Aromatische sigmatropische H-Verschiebungen und Claisen-Umlagerungen

Abstract

A general introduction into sigmatropic H- and C-shifts is given. In the following chapter aromatic sigmatropic H-transfers as well as O,C and C,C [3,3]- and [5,5]-rearrangements are discussed. Thermal [1,5s] H-transfers occur in ortho-vinyl phenols (cf. scheme 11). This can be demonstrated by deuterium labelling experiments. The thermal cis-trans isomerisation of ortho-propenyl phenols is also induced by [1,5s] H-shifts (cf. scheme 12). Thermal [1,5s] homosigmatropic H-shifts are observed in ortho-allyl substituted phenols (cf. scheme 14 and 15). [1,7a] sigmatropic H-transfers occur when ortho-(buta-1,3-dienyl) phenols are transformed thermally into Δ³-chromenes (cf. scheme 17 and 19). H,C/H,C [1,7a]-rearrangements cause the thermal equilibration of cis,cis- and cis,trans-ortho-dipropenyl benzene (cf. scheme 22).
Aromatic [3s,3s] sigmatropic rearrangements represent the first reaction step of the conversion of propargyl aryl ethers into Δ³-chromenes (cf. scheme 31 and 32).
The Claisen rearrangement of ortho-allyl cyclohexadienones, also a [3s, 3s] rearrangement, proceeds via a chair-like and boat-like geometry of the activated complex (cf. scheme 29 and 30). [5s,5s] sigmatropic rearrangements were established in the thermolysis of penta-2,4-dienyl phenyl ethers which are transformed into para-penta-2,4-dienyl-phenole (cf. scheme 33). The thermal behaviour of certain quinone ketals can also be explained by degenerated [5s,5s] rearrangements (cf. scheme 37).