A SmI2-mediated reductive cyclisation reaction using the trifluoroacetamide group as the radical precursor

A samarium(II)-mediated reductive cyclisation reaction with the aminoketyl radical from the trifluoroacetamide group for synthesising 2-trifluoromethylindolines was developed. This reaction is the first example of using an acyclic amide group, which is considered difficult to react with SmI2, in a reductive cyclisation. Additionally, the conversion of the obtained product into 2-trifluoromethylindole was achieved.

Diastereoselective Radical 1,4-Ester Migration: Radical Cyclizations of Acyclic Esters with SmI2

Reductive cyclizations of carbonyl compounds, mediated by samarium(II) diiodide (SmI₂, Kagan’s reagent), represent an invaluable platform to generate molecular complexity in a stereocontrolled manner. In addition to classical ketone and aldehyde substrates, recent advances in radical chemistry allow the cyclization of lactone and lactam-type substrates using SmI₂. In contrast, acyclic esters are considered to be unreactive to SmI₂ and their participation in reductive cyclizations is unprecedented. Here, we report a diastereoselective radical 1,4-ester migration process, mediated by SmI₂, that delivers stereodefined alkene hydrocarboxylation products via radical cyclization of acyclic ester groups in α-carbomethoxy δ-lactones. Isotopic labeling experiments and computational studies have been used to probe the mechanism of the migration. We propose that a switch in conformation redirects single electron transfer from SmI₂ to the acyclic ester group, rather than the “more reactive” lactone carbonyl. Our study paves the way for the use of elusive ketyl radicals, derived from acyclic esters, in SmI₂-mediated reductive cyclizations.

Deoxygenative Cross-Coupling of Aromatic Amides with Polyfluoroarenes

Considering the ubiquitous nature and ready synthesis of amides, and the great significance of organofluorine-containing species, the cross-coupling of amides and polyfluoroarenes, leading to new carbon-carbon bond-forming methodologies, would find useful applications in synthesis, late-stage functionalization, and rapid generation of molecular diversity. Herein, we present a novel synthesis of α-polyfluoroaryl amines via Sm/SmI₂ -mediated deoxygenative cross-coupling of aromatic amides with polyfluoroarenes through direct C-H functionalization. The structural and functional diversity of these readily available precursors provides a versatile and flexible strategy for the streamlined synthesis of α-polyfluoroaryl amines. Combining experimental and theoretical studies, a novel plausible mechanism of the α-aminocarbene-mediated C-H insertion has been revealed, which may stimulate future work for the development of novel methods in amine synthesis.

Samarium(II)-Electrocatalyzed Chemoselective Reductive Alkoxylation of Phthalimides

The unprecedented samarium-eletrocatalyzed reductive alkoxylation of phthalimides in a single step is presented. Under mild conditions, using electrogenerated Sm(II) with TMSCl (trimethyl chlorosilane), N-substituted 3-alkoxyl isoindolin-1-ones are isolated in good...

Palladium-Catalyzed Ring-Closing Reaction for the Synthesis of Saturated N-Heterocycles with Aminodienes and N,O-Acetals

An efficient palladium-catalyzed ring-closing reaction of aminodienes with N,O-acetals for the synthesis of saturated N-heterocycles is described. The reaction is consistently operated at room temperature and tolerates a wide range of functional groups with volatile MeOH as the sole byproduct. This method provides rapid and practical access to a broad range of saturated N-heterocycles with diverse structural backbones that are useful building blocks in natural product synthesis and drug discovery.

20 Years of Forging N-Heterocycles from Acrylamides through Domino/Cascade Reactions

Acrylamides are versatile building blocks that are easily obtained from readily available starting materials. During the last 20 years, these valuable substrates bearing a nucleophilic nitrogen atom and an electrophilic double bond have proven to be efficient domino partners, leading to a wide variety of complex aza-heterocycles of synthetic relevance. In this non-exhaustive review, metal-free and metal-triggered reactions followed by an annulation will be presented; these two approaches allow good modulation of the reactivity of the polyvalent acrylamides.

