Diels-Alder Reactions of (Trifluoromethyl)ethene and (Trifluoromethyl)styrenes with Functionalized Butadienes

The Diels-Alder reactions of (trifluoromethyl)ethene (1) with 2-(trimethylsiloxy)buta-1,3-diene (2), 2-[(trimethylsilyl)methyl]-1,3-butadiene (4), and 1-methoxy-3-(trimethylsiloxy)buta-1,3-diene (6) were carried out to give the corresponding [4 + 2] cycloadducts in 17–38% yields. It was found that the former two cycloadducts were a mixture of para (major) and meta (minor) isomers, while the latter was the para isomer exclusively. Similarly, β -(trifluoromethyl)-4-(methoxycarbonyl)styrene (9) and β -(trifluoromethyl)-4-nitrostyrene (10) were allowed to react with 4 and 6, giving the corresponding [4 + 2] cycloadducts in 56–90% yields. The regioselectivity of the reaction on using 6 as the diene turned out to be extremely high, leading to the formation of only one regioisomer. The substituent effect of the trifluoromethyl group in the Diels-Alder reaction in terms of regioselectivity is discussed.