Abstract
1-Mesitylallene (1), 1-mesityl-3-methylallene (2) and 1-mesityl-3,3-dimethyl allene (3) rearrange thermally at 150-190° in decane via [1,5s]sigmatropic H-shifts to yield the o-quinodimethanes 4, which cyclise to give the 1,2-dihydronaphthalenes 5 and 6 and/or undergo [1,7a]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 Table1. 1-Mesityl-1-methylallenc (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-isopropenylbenzene ((Z)-19, Scheme 7). Deuterium labelling experiments show that the rate determining step is an aromatic [1,5s] sigmatropic hydrogen shift from an sp3- to an sp-hybridised carbon atom. The primary kinetic isotopic effect (kH/kD) is 3.45, while the secondary beta-isotopic effect is 1.20 (Scheme 7 and Table 2).
Heimgartner, Heinz