Publication: Chiral Fullerenes from Asymmetric Catalysis
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2014-08
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ACS
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Abstract
Fullerenes are among the most studied molecules during the last three decades and, therefore, a huge number of chemical reactions have been tested on these new carbon allotropes. However, the aim of most of the reactions carried out on fullerenes has been to afford chemically modified fullerenes soluble in organic solvents or even water in the search for different mechanical, optical or electronic properties. Therefore, although a lot of effort has been paid to the chemical functionalization of these molecular allotropes of carbon, important aspects in the chemistry of fullerenes have not been properly addressed. In particular, the synthesis of 2 chiral fullerenes at will in an efficient manner by using asymmetric catalysis had never been previously addressed in fullerenes science. Thus, despite fullerenes chirality has always been considered a fundamental issue, the lack of a general stereoselective synthetic methodology has restricted the use of enantiopure fullerene derivatives, which have been usually obtained just after highly expensive HPLC isolation on specific chiral columns or prepared from a pool of chiral starting materials. In this Account, we describe the first stereodivergent catalytic enantioselective syntheses in fullerene science that have allowed synthesizing, in a highly efficient way, enantiomerically pure derivatives with a total control of the stereochemical result using either metallic catalysts and/or organocatalysts under very mild conditions. Theoretical DFT calculations strongly support the experimental findings for the assignment of the absolute configuration of the new stereocenters which has also been ascertained by application of the sector rule and single-crystal X-ray diffraction. The use of the curved double bond of fullerene cages as 2 component in a variety of stereoselective cycloaddition reactions represents a challenging goal considering that, in contrast to most of the substituted olefins used in these
reactions, pristine fullerene is a non-coordinating dipolarophile. The aforementioned features make the study of stereoselective 1,3-dipolar cycloadditions onto fullerenes a unique scenario to shed light onto important mechanistic aspects. On the other hand, the availability of achiral starting materials as well as the use of non-expensive asymmetric catalysts should open the access to chiral fullerenes and their further application to a variety of different fields. In this regard, in addition to the bio-medical applications, chiral fullerenes are of interest in less-studied areas such as materials science, organic electronics and nanoscience where the control of the order and morphology at the nanometer scale are critical issues for achieving better devices efficiencies.