Functionalization of C60 via organometallic reagents

The reaction of [60]fullerene with organolithium and Grignard reagents carrying orthoester, acetal or other end groups yielded adducts 3–5 at the 6–6 bond of C60 after quenching with trifluoroacetic acid. The adducts could be easily methylated or benzylated with methyl iodide or benzyl bromide in the presence of potassium tert-butoxide to yield exclusively the 1,4-disubstituted C60 6 and 7a,b. Cleavage of the orthoester, acetal and silylether groups gave the corresponding carboxylic acid 9, aldehydes 10a,b and 11 and alcohols 12 and 13a,b. The carboxylic acid 9 readily reacted with alanine ethyl ester under standard peptide coupling conditions to give 14 in 55% yield. Attempts to generate a silyl enol ether from the reaction of aldehyde 10b with TIPSOTf and triethylamine failed. Instead the reaction led to a cyclized ether 16a (or alcohol 16b in the absence of silylating agent) resulting from the addition of an initially formed fulleride anion to the aldehyde group. The corresponding acetal 4b reacted similarly. The reaction of aldehyde 10b with aniline also gave a cyclized product 19. Surprisingly, aldehyde 11, which no longer carried an acidic fullerene proton reacted with aniline to give a product 20 resulting from an intramolecular Diels–Alder reaction followed by aromatization of a primarily formed N-phenylimine. Alcohol 13b could be readily converted to the corresponding bromide using tetramethyl-a-bromoenamine. The bromide was reacted with the carbanion derived from the protected glycine derivative to yield the diastereomeric fullerene amino acid derivatives 1-benzyl-4-[a-propyl-tert-butylglycinate benzophenone imine] 1,4-dihydro[60]fullerenes 24a and 24b.