Antiaromaticity Gain Activates Tropone and Nonbenzenoid Aromatics as Normal-Electron-Demand Diels-Alder Dienes

Perturbing Pentalene: Aromaticity and Antiaromaticity in a Non-Alternant Polycyclic Aromatic Hydrocarbon and BN-Heteroanalogues

Pentalene (C8H6) and NN- and BB-bridged heterocyclic analogues (BN)4H6, derived by replacement of CC pairs with BN, are taken as paradigms for tuning of ring-current (anti)aromaticity by variation of π ${\pi }$ charge, electronegativity and substitution pattern. Ab initio calculation of maps for the π ${\pi }$ current density induced in these model systems by a perpendicular external magnetic field exhibits the full range of tropicity, from diatropic aromatic to nonaromatic to paratropic antiaromatic, with a ready rationalisation in terms of an orbital model. Further calculations on systems of varying charge in which these motifs are embedded in extended PAH systems with naphthalene and phenanthrene 'clamps' show promise for switching between current patterns and related opto-electronic properties. Particular sensitivity to charge is found for the experimentally accessible NN-bridged heteropentalene hybrids.

Substrate-Controlled [8 + 3] Cycloaddition of Tropsulfimides and Tropones with Zwitterionic Allenyl Palladium Species Derived from Vinylidenecyclopropane-diesters

A palladium-catalyzed regioselective [8 + 3] cycloaddition of tropsulfimides and tropones with vinylidenecyclopropane-diesters (VDCP-diesters) has been disclosed in this paper, affording decahydro-1H-cyclohepta[b]pyridine derivatives bearing an allene moiety or decahydro-1H-cyclohepta[b]pyran derivatives having a conjugated diene unit in moderate to good yields. The reactions proceed through a zwitterionic allenyl palladium species derived from VDCP-diesters. The substrate scopes have been investigated and the plausible reaction mechanisms have also been proposed according to the previous work, the first captured zwitterionic Pd-allenyl intermediate, and control experiments.

Antiaromaticity of Cycloheptatrienyl Anions: Structure, Acidity, and Magnetic Properties

Investigations of the nature and degree of antiaromaticity of cycloheptatrienyl anion derivatives using both experimental and computational tools are presented. The ground state of cycloheptatrienyl anion in the gas phase is triplet, planar and Baird-aromatic. In DMSO, it assumes a singlet distorted allylic form with a paratropic ring current. The other derivatives in both phases assume either allylic or diallylic conformations depending on the substituent pattern. A combination of experimental and computational methods was used to determine the pKa values of 16 derivatives in DMSO, which ranged from 36 to -10.7. We revealed that the stronger stabilization of the anionic system, which correlates with acidity, does not necessarily imply a lower degree of antiaromaticity in terms of magnetic properties. Conversely, the substitution pattern first affects the geometry of the ring through the bulkiness of the substituents and their better conjugation with a more distorted system. Consequently, the distortion reduces the cyclic conjugation in the π-system and thereby decreases the paratropic current in a magnetic field, which manifests itself as a decrease in the NICS. The triplet-state geometries and magnetic properties are nearly independent on the substitution pattern, which is typical for simple aromatic systems.

Quantum Chemical Study of the Cycloaddition Reaction of Tropone with 1,1-Diethoxyethene Catalyzed by B(C6F5)3 or BPh3

Cycloaddition reaction of tropone with 1,1-diethoxyethene catalyzed by Lewis acid (LA), B(C6F5)3 or BPh3, was examined by using ωB97X-D-level density functional theory (DFT) calculations. In the absence of LA, the reaction proceeds in a stepwise fashion to form two chemical bonds, first between the C2 atom in tropone and the C2 atom in ethene and then between the C5 atom in the former and the C1 atom in the latter. When B(C6F5)3 is attached to the O atom in tropone, the C5 atom in tropone is attacked preferentially by the C1 atom in ethene in the second stage. The attack of the O atom in tropone is shown to be less likely; thus, the [4 + 2] addition is favored in the B(C6F5)3-catalyzed reaction. In contrast, the attack of the O atom in the BPh3-attached tropone to the C1 atom in ethene is preferred over the attack of the C5 atom, indicating that the [8 + 2] cycloaddition instead of the [4 + 2] cycloaddition proceeds in the BPh3-catalyzed reaction. Whether the C1 atom in ethene is attacked by C5 or by O in the second bond formation step is shown in this study to be governed mainly by the nucleophilicity of σ-lone pair electrons of the carbonyl O atom of tropone in the presence of LA. These results are consistent with the experiments reported by Li and Yamamoto.

