Bridged benzocyclotrimers: concepts, synthesis, and applications

: Bridged polycyclic frameworks represent a unique tool to form curved units, the bicyclo[2.2.1]hepta-2,5-diene system being widely exploited to design and induce concave topologies. In particular, bridged benzocyclotrimers (BCTs) are characterized by a flat aromatic base decorated with bridged polycyclic motifs which provide the suitable curvature underlying the concave-convex topology. In the 1960s, these molecules attracted interest for their own chemical and physical properties. Later, the improvements of synthetic procedures to produce bridged BCTs have paved the way for their utilization to design and prepare molecular containers, bowls, cages, and baskets that are able to accommodate target molecules, recognize them, and modulate their functions. In this frame, we aim to describe the historical evolution of the concept, from the first bridged BCTs explored to confirm the existence of strained alkynes, and the phenomenon of bond alternation (Mills-Nixon hypothesis), to the most recent gated molecular baskets developed as dynamic synthetic receptors for molecular delivery. The main synthetic approaches which have been used to perform cyclotrimerization of bridged polycyclic alkenes, and related mechanisms, are also examined and discussed, with a specific focus on the syn/anti stereoselectivity issue and its consequences at a mechanistic level. The present review covers literature contributions published until mid 2021.

Role of hydrogen bond alternation and charge transfer states in photoactivation of the Orange Carotenoid Protein

Here, we propose a possible photoactivation mechanism of a 35-kDa blue light-triggered photoreceptor, the Orange Carotenoid Protein (OCP), suggesting that the reaction involves the transient formation of a protonated ketocarotenoid (oxocarbenium cation) state. Taking advantage of engineering an OCP variant carrying the Y201W mutation, which shows superior spectroscopic and structural properties, it is shown that the presence of Trp201 augments the impact of one critical H-bond between the ketocarotenoid and the protein. This confers an unprecedented homogeneity of the dark-adapted OCP state and substantially increases the yield of the excited photoproduct S*, which is important for the productive photocycle to proceed. A 1.37 Å crystal structure of OCP Y201W combined with femtosecond time-resolved absorption spectroscopy, kinetic analysis, and deconvolution of the spectral intermediates, as well as extensive quantum chemical calculations incorporating the effect of the local electric field, highlighted the role of charge-transfer states during OCP photoconversion.

On the Aromaticity and Acidity of 1-Hydroxy-4,5-benzotropylium Derivatives

<p> The aromaticity of 1-hydroxy-4,5-benzotropylium was assessed based on experimental and theoretical investigations. An X-ray crystallographic analysis revealed a decrease of bond alternation in the seven-membered ring of 1-hydroxy-4,5-benzotropylium derivatives compared with that of the parent 4,5-benzotropones, which is indicative of an increase in aromaticity upon protonation. NICS and AICD calculation also supported the increased aromaticity of 1-hydroxy-4,5-benzotropylium. The p<i>K</i>_a values for a series of 1-hydroxy-4,5-benzotropylium derivatives were also determined. </p>

On the Aromaticity and Acidity of 1-Hydroxy-4,5-benzotropylium Derivatives

<p> The aromaticity of 1-hydroxy-4,5-benzotropylium was assessed based on experimental and theoretical investigations. An X-ray crystallographic analysis revealed a decrease of bond alternation in the seven-membered ring of 1-hydroxy-4,5-benzotropylium derivatives compared with that of the parent 4,5-benzotropones, which is indicative of an increase in aromaticity upon protonation. NICS and AICD calculation also supported the increased aromaticity of 1-hydroxy-4,5-benzotropylium. The p<i>K</i>_a values for a series of 1-hydroxy-4,5-benzotropylium derivatives were also determined. </p>

Synthesis of azabenziporphyrinoids by SN Ar reactions

While most of porphyrins and related macrocycles have been synthesized by classical acid-catalyzed condensation and oxidation reactions, nucleophilic aromatic substitution reactions have recently emerged as a new protocol to construct novel porphyrinoids. [Formula: see text]-Dibromotripyrrin 3 was developed as a useful building block composed of three pyrrolic segments and two meso-carbons, which was allowed to react with various nucleophiles. Herein, we applied this strategy to synthesize carbaporphyrinoids containing two nitrogen atoms at the meso-positions. Reaction of [Formula: see text]-dibromotripyrrin 3 with meta- and para-phenylenediamine gave diazabenziporphyrins 8 and 9in good yields. Their oxidized species 12 and 13 were generated by facile oxidation with lead dioxide. While diaza-meta-benziporphyrin 12 exhibited nonaromatic characteristics, diaza-para-benziporphyrin 13 was revealed to be aromatic in view of [Formula: see text]H NMR, bond-alternation, and nucleus-independent-chemical-shift calculations. Curiously, aza-meta-benzicorrole 11 was obtained in a low yield by intramolecular cyclization reaction of [Formula: see text]-anilino-[Formula: see text]-bromotripyrrin with potassium carbonate.

C13 – a new empirical force field to characterize the mechanical behavior of carbyne chains

The C13 empirical potential is developed for accurate modeling of mechanical properties of carbyne specifically taking in account bond alternation.