The NMR spectra of the porphyrins: 23—Metalloporphyrins as diamagnetic shift reagents, chemical applications

Some applications illustrating the use of zinc (II) meso ‐tetraphenylporphyrin (ZnTPP) as a diamagnetic shift reagent are described. The complexation shifts (Δ P ) in a series of substituted pyridines are shown to be determined by both the basicity of the coordinating nitrogen atom and steric effects of ortho ‐substituents. The selectivity of ZnTPP ( N »O) in the simplification of the spectra of multifunctional ligands is demonstrated. Addition of the reagent neither affects the rotamer populations of the Inderal side‐chain nor produces any distortion of the 2‐benzylmorpholine ring. In addition, the substrate‐reagent solution may be shaken with D 2 O to identify exchangeable protons. The application of ZnTPP in structural problems involving substituted imidazo[2,1‐ b ]thiazoles and 2‐benzyl‐1,3‐dioxopyrrolo[3,4‐ c ]pyridines allows unambiguous identification of the possible structural isomers. To overcome the solvent limitations of ZnTPP, the more soluble Co(III)TPPBr can be used successfully as a shift reagent in DMSO‐ d 6 , CD 3 OD and acetone‐ d 6 solutions.