A Tale of Two Isomers: Enhanced Antiaromaticity/Diradical Character versus Deleterious Ring‐Opening of Benzofuran‐fused s‐Indacenes and Dicyclopenta[b,g]naphthalenes

Abstract

We examine the effects of fusing two benzofurans to s-indacene (indacenodibenzofurans,I DBFs) and dicyclopenta[b,g]naphthalene (indenoindenodibenzofurans, IIDBFs) to control the strong antiaromaticity and diradical character of these core units.S ynthesis via 3-functionalized benzofuran yields syn-IDBF and syn-IIDBF.s yn-IDBF possesses ah igh degree of paratropicity,e xceeding that of the parent hydrocarbon, which in turn results in strong diradical character for syn-IIDBF.I nt he case of the anti-isomers, synthesized via 2-substituted benzofurans,t hese effects are decreased;h owever,b oth derivatives undergo an unexpected ring-opening reaction during the final dearomatization step. All the results are compared to the benzothiophene-fused analogues and showt hat the increased electronegativity of oxygen in the syn-fused derivatives leads to enhancement of the antiaromatic core causing greater paratropicity.F or syn-IIDBF increased diradical character results from rearomatization of the core naphthalene unit in order to relieve this paratropicity.

Design and Synthesis of Kekulè and Non-Kekulè Diradicaloids via the Radical Periannulation Strategy: The Power of Seven Clar’s Sextets

“…Synthesis of extended polyaromatics often provides interesting synthetic challenges that span innovation. − In the present case, an attractive synthetic route to diradicaloids is opened by our recent work on radical alkyne periannulations (i.e., “zigzag annulations”) − where we developed a new strategy for expanding the scope of π-annulations at the zigzag edge of PAHs (Figure a). Fortuitously, the pericyclizations provide the necessary pieces for accomplishing the synthesis of new diradicals in a concise and modular fashion.…”

Tetra-tert-butyl-s-indacene is a Bond Localized C2h Structure: Even More Bad News for B3LYP

“…HF [a] (%) PG [b] ∆r [c] (Å) ∆Erel (kcal/mol) d H8 [d] (ppm) d H2 [d] (ppm) Some of us recently published a joint experimental and theoretical study of indacenodibenzofurans (IDBFs). [26] We found that syn-IDBF shows a high degree of paratropicity exceeding that of the parent s-indacene, while anti-IDBF exhibits weaker paratropicity. Indeed, experimental 1 H NMR signals for the hydrogens on the central six-membered ring of syn-IDBF are shifted upfield (Hsyn: 5.60 ppm, Figure 2A) compared to those of anti-IDBF (Hanti: 6.15 ppm, Figure 2B).…”

Photoswitching of Local (Anti)Aromaticity in Biphenylene-Based Diarylethene Molecular Switches

“…Aromaticity and antiaromaticity are two fundamental physical organic chemistry concepts that underlie several molecular properties of polycyclic conjugated systems. − Hence, a widely used approach to design such molecules with novel functions is to tune their electronic structure on the aromatic–nonaromatic–antiaromatic scale. , Apart from structure and reactivity, properties that are influenced by (anti)­aromaticity such as molecular stacking or conductance have recently been the subject of major interest. For example, aromatic systems assemble in a manner to minimize π-orbital repulsion, , while antiaromatic rings are predicted to prefer face-to-face stacking due to the emergence of transannular delocalization. − Furthermore, the increment in conductivity within aromatic, nonaromatic, and antiaromatic molecular wires has been rationalized based on their decreasing resistance to redistribute their π-electron systems upon charge transport. − As of now, the best approach to probe the effects of (anti)­aromaticity on molecular properties is to design and synthesize new compounds with different conjugation patterns, − among which reversible control of (anti)­aromaticity has rarely been reported. ,, …”