Crystalline Monomeric Allenyl/Propargyl Radical.

Reduction of alkynyl iminium salts derived from cyclic (alkyl)(amino)carbenes (CAACs) affords propargyl/allenyl radicals. Depending on the nature of the CAAC and alkyne substituents, these radicals can irreversibly dimerize, exist as monomers in solution but dimerize in the solid state, or can even remain monomeric as solids. The first characterization of an allenyl radical by single crystal X-ray crystallography is reported.