Abstract
The reactivity of a series of pyrrole complexes of the form [Os(NH3)5(4,5-?72-L)]2+(OTf)2 (L= pyrrole
and alkylated pyrroles) is surveyed with various electrophiles. The pyrrole ligand undergoes
alkylationor acylation with a wide variety of electrophiles (e.g., acids, alkyl triflates, anhydrides,
aldehydes, ketones, and Michael acceptors) predominately at the /3-position. Depending on reaction
conditions, the resulting products are either /3-substituted lif-pyrroleor 3if-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 occurs on the ring face anti to osmium
coordination. The osmium(II)—4,5-?/2-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-azanorbomene complexes may be ring-opened
with Lewis acids to generate -substituted 2//-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 3/f-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 3i/-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 heatingor by oxidation of the
metal (Ce™ or DDQ). Complexes of 2-pyrrolines are considerably more difficult to remove from
the metal; however, quaternizationor acylation of the nitrogen facilitates their decomplexation.
