies have led us to propose a common mechanism for the reduction of carboxylic acid derivatives using Sm(II), with only subtle changes observed as the carboxylic acid derivative and Sm(II) reagent system are varied. At the center of our postulated mechanism is the proposed reversibility of the first ET to the carbonyl of carboxylic acid derivatives, and this led us to devise several strategies that allow the radical anion intermediates to be exploited productively in efficient new processes. First, we have used internal directing groups in substrates to “switch on” productive ET to esters and amides and have exploited such an approach in tag-removal cyclization processes that deliver molecular scaffolds of significance in biology and materials science. Second, we have exploited external ligands to facilitate ET to carboxylic acid derivatives and have applied the strategy in telescoped reaction sequences. Finally, we have employed follow-up cyclizations with alkenes, alkynes, and allenes to intercept radical anion intermediates formed along the reaction path and have employed this strategy in complexity-generating cascade approaches to biologically significant molecular architectures. From our studies, it is now clear that Sm(II)-mediated ET to carboxylic acid derivatives constitutes a general strategy for inverting the polarity of the carbonyl, allowing nucleophilic carbon-centered radicals to be formed and exploited in novel chemical processes.
Sulfonium salts are playing an increasingly significant role in contemporary organic synthesis. In particular, the generation of radicals from sulfonium salts is a fundamental process in Nature and ...has been the subject of investigation for over 50 years. However, general synthetic methods that use sulfonium salts as radical precursors are rare. The advent of photoredox catalysis has triggered an upsurge of interest in the radical chemistry of sulfonium salts and this review surveys recent applications of aryl‐ and alkylsulfonium salts in light‐mediated, radical C−C bond formation.
Reactions proceeding through open‐shell, single‐electron pathways offer attractive alternative outcomes to those proceeding through closed‐shell, two‐electron mechanisms. In this context, samarium ...diiodide (SmI2) has emerged as one of the most important and convenient‐to‐use electron‐transfer reagents available in the laboratory. Recently, significant progress has been made in the reductive chemistry of other divalent lanthanides which for many years had been considered too reactive to be of value to synthetic chemists. Herein, we illustrate how new samarium(II) complexes and nonclassical lanthanide(II) reagents are changing the landscape of modern reductive chemistry.
Electron dance: Lanthanide(II)‐based electron‐transfer reagents, beyond the classic reagent samarium(II) iodide (SmI2, Kagan's reagent), are receiving increasing attention as new vehicles for the delivery of electrons and the orchestration of radical processes with power and precision (see scheme).
Highly chemoselective direct reduction of primary, secondary, and tertiary amides to alcohols using SmI2/amine/H2O is reported. The reaction proceeds with C–N bond cleavage in the carbinolamine ...intermediate, shows excellent functional group tolerance, and delivers the alcohol products in very high yields. The expected C–O cleavage products are not formed under the reaction conditions. The observed reactivity is opposite to the electrophilicity of polar carbonyl groups resulting from the nX → π*CO (X = O, N) conjugation. Mechanistic studies suggest that coordination of Sm to the carbonyl and then to Lewis basic nitrogen in the tetrahedral intermediate facilitate electron transfer and control the selectivity of the C–N/C–O cleavage. Notably, the method provides direct access to acyl-type radicals from unactivated amides under mild electron transfer conditions.
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