Synthesis of Nanographenes, Starphenes, and Sterically Congested Polyarenes by Aryne Cyclotrimerization

Abstract

In recent years, synthetic transformations based on aryne chemistry have become particularly popular, mostly due to the spread of methods to generate these highly reactive intermediates in a controlled manner under mild reaction conditions. In fact, aryne cycloadditions such as the Diels− Alder reaction are nowadays widely used for the efficient preparation of polycyclic aromatic compounds. In 1998, our group discovered that arynes can undergo transition metalcatalyzed reactions, a finding that opened new perspectives in aryne chemistry. In particular, Pd-catalyzed [2 + 2 + 2] cycloaddition of arynes allowed the straightforward synthesis of triphenylene derivatives such as starphenes or cloverphenes. We found that this reaction is compatible with different substituents and sterically demanding arynes as starting materials. This transformation is especially useful to increase the molecular complexity in one single step, transforming molecules formed by n cycles in structures with 3n + 1 cycles. In fact, we took advantage of this protocol to prepare a large variety of sterically congested polycyclic aromatic hydrocarbons such as helicenes or twisted polyarenes. Soon after the discovery of the reaction, the co-cyclotrimerization of arynes with other reaction partners, such as electron deficient alkynes, significantly expanded the potential of this transformation. Also the use of catalysts based on alternative metals besides Pd (e.g., Ni, Cu, Au) or the use of other strained intermediates such as cycloalkynes or cycloallenes added value to this reaction. In addition, we realized that the Pd-catalyzed aryne cyclotrimerization reaction is particularly useful for the bottom-up preparation of well-defined nanographenes by chemical methods. Although the extreme insolubility of these planar nanographenes hampered their manipulation and characterization by conventional methods, recent advances in single molecule on-surface characterization by atomic force microscopy (AFM) and scanning tunneling microscopy (STM) with functionalized tips under ultrahigh vacuum (UHV) conditions, permitted the impressive visualization of these nanographenes with submolecular resolution, together with the examination of the corresponding molecular orbital densities. Moreover, arynes have been shown to possess a rich on-surface chemistry. In particular, arynes have been generated and studied on-surface, showing that the reactivity is preserved even at cryogenic temperatures. On-surface aryne cyclotrimerization was also demonstrated to obtain large starphene derivatives. Therefore, it is expected that the combination of aryne cycloadditions and on-surface synthesis will provide notable findings in the near future, including the "àla carte" preparation of graphene materials or the synthesis of elusive molecules with unique properties.

Progress in Carbon Dots from the Perspective of Quantum Dots

“…当然近期在单 分散和窄谱带发射碳点方面取得了一些重要进展 [41,84] , 包括半高峰宽接近 20 nm 的三角形碳点. 对于单分散碳 点, 一个更有吸引力的极致替代策略是有机全合成, 即 制备具有大共轭分子核(含 100 个以上的碳原子)和烷基 表面的碳点, 能够实现原子级的尺寸均一度和结构可靠 性 [24,63,64] . 综合来说, 目前仍缺少对碳点合成过程系统 的重新审视和理性思考.…”

“…特别是碳点中存在大量表面缺陷、掺杂原子或其他共价 键合的功能基团, 这些杂原子化学键对能级结构的贡献 尚无明确结论 [44,63] . 同时, 碳核的强共价结构特征使其 结构调控和能带工程迥异于传统无机半导体, 导致与碳 点相关的掺杂、外延修饰和异质结设计存在很大困难, 而基于精细有机合成来设计和修饰碳点的核和表面结 构, 可能比简单借鉴无机半导体的杂原子掺杂更为可 取 [22,24,64] . 从一定意义上来说, 这是一个挑战, 同时也 是碳点的最大优势之一, 体现出碳点在传统碳材料和有 机分子领域间的桥连作用.…”

“…特别是碳点 的形成过程非常复杂, 详细的化学中间体和最终产物的 化学结构仍然不清楚. 碳点结构的澄清不应作为一个孤 立问题对待, 而应在传统无机量子点、纳米石墨、石墨 烯和多环芳烃等体系的基础上来综合考虑 [63][64][65][66][67] .…”

Rolf Huisgen's Classic Studies of Cyclic Triene Diels–Alder Reactions Elaborated by Modern Computational Analysis

“…4] exemplifies one such reaction discovered by R. C. Cookson et al . An enormous body of literature on thermal and metal‐catalyzed [2+2+2] cycloadditions and even genetically‐encoded [2+2+2] cycloadditions is now available.…”

The Diels–Alder Reaction for the Synthesis of Polycyclic Aromatic Compounds

“…Several approaches for the construction of PACs have been developed during the past years, including catalytic Friedel‐Crafts‐type cyclization of fluorovinyl‐substituted biaryls,, Pd‐catalyzed arylation, annulative π‐extension (APEX), aryne cyclotrimerization, flash vacuum pyrolysis of precursors, and many others. This review is devoted to the implication of the Diels–Alder reaction – a powerful synthetic method for the construction of six‐membered cyclic systems in one stage – for the synthesis of both linear and angular PACs with three or more condensed aromatic rings using various types of dienophiles.…”