1 Introduction

2 Metal-Free Annulations

2.1 Domino Reactions Triggered by a Michael Addition

2.2 Domino Reactions Triggered by an Aza-Michael Addition

2.3 Domino Processes Triggered by an Acylation Reaction

2.4 Domino Reactions Triggered by a Baylis–Hillman Reaction

2.5 Cycloadditions and Domino Reactions

2.6 Miscellaneous Domino Reactions

3 Metal-Triggered/Mediated Annulations

3.1 Zinc-Promoted Transformations

3.2 Rhodium-Catalyzed Functionalization/Annulation Cascades

3.3 Cobalt-Catalyzed Functionalization/Annulation Cascades

3.4 Ruthenium-Catalyzed Functionalization/Annulation Cascades

3.5 Iron-Catalyzed Functionalization/Annulation Cascades

3.6 Palladium-Catalyzed Functionalization/Annulation Cascades

3.7 Copper-Catalyzed Transformations

3.8 Transition Metals Acting in Tandem in Domino Processes

4 Radical Cascade Reactions

5 Conclusion

Additive Tuned Selective Synthesis of Bicyclo[3.3.0]octan-1-ols and Bicyclo[3.1.0]hexan-1-ols Mediated by AllylSmBr

The selective construction of bicyclo[3.3.0]octan-1-ols and bicyclo[3.1.0]hexan-1-ols was achieved by using an allylSmBr/additive(s) system. By employing HMPA as the only additive, the momoallylation/ketone-alkene coupling occurred preferably and afforded bicyclo[3.3.0]octan-1-ols in good yields with high diastereoselectivities. While the ester-alkene coupling predominated to generate bicyclo[3.1.0]hexan-1-ols in moderate yields with excellent diastereoselectivities in the presence of a proton source, such as pyrrole as the coadditive with HMPA. The tunable reactivity of allylSmBr by additive(s) would make it a versatile reagent in organic synthesis.

Reductive cyclisations of amidines involving aminal radicals

The first general study of aminal radical cyclisations, triggered by reduction of amidines with SmI₂, delivers quinazolinones with complete diastereocontrol.

Synthesis of pyridine trans-tetrafluoro-λ6-sulfane derivatives via radical addition

Pyridine-trans-SF₄-alkenes or -alkanes have been synthesized for the first time via the radical addition reactions of pyridine-trans-SF₄Cl to alkynes or alkenes in good to high yields.

Synthesis of Nitrogen Heterocycles Using Samarium(II) Iodide

Nitrogen heterocycles represent vital structural motifs in biologically-active natural products and pharmaceuticals. As a result, the development of new, convenient and more efficient processes to N-heterocycles is of great interest to synthetic chemists. Samarium(II) iodide (SmI₂, Kagan’s reagent) has been widely used to forge challenging C–C bonds through reductive coupling reactions. Historically, the use of SmI₂ in organic synthesis has been focused on the construction of carbocycles and oxygen-containing motifs. Recently, significant advances have taken place in the use of SmI₂ for the synthesis of nitrogen heterocycles, enabled in large part by the unique combination of high reducing power of this reagent (E_1/2 of up to −2.8 V) with excellent chemoselectivity of the reductive umpolung cyclizations mediated by SmI₂. In particular, radical cross-coupling reactions exploiting SmI₂-induced selective generation of aminoketyl radicals have emerged as concise and efficient methods for constructing 2-azabicycles, pyrrolidines and complex polycyclic barbiturates. Moreover, a broad range of novel processes involving SmI₂-promoted formation of aminyl radicals have been leveraged for the synthesis of complex nitrogen-containing molecular architectures by direct and tethered pathways. Applications to the synthesis of natural products have highlighted the generality of processes and the intermediates accessible with SmI₂. In this review, recent advances involving the synthesis of nitrogen heterocycles using SmI₂ are summarized, with a major focus on reductive coupling reactions that enable one-step construction of nitrogen-containing motifs in a highly efficient manner, while taking advantage of the spectacular selectivity of the venerable Kagan’s reagent.