Chemists Welcome More Dendralenes To The Family

Among the more exotic examples of hydrocarbon compounds are molecules with structures made up exclusively of sp²-hybridized carbons. One of these classes of compounds is the dendralenes, a set of linear, branched polyenes. Chemists have previously prepared [3]dendralene through [8]dendralene, which embody three to eight radiating C=C bonds. Mehmet F. Saglam, Thomas Fallon, and Michael S. Sherburn of Australian National University and Michael N. Paddon-Row of the University of New South Wales have now expanded that collection to include [9]dendralene through [12]dendralene (J. Am. Chem. Soc. 2015, DOI: 10.1021/jacs.5b11889). Dendralenes have never been a snap to make, but over time Sherburn’s group has devised a reliable palladium-catalyzed cross-coupling strategy to piece them together from various polyene building blocks. The team carried out extensive spectroscopic and computational studies on the complete set of dendralenes, showing that conformational preferences lead to odd-even differences in the properties and reactivity of the shorter versions, which diminish in the longer versions. Along the way, the researchers demonstrated that dendralenes can react with multiple dienophiles in Diels-Alder reactions to create complex cyclic molecules. Those reactions could prove to be handy for rapid construction of natural products and for preparing compounds with novel electronic properties.