Donor−Acceptor Chemistry at Heavy Chalcogen Centers

Stereoselective Entry into α,α'-C-Oxepane Scaffolds through a Chalcogen Bonding Catalyzed Strain-Release C-Septanosylation Strategy

The utility of unconventional noncovalent interactions (NCIs) such as chalcogen bonding has lately emerged as a robust platform to access synthetically difficult glycosides stereoselectively. Herein, we disclose the versatility of a phosphonochalcogenide (PCH) catalyst to facilitate access into the challenging, but biologically interesting 7-membered ring α,α'-C-disubstituted oxepane core through an α-selective strain-release C-glycosylation. Methodically, this strategy represents a switch from more common but entropically less desired macrocyclizations to a thermodynamically favored ring-expansion approach. In light of the general lack of stereoselective methods to access C-septanosides, a remarkable palette of silyl-based nucleophiles can be reliably employed in our method. This include a broad variety of useful synthons, such as easily available silyl-allyl, silyl-enol ether, silyl-ketene acetal, vinylogous silyl-ketene acetal, silyl-alkyne and silylazide reagents. Mechanistic investigations suggest that a mechanistic shift towards an intramolecular aglycone transposition involving a pentacoordinate silicon intermediate is likely responsible in steering the stereoselectivity.

Harnessing Multistep Chalcogen Bonding Activation in the α-Stereoselective Synthesis of Iminoglycosides

The use of noncovalent interactions (NCIs) has received significant attention as a pivotal synthetic handle. Recently, the exploitation of unconventional NCIs has gained considerable traction in challenging reaction manifolds such as glycosylation due to their capacity to facilitate entry into difficult-to-access sugars and glycomimetics. While investigations involving oxacyclic pyrano- or furanoside scaffolds are relatively common, methods that allow the selective synthesis of biologically important iminosugars are comparatively rare. Here, we report the capacity of a phosphonochalcogenide (PCH) to catalyze the stereoselective α-iminoglycosylation of iminoglycals with a wide array of glycosyl acceptors with remarkable protecting group tolerance. Mechanistic studies have illuminated the counterintuitive role of the catalyst in serially activating both the glycosyl donor and acceptor in the up/downstream stages of the reaction through chalcogen bonding (ChB). The dynamic interaction of chalcogens with substrates opens up new mechanistic opportunities based on iterative ChB catalyst engagement and disengagement in multiple elementary steps.

Exploiting π and Chalcogen Interactions for the β-Selective Glycosylation of Indoles through Glycal Conformational Distortion

Harnessing unconventional noncovalent interactions (NCIs) is emerging as a formidable synthetic approach in difficult-to-access glycosidic chemical space. C-Glycosylation, in particular, has gained a flurry of recent attention. However, most reported methods are restricted to the relatively facile access to α-C-glycosides. Herein, we disclose a β-stereoselective glycosylation of indoles by employing a phosphonoselenide catalyst. The robustness of this protocol is exemplified by its amenability for reaction at both the indolyl C- and N- reactivity sites. In contrast to previous reports, in which the chalcogens were solely involved in Lewis acidic activation, our mechanistic investigation unraveled that the often neglected flanking aromatic substituents of phosphonoselenides can substantially contribute to catalysis by engaging in π-interactions. Computations and NMR spectroscopy indicated that the chalcogenic and aromatic components of the catalyst can be collectively exploited to foster conformational distortion of the glycal away from the usual half-chair to the boat conformation, which liberates the convex β-face for nucleophilic attack.

Cooperative Bifurcated Chalcogen Bonding and Hydrogen Bonding as Stereocontrolling Elements for Selective Strain-Release Septanosylation

The exploitation of noncovalent interactions (NCIs) is emerging as a vital handle in tackling broad stereoselectivity challenges in synthesis. In particular, there has been significant recent interest in the harnessing of unconventional NCIs to surmount difficult selectivity challenges in glycosylations. Herein, we disclose the exploitation of an unconventional bifurcated chalcogen bonding and hydrogen bonding (HB) network, which paves the way for a robust catalytic strategy into biologically useful seven-membered ring sugars. Through 13C nuclear magnetic resonance (NMR) in situ monitoring, NMR titration experiments, and density functional theory (DFT) modeling, we propose a remarkable contemporaneous activation of multiple functional groups consisting of a bifurcated chalcogen bonding mechanism working hand-in-hand with HB activation. Significantly, the ester moiety installed on the glycosyl donor is critical in the establishment of the postulated ternary complex for stereocontrol. Through the 13C kinetic isotopic effect and kinetic studies, our data corroborated that a dissociative SNi-type mechanism forms the stereocontrolling basis for the excellent α-selectivity.

