THE MARCUS THEORY OF REACTIONS OF QUINOLINIUM IONS WITH BH4- AND OH-

The reactions of quinolinium ions with alkaline solutions of BH4- in a 4:1 mixture of 2-propanol and water were studied. Four 1-benzyl-3-cyanoquinolium ions and two 3-(aminocarbonyl)-1-benzylquinolinium ions were used. The quinolinium ions first reversibly add a nucleophile (mostly to the 4-position) and then are more slowly reduced to the 1,2- and 1,4-dihydroquinolines. The equilibrium constant for the addition of OH- or OR-, the rate constant for the addition, the rate constant for solvolysis of the adduct, and the rate constant for reaction of BH4- with the quinolinium ion were determined in each case. The quinolinium ion concentration was kept low enough so that only one of the hydridic hydrogens of BH4- was transferred to a quinolinium ion. Adduct formation and reduction by BH4- both have rates in the range 105-107 M-1 s-1, and the Bronsted a values are similar. Both sets of rate constants can, roughly, be fitted by the Marcus theory of atom and group transfer if the intrinsic barrier is assumed to arise exclusively from the reorganization of the quinolinium ion. Since the presumptive critical complex for H- transfer between BH3 units, B2H7-, isa known, stable ion, it is reasonable that the reorganization of BH4- does not contribute significantly to the intrinsic barrier for its reactions. The Marcus theory, and related ideas, such as the Leffler-Hammond principle, apparently can be usefully applied to reactions quite different from theoriginal model, but the exact values of the derived parameters quite possibly do not have their apparent physical significance. © 1990, American Chemical Society. All rights reserved.