Studies on electron transfer as mechanistic concept for [4 + 2] cycloadditions, 2. Cycloadditions of 2,3-bis(dimethylaminomethylene)bicyclo[2.2.1]heptane and 2,3-bis(dimethylaminomethylene)bicyclo[2.2.2]octane with electron-deficient olefins

摘要: Electron transfer as initiating step in [4 + 2] cycloadditions was studied with 2,3-bis(dimethylaminomethylene)bicyclo-[2.2.1]heptane (1) and 2,3-bis(dimethylaminomethylene)bicyclo[2.2.2]octane (2) models. These dienes, having the necessary synperiplanar configuration for a concerted cycloaddition were allowed to react dienophiles of increasing acceptor capacity. Slightly activated alkenes like methyl acrylate acrylonitrile undergo smooth contrast corresponding open-chain dienes where these could not be observed. The use dimethyl fumarate maleate lead identical products due an isomerization catalyzed by 1 2. Fumaronitrile maleonitrile proved stable under reaction conditions different stereoisomers introduction three four groups, combinations cyano methoxycarbonyl leads sudden increase reactivity towards At same time are no longer stereospecific those cases they checked, radical ions detected either ESR spectroscopy or stopped-flow visible spectroscopy. This applies particular dichloromethane solvent. Stopped-flow measurements combination rapid-scan unit reveals transient absorption at λ ∽ 420 nm acetonitrile 1,1-dicyano-2,2-bis(methoxycarbonyl)ethene dicyanofumarate which is ascribed zwitterionic intermediates comparison related systems. TCNE 2 thermally unstable adducts both cases. only observed solution but also isolated solid products, if placed polar environment KBr complete dissociation seems take place. relevance studies mechanism Diels-Alder reactions that suitable model systems can constructed electron Due loss stereospecificity it concluded principal their majority occur retention stereochemistry reactants.