Search

Whether tetra‐tert‐butyl‐s‐indacene is a symmetric D2h structure or a bond‐alternating C2h structure remains a standing puzzle. Close agreement between experimental and computed proton chemical shifts based on minima structures optimized at the M06‐2X, ωB97X‐D, and M11 levels confirm a bond‐localized C2h symmetry, which is consistent with the expected strong antiaromaticity of TtB‐s‐indacene. [ABSTRACT FROM AUTHOR]

The phosphaquinolinone scaffold has been previously studied as a modular core for a variety of fluorescent species where use of substituent effects has focused on increasing or decreasing electron density in the core rings. We now report the synthesis and analysis of several pyridine‐containing phosphaquinolinone species exhibiting notable linear conjugation from the aryl‐substituent to electron‐withdrawing pyridyl nitrogen. Varying the nature of the aryl substituent from electron‐withdrawing to electron‐donating leads to the generation of an internal charge‐transfer (ICT) band in the absorbance spectrum, which becomes the dominant absorbance in terms of intensity in the most electron‐rich ‐NMe2 example. This heterocycle exhibits improved photophysical properties compared to others in the set including high quantum yield and considerably red‐shifted emission. The enhanced ICT can be observed in the X‐ray data where a rare example of molecule co‐planarity is observed. Computational data show increased localization of negative charge on the pyridyl nitrogen as the electron‐donating character of the aryl‐substituent increases. [ABSTRACT FROM AUTHOR]

Coordinating solvents are commonly employed as ancillary ligands to stabilize late transition metal complexes and are conventionally considered to have little effect on the reaction products. Our work identifies that the presence of ancillary ligand in Pd‐diimine catalyzed polymerizations of α‐olefins can drastically alter reactivity. The addition of different amounts of acetonitrile allows for switching between distinct reaction modes: isomerization–polymerization with high branching (0 equiv.), regular chain‐walking polymerization (1 equiv.), and alkene isomerization with no polymerization (>20 equiv.). Optimization of the isomerization reaction mode led to a general set of conditions to switch a wide variety of diimine complexes into efficient alkene isomerization catalysts, with catalyst loading as low as 0.005 mol %. [ABSTRACT FROM AUTHOR]

The selective modification of nitrogen heteroaromatics enables the development of new chemical tools and accelerates drug discovery. While methods that focus on expanding or contracting the skeletal structures of heteroaromatics are emerging, methods for the direct exchange of single core atoms remain limited. Here, we present a method for 14 N → 15 N isotopic exchange for several aromatic nitrogen heterocycles. This nitrogen isotope transmutation occurs through activation of the heteroaromatic substrate by triflylation of a nitrogen atom, followed by a ring-opening/ring-closure sequence mediated by 15 N-aspartate to effect the isotopic exchange of the nitrogen atom. Key to the success of this transformation is the formation of an isolable 15 N-succinyl intermediate, which undergoes elimination to give the isotopically labeled heterocycle. These transformations occur under mild conditions in high chemical and isotopic yields.

Whether tetra-tert-butyl-s-indacene is a symmetric D 2h structure or a bond-alternating C 2h structure remains a standing puzzle. Close agreement between experimental and computed proton chemical shifts based on minima structures optimized at the M06-2X, ωB97X-D, and M11 levels confirm a bond-localized C 2h symmetry, which is consistent with the expected strong antiaromaticity of TtB-s-indacene., (© 2023 Wiley-VCH GmbH.)

