Diels-Alder reaction, isotope effects

Beno, B. R., Houk, K. N., Singleton, D. A., 1996, Synchronous or Asynchronous An Experimental Transition State from a Direct Comparison of Experimental and Theoretical Kinetic Isotope Effects for a Diels-Alder Reaction , J. Am. Chem. Soc., 118, 9984. 

A. Carbon-14 and Deuterium Isotope Effect Studies of the Diels-Alder Reaction... 

Fig. 10.12 (a) Transition state structures (C—C bond lengths) calculated at two levels for the concerted and step-wise Diels-Alder reaction shown in Fig. 10.11 (Houk, K. N., Gonzalez, J., and Li, Y,Accts. Chem. Res. 28, 81 (1995). The parenthesized values show results for calculations at a much higher (and much more expensive) level, (b) Calculated secondary deuterium isotope effects, kH/kD (per D) for the concerted and stepwise Diels-Alder reactions shown in Fig. 10.11 (Houk, K. N., Gonzalez, J., and Li, Y,Accts. Chem. Res. 28, 81 (1995). The parenthesized values show results for calculations at a much higher (and much more expensive) level)...

The behavior described above has been verified by experiment and calculation on numerous substituted dienes and dienophiles. For example Fig. 10.13 shows results for 2°-D isotope effects on Diels-Alder reactions of 2-methyl-butadiene with cyano-ethylene and 1,1-dicyano-ethylene. The calculated and experimental isotope effects are in quantitative agreement with each other and with the results on (butadiene + ethylene). In each case the excellent agreement between calculated and observed isotope effects validates the concerted mechanism and establishes the structure of the transition state as that shown at the bottom center of Fig. 10.11 and the left side of Fig. 10.12a. 

As measured by the criteria of stereospecificity, regioselectivity, kinetic isotope effects, and solvent effects [117-120, 541-543], 1,3-dipolar cycloaddition reactions represent orbital symmetry-allowed [n + n s] cycloadditions, which usually follow concerted pathways Diels-Alder reactions and 1,3-dipolar cycloadditions resemble each other, as demonstrated by the small solvent effects on their bimolecular rate constants. In going from nonpolar to polar solvents, the rate constants of 1,3-dipolar cycloadditions change only by a factor of 2... 10 [120, 131-134]. 

Goldstein, E. Beno, B. Houk, K. N. Density functional theory prediction of the relative energies and isotope effects for the concoted and stepwise mechanisms of the Diels-Alder reaction of butadiene and ethylene, 7. Am. Chem. Soc. 1996, 118, 6036-6043. 

Gajewski, J. J. Peterson, K. B. Kagel, J. R. Huang, Y. C. J. Transition-statestructure variation in the Diels-Alder reaction from secondary deuterium kinetic isotope effects. The reaction of nearly symmetrical dienes and dienophUes is nearly synchronous, 7. Am. Chem. Soc. 1989, 111, 9078-9081. 

In the Diels-Alder reaction shown in equation (2.42), rationalize the observation that there is no appreciable nC isotope effect at the 2- and 3-positions of 2-methylbuta-1,3-diene (16). 

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