Aromaticity in Semi-Condensed Figure-Eight Molecules.

Electron delocalization and aromaticity was comparatively evaluated in recently synthesized figure-eight molecules made of two condensed U-shaped polycyclic aromatic hydrocarbon moieties connected either by two single bonds or by two para-phenylene groups. The selected examples include molecules that incorporate eight-membered and sixteen-membered rings, as well as a doubly [5]helicene-bridged (1,4)cyclophane. We probe whether some electron delocalization could occur through the stereogenic single bonds in these molecules: Is aromaticity purely (semi-)local, or possibly also global in these molecules? It was concluded that the situation can go from a purely (semi-)local character when the dihedral angle at the connecting single bonds is large, such as in biphenyl, to a predominantly (semi-)local character with a minor global contribution when the dihedral angle is small, such as in the para-phenylene connectors of the [5] helicene-bridged cyclophane.

Conformational, Structural, and Chiroptical Properties of an Overcrowded Triply Fused Carbo[7]helicene.

Recently, the synthesis of the racemate of an overcrowded triply fused carbo[7]helicene of formula C66H36 with three carbo[7]helicenes fused within a central six-membered ring was described. This molecule was found to embed an extremely contorted central six-membered ring and two negative curvatures. We report herein the resolution of the corresponding enantiomers and their conformational, structural, photophysical, and chiroptical properties. The racemization of the triply fused carbo[7]helicene was determined to proceed at a rate of krac = 8.06 × 10-4 s-1 at 175 °C in ortho-dichlorobenzene, corresponding to a barrier to enantiomerization ΔGenant‡ = 140.4 kJ·mol-1, a value significantly lower than for pristine carbo[7]helicene. Interestingly, the crystalline structures of the racemic and enantiopure materials show some differences regarding the molecular geometry, with an increased negative curvature in the latter cases. This unusual curved delocalized π-conjugated system afforded notably green fluorescence at room temperature and far-red phosphorescence at low temperature. Finally, electronic circular dichroism and circularly polarized luminescence responses of the enantiopure compounds have been measured and showed very close absorption and emission dissymmetry factors, gabs and glum, respectively, of ca. 2.6 × 10-3, indicating a similar chiral rigid geometry for both ground and excited states.

Mechanistic insight into cobalt-mediated [2 + 2 + 2]-cycloaddition reactions with γ-alkylidenebutenolide and γ-alkylidenebuterolactam as 2π partners.

The molecular complexity of recently reported cobalt(III) polycyclic complexes, resulting from an intramolecular formal (2 + 2 + 3) cycloaddition reaction on an enediyne containing a lactone moiety, has prompted us to computationally review the mechanisms of cobalt cycloaddition reactions with γ-alkylidenebutenolide or γ-alkylidenebuterolactam as 2π partners. Computed mechanisms are compared, leading to either cobalt(III)- or cobalt(I)-spiro complexes depending of both the nature of the reaction (intra- vs. intermolecular pathway) and the nature of the 2π partner (γ-alkylidenebutenolide vs. γ-alkylidenebuterolactam). These proposed mechanisms are supported by experiments, allowing us to report the synthesis and characterization of the predicted compounds.

Overcrowded Triply Fused Carbo[7]helicene.

This paper presents the synthesis and comprehensive analysis of a highly contorted and doubly negatively curved multihelicene compound, composed of three carbo[7]helicene units fused within a central six-membered ring. The synthesis of this compound involved a [2 + 2 + 2] cycloaddition reaction of 13,14-picyne, employing a Ni(0) catalyst, which exhibited superior performance compared to conventional Pd(0) catalysts. The evaluation of aromaticity in this triple carbo[7]helicene, utilizing magnetic and electronic criteria, led to noteworthy insights challenging the limitations of Clar's model of aromaticity.

A Triply [5]Helicene-Bridged (1,3,5)Cyclophane.

A rigid propeller-shaped conjugated triple macrocycle consisting of two nearly perfectly stacked benzene rings and three linking [5]helicene moieties has been synthesized using a glyoxylic Perkin approach. Analysis of the electron delocalization in this atypical aromatic molecule revealed global aromaticity and a 78 π-electron circuit along the edge of its triple loop, to the detriment of the two 6 π-electron circuits in the two stacked benzene rings.

