Reactions of 1,1‐bis(dimethylamino)‐1,3‐butadiene with olefins – (4 + 2) cycloadditions and zwitterion formation

Publisher Website
Google Scholar
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.