This article is from the WeChat public account: X-MOLNews
High ring tension and anti-aromatism are two important factors that are not conducive to the stable existence of rings. In general, neither high-tension rings (e.g., phenylyne and isobenzene) nor anti-aromatic compounds (e.g., cyclobutadiene and pentylene) are stable at room temperature. Achieving both instability factors within a single ring poses a higher challenge for scientists. At the same time, the conversion chemistry of such unconventional compounds (such as skeleton rearrangement and aromatic transition) remains to be studied due to high ring tension and the stability of anti-aromatic compounds.
Carbolong Chemistry is a chemistry with Chinese labels discovered, founded and named by Professor Xia Haiping's research team, and has been officially included in the eighth edition of the international classic organic chemistry textbook March's Advanced Organic Chemistry [1]. After more than ten years of research, the synthesis, reactivity and application development of carbon dragon compounds have achieved comprehensive and systematic results, which have recently been summarized into two important reviews (Chem. Rev. 2020, 120, 12994; Acc. Chem. Res., 2023, DOI: 10.1021/acs.accounts.2c00750, cover article). Previous studies have focused on aromatic carbonosaur species, some with high ring tension. Antiaromatic chemistry is an important part of aromatic chemistry, and antiaromatic species have special photophysical properties, therefore, the study of antiaromatic carbonosaurus species is an important development direction of carbonosaur chemistry. Recently, Xia Haiping's research group reported two types of antiaromatic carbon dragon compounds: the first case of weak antiaromatic metal naphthalene (J. Am. Chem. Soc. 2021, 143, 15587) and the first high Craig antiaromatic species—the 8-carbonosaurus complex (PNAS 2023, 120, e2215900120, click to read more). Although important progress has been made in the study of aromatic and anti-aromatic carbon dragon complexes, they contain at most only one unstable factor (high cyclic tension or antiaromatic), and how to introduce two unstable factors into one molecule at the same time (i.e., construct antiaromatic rings with high tension) becomes a new challenge and may lead to new chemistry.
Recently, Xia Haiping's research group reported the first case of high-tensile and anti-aromatic carbon dragon compound-metal bridgehead indene. This discovery makes it possible for two unstable factors, high tension and antiaromatics, to exist simultaneously in a single metal heterocycle, in which the compound contains the smallest metallic ethylene bond angle within a six-membered ring (147.4°, Figure 1). Surprisingly, the compound remained stable to 180 °C. At the same time, the high tension and anti-aromatic properties make it rich in reactivity. For example, the backbone rearrangement of the antiaromatic ring can be achieved by nucleophilic addition (Figure 1 left), and the on-off regulation of aromaticity can be achieved by electrophilic addition (Figure 1 right). Related work was published in the Journal of the American Chemical Society (J. Smith). Am. Chem. Soc.）。
Figure 1. High-tensile and anti-aromatic metal bridgehead indene chemistry.
Direct chelation of metals by multidentated carbon chain ligands or ligand precursors can be used to efficiently prepare carbon dragon compounds (Nat. Commun., 2017, 8, 1912, click to read more). Based on the above new ideas, Xia Haiping's research group used 1,4,7-octanetriyne-3,6-diol (L) and osmium complexes to obtain the first case of metal bridgehead inddene compound 1-Cl containing metal vinylene elements (Figure 2a) by one-pot method under acidic conditions. Theoretical calculations show that the curved metal vinylene element in 1-Cl has high tension, and the introduction of metal increases the bond angle and reduces the ring tension compared with organic systems (Figure 2b). The introduction of quaternary phosphonium will weaken the antiaromatization of metal heterocycles and play a stabilizing role (Figure 2c/2d). The magnetic induced current density of the metal bridgehead indene ring is shown in a counterclockwise direction, and there is a paramagnetic ring current with antiaromatic properties (Figure 2e).
Figure 2. Synthesis, ring tension and aromatization of high-tensile and anti-aromatic metal indene.
Mechanistic calculations showed that the addition of triphenylphosphine (nucleophilic attack coordination alkyne group, C→D) and acid (promoting the formation of conjugated structures, D→F) played an important role in the formation of the product (Figure 3a). Therefore, the authors used polyyne ligand L to react directly with the osmium complex and separated the active intermediate 2 at a high yield. Intermediate 2 can be converted to metal bridgehead indene 1-Cl under the action of triphenylphosphine and acid (Figure 3B).