Photochemical Uncaging of Aldehydes and Ketones via Photocyclization/Fragmentation Cascades of Enyne Alcohols: An Unusual Application for a Cycloaromatization Process

We utilized a cycloaromatization reaction driven by relief of excited state antiaromaticity to photouncage aldehydes and ketones. We developed several synthetic routes towards the synthesis of photocaged carbonyls as allylically substituted 3-(2-(arylethynyl)phenyl)prop-2-en-1-ols. A library of photocaged aryl aldehydes and ketones containing donors and acceptors, as well as several photocaged fragrance aldehydes and the steroid 5α-cholestan- 3 -one, were synthesized and demonstrated photouncaging in good to excellent yields.

Not antiaromaticity gain, but increased asynchronicity enhances the Diels-Alder reactivity of tropone

Tropone is an unreactive diene in normal electron demand Diels-Alder reactions, but it can be activated via carbonyl umpolung by using hydrazone ion analogs. Recently, the higher reactivity of hydrazone ion analogs was ascribed to a raised HOMO energy induced by antiaromaticity (L. J. Karas, A. T. Campbell, I. V. Alabugin and J. I. Wu, Org. Lett., 2020, 22, 7083). We show that this is incorrect, and that the activation barrier is lowered by increased asynchronicity.

Localized Antiaromaticity Hotspot Drives Reductive Dehydrogenative Cyclizations in Bis- and Mono-Helicenes

We describe reductive dehydrogenative cyclizations that form hepta-, nona-, and decacyclic anionic graphene subunits from mono- and bis-helicenes with an embedded five-membered ring. The reaction of bis-helicenes can either proceed to the full double annulation or be interrupted by addition of molecular oxygen at an intermediate stage. The regioselectivity of the interrupted cyclization cascade for bis-helicenes confirms that relief of antiaromaticity is a dominant force for these facile ring closures. Computational analysis reveals the unique role of the preexisting negatively charged cyclopentadienyl moiety in directing the second negative charge at a specific remote location and, thus, creating a localized antiaromatic region. This region is the hotspot that promotes the initial cyclization. Computational studies, including MO analysis, molecular electrostatic potential maps, and NICS(1.7)_ZZ calculations, evaluate the interplay of the various effects including charge delocalization, helicene strain release, and antiaromaticity. The role of antiaromaticity relief is further supported by efficient reductive closure of the less strained monohelicenes where the relief of antiaromaticity promotes the cyclization even when the strain is substantially reduced. The latter finding significantly expands the scope of this reductive alternative to the Scholl ring closure.

Strength and Nature of Host-Guest Interactions in Metal-Organic Frameworks from a Quantum-Chemical Perspective

Metal-organic frameworks (MOFs) offer a convenient means for capturing, transporting, and releasing small molecules. Their rational design requires an in-depth understanding of the underlying non-covalent host-guest interactions, and the ability to easily and rapidly pre-screen candidate architectures in silico. In this work, we devised a recipe for computing the strength and analysing the nature of the host-guest interactions in MOFs. By assessing a range of density functional theory methods across periodic and finite supramolecular cluster scale we find that appropriately constructed clusters readily reproduce the key interactions occurring in periodic models at a fraction of the computational cost. Host-guest interaction energies can be reliably computed with dispersion-corrected density functional theory methods; however, decoding their precise nature demands insights from energy decomposition schemes and quantum-chemical tools for bonding analysis such as the quantum theory of atoms in molecules, the non-covalent interactions index or the density overlap regions indicator.

Tandem [5 + 1]/[8 + 2] cycloaddition reactions involving phosphiranes and tropones: facile access to 6,5,7-fused tricyclic skeletons

Tandem [5 + 1] carbonyl cyclization/[8 + 2] cycloaddition reactions of phosphiranes and tropones were developed as a straightforward method to access 6,5,7-fused tricyclic scaffolds.