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-pyrroline-3-carboxylic acid (TPCO) in an EtOAc solution under nitrogen atmosphere. From 1:1 EtOAc/hexane the compound crystallizes in triclinic space group P\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document} $$\overline 1$$ \end{document} with a = 11.078(2), b = 11.970(2), c = 15.746(3) Å, α = 105.41(3), β = 103.47(3), γ = 106.52(3)°, 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 Å implies the existence of a single electron π bond between N and O atoms, which stabilizes the radical molecule.