Figure 3. Study on the formation mechanism of metal bridgehead indene.
The curved metal ethylene structure results in high ring tension in the metal bridgehead, which implies rich chemical reactivity. For example, 1-Cl reacts with selenium dioxide and nucleophiles (hydroxide negative ions, generated from potassium tert-butoxide in situ with water) in an empty atmosphere to obtain oxidation products 3-BF4 and 4-Cl (Figure 4a). Interestingly, the main ring parts of 3-BF4 and 4-Cl are isomers. 3-BF4 is obtained by direct oxidation of metallic ethylene, while the production of 4-Cl undergoes a process of metal heterocyclic backbone rearrangement. Theoretical studies show that the six-membered ring still has some antiaromatization (Figure 4b). Therefore, the transformation of 1-Cl to 4-Cl not only undergoes the process of skeleton rearrangement, but also accompanied by the transfer of antiaromatics. In order to further explore the skeleton rearrangement process, the authors changed the reaction conditions from empty atmosphere to nitrogen atmosphere, and obtained the active intermediate 5 with high yield. Intermediate 5 contains a cumulative triene motif that can react with the nucleophiles ammonium chloride and tetrabutylammonium iodide to obtain skeleton rearrangement products 4-Cl and 6-I (Figure 4C).
Figure 4. Study on the reactivity of metallic ethylene in the indene ring of metal bridgehead.
Due to the anti-aromatic nature of metal bridgehead indene, under the action of electrophilic reagents (such as acid, liquid bromine and nitrostetrafluoroborate), the first case of metal indane compound (7~9, metal indane is a derivative of metal benzyne, with aromaticity, Figure 5a). Among them, aromatic compound 7 can regenerate anti-aromatic compound 9 under alkaline conditions. Based on this, the authors use acid-base reagents to achieve reversible regulation of aromatics, and explore the changes of UV-Vis absorption spectrum in this process. Experiments have shown that this conversion can be cycled multiple times, laying the foundation for the application of such anti-aromatic compounds (Figure 5b).
Figure 5. Conversion of anti-aromatic metal indene and aromatic metal indene.
brief summary
So far, Professor Xia Haiping's research group has reported for the first time high cyclic tension and anti-aromatic metal heterocyclic compounds, breaking the convention and realizing two unstable factors in a single ring at the same time. Through theoretical calculations and reactivity studies, the chemistry of this special high-tension and anti-aromatic species is enriched. And by using its chemical reaction characteristics, the regulation of photophysical properties is realized, which is expected to be applied to optical materials. Roald Hoffmann, Nobel Prize in Chemistry, once said, "The desire to make the molecule that violates the norm is part of that human struggle." （Angew. Chem. Int. Ed. 2008, 47, 4474）。 The authors believe that designing and synthesizing unconventional molecules can not only discover new chemistry, but often derive new materials, new theories, and even entirely new disciplines. The study of antiaromatic carbonosaur species not only opens a new window for carbonosaur chemistry, but also provides new ideas for the development of antiaromatic chemistry.
The work was supported by the National Natural Science Foundation of China, the Shenzhen Science and Technology Innovation Commission, the Guangdong Key Laboratory of Catalysis Fund, and the Shenzhen Outstanding Talent Training Fund.
Isolation, Reactivity, and Tunable Properties of a Strained Antiaromatic Osmacycle
Qian Li, Yuhui Hua, Chun Tang, Dafa Chen, Ming Luo, and Haiping Xia*
J. Am. Chem. Soc., 2023, DOI: 10.1021/jacs.3c00942
Resources:
1. China's original "Carbon Dragon Chemistry" entered the international classic textbook
https://www.x-mol.com/news/458637
Instructor Introduction:
Xia Haiping
https://www.x-mol.com/university/faculty/242394
Research group website
https://hpxia.xmu.edu.cn/index.htm
Postdoctoral fellows and research assistants are welcome to join the research group, and research directions: organic synthesis methodology (experience in chiral catalysis is preferred), organometallic chemistry, polymer chemistry, optoelectronic materials. Location: Shenzhen (Southern University of Science and Technology)