Abstract
In contrast to eta-1 and eta-5-pyrrole complexes, which generally react either at the nitrogen or at the s-positions, rl2-coordination of pyrrole by osmium(II)pentaammine activates the pyrrole ring toward regioselective protonation and electrophilic addition at the ~-position. Depending on the reaction conditions, the resulting products are either 1H-pyrrole or 3H-pyrrolium complexes, the latter of which are several orders of magnitude less acidic than their uncomplexed counterparts and resist rearomatization, rl2-Pyrrole complexes also undergo a dipolarcycloaddition reaction with certain electrophiles to generate a wide variety of 7-azanorbornene complexes. In most cases, the metal can be removed from the products by oxidation or heating to generate functionalized pyrroles, azanorbornanes, or pyrrolizinines. A series of I]-vinylpyrrole complexes can be synthesized via initial electrophilic addition to the pyrrole ring. These vinylpyrrole complexes readily undergo a Diels-Alder cycloaddition reaction with electron-deficient olefins to give functionalized 5,6,7,7a-tetrahydroindole complexes. These complexes, which are stable to isomerization, can be decomplexed and oxidized with DDQ to generate highly substituted indoles in moderate to good yields.
