Sigmatropic shifts 1,5-hexadiene

One of the double bonds of an allenic moiety can participate in Cope rearrangements. Thus, the bicyclic pentene derivative 1 undergoes a [1.3] sigmatropic shift at 220 =C to give 2. Cope rearrangement of 2 produces 3-methylene-l,5-hexadiene (4) at 300"C1107. 

Thus, the case for a non-concerted 3,3-shift via a cyclohexane-1,4-diyl is weak. Nonetheless, substituent effects on the rate of the 3,3-shift were intially interpreted in terms of the diyl species. In particular, Dewar found the 2-phenyl and 2,5,-diphenyl-l,5-hexadiene rearrange 40 and 1600 times, respectively, more rapidly than that of the parent diene. Further, semi-empirical MINDO/3 calculations supported the proposition that even the parent species proceeded via the chair-like cyclohexane-1,4-diyl. These observations and calculations provided stimulus for a substantial effort in the subsequent years to address the question of transition state structure in and the energy surface for the 3,3-sigmatropic shift of 1,5-hexadiene. 

The reverse of the ene reaction is the retroene reaction. Equation 11.40 shows the conversion of ds-l-methyl-2-vinylcyclopropane (90) to a s-1,4-hexadiene (91) by such a process. This reaction can also be described as a homo-[l,5] sigmatropic shift. 

TABLE 7. Transition state bond oiders for 3,3-sigmatropic shifts in 1,5-hexadienes. ... 

In 1962, W. von E. Doering and W. R Roth carried out a brilliant experiment that determined which of these two possible transition states was favored for this [3,3] sigmatropic shift. They used a stereolabeled molecule, 3,4-dimethyl-l,5-hexadiene (Fig. 20.54). Notice first that there are three forms of this molecule, a meso, achiral form and a racemic pair of enantiomers (p. 156). It all depends on how the methyl groups are attached. 

In the similar manner [3, 3]-sigmatropic shift of 1, 5-hexadiene is +0 +71 ] concerted cycloaddition. 

Table 10 Transition-stracture Geometries and Activation Energies of the [3,3]-Sigmatropic Shift of 1,5-Hexadiene...

Thermal sigmatropic 1,5 hydrogen shifts are quite common in certain allene and diene systems, cis- 1,3-Dienes have the proper geometric arrangement to undergo a thermally allowed suprafacial hydrogen shift, cis-1,3-Hexadiene, for instance, gives... 

The possibility of rearrangement in pentadienyl anions must be borne in mind when they are employed synthetically. When 1- or 5-alkyl groups are present, intramolecular 1,6-sigmatropic hydrogen shifts are possible and the stereochemistry follows Woodward-Hoffmann rules, being thermally antara-facial but photochemically suprafacial. Bates, for example, showed that the same equilibrium mixture of isomers results at 40°C from the deprotonation of either 5-methyl-1,4-hexadiene or 2-methyl-1,4-hexadiene (79). The tendency is to form isomers with fewer alkyl groups in the 1,3, and 5 positions of the delocalized system (50). 

This reaction is an example of a 1,3-shift that is suprafacial for both components and involves two 7i-systems, each with 3 electrons. The MOs of 1,5-hexadiene and the Cope-rearrangement transition state show the reacting orbitals. Table 4.3 gives the Woodward-Hoffmann rules for sigmatropic rearrangements between 7C-systems with I and / electrons. 

---