Pseudo-first-order rate constants for the reduction of the 2-methyl-3,4-dihydroisoquinoiinium cation (1) by l-(X-benzyl)-!,4-dihydronicotinamides (3) display kinetic saturation at high concentrations of 1 (20% CH3CN-80% H20, 25 °C, ionic strength 1.0). Association constants for 1:1 complex formation are independent of X (1.4 ± 0.2 M"* 1 *) and are most simply interpreted in terms of nonproductive complex formation. Pseudo-first-order rate constants for the reduction of 2-(Z-benzyl)-3,4-dihydroisoquinolinium cations (2) by 3 are linear for [2] up to approximately 60 mM. Hammett correlations for the second-order rate constants for these reactions give p, = -0.77 for the reduction of 2 (Z = 4-CN) by 3 and pz = 0.83 for the reduction of 2 by 3 (X = H). Comparisons of px and pz with equilibrium p values for closely related reactions indicate that the migrating hydrogen atom bears a charge of -0.33 and thus is clearly hydridic in character. These results are closely analogous to the conclusions of our earlier study of the 1,4-dihydronicotinamide reduction of 5-nitroisoquinolinium cations which have similar pKR+ values to those for 2. Thus similar reduction mechanisms apply to the reduction of aromatic and nonaromatic cations by 3. pKR+ values for pseudobase formation from 2 are correlated with a Hammett p = 1.72. The second-order rate constant for hydroxide ion attack on 2 (X = H) is fivefold larger than for the 2-benzyl-5nitroisoquinolinium cation, although the second-order rate constant for reduction by 3 (X = H) is 23-fold greater for the latter cation than for 2 (X = H). This is interpreted in terms of a poorer "fit" between 2 and 3 in the transition state for the reduction, relative to the better "fit" between the planar 5-nitroisoquinolinium cations and 3.
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