New concepts for regio- and stereoselective bis- and triscyclopropanations of C60.
The synthesis of isomerically multiple adducts of C(60) with a defined three-dimensional structure is still one of the most challenging tasks of exohedral fullerene chemistry. The inherent regioselectivity of successive additions of addends such as malonates to the fullerene's [6,6]-double bonds is only moderate. In most cases difficult-to-isolate mixtures of regioisomers are obtained. The regioselectivity can be significantly improved if multifunctional addends able to undergo two or more additions are allowed to react with C(60). Preorganization and minimization of strain energy within the addend skeleton reduce the number of sterically allowed addition patterns. Improved concepts for highly regio- and stereoselective bis- and triscyclopropanations of C(60) are described. Two examples of the bisadditions with complete regioselectivity leading to trans-2- and cis-2 are presented. Here, the two malonate binding sites are linked by rigid tetraphenylporphyrin and calix-[4]-arene spacers. Selective trisadditions were achieved with the easy-to-synthesize and easy-to-modify tripodal addends 5-7, where the malonates are held together by a focal aryl moiety. Another very elegant approach for bis- and trisadditions involves cyclo-[n]-alkylmalonates. Selection between addition patterns with and without rotational axes is possible by choosing the right combinations of the flexible alkyl chains connecting the malonates. If alkyl chains of identical lengths are used bis- and trisadducts such as 19-21 and 25 with rotational symmetry are formed with high regioselectivity. These addition patterns are avoided if cyclo-[n]-malonates containing alkyl chains of different lengths are employed. In this case adducts such as 26 and 27 with C(s)-symmetry are formed. The use of the chiral cyclo-[3]-malonate 28 allows for the regio- and stereoselective synthesis of the enantiomerically pure e,e,e-trisadducts 29 and 30 containing an inherently chiral addition pattern with C(3)-symmetry.
Hirsch
Andreas
A
Institut für Organische Chemie der Friedrich-Alexander-Universität Erlangen-Nürnberg, Henkestrasse 42, 91054 Erlangen, Germany. andreas.hirsch@chemie.uni-erlangen.de