PM3 study of the domino reaction of nitroalkenes with silyl enol ethers

The molecular mechanism of the domino reaction of nitroalkenes with silyl e nol ethers to give nitroso acetal adducts was characterized using computati onal procedures at the PM3 semiempirical level. The domino process comprise s three consecutive steps: the first and rate-determining step is the nucle ophilic attack of the silyl enol ether on the nitroalkene to give a zwitter ionic intermediate; closure of this intermediate leads to a nitronate inter mediate, which then affords the final nitroso acetal adduct through an intr amolecular [3 + 2] cycloaddition. The presence of both silicon and oxygen a toms in the silyl enol ether increases the nucleophilic character of the ca rbon-carbon double bond and favors the ionic character of the first step. T he presence of the Lewis acid promotes the delocalization of the negative c harge transferred in the nucleophilic attack of the silyl enol ether to the nitroalkene and decreases the activation energy of the rate-determining st ep. The inclusion of solvent effects predicts a stabilization of the first zwitterionic intermediate and therefore emphasizes the stepwise mechanism f or the first cycloaddition of this domino reaction. Copyright (C) 1999 John Wiley gr Sons, Ltd.