Isoprene dimers, functionalization

Maleic anhydride has been used in many Diels-Alder reactions (29), and the kinetics of its reaction with isoprene have been taken as proof of the essentially transoid stmcture of isoprene monomer (30). The Diels-Alder reaction of isoprene with chloromaleic anhydride has been analy2ed using gas chromatography (31). Reactions with other reactive hydrocarbons have been studied, eg, the reaction with cyclopentadiene yields 2-isopropenylbicyclo[2.2.1]hept-5-ene (32). Isoprene may function both as diene and dienophile in Diels-Alder reactions to form dimers. 

Further interesting possibilities to functionalize isoprene dimers are mentioned by Morikawa and Kitazume [120]. They started with 2,6-dime-... 

Palladium catalysts that are free of halide ions effect the dimerization and carboxylation of butadiene to yield 3,8-nonadienoate esters. Palladium acetate, solubilized by a tertiary amine or an aromatic amine, gives the best yields and selectivities (equation 57).87 Palladium chloride catalyzes the hydrocarboxylation to yield primarily 3-pentenoates.88 The hydrocarboxylation of isoprene and chloroprene is regio-selective, placing the carboxy function at the least-hindered carbon (82% and 71% selectively) minor amounts of other products are obtained (equation 58). Cyclic dienes such as 1,3-cyclohexadiene and 1,3-cyclooctadiene are similarly hydrocarboxylated. 

Dimerization of isoprene, if carried out regioselectively to give the head-to-tail dimer 97, would be useful for terpene synthesis. So far few reports on successful regioselective dimerization of isoprene have been given, and this remains an unsolved problem. 2,6-Dimethyl-1,3,6-octatriene (97), the head-to-tail dimer, can be prepared with high selectivity using Zr [36], Ni [37,38] and Pd [39] catalysts. However, selective functionalization of this dimer to the terpene alcohol 98 is not easy. 

With its concerted mechanism implying little charge distribution change along the pathway, the Diels-Alder reaction has been understood to have little rate dependence on solvent choice. For example, the relative rate of cyclopentadiene dimerization increases only by a factor of 3 when carried out in ethanol. The relative rate for the Diels-Alder reaction of isoprene with maleic anhydride (Table 7.1) varies by only a factor of 13 with solvents whose dielectric constants vary by almost a factor of 10, but the rate acceleration is not a simple function of the solvent polarity. Furthermore, the dimerizations of cyclopentadiene and 1,3-butadiene proceed at essentially identical rates in the gas and solution phases. ... 

Stereospecific polymerization of 1,3-dienes (10-18) (to butadiene) and isoprene homo- and copolymers), dimerization of propene (19) and recently stereospecific polymerization of acetylene (20) to high cis-content polyacetylene have all been reported using lanthanide catalysts. Sen (21) has reported the preparation of cationic europium systems (which perhaps function as cationic initiators) for polymerization of norbornadiene and 1,3-cyclohexa-diene. 

Palladium-catalysed reaction of trans-j8-farnesene, formed by catalytic tri-merization of isoprene, resulted in regioselective head-to-head coupling to a dimer which yielded squalene on hydrogenation. A detailed n.m.r. study of functionalized squalane derivatives and model compounds suggests the presence of precoiled conformations. ... 

The selective dimerization of isoprene followed by functionalization is of particularly great interest, since this reaction sequence enables the synthe-... 

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