Isoprene Friedel-Crafts reaction

The parent TMM complex (190 R = H) undergoes photochemical ligand substitution with trifluorophosphine or trimethylamine Al-oxide assisted substitution with tertiary phosphines or t-butyl isocyanide (Scheme 5A) Trimethylamine A-oxide assisted substitution using isoprene as the incoming ligand results in C-C bond formation to afford the bis-TT-allyl complex (197). An intramolecular version of this reaction is also known.The parent complex (190 R = H) reacts with electrophiles. Addition of HCl or Br2 gives the methallyl complexes (192) and (198), respectively. Tetrafluoroethylene adds across the Fe bond to afford (199) under photochemical conditions. Complex (190) undergoes Friedel-Crafts-type acylation with... 

Silicon has also been used to control the acylation of 1,3-dienes in a manner analogous to the situation with alkenes. Isoprenylation with 2-trimethylsilylmethylbuta-1,3-diene follows the course expected of acylation of the allylic silane. In these very rapid reactions, titanium tetrachloride seems to be one of the more efficient catalysts, as is aluminum chloride. The method was used in synthetic approaches to the terpenes ipsenol (14) and ipsdienol (15 Scheme 16). Of particular interest is the comparison of this iso-prenylating agent with isoprene itself. The examples of Friedel-Crafts acylations cited show the regio-control that can be achieved by suitable choice of substrate. 

It is possible to graft isobutene onto activated polymer molecules when they can be alkylated in an electrophilic reaction [602-604]. Polystyrene, for instance, is alkylated with Friedel-Crafts catalysts. The grafting is simplified by the presence of halogen atoms in the main polymer chain. Addition of diethylaluminumchloride or triethylaluminum leads to the formation of cations. The graft polymers usually contain between 10 and 60% of isobutene units and feature outstanding elasticity and dimensional stability at higher temperatures. Also, copolymers of isobutene and isoprene, butadiene, styrene, and so on, can be grafted in this manner [605-607]. 

Nafion-H is an efficent catalyst for Diels-Alder reactions (Table 3.39). The reactions of anthracene with maleic anhydride, dimethyl maleate, and dimethyl fuma late were carried out at 333 — 353 K in the presence of Nafion-H catalyst in either chloroform or benzene solvent. It should be noted that the reaction of dienophiles with very reactive dienes such as isoprene and 2,3-dimethylbutadiene can be carried out at room temperature to give the adduct in hi( yields. In usual systems, highly reactive dienophiles undergo polymerization during the desired reactions. In Diels-Alder reactions catalyzed by Friedel-Crafts Lewis acid catalysts, excess amounts of Lewis acid halides are required because of the formation of the complex between the halide and carbonyl oxygen atoms. Here again, Nafion-H catalysts allow easy and clean separation of products and the catalysts are not destroyed upon work up. 

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