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 ‐[G n ]TPP)Co II ] (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 ‐[G n ]TPP)Co II ] ( n =3 and 4), which gave [( m ‐[G n ]TPP)Co III C(CH 2 )R] ( n =3 and 4) exclusively. Overall reaction rates for [( m ‐[G n ]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 ‐[G n ]TPP)Co III C(CH 2 )Bu] ( n =0–4) demonstrated that the secondary transformation involves participation of [( m ‐[G n ]TPP)Co III H] ( n =0–4), derived from [( m ‐[G n ]TPP)Co II ] and AIBN, rather than [( m ‐[G n ]TPP)Co II ] alone. Crossover experiments using [( m ‐[G n ]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 .