Shift in 1,3-pentadiene

Rearrangement of (v) to (VI) exhibits large kinetic isotope effect of 12.2 at 25°C which is consistent with highly symmetrical transition state in a concerted process. [1, 5]-shift is preferred over [1, 3]-shift has been shown by Roth. The [1, 5]-shift takes of deuterium label in (VII) over all aromatic positions at high temperature, inspite of necessity to proceed through unstable isoindene. But base catalysed reaction involve [1, 3]-shift. 

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

This view also explains satisfactorily why a [1, 3] sigmatropic shift of a hydrogen in 1, 3 pentadiene will not be possible. In this case, a [1, 3] shift would have to be antarafacial and such a rearrangement would be structurally prohibited. 

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 thermal [1,5] sigmatropic hydrogen shift is an allowed suprafacial process, which has been investigated at the SCF level in 1,3-pentadiene, in -hydroxyacrolein and in cyclopentadiene. ... 

WORKED PROBLEM 20.12 Estimate the bond dissociation energy (BDE) for the migrating carbon—hydrogen bond in 1,3-pentadiene. Why is the observed activation energy of 36-38 kcal/mol for the [1,5] shift of hydrogen shown in Rgure 20.30a so much lower than your answer ... 

How are sigmatropic shifts formally described The first step is to identify the bond that is broken in the reaction and the bond that is made. In other words, write an arrow formalism. The bond that breaks in 1,3-pentadiene could he designated the l-l bond. Remember that these numbers have nothing to do with the lUPAC name. Next, note where the new bond is formed. In Figure 20.32a, the new bond is [1,5], in (b) it is [2,3], and in (c) it is [3,3].The numbers denoting the shift are always enclosed in brackets and separated by a comma. 

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 1,4 polymerization of trans-1.3-pentadiene has been studied by Natta, Porri and coworkers. Their results also show that the cis polymerization occurs with catalysts of ionicities in the middle region, while the trans structures come in the more ionic regions. The methyl group at the end of the diene systems shifts the transition points... 

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... 

Aryl substitution produces a bathochromic shift on 1,4-pentadienes so that standard reaction conditions (in solution at X > 250 nm) can be applied. The reaction of equation (4) shows one of the latest examples aimed at understanding the effect of an hydroxy or alkoxy function at the 3-position on the photorearrangement.In the case of the free hydroxy group, two secondary photoproducts were observed besides the two expected vinylcyclopropanes. At variance with this observation, no secondary photoproducts were detected for the phenyldimethylsilyloxy substituent. Direct or benzophenone-sen-sitized irradiation did not cause significant variation in the reaction products. 

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

The potential 3,3-shift product, which might be unstable with respect to starting material, could also be reluctant to tautomerize to the still more stable aromatic derivative so the pyrolyses were also conducted in deuterated tert-butyl alcohol solvent in the presence of its conjugate base. Under these circumstances, 1-phenyl-1,3-pentadiene was formed having deuterium distributed over the non-aromatic carbons (Scheme 12.18). 

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