Thermally Initiated Reactions of Allyl sec-Butyl Sulfone. Observation of a -Allylic Rearrangement

Article abstract of DOI:10.1021/jo00213a010

Allyl alkyl sulfones undergo thermal rearrangement to afford alkenes and sulfur dioxide.Details of the mechanism were investigated by studying the thermolysis of allyl sec-butyl sulfone (1).Gas-phase pyrolysis of 1 afforded propene, butenes, isopentane, 1,5-hexadiene, and 4-methyl-1-hexene (3) as major products.The activation energy for the process was determined to be 41.4 +/- 2.3 kcal/mol in the temperature range 220-272 deg C.As the temperature for the pyrolysis was lowered from 560 deg C to 192 deg C, the ratio of 1,5-hexadiene to 4-methyl-1-hexene (3) changed from 3.89 to 0.04.Furthermore, pyrolysis of allyl-α,α-d2 sec-butyl sulfone (1-d2) at 580 deg C gave a 1:1 mixture of 1,1- and 3,3-dideuterio-4-methyl-1-hexene, but the ratio changed to 1:4.2 at 278 deg C.Control experiments illustrated that at least some of the observed deuterium scrambling was due to the existence of a <1,3>-allylic rearrangement within the starting sulfone.Pyrolysis of optically active allyl sec-butyl sulfone ((R)-1^*) at 580 deg C and 278 deg C afforded racemic 4-methyl-1-hexene (3).A control experiment demonstrated that recovered sulfone was not racemized.These results suggest that the rearrangement changes from free radical to a more selective and perhaps concerted mechanism as the temperature for the pyrolysis is lowered.Taking this factor into account, it was possible to convert the observed rate constants into contribution from the two competing pathways, k_c and k_r.In this way, it was determined that the activation energies for the concerted and free radical pathways were 39.6 (log A = 11.8) and 48.1 kcal/mol (log A = 14.9), respectively.Kinetic simulation provided a good fit to the existing data and allowed a determination of the rate constant for the <1,3>-allylic rearrangement; good agreement with the existing literature value was obtained.