Eine thermische, intramolekulare [2 + 2]‐Cycloaddition eines Allenyl‐benzols; Synthese von Allenylbenzolen durch säurekatalysierte Dienol‐Benzol‐Umlagerung

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.