Intermolecular [2 + 2 + 1] cocyclization of isocyanates, alkynes,
and CO (1 atm) proceeded smoothly in the presence of a catalytic amount of Ru
3(CO)
12 (3.3 mol %) in mesitylene at 130
C for 3~42 h to give a variety of polysubstituted maleimides in excellent yields with high selectivity. The reaction may involve an azaruthenacyclopentenone intermediate derived from oxidative cyclization of an isocyanate
and an alkyne on an active ruthenium species.