Bingel-Hirsch cyclopropanation

The addition of a particularly useful reagent, bearing two ester functionalities, is obtained by means of the Bingel-Hirsch cyclopropanation [29]. Although the mechanism may not be of carbene-type, the result is formally equivalent to a carbene addition. The presence of the ester groups renders this derivative susceptible for further modifications. 

The Bingel-Hirsch Reaction One of the most valuable preparative methods applicable to fullerenes is the Bingel-Hirsch reaction. This is, in detail, the nucleophilic attack of a bromomalonate on the double bond of a fuUerene in the course of which a cyclopropane ring is generated. The reaction exclusively yields the meth-anofullerene with a bridged (6,6)-bond. It has first been described by C. Bingel in 1993, and A. Hirsch subsequently optimized the conditions, so today it is one of the best controllable reactions of fullerenes and can be performed with a multitude of reagents and substrates. 

The Bingel-Hirsch reaction is an important tool in fullerene functionalization. It yields cyclopropanated fullerenes by means of deprotonated bromomalonates. 

In 2007 two independent contributions made by Cordova and co-workers [14] and by W. Wang and co-workers [15] reported a simple and highly stereoselective cyclopropanation via the reaction of enals (15) and 2-bromomalonates (18). Catalyst VII allowed, based on a Michael addition and subsequent intramolecular a-alkylation (i.e., a Bingel-Hirsch reaction) of the enamine intermediate, the production of the cyclopropane motif 17 (Scheme 10.9, B). 

The two most commonly used derivatization methods for exohedral functionalization are the nucleophilic cyclopropanation with malonates (Bingel, 1993) and the formation of fulleropyrrolidines (Maggini et al., 1993). Both of these protocols have been used extensively to produce water-soluble fullerenes for biomedical applications. Other stable water-soluble fullerene adducts have also been reported (Hirsch and Brettreich, 2005). Sections 3.2.2-3.2.5 will give a short overview on the state-of-the-art of water-soluble fullerene derivatives and outline some general trends for designing such molecular structures. 

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