Shape Selective Alkane Hydroxylation by Metalloporphyrin Catalysts

A series of manganese and iron porphyrins with sterically protected pockets are shown to be shape selective alkane hydroxylation catalysts. With iodosobenzene as oxidant, good regioselectivity is observed for hydroxylation of alkanes at the least hindered methyl group by using the very sterically hindered (5,10,15,20-tetrakis(2’,4’,6’-triphenylphenyl)-porphyrinato)manganese(III) acetate (MnTTPPP(OAc)) as catalyst; the moderately hindered (5,10,15,20-tetrakis(2’,4’,6’-trimethoxyphenyl)porphyrinato)manganese(III) acetate shows little selectivity toward terminal CH₃ hydroxylation but does show enhancement for the adjacent, ω-1, CH₂ site. Primary selectivity is dependent on the size and shape of the alkane substrate, with more bulky substituents giving greater primary selectivity. Substituting pentafluoroiodosobenzene or m- chloroperbenzoic acid as oxidants yields similar selectivity, thus conclusively demonstrating metal based oxidation via a common intermediate for these three systems. In contrast, tert-butyl hydroperoxide or 2,2,2-trifluoroethanol solubilized pentafluoroiodosobenzene show no primary carbon selectivity, and reaction product ratios are independent of the metalloporphyrin catalyst; this demonstrates that the site of oxidation with these oxidants is not metal based. The iron porphyrin derivatives also show good primary selectivity, although to a lesser degree than with the Mn derivatives, proving that these oxidations too are metal based. The regioselectivities for alkane hydroxylation shown by TTPPP derivatives are comparable to or better than those found for some isozymes of cytochrome P-450 which are responsible for primary alcohol biosynthesis from steroids, fatty acids, and alkanes.