In
organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.Clayden, J.; ...
, helicenes are
ortho-condensed polycyclic aromatic compounds
Aromatic compounds, also known as "mono- and polycyclic aromatic hydrocarbons", are organic compounds containing one or more aromatic rings. The parent member of aromatic compounds is benzene. The word "aromatic" originates from the past groupin ...
in which
benzene rings or other aromatics are angularly
annulated
In organic chemistry annulation (from the Latin ''anellus'' for "little ring"; occasionally annelation) is a chemical reaction in which a new ring is constructed on a molecule.
:
Examples are the Robinson annulation, Danheiser annulation and cert ...
to give
helically-shaped
chiral molecules.
The chemistry of helicenes has attracted continuing attention because of their unique structural,
spectral
''Spectral'' is a 2016 3D military science fiction, supernatural horror fantasy and action-adventure thriller war film directed by Nic Mathieu. Written by himself, Ian Fried, and George Nolfi from a story by Fried and Mathieu. The film stars ...
, and
optical
Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviole ...
features.
Structure and properties
The
systematic naming for this class of compounds is based on the number of rings:
'n''elicene is the structure consisting of ''n'' rings. According to
IUPAC
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
, only structures where ''n'' is at least 5 are considered helicenes.
[ Some specific compounds also have alternate or ]trivial name
In chemistry, a trivial name is a nonsystematic name for a chemical substance. That is, the name is not recognized according to the rules of any formal system of chemical nomenclature such as IUPAC nomenclature of inorganic chemistry, IUPAC inor ...
s. 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
Steric effects arise from the spatial arrangement of atoms. When atoms come close together there is a rise in the energy of the molecule. Steric effects are nonbonding interactions that influence the shape ( conformation) and reactivity of ions ...
collisions. The resulting helix is chiral. For helicenes with six benzene units, a 360° turn is completed. In the helicene series the dihedral angle
A dihedral angle is the angle between two intersecting planes or half-planes. In chemistry, it is the clockwise angle between half-planes through two sets of three atoms, having two atoms in common. In solid geometry, it is defined as the uni ...
s between the extremities increases going from elicene (26°) to elicene (58°) and then decreases again for example in elicene (30°).
Helicenes are notable for having chirality despite lacking both asymmetric carbons and chiral center
In stereochemistry, a stereocenter of a molecule is an atom (center), axis or plane that is the focus of stereoisomerism; that is, when having at least three different groups bound to the stereocenter, interchanging any two different groups ...
s. Instead, there is axial chirality
Axial may refer to:
* one of the anatomical directions describing relationships in an animal body
* In geometry:
:* a geometric term of location
:* an axis of rotation
* In chemistry, referring to an axial bond
* a type of modal frame, in music
* ...
, 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
Optical rotation, also known as polarization rotation or circular birefringence, is the rotation of the orientation of the plane of polarization about the optical axis of linearly polarized light as it travels through certain materials. Circul ...
and right-handed helices are dextrorotatory
Optical rotation, also known as polarization rotation or circular birefringence, is the rotation of the orientation of the plane of polarization about the optical axis of linearly polarized light as it travels through certain materials. Circul ...
.
Synthesis
The first helicene structure was reported by Jakob Meisenheimer
Jakob Meisenheimer (14 June 1876 – 2 December 1934) was a German chemist. He made numerous contributions to organic chemistry, the most famous being his proposed structure for a group of compounds now named Meisenheimer complex. He also pro ...
in 1903 as the reduction product of 2-nitronaphthalene. elicene was synthesized in 1918 by Weitzenböck & Klingler. The first elicene (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
In organic chemistry, a carboxylic acid is an organic acid that contains a carboxyl group () attached to an R-group. The general formula of a carboxylic acid is or , with R referring to the alkyl, alkenyl, aryl, or other group. Carboxylic ...
