Hydrolysis of orthocarbonates. Evidence for charge imbalance in the transition state for the general acid catalyzed process

Article abstract of DOI:10.1021/ja00092a010

Catalytic constants have been measured for the hydrolysis of a range of aryl orthocarbonates, in which both the leaving group and trioxocarbenium ion moiety have been systematically varied, by neutral carboxylic acids (trichloroacetic, difluoroacetic, dichloroacetic, malonic, chloroacetic, and acetic) at 70.0°C in 60% water-40% acetonitrile, I = 1.0 M (KCI). Curvature cannot be detected in Br?nsted plots involving carboxylic acids only, but inclusion of the point for H₃O⁺ suggests downward curvature (i.e. p_x ≥ 0). β_1g Plots are curved downward (i.e. p_y′ = -?β_1g/?pK_1g > 0). Substitutent effects in the tris(aryloxy)carbenium ion fragment were quantitated by use of the experimental aqueous pK_a alues of the phenol (pK_rc), rather than Hammett σ values, since this gave better correlations for the spontaneous reactions (Kandanarachchi, P.; Sinnott, M. L. J. Am. Chem. Soc., preceding paper in this issue). Cross coefficients are large and not constant: p_xy′ (measured as ?α/?pK_1g rather than -?β_1g/?pK_HA) varies from 0.26 for (PhO)₃C⁺ to 0.16 for (PMeOC₆H₄O)₃C⁺. Likewise,p_xy (measured as -?α/?pK_rc rather than -?β_rc/?pK_HA) experiences large changes with the leaving group pK. Data to estimate p_yy′ (?β_rc/?pK_1g) are more limited, but it too changes with the pK_a of the catalyzing acid. These data indicate that a two-dimensional More O'Ferrall-Jencks diagram, with one axis representing both C-O cleavage and the ability of substituents in the tris(aryloxy)carbenium ion moiety to sense positive charge development, is inadequate to represent this reaction: separate axes are required for carbon-oxygen bond cleavage and development of carbon-oxygen double-bond character.