Site-Specific C(sp3 )-H Aminations of Imidates and Amidines Enabled by Covalently Tethered Distonic Radical Anions

Stereocontrolled and time-honored access to piperidine- and pyrrolidine-fused 3-methylenetetrahydropyrans using lactam-tethered alkenols

Polycyclic oxygen-heterocycles bearing the 3-methylenetetrahydropyran (i.e., 3-MeTHP) motif are resident in bioactive molecules such as hodgsonox and iridoid. Meanwhile, the δ- and γ-lactam topologies as well as their reduced variants (i.e., piperidines and pyrrolidines) are at the core of several pharmaceuticals and fragrances. A stereocontrolled, time-honored, and cost-effective strategy that merges a 3-MeTHP motif with the aforementioned azaheterocyclic scaffolds could exponentially expand the 3D-structural space for the discovery of new small molecules with medicinal value. In these studies, readily affordable lactam-tethered alkenols have been interrogated in two complementary cascade approaches, leading to the regioselective and stereocontrolled synthesis of lactam-fused 3-MeTHPs. The first approach hinges on regioselective 6-endo-trig bromoetherification of the alkenols and concomitant elimination to arrive at the desired 3-MeTHPs. The methylene portion of the 3-MeTHP is unveiled at a late stage, which is noteworthy since all existing approaches to 3-MeTHPs rely on early-stage introduction of the methylene group. The second strategy involves transition metal-catalyzed alkoxylation of the tethered alkenol followed by base-induced double bond isomerization. The lactam-fused 3-MeTHPs are obtained in high site- and diastereo-selectivities. Post-modification of the bicycles has led to the construction of 3-MeTHP-fused saturated piperidines and pyrrolidines as well as 3-MeTHPs bearing four contiguous stereocenters.

Revealing the nature of covalently tethered distonic radical anions in the generation of heteroatom-centered radicals: evidence for the polarity-matching PCET pathway

Recognition of the intermediacy and regulation of reactivity patterns of radical intermediates in radical chemistry have profound impacts on harnessing and developing the full potential of open-shell species in synthetic settings. In this work, the possibility of in situ formation of O/N-X intermediates from Brønsted base covalently tethered carbonyl hypohalites (BCTCs) for the generation of heteroatom-centered radicals has certainly been excluded by NMR experiments and density functional theory calculations. Instead, the spectroscopic analyses reveal that the BCTCs serve as precursors of tether-tunable distonic radical anions (TDRAs) which have been unequivocally substantiated to be involved in the direct cleavage of O/N-H bonds to generate the corresponding heteroatom-centered radicals. Meanwhile, a deep insight into the properties and reactivities of the resulting TDRAs indicates that the introduction of a tethered Brønsted base on the parent open-shell species reinforces their stabilities and leads to a reversal of electrophilicity. Moreover, the dual descriptor values and electrophilicity indices are calculated based on eleven reported radical reactions involving various electrophilic/nucleophilic radical species, further confirming their validity in the prediction of the polar effect and the polarity-matching consistency between nucleophilic TDRAs and protic O/N-H bonds. The additional halogen-free experiments mediated by the combination of phthaloyl peroxide and TEMPO also prove the feasibility of the TDRA-assisted philicity-regulation approach. Lastly, detailed intrinsic bond orbital (IBO) and Hirschfeld spin population analyses are employed to elucidate that the H-atom abstraction processes are the polarity-matching proton-coupled electron transfer (PCET) pathways, with a degree of oxidative asynchronicity.

Observation of SOMO-HOMO Inversion in a Neutral Polycyclic Conjugated Hydrocarbon

We report the generation of a nonbenzenoid polycyclic conjugated hydrocarbon, which consists of a biphenyl moiety substituted by indenyl units at the 4,4' positions, on ultrathin sodium chloride films by tip-induced chemistry. Single-molecule characterization by scanning tunneling and atomic force microscopy reveals an open-shell biradical ground state with a peculiar electronic configuration wherein the singly occupied molecular orbitals (SOMOs) are lower in energy than the highest occupied molecular orbital (HOMO).

Strategies for Accessing cis-1-Amino-2-Indanol

cis-1-amino-2-indanol is an important building block in many areas of chemistry. Indeed, this molecule is currently used as skeleton in many ligands (BOX, PyBOX…), catalysts and chiral auxiliaries. Moreover, it has been incorporated in numerous bioactive structures. The major issues during its synthesis are the control of cis-selectivity, for which various strategies have been devised, and the enantioselectivity of the reaction. This review highlights the various methodologies implemented over the last few decades to access cis-1-amino-2-indanol in racemic and enantioselective manners. In addition, the various substitution patterns on the aromatic ring and their preparations are listed.

Intramolecular chaperone-assisted dual-anchoring activation (ICDA): a suitable preorganization for electrophilic halocyclization

The halocyclization reaction represents one of the most common methodologies for the synthesis of heterocyclic molecules. Many efforts have been made to balance the relationship between structure, reactivity and selectivity, including the design of new electrophilic halogenation reagents and the utilization of activating strategies. However, discovering universal reagents or activating strategies for electrophilic halocyclization remains challenging due to the case-by-case practice for different substrates or different cyclization models. Here we report an intramolecular chaperone-assisted dual-anchoring activation (ICDA) model for electrophilic halocyclization, taking advantage of the non-covalent dual-anchoring orientation as the driving force. This protocol allows a practical, catalyst-free and rapid approach to access seven types of small-sized, medium-sized, and large-sized heterocyclic units and to realize polyene-like domino halocyclizations, as exemplified by nearly 90 examples, including a risk-reducing flow protocol for gram-scale synthesis. DFT studies verify the crucial role of ICDA in affording a suitable preorganization for transition state stabilization and X+ transfer acceleration. The utilization of the ICDA model allows a spatiotemporal adjustment to straightforwardly obtain fast, selective and high-yielding synthetic transformations.

