Recent Advances in Halogen Bonded Assemblies with Resorcin[4]arenes

Photocatalyst- and Transition Metal-Free Light-Induced Borylation Reactions

The increasing global warming concerns have propelled a surge in the demand for sustainable energy sources within the domain of synthetic organic chemistry. A particularly prominent area of research has been the development of mild synthetic strategies for generating heterocyclic compounds. Heterocyclic compounds containing boron have notably risen to prominence as pivotal reagents in a myriad of organic transformations, showcasing their wide-ranging applicability. This comprehensive review is aimed at collecting the literature pertaining to borylation reactions induced by light, specifically focusing on photocatalyst-free and transition metal-free methodologies. The central emphasis is on delving into selective mechanistic investigations. The amalgamation and analysis of these research insights elucidate the substantial potential inherent in eco-friendly approaches for synthesizing heterocyclic compounds, thus propelling the landscape of sustainable organic chemistry.

Investigation of the Effect of Solvents on the Synthesis of Aza-flavanone from Aminochalcone Facilitated by Halogen Bonding

It has been recognized that CBr4 can give rise to a noncovalent interaction known as halogen bond (XB). CBr4 was found to catalyze, in terms of XB formation, the transformation of 2'-aminochalcone to aza-flavanone through an intramolecular Michael addition reaction. The impact of XB and the resulting yield of aza-flavanone exhibited a pronounced dependence on the characteristics of the solvent. Notably, yields of 88% in ethanol and 33% in DMSO were achieved, while merely a trace amount of the product was detected in benzene. In this work, we use a computational modeling study to understand this variance in yield. The reaction is modeled at the level of density functional theory (based on the M06-2X exchange-correlation functional) with all-electron basis sets of triple-ζ quality. Grimme's dispersion correction is incorporated to account for the noncovalent interactions accurately. Harmonic frequency calculations are carried out to establish the character of the optimized structures (minimum or saddle point). Our calculations confirm the formation of an XB between CBr4 and the reacting species and its role in lowering the activation energy barrier. Stronger orbital interactions and significant lowering of the steric repulsion were found to be important in lowering the activation barrier. The negligible yield in the nonpolar solvent benzene may be attributed to the high activation energy as well as the inadequate stabilization of the zwitterionic intermediate. In ethanol, a protic solvent, additional H-bonding contributes to further lowering of the activation barrier and better stabilization of the zwitterionic intermediate. The combined effects of solvent polarity, XB, and H-bond are likely to give rise to an excellent yield of aza-flavanone in ethanol.

Stereoselective Synthesis of Z-Styryl Sulfides from Nucleophilic Addition of Arylacetylenes and Benzyl Thiols

The stereoselective synthesis of Z-anti-Markovnikov styryl sulfides via an anionic thiolate-alkyne addition reaction was achieved when the terminal alkynes and benzyl mercaptans were reacted using tBuOLi (0.5 equiv) in EtOH under ambient conditions. Exclusive stereoselectivity (ca. 100%) was achieved by stereoelectronic control via anti-periplanar and anti-Markovnikov addition of benzylthiolates to phenylacetylenes. Solvolysis of lithium thiolate ion pairs in ethanol significantly suppresses the competing formation of the E-isomer. A remarkable enhancement of the Z-selectivity under a longer reaction time was observed.

The "Nitrogen Effect": Complexation with Macrocycles Potentiates Fused Heterocycles to Form Halogen Bonds in Competitive Solvents

Weak intermolecular forces are typically very difficult to observe in highly competitive polar protic solvents as they are overwhelmed by the quantity of competing solvent. This is even more challenging for three-component ternary assemblies of pure organic compounds. In this work, we overcome these complications by leveraging the binding of fused aromatic N-heterocycles in an open resorcinarene cavity to template the formation of a three-component halogen-bonded ternary assembly in a protic polar solvent system.

