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Generalization of the Hückel rule predicts that the (anti)aromaticity of a neutral ring is qualitatively reverted upon a single twist of the π-orbital array (Möbius interconversion), and is preserved upon expansion of all the bonds by single C 2 units (ring carbo-merization). These opposite effects are addressed from quantitative theoretical and experimental standpoints, respectively. (i) According to most resonance energy (RE) schemes, the RE value of a Möbius ring is not the opposite of that of the Hückel version. This also applies to the Aihara's and Trinajstic's topological resonance energy (TRE), where a non-aromatic reference in the topological limit is defined as being "as identical as possible" to the parent ring but just "acyclic". In spite of its conceptual merits, the computing complexity and fictitious character of the TRE acyclic reference resulted in a disuse of TRE as a current energetic aromaticity index. Both the calculation and interpretation of TRE have been revisited in light of a cross-reference between the Hückel and Möbius rings within the Hückel molecular orbital (HMO) framework. Whereas the topological influence of triple bonds is currently neglected in the first-level HMO treatment of π-conjugated systems, a graph-theoretical analysis allows one to differentiate the TRE value of a [3n]annulene from those of the corresponding carbo-[n]annulene. The C 18 ring of carbo-benzene is thus predicted to be slightly more topologically aromatic than that of [18]annulene. (ii) Recent experimental and density functional theory (DFT) theoretical studies of quadrupolar carbo-benzene derivatives are presented. The results show that the "flexible aromaticity" of the p-C 18 Ph 4 bridge between donor anisyl substituents plays a crucial role in determining the intriguing chemical/spectroscopical/optical properties of these carbo-chromophores. theme of aromaticity. ‡ Corresponding author netic,…) are not uniquely determined [4], the rule remains qualitative in nature. Before addressing the problem of its quantitative transcription, it is noted that the rule is reversed by applying elementary molecular transformations altering not only the electronic configuration (one-electron spin inversion or two-electron ionization), but also the atomic configuration: one-bond 180°-twisting or one-bond C 2 -expansion, corresponding to Möbius conversion [5] and local carbo-merization, respectively [6]. The quantitative variation of aromaticity upon both latter types of transformations is hereafter addressed from both the theoretical and experimental standpoints. TWISTING THE RING CONTENT: THE MÖBIUS KEY TO AROMATICITYAfter a critical survey of various energetic measures of aromaticity proposed in the literature [7], the relative aromaticity of Hückel and Möbius rings will be scrutinized. Their complementary roles for the quantification of topological aromaticity is then investigated not only for cyclenes, but also for conjugated alkynes and cyclynes, and in particular for ring carbo-mers. Critical survey of HMO energetic a...

When targeting the quadrupolar p-dianisyltetraphenyl-carbo-benzene by reductive treatment of a hexaoxy-[6]pericyclyne precursor 3 with SnCl(2)/HCl, a strict control of the conditions allowed for the isolation of three C(18)-macrocyclic products: the targeted aromatic carbo-benzene 1, a sub-reduced non-aromatic carbo-cyclohexadiene 4A, and an over-reduced aromatic dihydro-carbo-benzene 5A. Each of them was fully characterized by its absorption and NMR spectra, which were interpreted by comparison with calculated spectra from static structures optimized at the DFT level. According to the nucleus-independent chemical shift (NICS) value (NICS≈-13 ppm), the macrocyclic aromaticity of 5A is indicated to be equivalent to that of 1. This is confirmed by the strong NMR spectroscopic deshielding of the ortho-CH protons of the aryl substituents, but also by the strong shielding of the internal proton of the endocyclic trans-CH=CH double bond that results from the hydrogenation of one of the C≡C bonds of 3. Both the aromatics 1 and 5A exhibit a high crystallinity, revealed by SEM and TEM images, which allowed for a structural determination by using an X-ray microsource. A good agreement with calculated molecular structures was found, and columnar assemblies of the C(18) macrocycles were evidenced in the crystal packing. The non-aromatic carbo-cyclohexadiene 4A is shown to be an intermediate in the formation of 1 from 3. It exhibits a remarkable dichromism in solution, which is related to the occurrence of two intense bands in the visible region of its UV/Vis spectrum. These properties could be attributed to the dibutatrienylacetylene (DBA) unit that occurs in the three chromophores, but which is not involved in a macrocyclic π-delocalization in 4A only. A versatile redox behavior of the carbo-chromophores is evidenced by cyclic voltammetry and was analyzed by calculation of the ionization potential, electron affinity, and frontier molecular orbitals.

