Catalytic [2+1] cycloaddition of diazo compounds to [60]fullerene

Abstract

A catalytic method for the efficient synthesis of 5,6 open and 6,6 closed (2π+1π) fullerene adducts by [2+1] cycloaddition of ethyl 2 alkyl(hetaryl) 2 diazoacetates to [60]fullerene in the presence of a three component catalyst Pd(acac) 2 -PPh 3 -Et 3 Al have been developed.Substituted cyclopropafullerenes are promising com pounds that could find applications in medicine and agri culture, as high energy fuel components and the heavy machinery lubricant additives. 1-13 Cyclopropafullerenes are usually obtained by cyclopropanation of fullerene with malonic acid derivatives following the Bingel proce dure, 14,15 as well as by thermal 16 or catalytic 17 [2+1] cy cloaddition of diazo compounds to [60]fullerene. The pub lished data 16,17 suggest that the known synthetic meth ods towards cyclopropafullerenes have low effectiveness or require stoichiometric amounts of such highly expen sive promoters as ruthenium complexes. Besides, the yields of the target compounds are low.The thermal cycloaddition reaction of diazoacetates to [60]fullerene yielded 5,6 open (homofullerenes) and 6,6 closed (methanofullerenes or cyclopropafullerenes) adducts in ~35% total yield. Separation of the reaction mixture into the individual adducts is very difficult and, in some cases, is not possible.Recently, 18 we have reported the synthesis of cyclo propafullerenecarboxylates with high yields by cycloaddi tion of ethyl diazoacetate to [60]fullerene in the presence of palladium complexes. In continuation of this research, in the present work we studied [2+1] cycloaddition of ethyl 2 alkyl(hetaryl) 2 diazoacetates and other substituted di azo compounds to [60]fullerene in the presence of palladi um complexes. The effect of the solvents and ratio of the catalytic system components as well as the bulkiness of the alkyl substituent in starting diazo compound on the yield and selectivity have also been studied. Results and DiscussionA three component catalyst prepared from Pd(acac) 2 , PPh 3 , and Et 3 Al turned out to be particularly efficient in the reaction under study showing the highest activity and selectivity among transition metal complexes tested. In this regard, all subsequent experiments were carried out using this catalytic system.It has been found that the reaction of ethyl 2 methyl 2 diazoacetate with [60]fullerene (molar ratio 5 : 1) in the presence of 20 mol.% of Pd(acac) 2 -PPh 3 -Et 3 Al (1 : 2 : 4) at 20 °C (18 h, o dichlorobenzene) resulted in a mixture of 6,6 closed (1) and 5,6 open (2) fullerene adducts along with the corresponding bis cyclopropane derivatives 3 and 4 in the ratio (1 + 2) : (3 + 4) ~3 : 1 in ~77% total yield (Scheme 1). No noticeable increase in the total yield (~79%) and changes in the ratio of adducts 1, 2 and 3, 4 were observed when the reaction time was prolonged up to 30 h. Similar results were obtained on increase in the reaction temperature to 40 and 80 °C with concomitant reduction of the reaction time from 18 to 1.5 and 0.5 h, respectively.The mixture of mono (1, 2) and bis adducts (3, 4) was separat...

Palladium-catalyzed selective cycloaddition of diazo compounds to [60]fullerene

“…5 In continuation of these studies and also to examine the effect of alkyl substituent size in the original diazoalkanes upon the selectivity of the reaction, we have carried out catalytic cycloaddition between C 60 and diazoalkanes generated in situ by oxidation of hydrazones of butiric, valeric and caproic aldehydes. The above reaction was found to provide the formation of the corresponding homofullerenes 1-3 (Scheme 1), while the yield of target [5,6]-open adducts decreases slightly with increasing the alkyl chain length of diazoalkane. Scheme 1.…”

“…Apparently, the decrease in the selectivity of the reaction is conditioned by the presence of the second alkyl substituent at the bridging C atom of the cycloadduct raising the probability of the stereoisomeric [5,6]-open adduct formation.…”

“…In our view, the reaction of fullerenes with diazo compounds due to its great synthetic potential is regarded as a very promising method to synthesize methanofullerenes and also allows, together with the [6,6]-closed adducts, obtaining the [5,6]-open isomeric ones.…”

“…It is accompanied by the formation of a mixture of [6,6]-closed (methanofullerenes) and stereoisomeric [5,6]-open (homofullerenes) adducts.…”

“…Over the past 5-10 years, we have been actively engaged in research of the cycloaddition reaction between diazo compounds and [60]fullerene under catalytic conditions and found that among the tested complexes and salts of Cu, Pd and Rh a three-component catalytic system Pd(acac) 2 -PPh 3 -Et 3 Al has the greatest efficiency and selectivity in the reactions of C 60 with the simplest diazoalkanes, 5 diazoacetates, 6,7 and diazoketones 8 to promote the selective formation of the corresponding homo-or methanofullerenes. Herein, we attempted to explore the application boundaries of the cycloaddition reaction of diazo compounds to C 60 in the presence of Pd(acac) 2 -PPh 3 -Et 3 Al catalyst on the example of symmetric and asymmetric diazoalkanes and also higher diazoacetates of various structure.…”