Nuclear Spin Hyperpolarization of NH2- and CH3-Substituted Pyridine and Pyrimidine Moieties by SABRE

Hyperpolarization of N-heterocycles with signal amplification by reversible exchange (SABRE) induces NMR sensitivity gains for biological molecules. Substitutions with functional groups, in particular in theortho-position of the heterocycle, however, result in low polarization using a typical Ir catalyst with a bis-mesityl N-heterocyclic carbene ligand for SABRE, presumably due to steric hindrance. With the addition of allylamine or acetonitrile as coligands to the precatalyst chloro(1,5-cyclooctadiene)[4,5-dimethyl-1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene] iridium, the(1)H signal enhancement increased in several substrates withorthoNH(2)substitutions. For example, for a proton in 2,4-diaminopyrimidine, the enhancement factors increased from -7 +/- 1 to -210 +/- 20 with allylamine or to -160 +/- 10 with acetonitrile. CH(3)substituted molecules yielded maximum signal enhancements of -25 +/- 7 with acetonitrile addition, which is considerably less than the corresponding NH(2)substituted molecules, despite exhibiting similar steric size. With the more electron-donating NH(2)substitution resulting in greater enhancement, it is concluded that steric hindrance is not the only dominant factor in determining the polarizability of the CH(3)substituted compounds. The addition of allylamine increased the signal enhancement for the 290 Da trimethoprim, a molecule with a 2,4-diaminopyrimidine moiety serving as an antibacterial agent, to -70.