Zwitterion from a cyclopropane with geminal donor and acceptor groups.
2,2-Dimethoxy-3,3-dicyanospiro[cyclopropane-1,9'-[9H]fluorene] reacted fast with methanol to afford 9-trimethoxymethyl-9-dicyanomethyl-9H-fluorene. Reaction with benzaldehydes also gave products of cyclopropane ring opening. Strong electron-donor p-substituents or a strong attractor enhanced the rate. Ring opening of the cyclopropane to a zwitterion that recloses or reacts with an aryl aldehyde, to form either a CO or a CC bond first, can explain the result. The former mode of closure is sensitive to p-substituents because they are directly conjugated to the positive charge at the benzylic carbon of the former aldehyde. The latter mode is sensitive to the ground-state electrophilicity of the carbonyl carbon of the former aldehyde. Thus, reaction of the cyclopropane with p-substituted aldehydes is accelerated by either electron-donor or -acceptor substituents. [reaction: see text].
Sliwinska
Anna
A
Department of Chemistry, McMaster University, Hamilton, Ontario L8S 4M1, Canada.
Czardybon
Wojciech
W
Warkentin
John
J