Synthesis and crystal structure of a pyrroline nitroxide radical, (N-2,2,5,5-tetramethyl-1-oxo-3-pyrroline-3-carbonyl) (N,N′-diisopropyl)urea

The title compound including a stable nitroxide radical was synthesized by a reaction of N,N'-diisopropylcarbondiimide (DIC) with 2,2,5,5-tetramethyl-1-oxo-3-pyrroline3-carboxylic acid (TPCO) in an EtOAc solution under nitrogen atmosphere. From 1:1 EtOAc/hexane the compound crystallizes in triclinic space group P (1) over bar with a = 11.078(2). b = 11.970(2), c = 15.746(3) Angstrom, alpha = 105.41(3), beta = 103.47(3), gamma = 106.52(3)degrees, and Z = 4. In an asymmetric unit two molecules assemble with each other in similar bond distances and angles but show crystallographically independent conformation. A normal single C-N bond is observed in the title compound, other than the partial double C=N bond shown in the parent urea. A longer N-O distance of 1.26 Angstrom implies the existence of a single electron 7 bond between N and O atoms, which stabilizes the radical molecule.