Cope Rearrangement of 1,2,6-Heptatriene

2 Cope Rearrangement of 1,2,6-Heptatriene Roth examined the Cope rearrangement of 1,2,6-heptatriene 40 to 3-methylenehexa-l,5-diene 41 and was able to trap a diradical intermediate with oxygen. He proposed that about half of 

Unlike the other pericycUc reactions discussed previously in this chapter, the diradical species here resides in a relatively deep well, greater than 12 kcal mol , and so the PES about the diradical cannot be described as a caldera. Nonetheless, this reaction exhibits nonstatistical behavior. 

Carpenter and Borden first computed two trajectories using direct dynamics on the CASSCF(8,8)/6-31G surface. Both trajectories were initiated at 43, the TS connecting reactant to the diradical intermediate. A linear-synchronous-transit (LST) path was computed connecting 43 to the intermediate 42 and one connecting 43 to 44. The vector corresponding to these two motions was used to assign the phases of the normal modes. The trajectory for the first case (the LST toward the intermediate) did, as expected, end at 42. The second trajectory (the LST towards 

In order to compute a ensemble of trajectories. Carpenter and Borden created an AMl-SRP fit to the CASSCF(8,8) energies. About 83 percent of the trajectories went into the region of the biradical. Of these, about 8 percent exited the biradical and proceed to product 41 in less than 500 fs, too short a time to be trapped under Roth s experiment. In addition, 17 percent of the trajectories completely 

Carpenter and Borden made two important conclusions that raise significant concerns about the traditional physical organic notions of reaction mechanisms. First, nonstatistical dynamics can occur even when intermediates exist in relatively deep potential energy wells, not just on flat caldera-like surfaces. Second, multiple products can be formed from crossing a single TS. The steepest descent path from a TS can only link to a single product, but reactions can follow nonsteepest descent paths that reach different products. 

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Much experimental and theoretical work has been performed with the two allenes 1,2,6-heptatriene (32) and 1,2,6,7-octatetraene (34). Thermal isomerization of 32 leads to 3-methylene-l,5-hexadiene (346), a process that at first sight looks like a typical Cope rearrangement. However, trapping experiments with either oxygen or sulfur dioxide have shown that at least half of the rearrangement passes through the diradical 345 (Scheme 5.52) [144],... 

Figure 4 Schematic representation of the Cope rearrangement of (left) 1,2,6-heptatriene via (middle) 2-methylenecyclohexane-l, 4-diyl to (right) 3-methylene-1,5-hexa-diene...

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