trans-1-(Dimethylamino)-1,3-butadiene - concerted and stepwise (4+2) cycloadditions.
Me2NCH:CHCH:CH2 is allowed to react with Me acrylate, acrylonitrile, maleo- and fumaronitrile, di-Me maleate and fumarate, and with di-Me dicyanomaleate and dicyanofumarate. In all cases except the last two mixts. (endo/exo) of cycloadducts are formed. Structures are detd. by a combination of one- and two-dimensional NMR spectroscopy. The E/Z isomers maleo- and fumaronitrile add stereospecifically to the diene. Cycloaddn. of di-Me maleate and fumarate leads to the same mixt. of isomers in almost the same ratio. The reaction of di-Me dicyanomaleate and di-Me dicyanofurmarate yields a single cycloadduct which seems to be the thermodynamically most stable one. The results are interpreted in such a manner that a concerted cycloaddn. takes place with maleo- and fumaronitrile, while stepwise reactions, presumably proceeding via zwitterions, are obsd. with di-Me maleate, fumarate, dicyanomaleate and dicyanofumarate. With increasing capability of the dienophile to stabilize a neg. charge the mechanism changes from a concerted to a stepwise reaction.
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