Pentadienes, 1,5-hydrogen shift

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. 

Figure E.6 Atom numbering for the transition structure for the [l,5]-hydrogen shift in 1,3-pentadiene...

Interestingly, benzonitrile oxide does not react with thiirene dioxide 19b even in boiling benzene, whereas the electron-rich diene l-piperidino-2-methyl-l, 3-pentadiene (177) does react under the same reaction conditions to give the expected six-membered [4 + 2] cycloadduct 178, accompanied by sulfur dioxide extrusion and 1,3-hydrogen shift to form the conjugated system 179175 (equation 70). 

With the low-valence iron pentacarbonyl, vinylcyclopropanes 22 are thermally transformed to the diene re-complexes, the (1,3-trans-pentadiene)iron carbonyl complexes 23, through bond fission, 1,2-hydrogen shift, and stereoselective coordination [15]. (Scheme 9)... 

The degenerate [1,5] hydrogen shift reaction of (Z)-l,3-pentadiene has been studied by a number of ab intio [67-69] and semiempirical [70-72] methods. Since the energy of concert [73] for this reaction is approximately 40 kcal/mol, there is no doubt that this reaction has a concerted mechanism with an aromatic... 

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. 

---