The discovery of new aromatic molecular frameworks has been one of the most attracting works for synthetic chemists. However, the design and synthesis of aromatic molecules with more than three fused-rings sharing a bridgehead atom is challenging and has never been achieved. Due to the inherent atomic orbital distribution, carbon atoms can only stand up to three aromatic fused-rings, but heteroatoms, for example metal atoms, could realize creative fused-ring aromatic frameworks.
In a study recently published in Science Advances (Sci. Adv. 2020, 6, eaay2535), researchers from Xiamen University and Southern University of Science and Technology have realized metal-bridged Tetracyclic Aromatics with four-leaf clover motif as shown blow.
The authors designed a tricyclic complex bearing MºC bond was properly as starting material to explore straightforward annulation strategy. Then, the metal-carbon triple bonds within metallacycles was directly employed to undergo the [3+2] cycloaddition reaction with an azide to construct the fourth fused aromatic ring. The annulation of this work can be regarded as the first example of [3+2] cycloaddition reaction between an azide and a late-transition-metal-carbyne, which exhibit excellent efficiency, good functional group tolerance and remarkable regioselectivity.
This work exhibits a considerable advance towards the assembly of aromatic metallacycles, which will provide useful insight for addressing unusual p-conjugation skeletons bearing highly congested metal centers. Of particular interest, these novel aromatic complexes exhibit broad absorption in the ultraviolet-visible-near-infrared region and notable photothermal performance, enabling their potential applications in photoelectric materials and biomedicine.
