Nonconvergent Cycloaddition Pathways for Thermal vs. Photochemical Reaction of N-Isobutenylpyrrolidine with Dimethyl Fumarate

The thermal reaction of the title compounds proceeds via a reversibly formed 1,4-zwitterion intermediate to yield the thermodynamically most stable [2 + 2] cycloadduct. The rate of the thermal reaction is substantially faster in moderately polar vs. nonpolar solvent. In contrast, the photochemical reaction of the title compounds occurs only in nonpolar solvent and yields a stereoisomer of the thermal adduct. Photochemical cycloaddition is not observed for several other α,β-unsaturated esters and enamines. The mechanism of the photochemical reaction and the failure to observe photochemical ionic [2 + 2] cycloaddition are discussed.