The phosphaquinolinone scaffold has been previously studied as a modular core for a variety of fluorescent species where use of substituent effects has focused on increasing or decreasing electron density in the core rings. We now report the synthesis and analysis of several pyridine-containing phosphaquinolinone species exhibiting notable linear conjugation from the aryl-substituent to electron-withdrawing pyridyl nitrogen. Varying the nature of the aryl substituent from electron-withdrawing to electron-donating leads to the generation of an internal charge-transfer (ICT) band in the absorbance spectrum, which becomes the dominant absorbance in terms of intensity in the most electron-rich -NMe 2 example. This heterocycle exhibits improved photophysical properties compared to others in the set including high quantum yield and considerably red-shifted emission. The enhanced ICT can be observed in the X-ray data where a rare example of molecule co-planarity is observed. Computational data show increased localization of negative charge on the pyridyl nitrogen as the electron-donating character of the aryl-substituent increases., (© 2023 Wiley-VCH GmbH.)

Inclusion of a second nitrogen atom in the aromatic core of phosphorus-nitrogen (PN) heterocycles results in unexpected tautomerization to a nonaromatic form. This tautomerization, initially observed in the solid state through X-ray crystallography, is also explained by computational analysis. We prepared an electron deficient analogue (2 e) with a fluorine on the pyridine ring and showed that the weakly basic pyridine resisted tautomerization, providing key insights to why the transformation occurs. To study the difference in solution vs. solid-state heterocycles, alkylated analogues that lock in the quinoidal tautomer were synthesized and their different 1 H NMR and UV/Vis spectra studied. Ultimately, we determined that all heterocycles are the aromatic tautomer in solution and all but 2 e switch to the quinoidal tautomer in the solid state. Better understanding of this transformation and under what circumstances it occurs suggest future use in a switchable on/off hydrogen-bond-directed receptor that can be tuned for complementary hydrogen bonding., (© 2022 Wiley-VCH GmbH.)

We examine the effects of fusing two benzofurans to s-indacene (indacenodibenzofurans, IDBFs) and dicyclopenta[b,g]naphthalene (indenoindenodibenzofurans, IIDBFs) to control the strong antiaromaticity and diradical character of these core units. Synthesis via 3-functionalized benzofuran yields syn-IDBF and syn-IIDBF. syn-IDBF possesses a high degree of paratropicity, exceeding that of the parent hydrocarbon, which in turn results in strong diradical character for syn-IIDBF. In the case of the anti-isomers, synthesized via 2-substituted benzofurans, these effects are decreased; however, both derivatives undergo an unexpected ring-opening reaction during the final dearomatization step. All the results are compared to the benzothiophene-fused analogues and show that the increased electronegativity of oxygen in the syn-fused derivatives leads to enhancement of the antiaromatic core causing greater paratropicity. For syn-IIDBF increased diradical character results from rearomati-zation of the core naphthalene unit in order to relieve this paratropicity., (© 2021 Wiley-VCH GmbH.)

Hydrogen bonding principles are at the core of supramolecular design. This overview features a discussion relating molecular structure to hydrogen bond strengths, highlighting the following electronic effects on hydrogen bonding: electronegativity, steric effects, electrostatic effects, π-conjugation, and network cooperativity. Historical developments, along with experimental and computational efforts, leading up to the birth of the hydrogen bond concept, the discovery of nonclassical hydrogen bonds (C-H…O, O-H…π, dihydrogen bonding), and the proposal of hydrogen bond design principles (e.g., secondary electrostatic interactions, resonance-assisted hydrogen bonding, and aromaticity effects) are outlined. Applications of hydrogen bond design principles are presented., Competing Interests: CONFLICT OF INTEREST The authors declare no potential conflict of interest.

The ability to alter optoelectronic and magnetic properties of molecules at a late stage in their preparation is in general a nontrivial feat. Here, we report the late-stage oxidation of benzothiophene-fused indacenes and dicyclopentanaphthalenes to their corresponding sulfone derivatives. We find that while such modifications increase the highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) energy gap to a small degree, other properties such as HOMO and LUMO energy levels, molecule paratropicity, and singlet-triplet energy gaps are influenced to a greater degree. The most surprising finding is a change of the bond alternation pattern within the s -indacene core of the sulfones. Computations corroborate the experimental findings and offer plausible explanations for these changes in molecular properties.