1,3-Dimethyl-2-phenylbenzimidazoline as a Novel and Efficient Reagent for Mild Reductive Dehalogenation of α-Halo Carbonyl Compounds and Acid Chlorides

Article abstract of DOI:10.1021/jo00376a064

1,3-Dimethyl-2-phenylbenzimidazoline (DMBI) has been found to be a powerful, chemoselective, and useful reducing agent for mild reductive dehalogenation of a variety of α-halo carbonyl compounds (halo = Br, Cl, F) and acid chlorides.The reduction of α-halo ketones, aldehydes, esters, lactones, and carboxylic acids with this new reagent in ether is quity selective and clean and generally proceeds in almost quantitative yields at moderate temperatures with no additives.The order of relative reactivities in a series of α-halo carbonyl compounds was Br>Cl>F (for halides), primary>secondary>tertiary (for substitution at the halogenated carbon), and cyclohexyl>cyclododecyl (for ring size).The reduction of α-bromocamphor with DMBI-2-d led stereospecifically to the formation of camphor-3-exo-d.Based on these results together with experiments with para-substituted DMBIs, the mechanism of the present dehalogenation reaction of α-halo carbonyl compounds is postulated to proceed via a simple linear transition state (direct S_N2 displacement) featuring an attack on the halogenated carbon center by hydrogen at the C-2 position of DMBI as a hydride.Reductive dechlorination of acid chlorides to the corresponding aldehydes or aldehyde-d with DMBI or DMBI-2-d has also been achieved most effectively in the presence of an acetic acid catalyst.