Ruthenium-Catalyzed Pauson-Khand Synthesis of Maleimides

Significance

A hetero-Pauson-Khand [2+2+1] cocyclization of isocyanates, alkynes, and CO to give polysubstituted maleimides in 82-97% yields is reported. Of the catalysts tested, Ru₃(CO)₁₂ shows the greatest reactivity and mesitylene is the best solvent. An excess (3 equiv) of isocyanates relative to alkynes is needed to avoid cyclotrimeri­zation of alkynes. Alkyl- or aryl-disubstituted alkynes were shown to be good substrates for the reaction while use of terminal alkynes led only to trace amounts (<10%) of products. No significant difference in reactivity for aryl- or alkyl-isocyanate substrates and no electronic effects for aryl isocyanates were observed. The reaction is postulated to proceed via aza-ruthenacyclopentenones A and B, for which inverse-gated ¹³C NMR evidence is provided. Dimaleimides, as shown in the box, can be generated from diisocyanates under the same reaction conditions.