Dendralenes
Encyclopedia
A dendralene is a discrete acyclic
cross-conjugated
polyene
. The simplest dendralene is buta-1,3-diene (1) or [2]dendralene followed by [3]dendralene (2), [4]dendralene (3) and [5]dendralene (4) and so forth. [2]dendralene (butadiene) is the only one not cross-conjugated.
The name dendralene is pulled together from the words dendrimer
, linear
and alkene
. The higher dendralenes are of scientific interest because they open up a large array of new organic compound
s from a relatively simple precursor especially by Diels-Alder chemistry. Their cyclic counterparts are aptly called radialene
s.
of a triacetate:
This compound reacts with two equivalents of maleic anhydride
in a tandem DA reaction:
With benzoquinone the reaction product was a linear polymer.
Several syntheses of substituted [3]dendralenes have been reported, one via an allene
, one via a Horner–Wadsworth–Emmons reaction , one via a cross-coupling reaction and one from an allylic carbonate .
One synthetic
route to [4]dendralene starts from chloroprene
. This compound is converted to a Grignard reagent by action of magnesium
metal which is then reacted with copper(I) chloride
to an organocopper intermediate which is in turn dimerized using copper(II) chloride
in an oxidative coupling reaction to give the butadiene dimer called [4]dendralene.
The [8]-dendralene compound was reported in 2009 :
in a successive Kumada–Tamao–Corriu coupling and Negishi coupling
.
units. The ultraviolet
absorption maxima equal that of butadiene itself. The dendralenes with an odd number of alkene
units are more reactive due to the presence of favorable s-cis diene conformation
s and Diels-Alder reaction
s take place more easily with a preference for the termini.
s. Several reaction schemes have been reported
[4]dendralene shows a tandem Diels-Alder reaction with the dienophile N-methyl-maleimide (NMM) . Complete site selectivity is possible with the addition of the Lewis acid
methyldichloroaluminium. With one set of premixing and 2 equivalents of NMM, the central diene group is targeted to the monoadduct 3. With another set and a larger amount of dienophile, the terminal groups react and the reaction proceeds from the monoadduct to the trisadducts 2 and 2b.
One reaction variation is cyclopropanation to a compound class called ivyanes with a reported synthesis in a Simmons–Smith reaction (diethyl zinc / trifluoroacetic acid
) of the first 6 members. . These 1,1-oligocyclopropanes are stable (except when exposed to acids) and have a large heat of combustion
with [6]ivyane exceeding that of cubane
. The oligocyclopropane chains adopt a helical conformation.
Acyclic
Acyclic can refer to:* In chemistry, a compound which is not cyclic, e.g. alkanes and acyclic aliphatic compounds* In mathematics:** A graph without a cycle, especially*** A directed acyclic graph...
cross-conjugated
Cross-conjugation
Cross-conjugation is a special type of conjugation in a molecule, when in a set of three Pi bonds only two pi-bonds interact with each other by conjugation, the third one is excluded from interaction...
polyene
Polyene
Polyenes are poly-unsaturated organic compounds that contain one or more sequences of alternating double and single carbon-carbon bonds. These double carbon-carbon bonds interact in a process known as conjugation, which results in an overall lower energy state of the molecule.Organic compounds with...
. The simplest dendralene is buta-1,3-diene (1) or [2]dendralene followed by [3]dendralene (2), [4]dendralene (3) and [5]dendralene (4) and so forth. [2]dendralene (butadiene) is the only one not cross-conjugated.
The name dendralene is pulled together from the words dendrimer
Dendrimer
Dendrimers are repetitively branched molecules. The name comes from the Greek word "δένδρον" , which translates to "tree". Synonymous terms for dendrimer include arborols and cascade molecules. However, dendrimer is currently the internationally accepted term. A dendrimer is typically symmetric...
, linear
Linear
In mathematics, a linear map or function f is a function which satisfies the following two properties:* Additivity : f = f + f...
and alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
. The higher dendralenes are of scientific interest because they open up a large array of new organic compound
Organic compound
An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon, and cyanides, as well as the...
s from a relatively simple precursor especially by Diels-Alder chemistry. Their cyclic counterparts are aptly called radialene
Radialene
[n]Radialenes are alicyclic organic compounds containing n cross-conjugated exocyclic double bonds. The double bonds are commonly alkene groups but those with a carbonyl group are also called radialenes...
s.
Synthesis
Vinylbutadiene ([3]dendralene) was first prepared in 1955 by pyrolysisPyrolysis
Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible...
of a triacetate:
This compound reacts with two equivalents of maleic anhydride
Maleic anhydride
Maleic anhydride is an organic compound with the formula C2H22O. It is the acid anhydride of maleic acid and in its pure state it is a colourless or white solid with an acrid odour....
in a tandem DA reaction:
With benzoquinone the reaction product was a linear polymer.
Several syntheses of substituted [3]dendralenes have been reported, one via an allene
Allene
An allene is a compound in which one carbon atom has double bonds with each of its two adjacent carbon centres. Allenes are classified as polyenes with cumulated dienes. The parent compound of allene is propadiene. Compounds with an allene-type structure but with more than three carbon atoms are...
