Di‐ and Dodeca‐Mitsunobu Reactions on C60 Derivatives: Post‐Functionalization of Fullerene Mono‐ and Hexakis‐Adducts

Abstract

The diverse addition patterns of fullerene adducts make them attractive molecules, generating widespread application. Firstly, in terms of the latter, mono-adducts are extensively used in opto-electronic devices, owing to the fullerene core s capacity to accept up to six electrons. A second important application involves the utilization of fullerene adducts as structural components in material sciences. With their octahedral topology, Th-symmetric hexakis-adducts, are particularly intriguing. Amongst others, the latter have been used in C60-based star polymers. [5] In the aforementioned contexts, the relatively stable methanofullerene adducts obtained through Bingel cyclopropanation reactions are of particular interest. However, this functionalization approach—primarily developed by A. Hirsch and later optimized by Y.-P. Sun—suffers from two critical weaknesses. One is its effectively exclusive limitation to malonates; only a few exceptions are known. The other is a structural concern. In fact, quite often only the use of fairly simple malonates leads to the desired fullerene adducts in reasonable yields without tedious purifications. In order to overcome these problems, several groups have developed post-functionalizations of methanofullerene adducts. For instance, J.F. Nierengarten adapted the copper-mediated Huisgen 1,3dipolar cycloaddition reaction and applied it to the preparation of complex hexasubstituted fullerenes. A. Hirsch developed a selective deprotection–functionalization sequence of fullerene e,e,e-trisadducts. Our own group derivatized hexakis methanofullerenes by using several organometallic cross-coupling reactions. This article outlines the use of the Mitsunobu reaction as an efficient tool for the post-functionalization of fullerene monoand hexakis-adducts. The mild, virtually neutral conditions under which this dehydrative coupling of an alcohol with a pronucleophile (generally an acid) proceeds, prompted our group to use this reaction for the derivatization of fullerene adducts. To this end, we have prepared a C60 mono-adduct bearing 2 hydroxyl groups and a hexakisadduct with 12 hydroxyl functions. The preparation of hexakis compound 4 is depicted in Scheme 1. Ethylene glycol was mono-protected as tert-butyldimethylsilyl ether according to a previously reported procedure. Subsequent diesterification of malonic acid was performed in the presence of DCC in 97% yield. Hexakis-adduct 3 was then readily synthesized by using the optimized conditions