Umlagerungsreaktionen von (2′‐Propinyl)cyclohexadienolen und ‐semibenzolen

Rearrangements of (2′‐Propinyl)cyclohexadienols and ‐semibenzenes The acid‐catalyzed dienol‐benzene rearrangement of 3‐ and 5‐methyl‐substituted (2′‐propinyl)cyclohexadienols has been investigated. Treatment of the dienols with CF 3 COOH in CCl 4 yields allenyl‐ and (2′‐propinyl)benzenes via [3,4]‐ and [1,2]‐sigmatropic rearrangements, respectively. The reaction with H 2 SO 4 in Et 2 O 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 H 2 SO 4 or POCl 3 ( Schemes 6 and 7 ). Under acidic conditions, the p‐(2′‐propinyl)semibenzenes 33–35 (Scheme 8) undergo [3,4]‐ and [1,2]‐sigmatropic 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. In contrast, the o ‐(2′‐propinyl)semibenzene b (Scheme 7) leads to (2′,3′‐butadienyl)benzene 32 via a thermal [3,3]‐sigmatropic rearrangement.