Modulating the Inclusive and Coordinating Ability of Thiacalix[4]arene and Its Antenna Effect on Yb3-Luminescence via Upper-Rim Substitution+

The present work introduces the series of thiacalix[4]arenes (H₄L) bearing different upper-rim substituents (R = H, Br, NO₂) for rational design of ligands providing an antenna-effect on the NIR Yb³⁺-centered luminescence of their Yb³⁺ complexes. The unusual inclusive self-assembly of H₃L⁻ (Br) through Br…π interactions is revealed through single-crystal XRD analysis. Thermodynamically favorable formation of dimeric complexes [2Yb³⁺:2HL³⁻] leads to efficient sensitizing of the Yb³⁺ luminescence for H₄L (Br, NO₂), while poor sensitizing is observed for ligand H₄L (H). X-ray analysis of the single crystal separated from the basified DMF solutions of YbCl₃ and H₄L(NO₂) has revealed the transformation of the dimeric complexes into [4Yb³⁺:2L⁴⁻] ones with a cubane-like cluster structure. The luminescence characteristics of the complexes in the solutions reveal the peculiar antenna effect of H₄L(R = NO₂), where the triplet level at 567 nm (17,637 cm⁻¹) arisen from ILCT provides efficient sensitizing of the Yb³⁺ luminescence.

t BuOLi-promoted terminal alkyne functionalizations by aliphatic thiols and alcohols

Selective radical addition to terminal alkynes is always a difficult task to achieve because it gives a mixture of stereo- and regioisomers. Herein we describe the selective addition of aliphatic thiols or alcohols to N-phenylpropiolamides (terminal alkynes) using lithium tert-butoxide (^tBuOLi) in ethanol as a promoter. Mechanistically, it has been shown that the reaction proceeded through the generation of a thiyl radical intermediate, and the amide group in N-phenylpropiolamide could help in the activation of the alkyne, which led to thioacetalization via the formation of a (Z)-selective anti-Markovnikov vinyl sulfide. The (Z)-selectivity during the formation of vinyl sulfides was controlled by an intramolecular sulfur⋯oxygen interaction.

Dimeric iodine(i) and silver(i) cages from tripodal N-donor ligands via the [N–Ag–N]+ to [N–I–N]+ cation exchange reaction

Complexation of tripodal ligands with silver(I) salts generated M3L2 cage complexes that encapsulated anions within their cavities. Subsequent [N–Ag–N]+ to [N–I–N]+ cation exchange with I2 resulted in the corresponding halogen-bonded iodine(I) cages.

Two Giant Calixarene-Like Polyoxoniobate Nanocups {Cu12 Nb120 } and {Cd16 Nb128 } Built from Mixed Macrocyclic Cluster Motifs

Cup-shaped molecules are of great interest due to their appealing architectures and properties. Compared with widely studied calixarenes, polyoxometalate-based cup-shaped molecules currently remain a virgin land waiting for exploration. In this work, we report the first discovery of two giant cup-shaped inorganic-organic hybrid polyoxoniobates (PONbs) of {Cu₁₂ Nb₁₂₀ } and {Cd₁₆ Nb₁₂₈ }. The former integrates three tricyclic Nb₂₄ clusters and a hexacyclic Nb₄₈ cluster into a cup-shaped molecule via a Cu₁₂ metallacalixarene, while the latter unifies two tricyclic Nb₂₄ clusters and a brand-new pentacyclic Nb₄₀ cluster into another cup-shaped molecule via a hybrid Cd₁₆ unit. With 132 and 144 metal centers, {Cu₁₂ Nb₁₂₀ } and {Cd₁₆ Nb₁₂₈ } show the largest two inorganic-organic hybrid PONbs known to date.

Disulfide metathesis via sulfur⋯iodine interaction and photoswitchability

The idea of constitutional dynamic chemistry (CDC) and dynamic combinatorial chemistry (DCC) is widespread in the literature using the chemistry of disulfides. The synthesis of unsymmetrical diaryl disulfides is challenging due to the presence of a weak S-S bond. We report herein the synthesis of unsymmetrical diaryl disulfides from two symmetrical disulfides via a cross-metathesis reaction which was controlled by a weak sulfur⋯iodine (S⋯I) interaction. The unsymmetrical disulfides were stable in acetonitrile solution in the presence of N-iodosuccinimide (NIS), and found to be reversibly photoswitchable to the symmetrical disulfides under visible light irradiation.