Reactions of 1,1-bis(dimethylamino)-1,3-butadiene with olefins. [4+2]-Cycloadditions and zwitterion formation.
1,1-Bis(dimethylamino)-1,3-butadiene (I) as a strong donor diene (E1/2 = 0.03 V vs. SCE, 1st IPv = 6.94 eV) is treated with acrylonitrile, di-Me dicyanofurmarate, and tetracyanoethylene. Cycloaddn. with acrylonitrile is slow and requires elevated temps. at which elimination of dimethylamine with formation of 1-cyano-2-(dimethylamino)-1,3-cyclohexadiene occurs. The reaction of I with TCNE a -40 Deg in acetonitrile is very fast and leads to the zwitterion (NC)2C-C(CN)2CH2CH:CHC+(NMe2)2 (II). At T >= -20 Deg, II eliminates HCN to give a highly colored (lmax = 489 nm, lg e = 4.716) merocyanine. A zwitterion, MeO2C(NC)C-C(CN)(CO2Me)CH2CH:CHC+(NMe2)2, generated from I and di-Me dicyanofumarate can be isolated in cryst. form. The structural anal. shows that the zwitterions are produced by attack of the olefin at the antiperiplanar conformation of I. Both zwitterions which can be trapped as cryst. picrates cannot be converted to cycloadducts. These results suggest that cycloaddn. is obsd. if zwitterion formation becomes energetically unfavorable.
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