compounds. Since then, several methods for synthesizing helicenes with different lengths and substituent
A substituent is one or a group of atoms that replaces (one or more) atoms, thereby becoming a moiety in the resultant (new) molecule. (In organic chemistry and biochemistry, the terms ''substituent'' and ''functional group'', as well as ''side ...
s are used. The oxidative photocyclization of a stilbene Stilbene may refer to one of the two stereoisomers of 1,2-diphenylethene:
* (''E'')-Stilbene (''trans'' isomer)
* (''Z'')-Stilbene (''cis'' isomer)
See also
* Stilbenoids, a class of molecules found in plants
* 1,1-Diphenylethylene
1,1-Diphenyl ...
-type precursor
Precursor or Precursors may refer to:
* Precursor (religion), a forerunner, predecessor
** The Precursor, John the Baptist
Science and technology
* Precursor (bird), a hypothesized genus of fossil birds that was composed of fossilized parts of u ...
is used most often as the key step. The longest helicene prepared by this method is 6elicene in 2015.
In one study, elicene was synthesized in an olefin metathesis
Olefin metathesis is an organic reaction that entails the redistribution of fragments of alkenes (olefins) by the scission and regeneration of carbon-carbon double bonds. Because of the relative simplicity of olefin metathesis, it often create ...
reaction of a divinyl compound (prepared from 1,1′-bi-2-naphthol (BINOL) in several steps), with Grubbs' second generation catalyst:
Other approach is also non-photochemical and is based on assembly of biphenylyl-naphthalenes and their platinum-catalyzed double cycloisomerization leading to various elicenes:
:
File:Tetrahelicene.jpg, elicene
File:Pentahelicene.jpg, elicene
File:Hexahelicene.jpg, elicene
File:Hexahelicene2.jpg, elicene, other chirality
Chirality is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object.
An object or a system is ''chiral'' if it is distinguishable from ...
File:Heptahelicene.jpg, elicene
File:Heptahelicene2.jpg, elicene, other chirality
File:Octahelicene.jpg, elicene
File:Nonahelicene.jpg, elicene
File:Decahelicene.jpg, 0elicene
File:Undecahelicene.jpg, 1elicene
File:Dodecahelicene.jpg, 2elicene
File:Tridecahelicene.jpg, 3elicene
File:Tetradecahelicene.jpg, 4elicene
File:Pentadecahelicene.jpg, 5elicene
File:Hexadecahelicene.jpg, 6elicene
File:Octadecahelicene.jpg, 8elicene
Applications
Helicenes have been studied with respect to nonlinear optics
Nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in ''nonlinear media'', that is, media in which the polarization density P responds non-linearly to the electric field E of the light. The non-linearity is typica ...
, CPL, organocatalysis
In organic chemistry, organocatalysis is a form of catalysis in which the rate of a chemical reaction is increased by an organic catalyst. This "organocatalyst" consists of carbon, hydrogen, sulfur and other nonmetal elements found in organic com ...
,[ conformational analysis,] chirality sensing
Chirality is a property of asymmetry important in several branches of science. The word ''chirality'' is derived from the Greek (''kheir''), "hand", a familiar chiral object.
An object or a system is ''chiral'' if it is distinguishable from ...
, chemical sensors
A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon.
In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
[ and hetero-atom substitution.]
See also
* Other configurations of consecutively-fused benzene rings:
** Acene
In organic chemistry, the acenes or polyacenes are a class of organic compounds and polycyclic aromatic hydrocarbons made up of benzene () rings which have been linearly fused. They follow the general molecular formula .
The larger represent ...
s, linear
** Phenacenes, zig-zag
** Circulene
A circulene is a macrocyclic arene in which a central polygon is surrounded and fused by benzenoids. Nomenclature within this class of molecules is based on the number of benzene rings surrounding the core, which is equivalent to the size of the ...
s, closed ring
References
; General references
*
{{PAHs
Polycyclic aromatic hydrocarbons
Geodesic polyarenes