β-Electrophilic Additions of Pentaammineosmium(II) β²-Pyrrole Complexes

The reactivity of a series of pyrrole complexes of the form [Os(NH3)5(4,5-2-L)]2+(OTf)2 (L = pyrrole and alkylated pyrroles) is surveyed with various electrophiles. The pyrrole ligand undergoes alkylation or acylation with a wide variety of electrophiles (e.g., acids, alkyl triflates, anhydrides, aldehydes, ketones, and Michael acceptors) predominately at the β-position. Depending on reaction conditions, the resulting products are either β-substituted 1H--pyrrole or 3H-pyrrolium complexes, the latter of which resist rearomatization due to the electron-donating properties of the metal. In all cases observed, the initial addition of the electrophile occuss on the ring face anti to osmium coordination. The osmium(II)-4,5-β2-pyrrole complexes are each in dynamic equilibrium with a minor isomer where the metal binds across C(3) and C(4). In this form, the uncoordinated portion of the pyrrole ring resembles an azomethine ylide, which can undergo a 1,3-dipolar cycloaddition reaction with certain electrophiles. The resulting 7-azanorbornene complexes may be ring-opened with Lewis acids to generate a-substituted 2H-pyrrolium complexes. As with the 3H-pyrrolium species, the 2H-pyrrolium complexes are stabilized by metal coordination and thereby resist rearomatization. The selectivity between Michael addition and dipolar cycloaddition depends on the pyrrole, electrophile, solvent, temperature, the presence of Lewis acids, and in some cases, concentration. The iminium carbon of both 2H- and 3H-pyrrolium tautomers is considerably less electrophilic than its organic analogs, but readily undergoes borohydride reduction to form complexes of 3- and 2-pyrrolines, respectively. When pyrrole complexes are combined with alkyne Michael acceptors, the intermediate enolate can be trapped by the iminium carbon of the 3H-pyrrolium species in DMSO to form a metalated cyclobutene derivative. Decomplexation of most pyrrole and 3-pyrroline derivatives can be accomplished in good yield either by heating or by oxidation of the metal (Ce or DDQ). Complexes of 2-pyrrolines are considerably moee difficult to remove from the metal; however, quaternization or acylation of the nitrogen facilitates their decomplexation. © 1995, American Chemical Society. All rights reserved.

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Author Jason Koontz Medicine, Cardiology