Tautomerizations, Protonations, and Electrophilic Additions of η²-Coordinated Pyrroles

A series of complexes is synthesized of the form [Os(NH3)5(2,3-η2-pyrrole)]2+, with pyrrole and various alkylated pyrroles. In contrast to the free ligands, these complexes can be protonated chemo- and stereoselectively at the β carbon, away from the metal, to produce pyrrolium species whose acidities range in pKa from 4.2 to 7.5. In the presence of a weak base, two of these 3H-pyrrolium species can be converted to the corresponding 2H-pyrrolium tautomer, [Os(NH3)5(3,4-η2-2H-pynolium)]3+. In the case of L = 2,5-dimethylpyrrole, the 2H-pyrrolium species can be deprotonated at nitrogen (pKa = 7.9), rendering a neutral 2H-pyrrole ligand. When L = 1-methylpyrrole, the 2H-pyrrolium species can be deprotonated at the α carbon, generating an unstable azomethine ylide complex bound through C3 and C4. Rapid rearrangement of this species yields the neutral 1H-pyrrole complex, [Os(NH3)5(2,3-η2-1-methylpyrrole)]2+ (pKep = 7.8 (overall process)). Through consideration of pKa and electrochemical data, the pyrrole/pyrrolenine isomerization energy (ΔG°) is found to decrease by about 16 kcal on osmium(II), to the point where these tautomers become virtually isoergic. In contrast, the pyrrole/pyrrolenine equilibrium is largely unaffected by coordination to Os(III), in comparison to the free ligand. © 1992, American Chemical Society. All rights reserved.

Duke Scholars

Author Jason Koontz Medicine, Cardiology