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.

The phosphonylation of C with HP(O)(OAlk) and Mn(OAc)·2HO has been considered to occur via a free radical (FR) path involving intermediate radicals ˙P(O)(OAlk). 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) for the phosphonate group and oxidation of C by the Mn(OAc)P(O)(OAlk) formed to a pair: (C˙, Mn(OAc)P(O)(OAlk)˙) 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)P(O)(OAlk) (Alk = Et and i-Pr) obtained make novel reagents for phosphonylation of fullerenes working by the OIT mechanism. The reactions of C in benzene with equimolar amounts of Mn(OAc)P(O)(OPr-i) or Hg[P(O)(OPr-i)] which is known as working by the FR mechanism since it produces radical ˙P(O)(OPr-i) under UV-irradiation, furnished the same radical ˙CP(O)(OPr-i). However, at a 20-fold molar excess of the reagent toward C, a single derivative C[P(O)(OPr-i)] and a mixture of derivatives bearing between two and eight phosphonyls were obtained in the former and latter cases, respectively. With C, the change of the mechanism produced a change in the regioselectivity: 5 and 3 isomers of ˙CP(O)(OPr-i) were obtained, respectively. DFT-calculations provided the hyperfine coupling (hfc) constants of the isomers and explained the regioselectivity change.