Aromatic olefin oxygenation with tetrahydroborate and dioxygen catalyzed by a manganese porphyrin

Chloro(meso-tetraphenylporphyrinato)manganese(III) catalyzed the oxygenation of aromatic olefins derived from styrene with molecular oxygen in the presence of sodium tetrahydroborate in methanol at ambient temperature. Styrenes afforded mainly the Markownikov ‘hydration’ products, e.g. benzyl alcohols. Substitution on the olefinic carbon by the bulky group resulted in the formations of various byproducts; α-substituted styrenes afforded dimerization products, e.g. 2,2,3,3-tetraphenylbutane from 1,1-diphenylethene, and both CC bond cleavage and α,β-diketone formation occurred in the case of β-substituted styrene, i.e. benzyl alcohol and benzil were respectively obtained from stilbene. Furthermore, α,β-di- or α,β,β-trisubstituted styrenes gave rise mainly to CC bond cleavage; the stereochemistry of the olefins thus substantially influenced the oxidation process. MnIII(TPP) was reduced to MnII(TPP) by the tetrahydroborate, and adduct formation of MnII(TPP) with molecular oxygen was retarded in the presence of olefin. A preference for the activation of olefin over the activation of molecular oxygen is thus suggested.