Eine thermische, intramolekulare [2 + 2]-Cycloaddition eines Allenyl-benzols; Synthese von Allenylbenzolen durch säurekatalysierte Dienol-Benzol-Umlagerung
✍ Scribed by Walter Summematter; Heinz Heimgartner
- Publisher
- John Wiley and Sons
- Year
- 1984
- Tongue
- German
- Weight
- 836 KB
- Volume
- 67
- Category
- Article
- ISSN
- 0018-019X
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✦ Synopsis
A thermal Intermolecular [2 + 2]‐Cycloaddition of an Allenyl‐Allyl‐Benzene; Synthesis of Allenylbenzenes via Acid‐Catalyzed Dienol‐Benzene Rearrangement
A few years ago, it has been shown that the acid‐catalyzed dienol‐benzene rearrangement of 2‐propinyl‐substituted cyclohexadienols is a convenient synthesis for allenyl‐substituted benzene derivatives. The cyclohexadienols 20 and 21 were prepared via C‐alkylation of the corresponding phenols with 2‐propinylbromide (Scheme 3), followed by reduction of the cyclohexadienone 13 and 17 with LiAlH~4~. Treatment of 20 and 21 with p‐toluenesulfonic acid in ether at −15°) yielded the desired allenyl benzenes 8 and 9, respectively, via [3,4]‐sigmatropic rearrangements (Scheme 4). The 2‐propinylbenzenes 22–24, formed via [1,2]‐sigmatropic shift of the 2‐propinylgroup, were found as by‐products. Thermolysis of allenyl benzene 8 in decane yielded two bicyclic (25 and 26) and two tricyclic products (27 and 28; Scheme 5). For the formation of 25 and 26, a pericyclic reaction mechanism (Scheme 6) as well as a mechanism via biradical intermediates (Scheme 7) is discussed. A [2 + 2]‐cycloaddition of the α,β‐allenic and the allylic C,C‐double bound of 8 led to the tricyclic products 27 and 28 (Scheme 9). All attempts to realize a [1,7]‐sigmatropic H‐shift in the allene 9 failed so far, and the starting material underwent a rapid polymerisation.