Helically Chiral Aromatics: The Synthesis of Helicenes by [2 + 2 + 2] Cycloisomerization of π-Electron Systems

Abstract

CONSPECTUS: Advanced molecular nanocarbons are now in the spotlight reflecting the basic discoveries of fullerenes, carbon nanotubes, and graphene. This research area includes also the chemistry, physics, and nanoscience of nonplanar polycyclic hydrocarbons, many of which exhibit helical chirality, such as iconic helicenes and their congeners. The combination of unique π-electron systems with the chirality phenomenon makes them highly attractive in various fields of science. Helicenes are polyaromatic compounds that are composed of all-angularly annulated benzene units, but other (hetero)cycles can also be embedded into their backbone. Even though they do not contain any stereogenic center, they are inherently chiral owing to the helical shape they adopt. Hexahelicene and higher homologues are conformationally stable within a reasonable range of temperatures and, therefore, can be obtained in an enantiopure form through a racemate resolution or asymmetric synthesis. An amazing array of synthetic methods for their preparation has been developed, but only a few of them have passed the tough scrutiny to be general, robust and practical methods such as traditional photocyclodehydrogenation of diaryl olefins and recently developed transition-metal-catalyzed [2 + 2 + 2] cycloisomerization of π-electron systems, which is discussed in this Account. Alkyne [2 + 2 + 2] cycloisomerization is a highly exergonic process and is therefore suitable for forming the strained helicene backbone, three (or more) cycles of which are closed in a single operation. The typical starting materials are aromatic triynes (optionally cyanodiynes or ynedinitriles) or tetraynes with diynes that undergo intramolecular or intermolecular cyclization, respectively, catalyzed by various complexes mainly of Ni 0 , Co I , or Rh I . Utilizing this synthetic methodology, various [5]-, [6]-, [7]-, [9]-, [11]-, [13]-, [16]-, [17]- , and [19]helicenes or their congeners, including functionalized derivatives, can be effectively prepared. Moreover, asymmetric synthesis (both catalytic and stoichiometric) of nonracemic helicenes has already been demonstrated. It relies on [2 + 2 + 2] cycloisomerization of centrally chiral triynes followed by an asymmetric transformation of the first order (controlled by the 1,3-allylic-type strain) or on enantioselective [2 + 2 + 2] cycloisomerization of alkynes catalyzed by chiral complexes mainly of Ni 0 or Rh I . Intriguingly, advanced helical architectures were formed such as the longest helicenes (up to oxa [19]helicene by closing 12 rings in a single synthetic operation) or laterally extended helicenes (e.g., pyreno[7]helicenes). Utilizing the aforementioned synthetic methodology, the tailor-made helical molecular nanocarbons are now better accessible to be applied in enantioselective catalysis, chirality sensing, spintronics (based on chirality induced spin selectivity), chiroptics (to produce circularly polarized light emission), organic/molecular electronics, or chiral single molecule devices.

Rhodium‐Catalyzed Enantioselective Synthesis of Highly Fluorescent and CPL‐Active Dispiroindeno[2,1‐c]fluorenes

“…In this context, helicenes, [15–19] heterohelicenes [19,20] and helical compounds [19,21–30] as representatives of SOMs have enjoyed a surge of interest as their twisted molecular structures and inherent helical chirality has been proven as a valuable structural motive for examining CPL properties [31,32] …”

Efficient Assembly of Molecular Complexity Enabled by Palladium‐Catalyzed Heck Coupling/C(sp2)−H Activation/ C(sp3)−H Activation Cascade

“…of 2 a were used. UPDATES asc.wiley-vch.de mones and benzyl bromides were evaluated and all reactions proceeded smoothly, affording the corresponding products (4 a', 4 c', 4 e', 4 l', 4 t', 4 z', and 4 ab') in up to 75% yield and 14:86 dr. [24][25][26] In general, two major diastereoisomers, including endo-isomer (4) and exo-isomer (4'), could be obtained in good isolated yields under the standard conditions with different solvents. The phenomenon of the switched diastereoselectivity in different solvent implied that the reaction may proceed with CH 3 CN as ligand or that solvent effects may control the diastereoselectivity.…”

“…The development of new synthetic strategy toward complex molecules via multiple bonds forming process in single operation with profound application in the synthesis of natural products, [1] bioactive molecules, [2] and functional materials [3] is an important aim in modern organic chemistry. [4] Recent decades, transition-metal catalyzed domino reaction has become a powerful synthetic protocol for the construction of complex polycycles from readily available starting materials. [5] In this regard, palladium-catalyzed norbornene (NBE)-mediated domino annulation, a Catellanitype transformation, represents an efficient strategy in the synthesis of fused aromatic compounds, which was pioneered by Lautens in 2000.…”

Synthesis and Properties of Partially Saturated Fluorenyl‐Derived [n]Helicenes Featuring an Overcrowded Alkene

“…[1f,7] At the heart of these blossoming applications lies the possibility to tune the chiroptical properties of helicenic compounds by modifying their structure. Starting from the basic carbo[n]helicenes, formally composed of orthofused benzene rings, several strategies were indeed found suitable to increase the diversity of helicenic scaffolds by: i) introducing main-group elements (B, Si, N, P …) leading to the design of heterohelicenes; [8] ii) varying the size and nature of the rings within the helix; iii) designing helicenoid or helicenelike systems formed of non-fully aromatic ortho-fused rings; [9] iv) inclusion of a positive or negative charge within the helical scaffold, [10] or v) incorporating helicenic ligands into organometallic complexes. [11] In this context, the development of efficient, flexible and convenient methods for the synthesis of helicenic compounds appears of utmost importance, as the field cannot grow without the advent of more practical, scalable and cost-efficient synthetic pathways.…”