Experimental Assessment of the π-Acidity of Anionic and Neutral Malonate-Derived N-Heterocyclic Carbenes

Modular Synthesis of Azines Bearing a Guanidine Core from N-Heterocyclic Carbene (NHC)-Derived Selenoureas and Diazo Reagents

N-Heterocyclic carbene (NHC)-derived selenoureas comprise a fundamentally important class of NHC derivatives, with key applications in coordination chemistry and the determination of NHC electronic properties. Considering the broad reactivity of chalcogen-containing compounds, it is surprising to note that the use of NHC-derived selenoureas as organic synthons remains essentially unexplored. The present contribution introduces a novel, straightforward transformation leading to azines bearing a guanidine moiety, through the reaction of a wide range of NHC-derived selenoureas with commercially available diazo compounds, in the presence of triphenylphosphine. This transformation offers a new approach to such products, having biological, materials chemistry, and organic synthesis applications. The guanidine-bearing azines are obtained in excellent yields, with all manipulations taking place in air. A reaction mechanism is proposed, based on both experimental mechanistic findings and density functional theory (DFT) calculations. A one-pot, multicomponent transesterification reaction between selenoureas, α-diazoesters, alcohols, and triphenylphosphine was also developed, providing highly functionalized azines.

Diamidocarbene-Based Thiele and Tschitschibabin Hydrocarbons: Carbonyl Functionalized Kekulé Diradicaloids

Herein, we report diamidocarbene (DAC)-based Thiele and Tschitschibabin hydrocarbons, diradicaloids that contain four carbonyl/amido functional groups. The impact of two different π-conjugated spacers, p-phenylene vs p,p'-biphenylene, has been realized. The quantum chemical calculations suggest diamidocarbene (DAC)-based Thiele hydrocarbon (p-phenylene bridged) closed-shell singlet is the ground state, whereas for the diamidocarbene (DAC)-based Tschitschibabin hydrocarbon (p,p'-biphenylene bridged), open-shell singlet is the ground state. The influence of two different π-conjugated spacers also has been reflected in their UV-vis spectra. To gain more information on the diamidocarbene (DAC)-based Thiele and Tschitschibabin hydrocarbons, we have also carried out cyclic voltammetry investigations along with UV-vis-NIR-spectroelectrochemical studies of their corresponding 2-e oxidized product.

Green synthesis of selenium based N-heterocyclic carbene compounds; structural, in-vitro anticancer and molecular docking studies

Benzimidazolium salts (3-6) were synthesized as stable N-Heterocyclic Carbene (NHC) precursors and their selenium-NHC compounds/Selenones (7-10) were prepared using water as a solvent. Characterization of each of the synthesized compounds was carried out by various analytical and spectroscopic (FT-IR, ¹H-, ¹³C NMR) methods. X-ray crystallographic analyses of single crystals obtained for salts 3 and 5 were carried out. Synthesized salts and their Se-NHCs were tested in-vitro for their anticancer potential against Cervical Cancer Cell line from Henrietta Lacks (HeLa), Breast cancer cell line (MDA-MB-231), Adenocarcinoma cell line (A549) and human normal endothelial cell line (EA.hy926). MTT assay was used for analysis and compared with standard drug 5-flourouracil. Benzimidazolium salts (3-6) and their selenium counter parts (7-10) were found potent anticancer agents. Salt 3-5 were found to be potent anticancer against HeLa with IC₅₀ values 0.072, 0.017 and 0.241 μM, respectively, which are less than standard drug (4.9 μM). The Se-NHCs (7-10) had also shown significant anticancer potential against HeLa with IC₅₀ values less than standard drug. Salts 3, 4 against EA.hy926, compounds 3,5,6, and 10 against MDA-MB-321, and compounds 4, 10 against A-549 cell line were found more potent anticancer agents with IC₅₀ values less than standard drug. Molecular docking for (7-10) showed their good anti-angiogenic potential having low binding energy and significant inhibition constant values with VEGFA (vascular endothelial growth factor), EGF (human epidermal growth factor), COX1 (cyclooxygenase-1) and HIF (hypoxia inducible factor).

Zero-, one-, two- and three-dimensional supramolecular architectures sustained by Se…O chalcogen bonding: A crystallographic survey

The Cambridge Structural Database was evaluated for crystals containing Se…O chalcogen bonding interactions. These secondary bonding interactions are found to operate independently of complementary intermolecular interactions in about 13% of the structures they can potentially form. This number rises significantly when more specific interactions are considered, e.g. Se…O(carbonyl) interactions occur in 50% of cases where they can potentially form. In about 55% of cases, the supramolecular assemblies sustained by Se…O(oxygen) interactions are one-dimensional architectures, with the next most prominent being zero-dimensional assemblies, at 30%.

Air-Stable Oxyallyl Patterns and a Switchable N-Heterocyclic Carbene

Oxyallyl derivatives are typically elusive compounds. Even recently reported "stabilized" 1,3-diaminooxyallyl species are still highly reactive and have short lifetimes at room temperature. Herein, we report the synthesis and preliminary study of mesoionic pyrimidine derivatives that feature 1,3-bis(dimethylamino)oxyallyl patterns with an unprecedented level of stabilization. The latter are not only insensitive towards air and moisture, but they are also compatible with the formation of an ancillary stable N-heterocyclic carbene moiety. As the oxyallyl pattern is proton-responsive, it allows the reversible switching of the electronic properties of the carbene, as a ligand.

N-Cyclopropenio-imidazol-2-ylidene: An N-heterocyclic carbene bearing an N-cationic substituent

A cationic NHC 1+ bearing an N-bound 2,3-bis(diisopropylamino)cyclopropenium group is reported. From an easily available dicationic imidazolium precursor, the coordination abilities and stereo-electronic properties of 1+ are evaluated by the formation of Pd(ii), Rh(i) and Au(i) complexes. The cationic gold(i) complex is implemented in representative intramolecular Au(i)-catalyzed cyclizations.

Purine-substituted imidazolium mesomeric betaines and their tautomeric N-heterocyclic carbenes. Formation of a cyclic borane adduct

6-Chloropurine and 2,6-dichloropurine were reacted with N1-substituted imidazoles to give purin-6-yl substituted imidazolium salts, respectively. Deprotonation of the 1-methylimidazolium derivative resulted in the formation of the corresponding stable conjugated mesomeric betaine, whereas the 1-phenyl,- 1-vinyl- and 1-(2-hydroxyethyl) derivatives proved to be unstable. In situ generation of the mesomeric betaines by caesium carbonate in the presence of sulfur and selenium, however, gave the thiones and the selenones of the tautomeric purine-substituted imidazol-2-ylidene, respectively. Its anionic N-heterocyclic carbene was formally trapped by reaction with triethylborane at high temperatures as a cyclic boron adduct which is the first representative of a new heterocyclic ring system. DFT calculations gained insight into the electronic properties of the N-heterocyclic carbenes substituted by π-electron donators. Results of a single crystal X-ray analysis of the boron adduct are presented.