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

摘要

The phosphonylation of C-60 with HP(O)(OAlk)(2) and Mn(OAc)(3)center dot 2H(2)O has been considered to occur via a free radical (FR) path involving intermediate radicals P-center dot(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 C-60 by the Mn(OAc)(2)P(O)(OAlk)(2) formed to a pair: (C-60(center dot+), Mn(OAc)(2)P(O)(OAlk)(2)(center dot-)) 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)(2)P(O)(OAlk)(2) (Alk = Et and i-Pr) obtained make novel reagents for phosphonylation of fullerenes working by the OIT mechanism. The reactions of C-60 in benzene with equimolar amounts of Mn(OAc)(2)P(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-center dot(O)(OPr-i) 2 under UV-irradiation, furnished the same radical (C60P)-C-center dot(O)(OPr-i)(2). However, at a 20-fold molar excess of the reagent toward C-60, a single derivative C-60[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 C-70, the change of the mechanism produced a change in the regioselectivity: 5 and 3 isomers of (C70P)-C-center dot(O)(OPr-i)(2) were obtained, respectively. DFT-calculations provided the hyperfine coupling (hfc) constants of the isomers and explained the regioselectivity change.