Helicene

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Hexahelicene Hexahelicene-from-xtal-3D-vdW.png
Hexahelicene
General structure formula of [n]helicene Helicenegeneral.svg
General structure formula of [n]helicene

In organic chemistry, helicenes are ortho-condensed polycyclic aromatic compounds in which benzene rings or other aromatics are angularly annulated to give helically-shaped chiral molecules. [1] The chemistry of helicenes has attracted continuing attention because of their unique structural, spectral, and optical features. [2] [3] [4] [5] [6] [7] [8]

Contents

Structure and properties

The systematic naming for this class of compounds is based on the number of rings: [n]helicene is the structure consisting of n rings. According to IUPAC, only structures where n is at least 5 are considered helicenes. [1] Some specific compounds also have alternate or trivial names. As the number of rings increases, starting at four, the structure becomes non-planar, but instead the planes of consecutive rings tilt to prevent steric collisions. For helicenes with six benzene units, a 360° turn is completed. In the helicene series the dihedral angles between the extremities increases going from [4]helicene (26°) to [6]helicene (58°) and then decreases again for example in [7]helicene (30°).

Helicenes are notable for having chirality despite lacking both asymmetric carbons and chiral centers. Instead, there is axial chirality, which results from the handedness of the helicity itself. The clockwise and counterclockwise helices are non-superposable. By convention a left-handed helix is minus and labeled (M), a right-handed helix is plus and labeled (P). Evidence from CD spectroscopy suggests left-handed helices are levorotatory and right-handed helices are dextrorotatory.

The stability of the two complementary helical enantiomers with respect to interconversion and the mechanism by which they interconvert depend on n. [9]

Synthesis

The first helicene structure was reported by Jakob Meisenheimer in 1903 as the reduction product of 2-nitronaphthalene. [10] [5]helicene was synthesized in 1918 by Weitzenböck & Klingler. [11] The first [6]helicene (also called hexahelicene) was synthesized by M. S. Newman and D. Lednicer in 1955 via a scheme that closed the two central rings by Friedel–Crafts cyclization of carboxylic acid compounds. [12] [13] Since then, several methods for synthesizing helicenes with different lengths and substituents are used. The oxidative photocyclization of a stilbene-type precursor is used most often as the key step. The longest helicene prepared by this method is [16]helicene in 2015. [14]

In one study, [15] [5]helicene was synthesized in an olefin metathesis reaction of a divinyl compound (prepared from 1,1′-bi-2-naphthol (BINOL) in several steps), with Grubbs' second generation catalyst:

Helicene synthesis by olefin metathesis Helicene olefin Metathesis.png
Helicene synthesis by olefin metathesis

Other approach is also non-photochemical and is based on assembly of biphenylyl-naphthalenes and their platinum-catalyzed double cycloisomerization leading to various [6]helicenes: [16]

Helicene Cycloisomerization.png

Applications

Helicenes have been studied with respect to nonlinear optics, [17] CPL, [18] [19] organocatalysis, [20] conformational analysis, [21] chirality sensing, [22] chemical sensors [23] and hetero-atom substitution. [24] [25] [26] [27]

See also

Related Research Articles

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<span class="mw-page-title-main">Elias James Corey</span> American chemist (born 1928)

Elias James Corey is an American organic chemist. In 1990, he won the Nobel Prize in Chemistry "for his development of the theory and methodology of organic synthesis", specifically retrosynthetic analysis. Regarded by many as one of the greatest living chemists, he has developed numerous synthetic reagents, methodologies and total syntheses and has advanced the science of organic synthesis considerably.

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<span class="mw-page-title-main">Organolithium reagent</span> Chemical compounds containing C–Li bonds

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<span class="mw-page-title-main">Cycloparaphenylene</span>

A cycloparaphenylene is a molecule that consists of several benzene rings connected by covalent bonds in the para positions to form a hoop- or necklace-like structure. Its chemical formula is [C6H4]n or C
6n
H
4n
Such a molecule is usually denoted [n]CPP where n is the number of benzene rings.

<span class="mw-page-title-main">Michal Hocek</span> Czech chemist

Michal Hocek is a Czech chemist. He is a group leader at the Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences and a professor of organic chemistry at Charles University in Prague. He specializes in the chemistry and chemical biology of nucleosides, nucleotides, and nucleic acids.

<span class="mw-page-title-main">Rick L. Danheiser</span> American organic chemist

Rick L. Danheiser is an American organic chemist and is the Arthur C. Cope Professor of Chemistry at the Massachusetts Institute of Technology and chair of the MIT faculty. His research involves the invention of new methods for the synthesis of complex organic compounds. Danheiser is known for the Danheiser annulation and Danheiser benzannulation reactions.

References

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General references