Stereoselective 1,3-debromination reactions

The metallate, PPN+Cr(CO)4NO−, was used for the 1,3-debrominative reductive cyclization of the (±) and meso isomers of dimethyl 2,4-dibromoglutarate and dimethyl 2,4-dibromo-2,4-dimethylglutarate. In both (±) isomers, the reaction is stereospecific in giving the trans cyclopropane product. In the meso case, the reaction is unselective in the first case, but distinctly favors the cis cyclopropane isomer in the second set of compounds. This (±) and meso pair thus represent the first example of near stereospecificity in the debrominations of both 1,3-dibromo diastereomers. Using an enantiomerically enriched dimethyl 2,4-dibromoglutarate, and determining the absolute stereochemistry of the cyclopropane product, it was found that a strict double inversion mechanism is involved in the (±) debromination reaction and presumably also in the near-stereospecific meso case reported above.