Catalytic Homogeneous Functionalization of Adamantane. Influence of Electronic and Structural Features of the Metalloporphyrin Catalyst on Atom Transfer Selectivity (Oxygenation versus Azidification/Halogenation)

Article abstract of DOI:10.1021/jo00259a028

Upon treatment of the two-phase systems, Mn tetraarylporphyrin, and alkane (organic phase)/Na⁺X^- (aqueous phase), with iodosylarenes, both alcohols and alkyl azides (or halides), X^- = halide or azide, are formed from the alkane substrates.The Mn porphyrin functions as a catalyst for alkane oxygenation and a phase transfer catalyst for X^-.Catalytic functionalization of the exemplary caged alkane, adamantane, by a variety of these two-phase systems as function of the reaction conditions has been examined.The results reported here allow, for the first time, an assessment of the relathionship between the electronic and structural features of the metalloporphyrin catalysts and their selectivity with respect to the replacement of unactivated alkane carbon-hydrogen bonds with oxygen versus non-oxygen (halide or azide) functional groups.Of the first-row transition metal matalloporphyrins, only those of manganese are active for both the cleavage of unactivated alkane C-H bonds and replacement of these bonds by halogen or nitrogen-based groups.The oxygen donors that give the highest yields of these non-oxygenated products are the iodosylarenes.Examination of adamantane functionalization by iodosylarenes catalyzed by eight different manganese tetraphenylporphyrin derivatives, whose porphyrin ligands vary widely in electron-donating ability, establishes that the relative tertiary-secondary C-H cleavage selectivities are minimally affected by such electronic effects.In contrast, the selectivity for incorporation of the non-oxygen versus oxygen functions is substantially affected by the electron-donating ability of the catalyst porphyrin rings.The more electron withdrawing the porphyrin ring, and consequently the more anodic the potential of the ligated, S = 2, manganese(III) ion, the lower the selectivity for incorporation of the non-oxygen functions.Functionalization of adamantane catalyzed by the most electron poor manganese porphyrin complex, Mn^III(F20TPP)X, is effectively selective for oxygenation.All the metalloporphyrins examined here eventually succumb to deactivation by irreversible oxidative degradation of the organic porphyrin ligand.