Isoprene zirconocene complex

A cationic zirconocene complex formed by treatment of Cp2ZrCl2 with AgC104 has been found to catalyze Diels-Alder reactions [24]. For example, reaction of epoxy ester 55 with isoprene presumably proceeds via the formation of dioxolenium ion 56 then cyclization (Eq. 24). Hydrolysis gave cyclohexenecarboxylic acid 57. 

As a case in point, 3,3-dimethylbutan-2-one (139 Scheme 16) upon reaction with the zirconocene complex of isoprene in benzene at 60 C for 2 h affords (140a), whereas irradiation of the reaction mixture at low temperature in toluene leads to (140b). Mixtures of (140a) and (140b) are obtained by irradiation of isolated (138) and ketone (139) at higher temperatures. As the reaction temperature is lowered, an increasing amount of (140b) is formed. Therefore, it is reasoned that the zirconocene complex reacts in the cis form (138), under thermal conditions, and in the trans form under photochemical eolations. 

Zirconocene complexes (with s-cis geometry) of isoprene, 2,3-dimethylbutadiene, and 3-methyI-I,3-pentadiene are reported to give exclusively 1 1 addition with carbonyl substrates even if these are used in excess and the reaction temperature is fairly high (ca. 100 C). On the contrary, zirconocene complexes of 5-ci5-butadiene, 1,3-pentadiene, and 2,4-hexadiene ca, 1 1 mixture of the s-cis and s-trans isomers) easily accept (equation 53) 2 equiv. of either butanal or 3-pentanone, at low temperature ca. 30 C) in high yields (95%). This can be exploited for the stepwise insertion of two different electrophiles... 

Preparation of (l,4-diphenyl-l,3-butadiene)ZrCp2 (33) may be accomplished by photolysis of (/l-styryl)(benzyl)zirconocene (34)67 or by displacement of isoprene from (isoprene)ZrCp2 (35) at elevated temperature (Scheme 6)17. For complex 33, the s-trans/s-cis equilibrium lies far toward the s-trans isomer (95 5). 

Similarly, Cp2M( 1,3-diene) (M = Zr, Hf 1,3-diene = isoprene, 2,3-dimethylbutadiene, 1,4-diphenylbutadi-ene) are prepared [212]. The ratio of cis and trans isomers depends on the diene ligand as well as crystallization condition isomer turns thermally trans one). The title complex can be alternatively synthesized by the photolysis of diphenyl zirconocene in the presence of butadiene [213,214] and treatment of CP2MCI2 (M = Zr, Hf) with 2 equivalents, of vinyl lithium [215,216],... 

Bochmann and coworkers [77] carried out polymerizations of isobutylene and copolymerizations with isoprene using cationic zirconocene hydride complexes. The combination of [CpaZrH] with various trityl salts of weakly coordinating anions gives binuclear cationic hydrides [Cp 4Zr2H (p,-H)2] X which are powerful initiators for the polymerization of isobutene and its copolymerization with isoprene. The temperature dependence of M is indicative of a cationic mechanism. The highest molecular weights are obtained only under scmpulously dry conditions. 

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