Design, Synthesis, EPR-Studies and Conformational Bias of Novel Spin-Labeled DCC-Analogues for the Highly Regioselective Labeling of Aliphatic and Aromatic Carboxylic Acids.
Novel types of spin-labeled N,N'-dicyclohexylcarbodiimides (DCC) are reported that bear a 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) residue on one side and different aromatic and aliphatic cyclohexyl analogues on the other side of the diimide core. These readily available novel reagents add efficiently to aliphatic and aromatic carboxylic acids, forming two possible spin-labeled amide derivatives with different radical distances of the resulting amide. The addition of aromatic DCC analogues proceeds with excellent selectivity, giving amides where the carboxylic acid is exclusively connected to the aromatic residue, while little or no selectivity was observed for the aliphatic congeners. The usefulness of these adducts in structural studies was demonstrated by EPR (electron paramagnetic resonance) measurements of biradical adducts of biphenyl-4,4'-dicarboxylic acids. These analyses also reveal high degrees of conformational bias for aromatic DCC derivatives, which further underlines the powerfulness of these novel reagents. This observation was further corroborated by quantum chemical calculations, giving a detailed understanding of the structural dynamics, while detailed information on the solid state structure of all novel reagents was obtained by X-ray structure analyses.
Kekulé-Institut für Organische Chemie und Biochemie, Universität Bonn, Gerhard-Domagk-Str. 1, 53121, Bonn, Germany.
Jan Philipp
http://orcid.org/0000-0003-0023-3413
JP
Gölz
Institut für Physikalische und Theoretische Chemie, Universität Bonn, Wegelerstr. 12, 53115, Bonn, Germany.
Yaser
Y
NejatyJahromy
Mulliken Center for Theoretical Chemistry, Institut für Physikalische und Theoretische Chemie, Universität Bonn, Beringstraße 4, 53115, Bonn, Germany.
Mirko
M
Bauer
Kekulé-Institut für Organische Chemie und Biochemie, Universität Bonn, Gerhard-Domagk-Str. 1, 53121, Bonn, Germany.
Ashraf
A
Muhammad
Institut für Anorganische Chemie, Universität Bonn, Gerhard-Domagk-Straße 1, 53121, Bonn, Germany.
Gregor
G
Schnakenburg
Mulliken Center for Theoretical Chemistry, Institut für Physikalische und Theoretische Chemie, Universität Bonn, Beringstraße 4, 53115, Bonn, Germany.
Stefan
S
Grimme
Institut für Physikalische und Theoretische Chemie, Universität Bonn, Wegelerstr. 12, 53115, Bonn, Germany.
Olav
O
Schiemann
Kekulé-Institut für Organische Chemie und Biochemie, Universität Bonn, Gerhard-Domagk-Str. 1, 53121, Bonn, Germany. dirk.menche@uni-bonn.de.
Dirk
http://orcid.org/0000-0002-4724-8383
D
Menche
EPR spectroscopy; N,N′-dicyclohexylcarbodiimides; carboxylic acids; radicals; spin-labels