The synthesis of quadrupolar tetraphenyl-carbo-benzene derivatives is envisioned through the corresponding tetraphenyl-tetramethoxy-[6]pericyclynedione last-but-one precursor. The latter was thus prepared in 9 steps and 7% overall yield via the corresponding [6]pericyclynediol, itself obtained by a [8 þ 10] ring formation process between a C 8 octatriyne dinucleophile and a C 10 decatriynedial dielectrophile. In the search for even shorter alternative accesses, the preparation and attempted uses of the corresponding C 10 decatriynediol and diethyl decatriynedioic diester are also described. While the pericyclynediol possesses 14 diastereoisomers proving unseparable, the pericyclynedione possesses five diastereoisomers only that could be resolved by semipreparative HPLC techniques. Reaction of the pericyclynedione with trimethylsilylethynylmagnesium bromide afforded the alkynyl-[6]pericyclyneketol mono-adduct in 22% yield, along with the corresponding [6]pericyclynediol bis-adduct in 43% yield. One of the 14 diastereoisomers of the latter could be separated by column chromatography, and its configuration could be assigned to one of only two possibilities in accordance with the observed number of non-equivalent 1 H and 13 C NMR signals. Since a regioisomer of the dialkynyl-[6]pericyclynediol bis-adduct was shown to undergo ''efficient'' reductive aromatization, these results demonstrate the potential value of the pericyclynedione as a pivotal precursor of p-disubstituted tetraphenyl-carbobenzenes.

The π-electron-rich C(8)-conjugated sequence of 1,4-dialkynylbutatrienes is identified as a fragile and fascinating motif occurring in carbo-benzene derivatives, and in Diederich's 1,4-bis(arylethynyl)- or 1,4-bis(triisopropylsilylethynyl)butatriene "capped" representatives, in particular, in tetraalkynylbutatriene. The family of symmetrical 1,4-dialkynylbutatrienes (E-C≡C)RC=C=C=CR(C≡C-E) is extended to functional caps (E=H, CH(3), C≡CPh, CPh=CHBr, or CPh=CBr(2)) with non-alkynyl substituents at the sp(2) vertices (R=Ph or CF(3)). The targets were selected for their potential in appealing retrosynthetic routes to carbo-benzenes, in which the aromatic C(18) macrocycle would be directly generated by sequential metathesis or reductive coupling processes. The functional 1,4-dialkynylbutrienes were synthesized by either classical methods used for the preparation of generic butatrienes (R'Li/CuX-mediated reductive coupling of gem-dihaloenynes or SnCl(2)/HCl-mediated reduction of 3,6-dioxy-octa-1,4,7-triyne precursors). Their spectroscopic and electrochemical properties are compared and analyzed on the basis of the relative extent of total conjugation.

The industrial market depends intensely on wood-based composites for buildings, furniture, and construction, involving significant developments in wood glues since 80% of wood-based products use adhesives. Although biobased glues have been used for many years, notably proteins, they were replaced by synthetic ones at the beginning of the 20th century, mainly due to their better moisture resistance. Currently, most wood adhesives are based on petroleum-derived products, especially formaldehyde resins commonly used in the particleboard industry due to their high adhesive performance. However, formaldehyde has been subjected to strong regulation, and projections aim for further restrictions within wood-based panels from the European market, due to its harmful emissions. From this perspective, concerns about environmental footprint and the toxicity of these formulations have prompted researchers to re-investigate the utilization of biobased materials to formulate safer alternatives. In this regard, proteins have sparked a new and growing interest in the potential development of industrial adhesives for wood due to their advantages, such as lower toxicity, renewable sourcing, and reduced environmental footprint. This work presents the recent developments in the use of proteins to formulate new wood adhesives. Herein, it includes the historical development of wood adhesives, adhesion mechanism, and the current hotspots and recent progress of potential proteinaceous feedstock resources for adhesive preparation.

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