Aromatische [1,5s]‐sigmatropische H‐Verschiebungen in Arylallenen

Abstract 1‐Mesityl allene ( 1 ), 1‐mesityl‐3‐methyl allene ( 2 ) and 1‐mesityl‐3,3‐dimethyl allene ( 3 ) rearrange thermally at 150–190° in decane via [1,5 s ]sigmatropic H‐shifts to yield the o ‐quinodimethanes 4 , which cyclise to give the 1,2‐dihydronaphthalenes 5 and 6 and/or undergo [1,7 a ]sigmatropic H‐shifts to give 1‐mesityl‐( Z )‐buta‐1, 3‐dienes ( Z )‐ 7 and ( Z )‐ 8 , respectively ( Schemes 1,3,4 and 5 ) in almost quantitative yields. The activation parameters of these isomerisations are given in Table 1. 1‐Mesityl‐1‐methyl allene ( 9 ) isomerises at 190° to give 4,5,7‐trimethyl‐1,2‐dihydronaphthalene ( 17 ) in 50% yield ( Scheme 6 ). 2′‐Isopropylphenyl allene ( 10 ) in decane rearranges at 170° to 1‐( Z )‐propenyl‐2‐isopropenyl‐benzene (( Z )‐ 19 , Scheme 7 ). Deuterium labelling experiments show that the rate determining step is an aromatic [1,5s]sigmatropic hydrogen shift from an sp 3 ‐ to an sp‐hybridised carbon atom. The primary kinetic isotopic effect (k H /k D ) is 3.45, while the secondary βisotopic effect is 1.20 (Scheme 7 and Table 2).