A mechanistic study of manganese(iii) acetate-mediated phosphonyl group additions to [60]- and [70]-fullerenes: the oxidative-ion-transfer mechanism vs. free radical addition

Abstract

The phosphonylation of C₆₀ with HP(O)(OAlk)₂ and Mn(OAc)₃·2H₂O has been considered to occur via a free radical (FR) path involving intermediate radicals ˙P(O)(OAlk)₂. The present study provides evidence in support of another mechanism for the reactions, oxidative-ion-transfer (OIT). The mechanism involves the change of an acetate group in Mn(OAc)₃ for the phosphonate group and oxidation of C₆₀ by the Mn(OAc)₂P(O)(OAlk)₂ formed to a pair: (C₆₀˙⁺, Mn(OAc)₂P(O)(OAlk)₂˙⁻) followed by the transfer of the phosphonate anion to give the monophposphonylfullerenyl radical. It undergoes reversible dimerization. The polyaddition occurs analogously. Moreover, the compounds Mn(OAc)₂P(O)(OAlk)₂ (Alk = Et and i-Pr) obtained make novel reagents for phosphonylation of fullerenes working by the OIT mechanism. The reactions of C₆₀ in benzene with equimolar amounts of Mn(OAc)₂P(O)(OPr-i)₂ or Hg[P(O)(OPr-i)₂]₂ which is known as working by the FR mechanism since it produces radical ˙P(O)(OPr-i)₂ under UV-irradiation, furnished the same radical ˙C₆₀P(O)(OPr-i)₂. However, at a 20-fold molar excess of the reagent toward C₆₀, a single derivative C₆₀[P(O)(OPr-i)₂]₄ and a mixture of derivatives bearing between two and eight phosphonyls were obtained in the former and latter cases, respectively. With C₇₀, the change of the mechanism produced a change in the regioselectivity: 5 and 3 isomers of ˙C₇₀P(O)(OPr-i)₂ were obtained, respectively. DFT-calculations provided the hyperfine coupling (hfc) constants of the isomers and explained the regioselectivity change.