, one via a Horner–Wadsworth–Emmons reaction , one via a cross-coupling reaction and one from an allylic carbonate .
One synthetic
Organic synthesis
Organic synthesis is a special branch of chemical synthesis and is concerned with the construction of organic compounds via organic reactions. Organic molecules can often contain a higher level of complexity compared to purely inorganic compounds, so the synthesis of organic compounds has...
route to [4]dendralene starts from chloroprene
Chloroprene
Chloroprene is the common name for the organic compound 2-chlorobuta-1,3-diene, which has the formula CH2=CCl-CH=CH2. This colorless liquid is the monomer for the production of the polymer polychloroprene, a type of synthetic rubber...
. This compound is converted to a Grignard reagent by action of magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...
metal which is then reacted with copper(I) chloride
Copper(I) chloride
Copper chloride, commonly called cuprous chloride, is the lower chloride of copper, with the formula CuCl. The substance is a white solid sparingly soluble in water, but very soluble in concentrated hydrochloric acid...
to an organocopper intermediate which is in turn dimerized using copper(II) chloride
Copper(II) chloride
Copper chloride is the chemical compound with the formula CuCl2. This is a light brown solid, which slowly absorbs moisture to form a blue-green dihydrate. The copper chlorides are some of the most common copper compounds, after copper sulfate....
in an oxidative coupling reaction to give the butadiene dimer called [4]dendralene.
The [8]-dendralene compound was reported in 2009 :
in a successive Kumada–Tamao–Corriu coupling and Negishi coupling
Negishi coupling
The Negishi coupling is a cross coupling reaction in organic chemistry involving an organozinc compound, an organic halide and a nickel or palladium catalyst creating a new carbon-carbon covalent bond:* The halide X can be chloride, bromine or iodine but also a triflate or acetyloxy group with as...
.
Properties
Even-membered dendralenes (e.g. [6]dendralene, [8]dendralene) tend to behave as chains of decoupled and isolated dieneDiene
In organic chemistry a diene or diolefin is a hydrocarbon that contains two carbon double bonds.Conjugated dienes are functional groups, with a general formula of CnH2n-2. Dienes and alkynes are functional isomers...
units. The ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
absorption maxima equal that of butadiene itself. The dendralenes with an odd number of alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
units are more reactive due to the presence of favorable s-cis diene conformation
Conformation
Conformation generally means structural arrangement and may refer to:*Carbohydrate conformation, the three-dimensional shape of a carbohydrate...
s and Diels-Alder reaction
Diels-Alder reaction
The Diels–Alder reaction is an organic chemical reaction between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene system. The reaction can proceed even if some of the atoms in the newly formed ring are not carbon...
s take place more easily with a preference for the termini.
Reactions
With simple dienophiles , dendrales can give quick access to complex molecules in Diels-Alder reactionDiels-Alder reaction
The Diels–Alder reaction is an organic chemical reaction between a conjugated diene and a substituted alkene, commonly termed the dienophile, to form a substituted cyclohexene system. The reaction can proceed even if some of the atoms in the newly formed ring are not carbon...
s. Several reaction schemes have been reported
[4]dendralene shows a tandem Diels-Alder reaction with the dienophile N-methyl-maleimide (NMM) . Complete site selectivity is possible with the addition of the Lewis acid
Lewis acid
]The term Lewis acid refers to a definition of acid published by Gilbert N. Lewis in 1923, specifically: An acid substance is one which can employ a lone pair from another molecule in completing the stable group of one of its own atoms. Thus, H+ is a Lewis acid, since it can accept a lone pair,...
methyldichloroaluminium. With one set of premixing and 2 equivalents of NMM, the central diene group is targeted to the monoadduct 3. With another set and a larger amount of dienophile, the terminal groups react and the reaction proceeds from the monoadduct to the trisadducts 2 and 2b.
One reaction variation is cyclopropanation to a compound class called ivyanes with a reported synthesis in a Simmons–Smith reaction (diethyl zinc / trifluoroacetic acid
Trifluoroacetic acid
Trifluoroacetic acid is the simplest stable perfluorinated carboxylic acid chemical compound, with the formula CF3CO2H. It is a strong carboxylic acid due to the influence of the electronegative trifluoromethyl group. TFA is almost 100,000-fold more acidic than acetic acid...
) of the first 6 members. . These 1,1-oligocyclopropanes are stable (except when exposed to acids) and have a large heat of combustion
Heat of combustion
The heat of combustion is the energy released as heat when a compound undergoes complete combustion with oxygen under standard conditions. The chemical reaction is typically a hydrocarbon reacting with oxygen to form carbon dioxide, water and heat...
with [6]ivyane exceeding that of cubane
Cubane
Cubane is a synthetic hydrocarbon molecule that consists of eight carbon atoms arranged at the corners of a cube, with one hydrogen atom attached to each carbon atom. A solid crystalline substance, cubane is one of the Platonic hydrocarbons. It was first synthesized in 1964 by Philip Eaton, a...
. The oligocyclopropane chains adopt a helical conformation.