Abstract
The acid-catalyzed dienol-benzene rearrangement of 3- and 5-methyl-substituted (2’-propiny1)cyclohexadienols has been investigated. Treatment of the dienols with CF3COOH in CCl4 yields allenyl- and (2’-propiny1)benzenes via [3,4]- and [1,2]-sigmatropic rearrangements, respectively. The reaction with H2SO4 in Et2O leeds to a mixture of allenyl-, 2’-propinyl-, 3’-butinyl- and (2’,3’-butadienyl)benzenes (Scheme 3). The latter are products of a thermal semibenzene-benzene rearrangement (cf: Scheme 9). The corresponding semibenzenes have been prepared by dehydration of the cyclohexadienols with H2SO4 or POCl3 (Schemes 6 and 7). Under acidic conditions, the p-(2’-propinyl)semibenzenes 33-35 (Scheme 8) undergo [3,4]- and [1,2]-slgmatropic rearrangements to give again allenyl- and (2’-propinyl)benzenes, whereas the thermal rearrangements to the 3‘-butinyl- and (2’,3’-butadienyl)benzenes (Scheme 9) involves a radical mechanism. Incontrast, the o-(2’-propiny1)semibenzene b (Scheme 7) leads to (2’,3‘-butadieny1)benzene 32 via a thermal [3,3]-sigmatropic rearrangement.
Lukac, Josef