Photodimers of N-alkyl-3,4-dimethylmaleimides—Product ratios and reaction mechanism

The photodimerization of three N-alkyl-3,4-dimethylmaleimides (DMI) – frequently used to form crosslinks in polymers – was examined. N-Isobutyl-3,4-dimethylmaleimide, N-(2-acetamido)ethyl-3,4-dimethylmaleimide, N-(3-propionic acid)-3,4-dimethylmaleimide were studied by steady-state and time-resolved photolysis in various solvents. Unexpectedly, under all conditions a single-bonded dimer distinct from the well-known cyclobutane fused cyclic dimer was identified. Depending on the solvent the single-bonded dimer amounts to 5–50% of the product yield, allowing for considerable selectivity. Dimerization quantum yields, Φdim, range from 0.01 to 0.5. Results from time-resolved experiments establish the involvement of the triplet state in the photodimerization mechanism, the quantum yield of singlet molecular oxygen formation accounts for the Φdim values. Electron transfer and hydrogen abstraction reactions of DMI in polar and protic solvents, in contrast to N-alkylmaleimides, are inefficient processes.