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Solvent-Dependance in Organocatalyzed Diels-Alder Reactions
J. B. Brazier, K. M. Jones, J. A. Platts*, N. C. O. Tomkinson*
Cardiff University, UK
18 March 2011 (online)
The authors investigated the role of protic solvent systems for the imidazolidinone 1 catalyzed Diels-Alder reaction of cinnamaldehyde and cyclopentadiene. In aprotic solvents the addition of water presumably accelerates iminium ion formation (shown by kinetic measurements), as well as product liberation by faster hydrolysis, prior to counterproductive and race-mizing retro-Diels-Alder reaction. In alcoholic solvents, such as methanol, the role of water appears to be somewhat more complex (see Scheme). The equilibrium of the parent aldehyde and its dimethyl acetal appears to be shifted to the aldehyde side, thus facilitating more efficient iminium ion formation and faster Diels-Alder reaction.