Synthesis, Photophysical Properties, and Aromaticity of Pyrazine-Fused Tetrazapentalenes.

Pyrazine-fused 1,2,6,6a-tetrazapentalenes (PyTeAP) are zwitterionic tricyclic compounds exhibiting an original pattern with four consecutive nitrogen atoms. They were obtained by a challenging cyclization through the formation of a N-N bond under thermolytic conditions. Ten derivatives were synthesized, and the original scaffold of PyTeAP was confirmed by single-crystal X-ray diffraction analysis of one derivative. Examination of their photophysical properties in solution revealed blue fluorescence with λem = 416-426 nm. Theoretical investigations of the aromaticity in these compounds through magnetic criteria evidenced the presence of a dominant 14-electron circuit at the periphery.

Enantiospecific Syntheses of Congested Atropisomers through Chiral Bis(aryne) Synthetic Equivalents.

The synthetic equivalents of the enantiopure binaphthyl bis(aryne) atropisomers derived from BINOL (1,1'-bi-2,2'-naphtol) featuring a stereogenic axis vicinal to the two reactive triple bonds can be generated for the first time in solution in an enantiospecific manner. Using a two-step sequence based on the bidirectional [4+2] cycloaddition of furan derivatives followed by an aromatizative deoxygenation reaction, several 9,9'-bianthracenyl-based atropisomers could be prepared enantiospecifically in high enantiomeric purity. Alternatively, bidirectional reactions with anthracene, 2-bromostyrene, and perylene as the arynophiles afforded very congested bis(benzotriptycene), bis(tetraphene) and bis(anthra[1,2,3,4-ghi]perylene) nanocarbon atropisomers in equally high enantiomeric purity. In complement, cross reactions with two different arynophiles revealed possible. The unusual atropisomer prototypes described in this study open the way to enantiopure nanographene atropisomers designed for functions.

Determination of the Rate Constant of the [4 + 2] Cycloaddition Between an Aryne Atropisomer and Furan in Solution.

Using a specially designed prototype of a nonracemic aryne atropisomer with a low barrier to enantiomerization (ca. 36 kJ·mol-1), it was possible to determine the kinetic constant of its cycloaddition with furan in solution by a combination of theoretical calculations and experimental measurements. It was found that the reaction half-life of this aryne atropisomer in solution with 100 equivalents of furan as the trapping reagent is <150 ns at temperatures above -20 °C.

BenzAI: A Program to Design Benzenoids with Defined Properties Using Constraint Programming.

The BenzAI program can automatically generate benzenoids with defined structural constraints, like the number of hexagons, the number of carbon and/or hydrogen atoms, the existence of symmetries, the number of Kekulé structures, their diameter, or more elaborate criteria like their irregularity. BenzAI allows both automatic generation and manual building of benzenoids including or excluding defined patterns. For all benzenoids with less than 10 rings, it can extract IR spectra from a database that we generated. It computes local aromaticity for closed-shell and monoradical species using circuit-based algorithms. We present practical examples of what can be done with BenzAI for a diversity of cases.

Synthesis and Electron Accepting Properties of Two Di(benz[f]indenone)-Fused Tetraazaanthracene Isomers.

We designed and synthesized a novel di(benz[f]indenone)-fused tetraazaanthracene derivative and isolated its two isomers, 1a and 1s, having anti and syn configurations, respectively. Their structure and that of the condensation reaction intermediates, anti-2a and syn-2s, were fully characterized using one- and two-dimensional nuclear magnetic resonance spectroscopy and single-crystal X-ray diffraction. The optical and electronic properties of 1a and 1s were investigated using ultraviolet-visible absorption and fluorescence spectroscopies, cyclic voltammetry, and time-dependent density functional theory calculations. The presence of the carbonyl and ethynyltris(isopropyl)silane groups endows the di(benzoindenone)-fused azaacene derivatives with a strong electron accepting character. With an electron affinity of approximately -3.7 eV, the two isomers represent attractive electron-deficient molecular systems for the generation of n-channel semiconducting materials. Organic field effect transistors of 1a and 1s showed electron transport, and organic solar cells gave a proof of concept of the potential of the two compounds as electron acceptor materials when they are paired with an electron donor polymer.

