Formation of C₆₀ adducts with two different alkyl groups via combination of electron transfer and S(N)2 reactions

The formation of organofullerenes of the type R₂C₆₀ and R(R')C₆₀ from C₆₀^2- and alkyl halides (RX or R'X) in benzonitrile was mechanistically investigated for 15 different alkyl halides which vary in electrophilicity and electron acceptor ability. The first step in the reaction leads to RC₆₀^- via an electron-transfer mechanism, followed by formation of R₂C₆₀ or R(R')C₆₀ via an S(N)2 mechanism. Evidence of the mechanism comes from comparison of rate constants for the stepwise addition of two R groups to C₆₀2^- with rate constants for the genuine electron transfer and S(N)2 reactions. The formation of t-BuC₆₀^- and PhCH₂C₆₀^- after the first R group addition was confirmed by electrospray ionization mass spectroscopy. The t-BuC₆₀^-derivative will not react further with excess t-BuI, but this is not the case for the less sterically hindered PhCH₂Br, which adds to t-BuC₆₀^- in benzonitrile to give t- Bu(PhCH₂)C₆₀. A protonation of t-BuC₆₀^- with trifluoroacetic acid can also occur to give 1,4-t-Bu(H)C₆₀, which rearranges rapidly to yield 1,2- t-Bu(H)C₆₀. Rate constants for the second alkylation of t-BuC₆₀^- with a variety of different alkyl halides are compared with values of genuine S(N)2 reactions and indicate that the second step in the fullerene alkylation reaction proceeds via an S(N)2 mechanism. The rate constants of electron transfer from C₆₀^2- to RX span a range of 10⁵, but are insensitive to the steric effect of the alkyl group, i.e., they depend only on the electron- acceptor ability of RX. In contrast, the S(N)2 rate constants of t-BuC₆₀^- with RX are highly susceptible to the steric effect of the alkyl group and no reaction at all takes place between t-BuC₆₀^- and t-BuI. Thus, the first addition of one sterically hindered alkyl group to C₆₀^2- occurs via electron transfer and cannot be followed by further addition of a second sterically hindered group (via an S(N)2 reaction). This is not the case for less sterically hindered alkyl groups such as benzyl bromide which can add via an S(N)2 reaction to yield C₆₀ adducts with two different alkyl groups.