Dendralenes

Dendralenes are discrete acyclic cross-conjugated polyenes. The simplest dendralene is butadiene (1) or [2]dendralene followed by [3]dendralene (2), [4]dendralene (3) and [5]dendralene (4) and so forth.



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 compounds from a relatively simple precursor especially by Diels-Alder chemistry. Their cyclic counterparts are aptly called radialenes. 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 in an oxidative coupling reaction to the butadiene dimer with copper(II) chloride.



Diels-Alder reactions
[4]dendralene shows a tandem Diels-Alder reaction with the dienophile N-methyl-maleimide or 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.