Preparation, crystal structure and reactions of a new spin-labelling reagent, cis-3,5-dibromo-4-oxo-2,2,6,6- tetramethylpiperidin-1-yloxy : Crystal structure of a derived bis-nitroxide

Abstract

Methods for the preparation of cis - 3,5 - dibromo - 4 - oxo - 2,2,6,6 - tetramethylpiperidin -1 - yloxy ( 2b) and its use as a convenient, selective spin-labelling reagent for amino functions, are described. The crystal and molecular structures of 2b and the bis-nitroxide 4 - [(2',2',5',5' - tetramethylpyrrolin -1 - yloxy) - 3' - carbonyloxy] - 2, 2,6,6 - tetramethyl - 3,5 - dibromo - 3,4 - dehydropiperidin - 1 - yloxy ( 3) [obtained on treatment of 2b with triethylamine] have been determined by conventional heavy-atom techniques. In 2b the individual molecules have mirror symmetry with the bromine atoms equatorial. The N-O bond length is 1.278(5) Å, CN̂C is 125.1(3)° and the N-O bond makes an angle of 24.2° with the CN̂C plane. In 3 there are two different nitroxide groups. The pyrrolinyloxy ring is virtually planar with a N-O bond length of 1.252(13) Å and CN̂C of 113.1(10)°. The 6 -membered ring exists in a sofa conformation with a pseudoaxial Br atom. Its nitroxide group is tetrahedrally distorted. The N-O bond length is 1.268(15) Å, CN̂C is 121.7(10)° and the N-O bond makes an angle of 16.0° with the CN̂C plane. The packing of 3 is dictated by a short Br... .O secondary bond of 3.09(1) Å, 0.28 Å less than the sum of the van der Waal's radii resulting in dimeric units related by a centre of symmetry.