Palladium-catalyzed asymmetric hydrogenation of furan carboxylic acids

摘要

Enantioseletive hydrogenation of aromatic and heteroaromatic compounds is the fiels where chirally modified metal hydrogenation catalysts have the biggest potential compared to homogeneous chiral tranistion metal complexes Here we report the hydrogenation of furan and benzofuran carboxylic acids over a cinchonidine-modified 5 wt% Pd/Al_2O_3 catalyst(S)-Tetrahydrofuran -2-carboxylic acid was synthesized in 4 h at rt and 30 bar with 95% yield and 32% ee.The ee was lower in the hydrogenation of methylfuran carboxylic acids but up to 100% de was achieved In the slow hydrogenatin of benzofuran-2-carboxylic acid the ee went up to 50% yield The potential applicatin of the method is limited by the competing hydrogenatin of the quinoline rings of cinchonidine in the latter reactin necessitating the feeding of small amounts of cinchonidine during reaction Still this simples method using an easily available chiral modifier and catalyst affords the highest rate and ee reported so far in the catalytic asymmdtric hydrogenatin of furan and benzofuran carboxylic acids and it may be an attractive route in combination with optical resolution We assume that the reaction mechanism is analogous to the described for alpha,beta-unsaturated carboxylic acids over the samecatalyst involving a 1:2-type interaction between the cinchonidine and the acid dimer.