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

Abstract

A Thermal Intramolecular [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 cyclohexadienones 13 and 17 with LiAIH4. 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-propinyl group, 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 alpha,beta-allenic and the
allylic C,C-double bond of 8 led to the tricyclic products 27 and 28 (Scheme 8). In this case too, a mechanism via biradical intermediates can be formulated (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.