FORMATION OF C-60 ADDUCTS WITH 2 DIFFERENT ALKYL-GROUPS VIA COMBINATION OF ELECTRON-TRANSFER AND S(N)2 REACTIONS

The formation of organofullerenes of the type R2C60 and R(R')C-60 from C-60(2-) and alkyl halides (RX or R'X) in benzonitrile was mechanisti cally investigated for 15 different alkyl halides which vary in electr ophilicity and electron acceptor ability. The first step in the reacti on leads to RC60- via an electron-transfer mechanism, followed by form ation of R2C60 Or R(R')C-60 via an S(N)2 mechanism. Evidence of the me chanism comes from comparison of rate constants for the stepwise addit ion of two R groups to C602- With rate constants for the genuine elect ron transfer and SN2 reactions. The formation of t-BuC60- and PhCH2C60 - after the first R group addition was confirmed by electrospray ioniz ation mass spectroscopy. The t-BuC60- derivative will not react furthe r 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)C-60. A protonation of t-BuC60- with trifluoroacetic acid can al so occur to give 1,4-t-Bu(H)C-60, which rearranges rapidly to yield 12 -t-Bu(H)C-60. Rate constants for the second alkylation of t-BuC60- wit h a variety of different alkyl halides are compared with values of gen uine S(N)2 reactions and indicate that the second step in the fulleren e alkylation reaction proceeds via an S(N)2 mechanism. The rate consta nts of electron transfer from C602- to RX span a range of 10(5), but a re insensitive to the steric effect of the alkyl group, i.e., they dep end 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 ste ric effect of the alkyl group and no reaction at all takes place betwe en t-BuC60- and t-BuI. Thus, the first addition of one sterically hind ered alkyl group to C-60(2-) occurs via electron transfer and cannot b e followed by further addition of a second sterically hindered group ( via an S(N)2 reaction). This is not the case for less sterically hinde red alkyl groups such as benzyl bromide which can add via an S(N)2 rea ction to yield C-60 adducts with two different alkyl groups.