Catalysis of Organic..

3382,5-Dimethyl-2,4-Hexadieneharvested shortly after blooming (8). To by-pass this problem, the Australianvariety has been genetically modified to ensure that all the flowers grow at thesame speed, thus reducing the harvest costs. To overcame the availabilityproblems, many chemical companies started to manufacture the syntheticequivalents (pyrethroids), which are esters of different acid and alcohol moieties(1-7,9). Pyrethroids may be classified on the basis of their biological and physicalproperties (knock down or killing effect, UV-resistant or non UV-resistant, forfarming or domestic uses, respectively), which depend on chemical structures andstereoisomerisms.The main intermediate for preparing pyrethroids is the chrysanthemic acid(CHA), which is currently obtained by the reaction of an appropriate diazo-aceticester with 2,5-dimethyl-2,4-hexadiene (tetramethyl-butadiene or TMBD). Thelatter may be prepared in different ways (10-20), although the most relevant onesappear to be both the vapour phase reaction of i-butyralde hyde with i-butene(Prins reaction) on different oxides [14-16,18] and the liquid or vapour phasedehydration of 2,5-dimethyl-2,5-hexandiol (DMED) (Fig. 1) [11,12,19]. However,the Prins reaction shows low conversion values and the formation of high amountsof by-products, with a rapid catalyst deactivation [14,15], while in the vapourphase dehydration the surface acidity has to be finely tuned [19] in order toprevent the formation of huge amounts of useless isomers. In this study, a neweconomical and environmentally friendly synthesis of TMBD, by selective vapourphase dehydration of DMED on different commercial solid acids is reported. Theresults obtained were significantly better than those reported in literature [12],demonstrating the possibility to use cheap and commercially available raw claysor clay mixtures.OH2 H 2 OOHFigure 1. Dehydration of 2,5-dimethyl-2,5-hexandiol (DMED) to 2,5-dimethyl-2,4-hexadiene (TMBD).