ALPHA-DEUTERIUM AND C-13 ISOTOPE EFFECTS FOR A SIMPLE, INTER-MOLECULAR SULFUR-TO-OXYGEN METHYL-TRANSFER REACTION - TRANSITION-STATE STRUCTURES AND ISOTOPE EFFECTS IN TRANSMETHYLATION AND TRANSALKYLATION

The transmethylation reaction of isotopically labeled S-methyldibenzothiophenium (l2CH3,-SC12H8 + 13CH3-SC12H8 +, and 12CD3-SC12H8 +) tetrafluoroborates with methoxide ion in methanol at 25°C shows k3H/k3D= 0.97±0.02 and k12/k13= 1.08±0.02. The large carbon isotope effect is similar to that for enzymic transmethylation and is consistent with a central, roughly planar methyl group in the transition state. The α-D effect is compared with others for enzymic and nonenzymic transmethylation and transalkylation reactions by estimating the transition-state fractionation factor ϕTrelative to ethane, using the calculated factors of Hartshorn and Shiner. For 22 transmethylation reactions, ϕTis closely described in terms of reactant (ϕR) and product (ϕP) factors by ϕT= O.99(ϕRϕP)0.70. Transmethylation transition states appear structurally implastic with a roughly constant, probably high valency to methyl. Transalkylation transition states appear to be looser, relative to reactants and products, than transmethylation transition states and also far more structurally plastic. The enzymic transition state has an unusually large fractionation factor, consistent with transition-state compression as a mechanism of enzymic catalysis. © 1979, American Chemical Society. All rights reserved.