BETA-ELECTROPHILIC ADDITIONS OF PENTAAMMINEOSMIUM(II) ETA(2)-PYRROLE COMPLEXES

The reactivity of a series of pyrrole complexes of the form [Os(NH3)(5 )(4,5-eta(2)-L)](2+)(OTf)(2) (L = pyrrole and alkylated pyrroles) is s urveyed with various electrophiles. The pyrrole ligand undergoes alkyl ation or acylation with a wide variety of electrophiles (e.g., acids, alkyl triflates, anhydrides, aldehydes, ketones, and Michael accepters ) predominately at the -position. Depending on reaction conditions, th e resulting products are either beta-substituted 1H-pyrrole or 3H-pyrr olium 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 occurs on the ring face anti to osmium coordination. The osmium(II)-4,5-eta(2)-pyrrole complexes are e ach in dynamic equilibrium with a minor isomer where the metal binds a cross C(3) and C(4). In this form, the uncoordinated portion of the py rrole ring resembles an azomethine ylide, which can undergo a 1,3-dipo lar cycloaddition reaction with certain electrophiles. The resulting 7 -azanorbornene complexes may be ring-opened with Lewis acids to genera te a-substituted 2H-pyrrolium complexes. As with the 3H-pyrrolium spec ies, the 2H-pyrrolium complexes are stabilized by metal coordination a nd thereby resist rearomatization. The selectivity between Michael add ition 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 tautome rs is considerably less electrophilic than its organic analogs, but re adily undergoes borohydride reduction to form complexes of 3- and 2-py rrolines, respectively. When pyrrole complexes are combined with alkyn e Michael accepters, the intermediate enolate can be trapped by the im inium carbon of the 3H-pyrrolium species in DMSO to form a metalated c yclobutene derivative. Decomplexation of most pyrrole and S-pyrroline derivatives can be accomplished in good yield either by heating or by oxidation of the metal (Ce-IV or DDQ). Complexes of 2-pyrrolines are c onsiderably more difficult to remove from the metal; however, quaterni zation or acylation of the nitrogen facilitates their decomplexation.