Visualizing electron delocalization in contorted polycyclic aromatic hydrocarbons.

Electron delocalization in contorted polycyclic aromatic hydrocarbon (PAH) molecules was examined through 3D isotropic magnetic shielding (IMS) contour maps built around the molecules using pseudo-van der Waals surfaces. The resulting maps of electron delocalization provided an intuitive, yet detailed and quantitative evaluation of the aromatic, non aromatic, and antiaromatic character of the local and global conjugated cyclic circuits distributed over the molecules. An attractive pictural feature of the 3D IMS contour maps is that they are reminiscent of the Clar π-sextet model of aromaticity. The difference in delocalization patterns between the two faces of the electron circuits in contorted PAHs was clearly visualized. For π-extended contorted PAHs, some splits of the π system resulted in recognizable patterns typical of smaller PAHs. The differences between the delocalization patterns of diastereomeric chiral PAHs could also be visualized. Mapping IMS on pseudo-van der Waals surfaces around contorted PAHs allowed visualization of their superimposed preferred circuits for electron delocalization and hence their local and global aromaticity patterns.

Periselectivity in the aza-Diels-Alder Cycloaddition between α-Oxoketenes and N-(5-Pyrazolyl)imines: A Combined Experimental and Theoretical Study.

The thermal 6π aza-Diels-Alder cycloadditions between α-oxoketenes, in situ derived from a thermally induced Wolff rearrangement of 2-diazo-1,3-diketones, and N-(5-pyrazolyl)imines as prototypical electron-rich 2-azadienes lead to two distinct sets of products, essentially as a function of the nature of the α-oxoketenes involved. For instance, cyclic five-membered α-oxoketenes lead preferentially to spiro hydropyridin-4-ones, which involves the α-oxoketenes as the 2π partners at their C═C double bond and the N-(5-pyrazolyl)imines as the 4π partners at their 2-azadiene moiety. In contrast, other cyclic and acyclic α-oxoketenes lead preferentially to 1,3-oxazin-4-ones, which now involves the α-oxoketenes as the 4π partners at their 1-oxadiene moiety and the N-(5-pyrazolyl)imines as the 2π partners at their C═N double bond. A computational modeling study using DFT methods allowed rationalizing this change of periselectivity: the formation of spiro hydropyridin-4-ones is under thermodynamic control while the formation of 1,3-oxazin-4-ones is kinetically controlled, and slightly thermodynamically disfavored in the five-membered ring series. The competing cyclodimerization of the α-oxoketenes is also studied in detail.

Stereoselective Syntheses, Structures, and Properties of Extremely Distorted Chiral Nanographenes Embedding Hextuple Helicenes.

We report a molecular design and concept using π-system elongation and steric effects from helicenes surrounding a triphenylene core toward stable chiral polycyclic aromatic hydrocarbons (PAHs) with a maximal π-distortion to tackle their aromaticity, supramolecular and molecular properties. The selective syntheses, and the structural, conformational and chiroptical properties of two diastereomeric large multi-helicenes of formula C90 H48 having a triphenylene core and embedding three [5]helicene units on their inner edges and three [7]helicene units at their periphery are reported based on diastereoselective and, when applicable, enantiospecific Yamamoto-type cyclotrimerizations of racemic or enantiopure 9,10-dibromo[7]helicene. Both molecules have an extremely distorted triphenylene core, and one of them exhibits the largest torsion angle recorded so far for a benzene ring (twist=36.9°).

When cobalt-mediated [2 + 2 + 2] cycloaddition reaction dares go astray: synthesis of unprecedented cobalt(iii)-complexes.

In sharp contrast with the standard [2 + 2 + 2] cycloaddition reaction of diyne/ene, cobalt-mediated cycloadditions with γ-alkylidenebutenolide led to unprecedented cobalt(iii) polycyclic complexes. A plausible mechanism supported by a computational study based on an unusual fragmentation of the butenolide moiety was postulated to account for this original reaction.

Cyclobishelicenes: Shape-Persistent Figure-Eight Aromatic Molecules with Promising Chiroptical Properties.

