Cobalt(II) complexes of poly(aryl ester) dendrimer porphyrins (m-[Gn]TPP)Co-II and (p-[Gn]TPP)Co-II (n = 0-3) underwent AIBN-initiated alkylation (AlBN = 2,2'-azobis(isobutyronitrile)) at the metal center with propargyl alcohol in CDCl3 at 60 degreesC, where the dendritic substituents did not affect the overall conversion rate but selectivity of the alkylation. With the largest (m-[G3]TPP)Co-II, a single organocobalt(III) species (Co-III-C(=CH2)CH2OH, 4) was selectively formed in 91% yield, due to a steric protection of 4 by the large dendrimer cage from the access of another molecule of cobalt porphyrin species. In contrast, with other cobalt(II) porphyrins, isomerized compounds such as Co-III-C(CH3)=CHOH (5) and Co-III-CH(CH3)CHO (6) were formed in addition to 4. A stereochemical investigation with (m-[G3]TPP)Co-II using AlBN-d(12), in place of nondeuterated AlBN, demonstrated that the alkylation (cobalt(III) hydride addition to propargyl alcohol) is selective to a trans adduct. Results also indicated that this addition step does not involve external activation of propargyl alcohol.