Rate-controlling two-proton transfer coupled with heavy-atom motion in the 2-pyridinone-catalyzed mutarotation of tetramethylglucose. Experimental and calculated deuterium isotope effects

Primary and secondary deuterium isotope effects have been measured by polarimetry, and primary isotope effects have been calculated for the classical bifunctional catalysis: 2-pyridinone-catalyzed mutarotation of 2,3,4,6-tetra-O-methyl-..cap alpha..-D-glucopyranose (..cap alpha..-TMG) in benzene. From the positively curved plot of the specific rate of epimerization vs. the mole fraction of /sup 2/H in the ''pool'' of OH and NH hydrogens, the isotope effects k/sub HH//k/sub DD/ = 3.66 +/- 0.09, k/sub HH//k/sub DH/ = 1.5, and k/sub HH//k/sub HD/ = 2.4 have been calculated. A secondary isotope effect of 1.14 +/- 0.02 has been measured by using ..cap alpha..-TMG and (1-/sup 2/H)-2,3,4,6-tetra-O-methyl-..cap alpha..-D-glucopyranose ((l-/sup 2/H)-..cap alpha..-TMG), the synthesis of which is described in detail, together with those for (N-/sup 2/H)-2-pyridinone and (1-O-/sup 2/H)-2,3,4,6-tetra-O-methyl-..cap alpha..-D-glucopyranose ((1-O-/sup 2/H)-..cap alpha..-TMG). The rate data obtained have also been analyzed by fractionation theory, yielding approximately equal fractionation factors (0.5). The interpretation of the results has been assisted by calculations of the primary deuterium isotope effects using the BEBOVIB IV program. Two models involving small and considerable coupling, respectively, of the transferring protons to heavy-atom motion have been considered. In the favored structure for the transition state of the rate-limiting step, two protons are in transit, and their motion is governed either by a potential with a barrier or by one without. Their motion is considerably coupled to the heavy-atom motion (i.e., the breakage of the ring C-O bond), and tunnel corrections to the isotope effects are found to be negligible.