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

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Abstract
Cobalt(II) complexes of poly(aryl ester) dendrimer porphyrins [(m-[Gn]TPP)CoII] (generation number n=0–4), in the presence of azobisisobutyronitrile (AIBN) at 60 °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)CoII] (n=3 and 4), which gave [(m-[Gn]TPP)CoIIIC(CH2)R] (n=3 and 4) exclusively. Overall reaction rates for [(m-[Gn]TPP)CoII] (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)CoII]. Mechanistic studies on double-bond migration with pure [(m-[Gn]TPP)CoIIIC(CH2)Bu] (n=0–4) demonstrated that the secondary transformation involves participation of [(m-[Gn]TPP)CoIIIH] (n=0–4), derived from [(m-[Gn]TPP)CoII] and AIBN, rather than [(m-[Gn]TPP)CoII] alone. Crossover experiments using [(m-[Gn]TPP)CoIIIC(CH2)Bu] (n=2–4), in combination with nondendritic [(m-[G0]TPP)CoII] and AIBN, indicated a high level of steric protection of the active center by a robust [G4]-dendritic cage, as suggested by a 1H NMR pulse relaxation time profile of m-[G4]TPPH2.

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