Meso-2,3-Dimethylsuccinic acid

The formation of meso-dimethylsuccinic acid from dimethylmaleic acid and the racemic mixture from dimethylfumaric acid implies that both hydrogen atoms add to the same side of the unsaturated molecule 11). Bourguel (12) also noted that disubstituted acetylenes yielded initially cis-ethylenes but that trans isomers were formed if the hydrogenations were protracted. 

Diethyl acetylenedicarboxylate is reduced to diethyl fumarate. Dime thy Ifumaric acid is reduced to meso-dimethylsuccinic acid, and dimethylmaleic acid is reduced to racemic dimethylsuccinic acid. 

A Ru complex with a BINAP derivative covalently bonded to an aminomethylated polystyrene resin is also usable, though both the rate and enantioselectivity are decreased [57]. 2,3-Dimethylenesuccinic acid is hydrogenated by an (R)-BINAP-Ru complex at 3 atm of H2 to give a 98.8 1.2 mixture of (2S,3S)-dimethylsuccinic acid with 96% e.e. and the meso isomer [58]. 

Since olefin molecules can have stereoisomers (cis and trails Z and E), the interesting stereochemistry, which is not only stereoselectivity but also stereospecificity, of their hydrogenation should be observed when the products also have stereoisomers d, I and meso threo and erythro). However, only in a few works could stereospecificity be observed. Dimethylmaleic and dimethylfumaric acids are reduced at a mercury cathode to form d, I and meso a, a -dimethylsuccinic acids, respectively. This is direct evidence that this hydrogenation proceeds through Ivans addition of hydrogen. 

The second model compound, 2,5 dimethyltetrahydrofuran-3,4-dicarboxylic acid, was found to consist of three isomers (plus a partially hydrogenated material). We were unable to assign the stereochemistry of this model unequivocally. We also used as a model 2,3-dimethylsuccinic acid which was a mixture of the racemic and meso forms. On this basis, one can show that the resonances of the carbons al a to the anhydride or acid group along the backbone occur at 44.6 ppm and are approximately equal to that found in the copolymer. 

A study of the stereochemistry of a reaction yields information about the geometrical arrangement of reactants at some critical stage or stages of the transformation. For the problem at hand, this type of study has revealed that the principal product of the hydrogenation of an unsaturated compound is formed by the addition of two atoms of hydrogen to the same side of the molecule. This is deduced from the formation of cis olefins from disubstituted acetylenes and meso-1,2-dimethylsuccinic acid from dimethyl-maleic acid (1). 

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