Substituent effects in dehalogenation of aryl bromides with NaAlH2(OCH2CH2OCH3)(2)

Kinetics of dehalogenation of the bromobenzenes 4-RC6H4Br (R=F, Cl, H, CH3, Cn(3)O and N(CH3)(2)) with the title hydride (SDMA) and with SDMA in the p resence of Co(acac)(2) have been examined. The rate constants of the debrom ination were correlated with Hammett substituent constants sigma (p) and E- 1/2 reduction potentials of the bromobenzenes. rho values (+4.3 for SDMA re duction and +1.9 for the Co-catalyzed debromination) imply a negative charg e concentration effect in transition states of both debrominations. Differe nces in the course of these reactions are discussed.