Lm. Hodges et al., BETA-ELECTROPHILIC ADDITIONS OF PENTAAMMINEOSMIUM(II) ETA(2)-PYRROLE COMPLEXES, Journal of organic chemistry, 60(7), 1995, pp. 2125-2146
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.