Elimination reactions tunnelling effects

Lin, S., Saunders, W. H. (1994) Tunneling in elimination reactions -structural effects on the secondary beta-tritium isotope effect, J. Am. Chem. Soc. 116, 6107-6110. 

A very large deuterium isotope effect has been observed240 by ESR at 77 K on hydrogen-deuterium elimination reaction from 2,3-dimethylbutane (H-DMB)-SFg and 2,3-dimethylbutane-2,3-D2 (D-DMB)-SFg (0.6 mol% mixtures), /-irradiated at 70 K and then stored at 77 K. The significant isotope effect, h2 Ad2 = 1-69 x 104 at 77 K, has been explained by tunnelling elimination of hydrogen (H2) molecules from a DMB+ ion240. 

Kaldor, S.B., Eredenburg, M.E. and Saunders, W.H. (1980). Mechanisms of elimination reactions 32. Tritium isotope effects and tunnel effects in the reaction of 2,2-diphenylethyl-2-t derivatives with various bases. J. Am. Chem. Soc. 102, 6296-6299... 

Lately, it was observed that the measured a-D KIE considerably exceeds the maximum value of the expected isotope effect. Thus, the a-D KIE for proton dissociation in some elimination reactions was found to be much higher than the expected maximum. Such results are ascribed to tunneling. That tunneling can contribute not only to primary but also to secondary KIE is sup-... 

We should remember (1) that the activation energy of eh reactions is nearly constant at 3.5 0.5 Kcal/mole, although the rate of reaction varies by more than ten orders of magnitude and (2) that all eh reactions are exothermic. To some extent, other solvated electron reactions behave similarly. The theory of solvated electron reaction usually follows that of ETR in solution with some modifications. We will first describe these theories briefly. This will be followed by a critique by Hart and Anbar (1970), who favor a tunneling mechanism. Here we are only concerned with fe, the effect of diffusion having been eliminated by applying Eq. (6.18). Second, we only consider simple ETRs where no bonds are created or destroyed. However, the comparison of theory and experiment in this respect is appropriate, as one usually measures the rate of disappearance of es rather than the rate of formation of a product. 

There are two ways in which an enzymic reaction can fail to satisfy the Swain-Schaad relationship, one of which is if tunneling occurs. In order to use violations of this rule to diagnose the presence of tunneling, it is necessary to eliminate the other possible reason for a violation, namely, limitation of the rate by more than one step. The derivation of the Swain-Schaad equation in Chart 3 assumes that the step that produces the isotope effect is fully rate-limiting, and if this should be untrue, then the relationship should fail without any significance for tunneling. 

The secondary Hke/T H KIE in the eliminations of 373, 374 and 375 presented above which are higher than this maximum possible secondary IE value, are taken as strongly implicating tunnelling. This conclusion has been supported also by intercomparison of secondary H/T and D/T isotope effects in E2 reactions of RNM3 1 Br at 50 °C. The secondary IE is depressed markedly when deuterium rather than proton is transferred, which also implicates tunnelling ... 

In view of the complications imposed on interpretation of kinetic isotope effects by quantum mechanical tunnelling and a variable profile of isotope effect with proton transfer to different bases, a more certain prediction would seem most probable if comparisons are restricted to reactions of a series of similar substrates within a given reaction medium. Within this framework it is possible to make reasonable predictions of the effect of substrate structure on the nature of the transition state for elimination using only primary kinetic hydrogen isotope effects. 

The elimination of a selenoxide is a common, mild method for the formation of a carbon-carbon double bond, occuring via a syn-elimination pathway as shown to the right. The activation energy difference between the H- and D-substituted compounds was found to be 2.52 kcal / mol, giving an isotope effect of 74. Other experimental parameters were also found to support tunneling. For example, the A l Aq is 0.092, considerably different from 1, Furthermore, a barrier width for the reaction was calculated to be 0.82 A, definitely less than the length of a common C-H bond (1.1 A). 

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