A novel dimerization mode of a cyclic ketene imine

The strained seven-membered cyclic ketene imine 9, obtained by cycloaddition of thiocarbonyl ylide 6 with 2,3-bis(trifluoromethyl)fumaronitrile (7), underwent base-catalyzed dimerization at room temperature on treatment with KCN in acetonitrile or with proton sponge in acetonitrile or CDCl(3). Two diastereoisomeric dimers, (6SR,3'RS)-13 and (6SR,3'SR)-13, were formed in 1:1 ratio in high yield. X-ray analysis revealed a deep-seated structural change which is unrelated to known dimerization pathways of ketene imines. In 13, one of the seven-membered rings is opened, and attached to the second unit by a thioimidate group. An ionic chain reaction with a formal fluoride ion as transfer agent offers a rationalization.