Anomalous effect of non-alternant hydrocarbons on carbocation and carbanion electronic configurations

Synthesis and Properties of Twisted and Helical Azulene Oligomers and Azulene-Based Polycyclic Hydrocarbons

The construction of 1,2-position-connected azulene oligomers was achieved. In the crystal packing structure of the terazulene, two molecules of (Ra )- and (Sa )-configurations formed a pair. Variable temperature NMR measurements and theoretical calculations of the quaterazulene suggest that the helical and syn-type structure with terminal azulene overlap is more stable. Two kinds of fused terazulenes (1,2''-closed and 1,8''-closed) were also synthesized by intramolecular Pd-catalyzed C-H/C-Br arylation of the terazulene moieties. X-ray structure analysis of 1,2''-closed terazulene revealed a planar structure, while an analysis of 1,8''-closed terazulene performed on a C60 co-crystal revealed a curved structure forming a 1 : 1 complex covering the co-crystal. Nucleus-independent chemical shift (NICS) calculations carried out for the central seven-membered ring of 1,8''-closed terazulene showed a positive value, suggesting anti-aromatic properties.

Steric Hindrance Favors σ Dimerization over π Dimerization for Julolidine Dicyanomethyl Radicals

Metastable radicals exist in a steady-state equilibrium in solution with dimers, which can be either σ dimers or π dimers. Here, we show that steric hindrance at the para position causes julolidine-derived dicyanomethyl radicals to form σ dimers rather than π dimers, the opposite behavior as seen in other carbon-centered radicals, where steric hindrance typically favors pimerization. The change in dimerization mode can be attributed to weaker London dispersion forces and a decreased orbital overlap in the sterically hindered dicyanomethyl radical π dimers, while the bulky groups exert relatively little effect on the energy of the σ dimer.

Anti-Aromaticity Relief as an Approach to Stabilize Free Radicals

A new strategy to stabilize free radicals electronically is described by conjugating formally antiaromatic substituents to the free radical. With an antiaromatic substituent, the radical acts as an electron sink to allow configuration mixing of a low-energy zwitterionic state that provides antiaromaticity relief to the substituent. A combination of X-ray crystallography, VT-EPR and VT-UV/Vis spectroscopy, as well as computational analysis, was used to investigate this phenomenon. We find that this strategy of antiaromaticity relief is successful at stabilizing radicals, but only if the antiaromatic substituent is constrained to be planar by synthetically imposed conformational restraints that enable state mixing. This work leads to the counterintuitive finding that increasing the antiaromaticity of the radical substituent leads to greater radical stability, providing proof of concept for a new stereoelectronic approach for stabilizing free radicals.

Azulene Moiety as Electron Reservoir in Positively Charged Systems; A Short Survey

The non-alternant aromatic azulene, an isomer of alternant naphthalene, differs from the latter in peculiar properties. The large polarization of the π-electron system over the seven and five rings gives to azulene electrophile property a pronounced tendency to donate electrons to an acceptor, substituted at azulene 1 position. This paper presents cases in which azulene transfers electrons to a suitable acceptor as methylium ions, positive charged heteroaromatics and examples of neutral molecules that can accept electrons. The proposed product synthesis was outlined and the expected electron transfer was highlighted by analyzing the NMR, UV-Vis spectra and the pKR+ values.

High-Spin Diradical Dication of Chiral π-Conjugated Double Helical Molecule

We report an air-stable diradical dication of chiral D₂-symmetric conjoined bis[5]diazahelicene with an unprecedented high-spin (triplet) ground state, singlet triplet energy gap, ΔE_ST = 0.3 kcal mol^-1. The diradical dication possesses closed-shell (Kekulé) resonance forms with 16 π-electron perimeters. The diradical dication is monomeric in dibutyl phthalate (DBP) matrix at low temperatures, and it has a half-life of more than 2 weeks at ambient conditions in the presence of excess oxidant. A barrier of ∼35 kcal mol^-1 has been experimentally determined for inversion of configuration in the neutral conjoined bis[5]diazahelicene, while the inversion barriers in its radical cation and diradical dication were predicted by the DFT computations to be within a few kcal mol^-1 of that in the neutral species. Chiral HPLC resolution provides the chiral D₂-symmetric conjoined bis[5]diazahelicene, enriched in (P,P)- or (M,M)-enantiomers. The enantiomerically enriched triplet diradical dication is configurationally stable for 48 h at room temperature, thus providing the lower limit for inversion barrier of configuration of 27 kcal mol^-1. The enantiomers of conjoined bis[5]diazahelicene and its diradical dication show strong chirooptical properties that are comparable to [6]helicene or carbon-sulfur [7]helicene, as determined by the anisotropy factors, |g| = |Δε|/ε = 0.007 at 348 nm (neutral) and |g| = 0.005 at 385 nm (diradical dication). DFT computations of the radical cation suggest that SOMO and HOMO energy levels are near-degenerate.

Demonstration of Baird's rule complementarity in the singlet state with implications for excited-state intramolecular proton transfer

Greater aromaticity in the ground state leads to greater antiaromaticity in the excited state (and vice versa) which helps rationalize previously unexplained behavior of ESIPT fluorophores.

Aryl Nitrenium and Oxenium Ions with Unusual High-Spin π,π* Ground States: Exploiting (Anti)Aromaticity

Nitrenium and oxenium ions are important reactive intermediates in synthetic and biological processes, and their ground electronic configurations are of great interest due to having distinct reactivities and properties. In general, the closed-shell singlet state of these intermediates usually react as electrophiles, while reactions of the triplet states of these ions react like typical diradicals (e.g., H atom abstractions). Nonsubstituted phenyl nitrenium ions (Ph-NH⁺) and phenyl oxenium ions (Ph-O⁺) have closed-shell singlet ground states with large singlet-triplet gaps resulting from a strong break in the degeneracy of the p orbitals on the formal nitrenium/oxenium center. Remarkably, we find computationally (CBS-QB3 and G4MP2) that azulenyl nitrenium and oxenium ions can have triplet ground states depending upon the attachment position on the azulene core. For instance, CBS-QB3 predicts that 1-azulenyl nitrenium ion and 1-azulenyl oxenium ion are singlet ground-state species with considerable singlet-triplet gaps of -47 and -45 kcal/mol to the lowest-energy triplet state, respectively. In contrast, 6-azulenyl nitrenium ion and 6-azulenyl oxenium ion have triplet ground states with a singlet-triplet gap of +7 and +10 kcal/mol, respectively. Moreover, the triplet states are π,π* states, rather than the typical n,π* states seen for many aryl nitrenium or oxenium ions. This dramatic switch in favored electronic states can be ascribed to changes in ring aromaticity/antiaromaticity, with the switch from ground-state singlet ions to triplet-favored ions resulting from both a destabilized singlet state (Hückel antiaromatic) and a stabilized triplet (Baird aromatic) state. Density functional theory (UB3LYP/6-31+G(d,p)) was used to determine substituent effects on the singlet-triplet energy gap for azulenyl nitrenium and oxenium ions, and we find that the unusual ground triplet states can be further tuned by employing electron-donating or -withdrawing groups on the azulene ring. This work demonstrates that azulenyl nitrenium and oxenium ions can have triplet π,π* ground states and provides a simple recipe for making ionic intermediates with distinct electronic configurations and consequent prediction of unique reactivity and magnetic properties from these species.