Alkyne‐Based Syntheses of Carbo‐ and Heterohelicenes

Abstract

[n]Helicenes are polycyclic aromatic molecules in which n ortho‐condensed benzene or other aromatic rings give rise to helical chiral and highly conjugated structures. The unique screw shape, helically extended π‐system and intriguing electronic and chirooptical properties of these molecules have attracted scientific interest for decades. The applications of helicenes have now expanded into several fields including material science, nanoscience, biochemistry, supramolecular and polymer chemistry. Despite the great theoretical and practical interest in helicenes, their synthesis remains a challenge. Among synthetic approaches to helicenes, alkyne cyclizations have great potential, since carbon‐rich high‐energy alkynes can form aromatic rings via cycloisomerizations. These processes are atom‐economic. In addition, alkyne reactions are highly favorable thermodynamically. This review focuses on alkyne‐based syntheses of carbo‐ and heterohelicenes (2000–2022). These syntheses are obviously multistage. In the review, they are systematized in terms of the key step leading to the formation of a helical shape. The methods are grouped into two categories: (a) metal‐free syntheses and (b) transition‐metal‐catalyzed syntheses. This division is very arbitrary, since catalytic Sonogashira and Suzuki‐Miyaura reactions and others are often used to prepare starting materials in the syntheses of the first group. Most of the publications on the topic are related to transition‐metal‐catalyzed syntheses of helicenes.

Electrochemical Synthesis of Hetero[7]helicenes Containing Pyrrole and Furan Rings via an Oxidative Heterocoupling and Dehydrative Cyclization Sequence

“…Helicenes and other related helical molecules with screw-shaped skeletons have helical chirality caused by the steric repulsion of terminal aromatic rings. [1] In recent few decades, helicene derivatives have attracted significant attention in synthetic organic and physical chemistry due to their use in high-potential materials such as catalysts, liquid crystals, and molecular devices. [2] Helicenes have traditionally been synthe-sized by photochemical reactions using stilbene-type molecules or multi-step syntheses.…”

“…[3] In modern organic chemistry, ground breaking synthetic methods using transition metal catalysts have been developed for the construction of helicenes. [1,2] These methodologies shorten the process and establish a stereoselective synthesis. [3f] However, several issues still remain, such as the preparation of complex starting substrates, high catalyst loading, and high reaction temperature.…”

New advances in chiral nanographene chemistry

“…[7][8][9][10] One particular area of interest is directed toward the development of non-planar, contorted, or twisted structures that exhibit inherent chirality. Many synthetic techniques have been employed for the development of these chiral, nonplanar nanographenes, including, but not limited to, alkyne-based methods 11,12 and Scholl oxidation. Scholl chemistry in particular has been utilized extensively in the preparation of chiral nanographenes, and has recently been thoroughly reviewed by Miao et al in the context of preparing contorted structures.…”

Divergent Synthesis of Helical Ketone Enabled by Rearrangement of Spiro Carbocation

“…The fused nonplanar conjugated organic molecules (NPCOMs) are important compounds, which have been designed as molecular rotors, light emitters, and semiconductors . Among all NPCOMs, helicenes represent a unique class of nonplanar screw-shaped compounds with the main skeleton of ortho -fused benzene − or other aromatic rings, − which now have been extended to non-aromatic systems . The helical distortion significantly depends upon the length of helicenes and their substituents, which show a variety of fascinating molecular packing.…”