STABILIZATION OF A PUTATIVE CYCLOHEXANE-1,4-DIYL INTERMEDIATE ELICITS AN ANTARAFACIAL COPE REARRANGEMENT VIA A STEPWISE MECHANISM - PYROLYSIS OF (R,E)-5-METHYL-1,2,6-OCTATRIENE TO 4-METHYL-3-METHYLENE-1,5-HEPTADIENE

The pyrolysis of (E)-5-methylocta-1,2,6-triene (1) has been designed to bring to light a stepwise Cope rearrangement by stabilization of the putative biradical intermediate. Compound 1 rearranges to (E)-4-methyl-3-methylene-1,5-heptadiene ((E)-3) with the Arrhenius parameters E(a) = 31.7 kcal/mol and log A 10.6 (A in s 1). (R,E)-5-Methylocta-1,2,6-triene ((R,E)-1), 90.8 +/- 0.8% enantiomerically pure, upon pyrolysis at 440 K gives (R,E)-3 with 62% ee, which corresponds to 68% retention of enantiomeric specificity in the rearrangement to E-product. Antarafacial participation of one allylic unit thus accounts for about 16% of the reaction. The simplest mechanistic interpretation involves two competitive chairlike pathways which lead respectively to the axial and equatorial biradicals 4 and 6. Products are formed by cleavage of the C3-C4 bond of the biradicals, either before or after internal rotations to conformational isomers 5 and 7.