Cyclobis[n]helicenes (n=3 or 5) are chiral D2 -symmetric π-conjugated macrocycles with stable lemniscular, or figure-eight, shapes. The conformational analysis of five different cyclobis[n]helicenes revealed that these molecules can only exist as their lemniscular conformers with high barriers to enantiomerization (>200 kJ mol-1 ). The enantiomers of a cyclobis[5]helicene were resolved by HPLC and their unusual chiroptical properties were attributed to the inherent chirality of their macrocyclic figure-eight.

Reply to the 'Comment on "trans-1,2-Disiloxybenzocyclobutene, an adequate partner for the auto-oxidation: EPR/spin trapping and theoretical studies"' by H.-G. Korth, P. Mulder and T. Paul, Phys. Chem. Chem. Phys., 2017, 19, C6CP04187F.

The proposal and the comments made by Korth et al. on a biradical intermediate along the isomerization path of the reaction of trans-1,2-tert-butyldimethyldisilyloxybenzocyclobutene 1 with dioxygen are unsuitable in our case. The mechanism scenario that we proposed is in agreement with our experimental observations. Moreover, new calculations were able to give an answer to the crucial question, "is the biradical an intermediate or a transition state?" together with the localization of the two radicals. This full article is a response to the "Comment on "trans-1,2-Disiloxybenzocyclobutene, an adequate partner for the auto-oxidation: EPR/spin trapping and theoretical studies" by J. Drujon et al., Phys. Chem. Chem. Phys. 2014, 16, 7513".

Theoretical Studies of Autoxidation of 2-Alkylidene-1,3-cyclohexadione Leading to Bicyclic-Hemiketal Endoperoxides.

Mechanism of the addition of molecular oxygen on the dienolic form of the 2-alkylidene-1,3-cyclohexadione was investigated by quantum chemical calculations using the approximate projection method developed by Yamaguchi. The complete reaction pathway of the formation of the endoperoxide is described. The crossing between triplet and singlet potential energy surfaces has been located. A multireference complete active space self-consistent field calculation has been performed to strengthen the results.

Recasting wave functions into valence bond structures: A simple projection method to describe excited states.

A method is proposed to obtain coefficients and weights of valence bond (VB) determinants from multi configurational wave functions. This reading of the wave functions can apply to ground states as well as excited states. The method is based on projection operators. Both energetic and overlap-based criteria are used to assess the quality of the resulting VB wave function. The approach gives a simple access to a VB rewriting for low-lying states, and it is applied to the allyl cation, to the allyl radical and to the ethene (notably to the V-state). For these states, large overlap between VB and multi reference wave functions are easily obtained. The approach proves to be useful to propose an interpretation of the nature of the V-state of ethene.

On the ring-opening of substituted cyclobutene to benzocyclobutene: analysis of π delocalization, hyperconjugation, and ring strain.

The influence of several substituents on the ring-opening elementary step of cyclobutene-like systems is analyzed computationally in detail. We focus on trans-1,2-disiloxycyclobutene-like molecules. Electronic effects (hyperconjugation and π delocalization) and geometrical constraints are decoupled and allow for an instructive analysis. It is found that the energy difference between closed and open forms is dictated mainly by the electronic structure of the open form, in which the rotation along the resulting simple C-C bond drives the electronic delocalization. Our calculations led us to quantify effects that determine the energy difference in the special case of disubstituted benzocyclobutene with respect to the disubstituted o-xylylene (aromaticity, π delocalization, ring strain). The relevant role of the siloxy-substituents is rationalized by an analysis of the molecular orbital interaction in an original manner. Finally, calculations are presented and show that the PBE0 functional must be preferred to the popular B3LYP functional for computations on substituted cyclobutene-like rings.

trans-1,2-Disiloxybenzocyclobutene, an adequate partner for the auto-oxidation: EPR/spin trapping and theoretical studies.

The auto-oxidation of trans-1,2-disiloxybenzocyclobutene was found to be very efficient, giving endo-peroxide in quantitative yield. Each step of the mechanism of spin-forbidden addition of triplet oxygen O2((3)Σg) was studied by both EPR/spin trapping and theoretical studies.