Carbolong Chemistry: Planar CCCCX-Type (X = N, O, S) Pentadentate Chelates by Formal [3+1] Cycloadditions of Metalla-Azirines with Terminal Alkynes
Ming Luo, Yuhui Hua, Kaiyue Zhuo, Lipeng Long, Xinlei Lin, Zhihong Deng, Zhenyang Lin, Hong Zhang, Dafa Chen, Haiping Xia
Abstract
Three-membered unsaturated rings have been widely utilized in organic synthesis due to their inherent highly strained structures. Although much research has been applied to ring-expansion reactions of such strained ring systems, pathways other than [3+2] cycloadditions have rarely been observed in their reactions with alkynes, and to date, no [3+1] cycloadditions of isolated three-membered unsaturated rings with alkynes have been reported. Here we demonstrate an unprecedented [3+1] cycloaddition in which a fused metalla-azirine is used as a template to generate a series of planar CCCCX-type (X = N, O, S) pentadentate carbolong complexes 3–6. A series of substituted alkynes was tested for further exploration of the scope and mechanism for the [3+1] cycloaddition reactions. Complexes 3– 6 represent rare examples of planar pentadentate complexes that assemble four π-conjugated fused rings by one bridgehead atom. These novel complexes are thermally stable in air and absorb ultraviolet, visible, and near-infrared light, and thus are potentially applicable in photoelectric devices and pharmaceuticals.