Methyl-2,4-pentadienoate dimerization

Another alternative method to produce sebacic acid iavolves a four-step process. First, butadiene [106-99-0] is oxycarbonylated to methyl pentadienoate which is then dimerized, usiag a palladium catalyst, to give a triply unsaturated dimethyl sebacate iatermediate. This unsaturated iatermediate is hydrogenated to dimethyl sebacate which can be hydrolyzed to sebacic acid. Small amounts of branched chain isomers are removed through solvent crystallizations giving sebacic acid purities of greater than 98% (66). 

Methyl 2, -pentadienoate self-dimerization as a potential route to sebacic acid precursors was extensively evaluated. Homogeneous metal catalyzed dimerization of compounds containing conjugated double bonds is known to give cyclic, branched, and linear dimers(43-45). At higher temperatures or in the presence of many zero-valent metal catalysts, cyclodimerization (Equation 12.) is the only observable reaction. 

In high polarity solvents, such as acetonitrile, dimethyl sulfoxide, methyl acetate, and methanol, the branched dimer, 4-methyl azelate precursor, is formed in high yield. In methanol, a methoxy dimer (CH302CCgH] 4(0013)0020113) is also formed in moderate yield. Heavies contain both an acyclic methyl, 4-pentadienoate trimeric product and high molecular weight methyl, 4-pentadienoate homopolymer. Polymerization in the absence of air(48) appears to be catalyzed by traces of the tertiary phosphine which is used to prepare the palladium dimerization catalyst. 

While the intermolecular reactions of butadiene and related methyl or simple alkyl-substituted dienes have been investigated extensively, relatively few examples of the Pd-catalyzed linear dimerization of higher dienes have been reported. Brun and co-work-ers in an isolated paper reported that, under palladium catalysis, reaction of methyl 2,4-pentadienoate (Scheme 9, 28) affords the linear dimer 29 in high yield (95%). Two aspects of this reaction are of particular interest, (i) The dimerization yields essentially only the tail-to-tail dimer 29, not the head-to-tail or head-to-head isomers (30 or 31, respectively). This is in contrast to the behavior of alkyl-substituted dienes under similar conditions vide infra), (ii) Although no details are given, the authors imply that 29 is... 

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