Reactions of 1,1-bis(dimethylamino)-1,3-butadiene with olefins – (4 + 2) cycloadditions and zwitterion formation |
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Authors: | Reiner Sustman Monika Rogge Ursula Nüchter Heinz Bandmann |
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Affiliation: | Institut für Organische Chemie der Universität Essen, Universitätstraße 5, W-4300 Essen 1 (F.R.G.) |
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Abstract: | 1,1-Bis(dimethylamino)-1,3-butadiene ( 1 ) as a strong donor diene (E1/2 = 0.03 V vs. SCE, 1st IPv = 6.94 eV) is treated with acrylonitrile, dimethyl dicyanofumarate, and tetracyanoethylene. Cycloaddition with acrylonitrile is slow and requires elevated temperatures at which elimination of dimethylamine with formation of 1-cyano-2-(dimethylamino)-1,3-cyclohexadiene occurs. The reaction of 1 with TCNE at −40°C in acetonitrile is very fast and leads to the zwitterion 5 . At T ⩾ −20°C, 5 eliminates hydrogen cyanide to give a highly colored (λmax = 489 nm, lgϵ = 4.716) merocyanine. A zwitterion 9 generated from 1 and dimethyl dicyanofumarate can be isolated in crystalline form. The structural analysis shows that the zwitterions are produced by attack of the olefin at the antiperiplanar conformation of 1 . Both zwitterions which can be trapped as crystalline picrates cannot be converted to cycloadducts. These results suggest that cycloaddition is observed if zwitterion formation becomes energetically unfavorable. |
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Keywords: | (4 + 2) Cycloaddition Zwitterion formation Ketene aminal |
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