Characteristics of Organic Transformations in a Confined Dendritic Core: Studies on the AIBN-Initiated Reaction of Dendrimer Cobalt(II) Porphyrins with Alkynes

Cobalt(II) complexes of poly(aryl ester) dendrimer porphyrins [(m-[Gn]TPP)Co(II)] (generation number n=0-4), in the presence of azobisisobutyronitrile (AIBN) at 60 degrees C, underwent alkenylation with several alkynes at the metal center. A complete inhibition of double-bond migration (secondary transformation) was observed for [(m-[Gn]TPP)Co(II)] (n=3 and 4), which gave [(m-[Gn]TPP)Co(III)-C(=CH(2))R] (n=3 and 4) exclusively. Overall reaction rates for [(m-[Gn]TPP)Co(II)] (n=0-3) were hardly dependent on the size of the dendritic substituents, while a notable retardation was observed for the largest dendrimer, [(m-[G4]TPP)Co(II)]. Mechanistic studies on double-bond migration with pure [(m-[Gn]TPP)Co(III)-C(=CH(2))Bu] (n=0-4) demonstrated that the secondary transformation involves participation of [(m-[Gn]TPP)Co(III)H] (n=0-4), derived from [(m-[Gn]TPP)Co(II)] and AIBN, rather than [(m-[Gn]TPP)Co(II)] alone. Crossover experiments using [(m-[Gn]TPP)Co(III)-C(=CH(2))Bu] (n=2-4), in combination with nondendritic [(m-[G0]TPP)Co(II)] and AIBN, indicated a high level of steric protection of the active center by a robust [G4]-dendritic cage, as suggested by a (1)H NMR pulse relaxation time profile of m-[G4]TPPH(2).