Aryne Atropisomers: Chiral Arynes for the Enantiospecific Synthesis of Atropisomers and Nanographene Atropisomers

The basics about arynes and their applications in synthetic organic chemistry are briefly presented, and the concept of atropisomerism is defined, highlighting that it is a time-dependent form of isomerism and chirality. It is remembered that racemization is a macroscopic and statistical irreversible process, while enantiomerization is a nanoscopic reversible process that occurs at the molecular scale, with racemization being twice as fast as enantiomerization. The concept of aryne atropisomers is introduced with a naive question: Can synthetically useful nonracemic aryne atropisomers having a triple bond ortho to the stereogenic single bond exist in solution? It was found that such aryne atropisomers can be generated in solution from easily available ortho-iodoaryl triflate precursors and excess trimethylsilylmethylmagnesium chloride. Analysis of the barriers to enantiomerization of some aryne atropisomers by computational modeling revealed the key contribution to the configurational stability of the H atom in tris-ortho-substituted biphenyl-based atropisomers. Using a specially designed prototype of aryne atropisomer, for which the barrier to enantiomerization was accurately evaluated by advanced computational modeling, the kinetic parameters of its reaction with furan were experimentally determined. From these measurements, it was concluded that any aryne atropisomer with a barrier to enantiomerization ΔG_enant^⧧ equal to or higher than 50 kJ mol^-1 would lead to fully enantiospecific reactions. The synthetic applications of two structurally distinct aryne atropisomers built on a 1-phenylnaphthalene platform are described: one has the aryne triple bond embedded in the naphthyl moiety, and the other has the aryne triple bond embedded in the phenyl moiety. Both aryne atropisomers allowed for the fully enantiospecific, and possibly overall enantioselective, syntheses of original atropisomers based on standard aryne chemistry. For instance, reactions with anthracene and perylene afforded triptycene and nanographene atropisomers, respectively, in high enantiomeric excesses. A bis(aryne) atropisomer synthetic equivalent prepared from either enantiomer of BINOL is described for 3D bidirectional reactions with a single handedness. Its 2-fold reactions with anthracene and perylene afforded the corresponding severely congested bis(benzotriptycene) (99% ee) nanocarbon atropisomer and bis(anthra[1,2,3,4-ghi]perylene) (98% ee) nanographene atropisomer, respectively. This allowed the discovery of bis(twistacene) atropisomers as a new class of polycyclic aromatic hydrocarbons (PAH) with multiple stereogenicities. Cross reactions with the bis(aryne) atropisomer synthetic equivalent and two different arynophiles proved feasible, providing a nanographene atropisomer with a benzotriptycene unit and an anthra[1,2,3,4-ghi]perylene unit assembled around a stereogenic axis as a unique chiral PAH (99% ee). Overall, because the concept is simple and its implementation is easy, aryne atropisomers is an attractive approach to the synthesis of atropisomers in a broad meaning. Applications to the synthesis of large PAH atropisomers with single handedness are particularly promising.

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.

A Large Starphene Comprising Pentacene Branches

Starphenes are attractive compounds due to their characteristic physicochemical properties that are inherited from acenes, making them interesting compounds for organic electronics and optics. However, the instability and low solubility of larger starphene homologs make their synthesis extremely challenging. Herein, we present a new strategy leading to pristine [16]starphene in preparative scale. Our approach is based on a synthesis of a carbonyl-protected starphene precursor that is thermally converted in a solid-state form to the neat [16]starphene, which is then characterised with a variety of analytical methods, such as ¹³ C CP-MAS NMR, TGA, MS MALDI, UV/Vis and FTIR spectroscopy. Furthermore, high-resolution STM experiments unambiguously confirm its expected structure and reveal a moderate electronic delocalisation between the pentacene arms. Nucleus-independent chemical shifts NICS(1) are also calculated to survey its aromatic character.

A simple Hückel model-driven strategy to overcome electronic barriers to retro-Brook silylation relevant to aryne and bisaryne precursor synthesis

ortho-Silylaryl triflate precursors (oSATs) have been responsible for many recent advances in aryne chemistry and are most commonly accessed from the corresponding 2-bromophenol. A retro-Brook O- to C-silyl transfer is a key step in this synthesis but not all aromatic species are amenable to the transformation, with no functionalized bisbenzyne oSATs reported. Simple Hückel models are presented which show that the calculated aromaticity at the brominated position is an accurate predictor of successful retro-Brook reaction, validated synthetically by a new success and a predicted failure. From this, the synthesis of a novel difunctionalized bisaryne precursor has been tested, requiring different approaches to install the two C-silyl groups. The first successful use of a disubstituted o-silylaryl sulfonate bisbenzyne precursor in Diels-Alder reactions is then shown.

Exploration of Kobayashi's aryne precursor: a potent reactive platform for the synthesis of polycyclic aromatic hydrocarbons

Arynes due to their transient nature leads to remarkable and versatile applications in the synthetic world. Apparently, researchers have focused on the construction of simple to complex π-conjugated systems using arynes as the reactive platform. In this regard, Kobayashi's aryne precursor has shown a great extent of reactivity and afforded significant advancement in the synthesis of polycyclic aromatic systems with wide practical utility. This review emphasizes the extensive utilization of Kobayashi's aryne intermediates and their derivatives for the synthesis of different classes of polycyclic aromatic hydrocarbons (PAHs).