Hydrogen shift in pentadiene

Table 7. [1,5] Transition structure geometries, activation energies, and reaction energies for transition structures of [l,5]-hydrogen shift in pentadiene...

The greater flexibility of the linear dienes should - and does - show up in a negative entropy of activation, but should not raise the energy of activation unless the transition state is quite strained that for a suprafacial hydrogen shift in pentadiene is not. 

Let us finally consider two Z-matrices for optimization to transition structures, the Diels-Alder reaction of butadiene and ethylene, and the [l,5]-hydrogen shift in Z-1,3-pentadiene. To enforce the symmetries of the TSs (Cj in both cases) it is again advantageous to use dummy atoms. 

The [l,5]-hydrogen shift in Z-l,3-pentadiene is an example of a narcissistic reaction, with the reactant and product being identical. The TS is therefore located exactly at half-way, and has a symmetry different from either the reactant or product. By suitable constraints on the geometry the TS may therefore be located by a minimization within a symmetry consti ained geometry. 

In a related study, Borden and coworkers have examined whether other pericyclic reactions might express chameleonic behavior. Using B3LYP/6-31G calculations, they located transition states for the 1,5-hydrogen shift in phenyl-substituted 1,3-pentadienes (21-23). The activation enthalpy for the... 

Hayase, S. Hrovat, D. A. Borden, W. T. A B3LYP study of the effects of phenyl substituents on 1,5-hydrogen shifts in 3-(Z)-l,3-pentadiene provides evidence against a chameleonic transition structure, J. Am. Chem. Soc. 2004,126,10028-10034. 

The kinetics of [1,5] hydrogen shifts in some alkyl-substituted (Z)-l,3-pentadienes have been determined23 50-52. From kinetic analyses53,54 of substituent effects on the reaction, several trends are evident. In the temperature range studied (130-250 l C), the [1,5] sigmatropic hydrogen shift is usually reversible. Only if there is an appreciable thermodynamic stabilization of the product is the reaction found to be unidirectional (Table 2, entries 7, 13, 16). 

Table 2. Activation Parameters and Relative Rate Constants for [1,5] Sigmatropic Hydrogen Shifts in Selected Acyclic 1,3-Pentadienes...

From the kinetic parameters it is evident that the [1,5] hydrogen shift in 1.3-cyclohexadienes demands a substantially higher activation energy (by 12-15 keal/mol)34-36 than the analogous reaction in 1,3-cyclopentadiene (vide supra), but the amount required is similar to that in acyclic (Z)-l,3-pentadienes. 

The 1,5-hydrogen shift in c 5 -l,3-pentadienes is a facile thermal process first described by Wolinsky (Scheme 6.41). ... 

Okamura determined the primary deuterium kinetic isotope effect in a vitamin D-like model system as a function of temperature to obtain d - h = 1. 62 kcal/mol and Ah/Ad = 0.835 (Scheme 7.56). These results suggest a normal primary KIE without tunneling, not unlike those determined by Roth for the 1,5-hydrogen shift in cw-l,3-pentadiene (see Chapter 6, Section 2). 

FIGURE 3.7 [l,5]-Sigmatropic hydrogen shift in cis-1,3-pentadiene. Temperature dependence of the rate constant for the dideuterated compound. 

---