Kinetics of the Reduction of 3,4-Dihydroisoquinolinium Cations by 1,4-Dihydronicotinamides

Article abstract of DOI:10.1021/jo00133a015

Pseudo-first-order rate constants for the reduction of 2-methyl-3,4-dihydroisoquinolinium cation (1) by 1-(X-benzyl)-1,4-dihydronicotinamides (3) display kinetic saturation at high concentrations of 1 (20percent CH3CN-80percent H2O, 25 deg 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 ρ_x = -0.77 for the reduction of 2 (Z = 4-CN) by 3 and ρ_z = 0.83 for the reduction of 2 by 3 (X = H).Comparisons of ρ_x and ρ_z with equilibrium ρ 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 pK_R+ values to those for 2.Thus similar reduction mechanisms apply to the reduction of aromatic and nonaromatic cations by 3. pK_R+ values for pseudobase formation from 2 are correlated with a Hammett ρ = 1.72.The second-order rate constant for hydroxide ion attack on 2 (X = H) is fivefold larger than for the 2-benzyl-5-nitroisoquinolinium 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.