Die Dienol‐Benzol‐Umlagerung von Allyldienolen: aromatische [1,2]‐, [3,3]‐ und [3,4]‐sigmatropische Umlagerungen

Abstract

Abstract The dienol‐benzene rearrangement of syn and anti ‐4‐allyl‐4‐methylcyclohexa‐2,5‐dien‐1‐ol ( syn and anti 15) occurs by formation of a benzonium ion intermediate in p ‐toluene‐sulphonic acid in ether below 0° and leads to a mixture of 2‐, 3‐ and 4‐allyltoluenes in the ratio 54:10:36. By the introduction of 14 C‐, D‐ and methyl labelled dienols it is shown that only the allyl group migrates and that this rearrangement is an intramolecular, one‐step process. The formation of 2‐allyltoluene occurs with retention, whereas the 3‐ and 4‐allyltoluenes are formed by inversion of the carbon skeleton of the migrating allyl group. These rearrangements can be therefore classified as suprafacial, aromatic sigmatropic reactions of the order [1,2], [3,3] and [3,4]. The transition state can be postulated as representing a positively charged complex consisting of interacting allyl and tolyl radicals. The interaction of the two parts is controlled by the symmetry of the highest occupied π‐orbitals (ψ 3 for toluene and ψ 2 for the allyl group) in agreement with the Woodward‐Hoffmann rules. The better “distribution” of the charge in the transition state of these reactions in comparison to the ground state is chiefly responsible for the CoPE‐like [3,3] sigmatropic reaction occurring at low temperatures. In general, sigmatropic reactions in charged systems are faster. The rearrangement of syn and anti 2‐allyl‐2‐methylcyclohexa‐3,5‐dien‐1‐ol ( syn and anti 28) gives results similar to those obtained with the para ‐allyldienols. The thermal rearrangement of 15 and 28 gives 3‐allyltoluene by a [3,3] sigmatropic Cope rearrangement followed by elimination of water.