No abstract

SHORT COMMUNICATIONS Suzuki et al. [1] were the first to report in 1991 on cycloaddition of diphenyldiazomethane to fullerene C 60 . Since that time, numerous publications have appeared on reactions of fullerene with mono-and disubstituted diazomethanes, cyclic diazo compounds, and diazoacetates [2,3]. All these reactions may be divided into two groups, the first of which includes addition of diazoalkanes to C 60 with formation of mixtures of [5,6]-fulleroids and [6,6]-methanofullerenes, while the second is represented by thermal reactions of fullerene C 60 with diazo compounds generated in situ from the corresponding p-tolylsulfonylhydrazone lithium salts; the latter reactions lead to selective formation of [5,6]-open adducts. Disadvantages of the above procedures are low selectivity, experimental difficulties related to extreme instability of substituted diazoalkanes, and low yield of the target cycloadducts.We previously reported [4] on selective cycloaddition of diazomethane (generated in situ) to C 60 in the presence of palladium complexes. In continuation of these studies, the present communication reports on cycloaddition of alkyl(aryl)-substituted diazoalkanes to C 60 in the presence of palladium complexes, which was examined with a view to reveal the effect of diazoalkane structure on the reaction direction and develop efficient procedures for the synthesis of substituted [5,6]-fulleroids.We found that three-component catalyst prepared from Pd(acac) 2 , PPh 3 , and Et 3 Al showed the highest catalytic activity and selectivity in reactions of C 60 with diazoacetates [5,6]. Preliminary experiments showed that palladium complexes were also more efficient than copper or rhodium compounds in reactions with diazoalkanes. The reaction of C 60 with diazoethane (molar ratio 1.5 : 1) generated in situ by oxidation of acetaldehyde hydrazone with Ag 2 O was carried out in o-dichlorobenzene at 20°C (reaction time 20 min) in the presence of 20 mol % of Pd(acac) 2 -PPh 3 -Et 3 Al (1 : 2 : 4) gave [5,6]-open adduct Ia in ~70% yield with high selectivity. Increase of the reaction time to 1 h did not result in appreciable improvement of the yield of Ia (~75%). Following an analogous procedure, fullerene C 60 was brought into reactions with 2-diazopropane, phenyldiazomethane, and 1-diazo-1-phenylethane, which led to the formation of 55-70% of [5,6]-open adducts Ib-Id. It is seen that the yield of the adducts decreases as the size of substituents in the initial diazoalkanes increases. R = H, R′ = Me (a); R = R' = Me (b); R = H, R′ = Ph (c); R = Me, R′ = Ph (d).

Methanofullerenes were directly synthesized under mild conditions by cycloaddition to fullerene C 60 of diazoalkanes generated in situ by oxidation of ketone hydrazones containing a heterocyclic fragment with manganese(IV) oxide in the presence of the catalytic system Pd(acac) 2 -2 PPh 3 -4 Et 3 Al.Methanofullerenes available via Bingel-Hirsch reaction constitute now one of the most promising classes of fullerenes from the view point of practical applications. Methanofullerenes can be used in the manufacture of highly efficient solar batteries and in the preparation of drugs for the treatment of most dangerous human diseases [1,2]. Apart from the BingelHirsch reaction, methods based on cycloaddition of diazo compounds to carbon clusters have found wide application in synthetic practice; however, these procedures generally lead to the formation of mixtures of stereoisomeric homo-and methanofullerenes. The use of metal complex catalysts makes it possible to direct the cycloaddition of diazo compounds to fullerenes toward formation of individual methanofullerenes. Complexes based on palladium and rhodium were reported to successfully catalyze reactions of fullerene C 60 with diazomethanes [3], diazoacetates [4][5][6][7], and diazo ketones [8-10], and the corresponding cyclopropafullerenes were obtained with high selectivity.We recently showed that homofullerenes and spiro homofullerenes can be synthesized with high selectivity by reaction of C 60 with monosubstituted diazomethanes and cyclic diazoalkanes generated in situ by oxidation of the corresponding hydrazones in the presence of Pd(acac) 2 -PPh 3 -Et 3 Al as catalytic system [11][12][13][14][15][16]. We thus obtained pure [5,6]-open cycloadducts in which bulkier substituent was located above the five-membered fragment of C 60 . Analogous reactions with disubstituted diazomethanes generated in situ from unsymmetrical ketone hydrazones were characterized by lower selectivity, and mixtures of stereoisomeric homofullerenes were obtained [12, 13,15]. The proposed catalytic procedure is considerably more advantageous than known thermal cycloadditions of diazoalkanes to fullerene C 60 [17][18][19][20][21]. Reactions of carbon clusters with diazo compounds were reviewed in detail in [2].In continuation of our studies on cycloaddition of diazo compounds to fullerenes, in the present work we Me O I Scheme 1.