Substrate-Directed Enantioselective Aziridination of Alkenyl Alcohols Controlled by a Chiral Cation

Alkene aziridination is a highly versatile transformation for the construction of chiral nitrogen-containing compounds. Inspired by the success of analogous substrate-directed epoxidations, we report an enantioselective aziridination of alkenyl alcohols, which enables asymmetric nitrene transfer to alkenes with varied substitution patterns, including those not covered by the current protocols. We believe that our method is effective because it is substrate-directed, exploiting a network of attractive non-covalent interactions between the substrate, an achiral dianionic rhodium(II,II) tetracarboxylate dimer, and its two associated cinchona alkaloid-derived cations. It is these cations that provide a defined chiral pocket in which the aziridination can occur. In addition to a thorough evaluation of compatible alkene classes, we advance a practical mnemonic to predict reaction outcome and disclose a range of post-functionalization protocols that highlight the unique synthetic potential of the enantioenriched aziridine-alcohol products.

Organic radicals with inversion of SOMO and HOMO energies and potential applications in optoelectronics

Organic radicals possessing an electronic configuration in which the energy of the singly occupied molecular orbital (SOMO) is below the highest doubly occupied molecular orbital (HOMO) level have recently attracted significant interest, both theoretically and experimentally. The peculiar orbital energetics of these SOMO-HOMO inversion (SHI) organic radicals set their electronic properties apart from the more common situation where the SOMO is the highest occupied orbital of the system. This review gives a general perspective on SHI, with key fundamental aspects regarding the electronic and structural factors that govern this particular electronic configuration in organic radicals. Selected examples of reported compounds with SHI are highlighted to establish molecular guidelines for designing this type of radical, and to showcase the potential of SHI radicals in organic spintronics as well as for the development of more stable luminescent radicals for OLED applications.

Recent advances in visible light-induced C(sp3)–N bond formation

Synthetic chemists have long focused on selective C(sp ³)-N bond-forming approaches in response to the high value of this motif in natural products, pharmaceutical agents and functional materials. In recent years, visible light-induced protocols have become an important synthetic platform to promote this transformation under mild reaction conditions. These photo-driven methods rely on converting visible light into chemical energy to generate reactive but controllable radical species. This Review highlights recent advances in this area, mostly after 2014, with an emphasis placed on C(sp ³)-H bond activations, including amination of olefins and carbonyl compounds, and cross-coupling reactions.

Complementary Copper-Catalyzed and Electrochemical Aminosulfonylation of O-Homoallyl Benzimidates and N-Alkenyl Amidines with Sodium Sulfinates

A complementary copper-catalyzed and electrochemical aminosulfonylation of O-homoallyl benzimidates and N-alkenyl amidines with sodium sulfinates was developed. The terminal alkene substrate produced sulfone-containing 1,3-oxazines and tetrahydropyrimidines in the presence of Cu(OAc)₂, Ag₂CO₃, and DPP, and under similar reaction conditions, sulfonylated tetrahydro-1,3-oxazepines were prepared from 1-aryl-substituted O-homoallyl benzimidates in moderate to good yields. For certain electron-rich 1,1-diaryl-substituted alkene substrates, the corresponding tetrahydro-1,3-oxazepines could also be obtained in similar or even higher yields via a green electrochemical technique.

Radical Alkene-Trifluoromethylation-Triggered Nitrile Insertion/Remote Functionalization Relay Processes: Diverse Synthesis of Trifluoromethylated Azaheterocycles Enabled by Copper Catalysis

A copper-catalyzed alkene-trifluoromethylation-triggered nitrile insertion/remote functionalization relay process has been achieved, in which "interrupted" remote 1,n-difunctionalizations of alkenes with nitrile insertion can deliver iminyl radical intermediates instead of C-based radicals, followed by subsequent 1,n-HAT to furnish corresponding remote functionalization. This relay protocol enables a straightforward approach to streamline the assembly of structurally diverse trifluoromethylated azaheterocycles.

Copper-Catalyzed Difluoroalkylation of Alkene/Nitrile Insertion/Cyclization Tandem Sequences: Construction of Difluorinated Bicyclic Amidines

A copper-catalyzed difluoroalkylation of an alkene/nitrile insertion/cyclization tandem sequence of N-cyanamide alkene was described, which provided a convenient synthetic approach for accessing difluorinated bicyclic amidines bearing imine moieties in a sustainable fashion. This protocol is characterized by high yields, a broad substrate scope, and good functional group compatibility. In addition, the desired product can be readily converted into other valuable functionalized fluorinated aza-heterocycles.

Visible Light Driven and Copper-Catalyzed C(sp3)-H Functionalization of O-Pentafluorobenzoyl Ketone Oximes

The C(sp³)-H functionalization of O-pentafluorobenzoyl ketone oximes was implemented under visible light irradiation with copper complexes as catalysts. The reactions involve iminyl-radical-mediated intramolecular hydrogen atom transfer as the key step, with the iminyl radicals being generated via copper-effected N-O cleavage. The reaction afforded 3,4-dihydro-2H-pyrroles under the conditions of [Cu(DPEphos)(bcp)]PF₆ and DABCO, while γ-pentafluorobenzoyloxy ketones were produced predominantly when [Cu(dpp)₂]PF₆ and InCl₃·4H₂O were used as catalysts.