Halide Complexes of 5,6-Dicyano-2,1,3-Benzoselenadiazole with 1 : 4 Stoichiometry: Cooperativity between Chalcogen and Hydrogen Bonding

The [M4 -Hal]- (M=the title compound; Hal=Cl, Br, and I) complexes were isolated in the form of salts of [Et4 N]+ cation and characterized by XRD, NMR, UV-Vis, DFT, QTAIM, EDD, and EDA. Their stoichiometry is caused by a cooperative interplay of σ-hole-driven chalcogen (ChB) and hydrogen (HB) bondings. In the crystal, [M4 -Hal]- are connected by the π-hole-driven ChB; overall, each [Hal]- is six-coordinated. In the ChB, the electrostatic interaction dominates over orbital and dispersion interactions. In UV-Vis spectra of the M+[Hal]- solutions, ChB-typical and [Hal]- -dependent charge-transfer bands are present; they reflect orbital interactions and allow identification of the individual [Hal]- . However, the structural situation in the solutions is not entirely clear. Particularly, the UV-Vis spectra of the solutions are different from the solid-state spectra of the [Et4 N]+ [M4 -Hal]- ; very tentatively, species in the solutions are assigned [M-Hal]- . It is supposed that the formation of the [M4 -Hal]- proceeds during the crystallization of the [Et4 N]+ [M4 -Hal]- . Overall, M can be considered as a chromogenic receptor and prototype sensor of [Hal]- . The findings are also useful for crystal engineering and supramolecular chemistry.

Chalcogen⋅⋅⋅π Bonding Catalysis

While the presence of sulfur⋅⋅⋅π bonding interaction is a general phenomenon in the biological systems, the exploitation of this noncovalent force in a chemical process yet remains elusive. Herein, we describe the concept of chalcogen⋅⋅⋅π bonding catalysis that activates molecules of π systems through the interaction between chalcogen and π-electron cloud. The proof-of-concept studies using a vinylindole-based Diels-Alder benchmark reaction demonstrate that S⋅⋅⋅π and Se⋅⋅⋅π bonding interaction can drive the cycloaddition reaction efficiently. Experimental results suggest that a simultaneously double Se⋅⋅⋅π bonding interaction directs the stereoselectivity in this cycloaddition process.

Synthetic and structural investigations of bis(N-alkyl-benzoselenadiazolium) cations

A variety of supramolecular structures is formed by selenadiazolium cations linked by hydrocarbon bridges.

Donor-Acceptor Complexes between 1,2,5-Chalcogenadiazoles (Te, Se, S) and the Pseudohalides CN- and XCN- (X=O, S, Se, Te)

Donor-acceptor (D-A) complexes between 3,4-dicyano-1,2,5-chalcogenadiazoles [chalcogen=Te (1 a), Se (1 b), S (1 c)] and the pseudohalides CN^- and XCN^- (X=O, S, Se, Te) were studied experimentally and theoretically. For 1 a, they were isolated as [K(18-crown-6)][1 a-CN] (2), [K(18-crown-6)][1 a-NCO] (3), [K(18-crown-6)][1 a-SCN] (4), [K(18-crown-6)][1 a-SeCN] (5), and [K][1 a-NCSe] (6) and characterized by X-ray diffraction (XRD), UV/Vis and NMR spectroscopy, and DFT and QTAIM calculations. For 1 b and 1 c, the complexes were not isolated due to unfavorable thermodynamics. In all isolated complexes, the D-A bonds, stabilized by negative hyperconjugation, were longer than the sum of the covalent radii and shorter than the sum of the van der Waals radii of the bonded atoms. In mixtures of 1 a, F^- , and SeCN^- , the complex [1 a-F]^- was selectively formed in accordance with thermodynamics. The reaction of 1 a with SeCN^- and the cyclic trimeric perfluoro-ortho-phenylene mercury afforded the complex [K(18-crown-6)][SCN]⋅(o-C₆ F₄ Hg)₃ , which was characterized by XRD.

NHCs in Main Group Chemistry

Since the discovery of the first stable N-heterocyclic carbene (NHC) in the beginning of the 1990s, these divalent carbon species have become a common and available class of compounds, which have found numerous applications in academic and industrial research. Their important role as two-electron donor ligands, especially in transition metal chemistry and catalysis, is difficult to overestimate. In the past decade, there has been tremendous research attention given to the chemistry of low-coordinate main group element compounds. Significant progress has been achieved in stabilization and isolation of such species as Lewis acid/base adducts with highly tunable NHC ligands. This has allowed investigation of numerous novel types of compounds with unique electronic structures and opened new opportunities in the rational design of novel organic catalysts and materials. This Review gives a general overview of this research, basic synthetic approaches, key features of NHC-main group element adducts, and might be useful for the broad research community.

Building new discrete supramolecular assemblies through the interaction of iso-tellurazole N-oxides with Lewis acids and bases

The supramolecular macrocycles spontaneously assembled by iso-tellurazole N-oxides are stable towards Lewis bases as strong as N-heterocyclic carbenes (NHC) but readily react with Lewis acids such as BR₃ (R = Ph, F). The electron acceptor ability of the tellurium atom is greatly enhanced in the resulting O-bonded adducts, which consequently enables binding to a variety of Lewis bases that includes acetonitrile, 4-dimethylaminopyridine, 4,4'-bipyridine, triphenyl phosphine, a N-heterocyclic carbene and a second molecule of iso-tellurazole N-oxide.

Synthetic, structural, and computational investigations of N-alkyl benzo-2,1,3-selenadiazolium iodides and their supramolecular aggregates

Alkyl benzo-2,1,3-selenadiazolium cations are conveniently prepared by cyclo-condensation ofN-alkyl-phenylenediamines with selenous acid.

Donor-acceptor chemistry in the main group

This Perspective article summarizes recent progress from our laboratory in the isolation of reactive main group species using a general donor-acceptor protocol. A highlight of this program is the use of carbon-based donors in combination with suitable Lewis acidic acceptors to yield stable complexes of parent Group 14 element hydrides (e.g. GeH2 and H2SiGeH2). It is anticipated that this strategy could be extended to include new synthetic targets from throughout the Periodic Table with possible applications in bottom-up materials synthesis and main group element catalysis envisioned.

Phosphine complexes of lone pair bearing Lewis acceptors

An overview of the synthesis, structures and reaction chemistry of coordination complexes featuring an acceptor with at least one lone pair and at least one phosphine donor is presented. One or more examples of complexes have been structurally-characterized for the majority of p-block elements but few are known for most elements. The unusual condition of a p-block element centre accommodating a lone pair of electrons and offering a low energy LUMO gives the element centre the potential to behave as both a Lewis acid and a Lewis Base. The structural diversity and reactivity of the phosphine complexes highlights new directions in main group chemistry and by comparison with transition metal coordination chemistry, the featured complexes demonstrate significant configurational and stereochemical flexibility. Ligand exchange, oxidation and reduction chemistry at the lone pair bearing acceptor centre reveals unusual reactivity and an interesting class of ligands and inorganic reagents, with new possibilities for catalysis or small molecule activation.

Breathing some new life into an old topic: chalcogen-nitrogen π-heterocycles as electron acceptors

Recent progress in the design, synthesis and characterization of chalcogen-nitrogen π-heterocycles, mostly 1,2,5-chalcogenadiazoles (chalcogen: S, Se and Te) and their fused derivatives, possessing positive electron affinity is discussed together with their use in preparation of charge-transfer complexes and radical-anion salts-candidate building blocks of molecule-based electrical and magnetic functional materials.

The role of the Lewis acid−base properties in the supramolecular association of 1,2,5-chalcogenadiazoles

The secondary bonding interactions that link the supramolecular structures assembled by 1,2,5-chalcogenadiazoles were analyzed through explicit orthogonalization of molecular orbitals (NBO), topological analysis of the electron density (AIM), and the electron localization function (ELF). The results of these analyses are consistent with a bonding description that attributes important covalent and electrostatic character to these interactions. Application of these analyses to the individual molecules highlighted the structural features from which each of those contributions originates, namely the polarity and modest strength of the E–N bond. Both of these effects increase along the series S, Se, Te. Perturbations to the heterocycle electronic structure that result in a weaker and more polar E–N bond cause an increase in the Lewis acidity at the chalcogen centre, which in turn leads to stronger secondary bonding interactions with Lewis bases. Additionally, the contribution of dispersion forces is not negligible and is most important in the case of sulfur.