Stereodivergent‐at‐Metal Synthesis of [60]Fullerene Hybrids

Abstract

Chiral fullerene-metal hybrids with complete control over the four stereogenic centers, including the absolute configuration of the metal atom, have been synthesized for the first time. The stereochemistry of the four chiral centers formed during [60]fullerene functionalization is the result of both the chiral catalysts employed and the diastereoselective addition of the metal complexes used (iridium, rhodium, or ruthenium). DFT calculations underpin the observed configurational stability at the metal center, which does not undergo an epimerization process.Preparation of fullerenes in multigram amounts by Krätschmer et al. in 1990 [1] permitted the study of the reactivity of the new carbon allotropes with transition-metal complexes, which subsequently arose as one of the main fields in fullerene science. [2] To date, interest in fullerene-transitionmetal complexes has been renewed in the search for new applications and synergies arising from the complementary properties of transition metals and fullerenes. [3] Transition metal involvement in organic transformations drives the investigation of new complexes with catalytic activity. In particular, preparative methods for chiral metal complexesspecifically chiral-at-metal complexes-are highly sought after in current organometallic chemistry because of their relevance in enantioselective metal-mediated processes. [4] Despite the wealth of metal-fullerene hybrids reported to date, only a few are optically active. [2,5] To the best of our knowledge, only a single case presents a stereogenic metal center, [6] while in all examples the chiral information stems from the stoichiometric use of chiral ligands. [7] In this regard, the use of asymmetric organo-and metal-catalysis in fullerene chemistry has provided an easy access to optically active derivatives by precise stereocontrol of the newly formed asymmetric carbon atoms. [8] Conversely, control of the metalcentered chirality is not a trivial issue because of the configurational lability of the metals, which often undergo rapid ligand exchange. [4] Herein, we report the first enantioselective synthesis of fullerene hybrids endowed with a stable stereogenic metal center whose configuration can be defined at will by the proper choice of chiral catalyst. In these reactions, iridium, rhodium, and ruthenium pyrrolidino [3,4:1,2][60]fullerene half-sandwich complexes with four new stereocenters are formed; namely the two C2 and C5 chiral carbon atoms at the pyrrolidine ring, the asymmetric nitrogen atom, and the transition-metal center (Scheme 1). The functionalization and stereochemistry of the chiral fullerene ligand determines the final configuration of the metal center. Interestingly, the chiral metal center does not undergo any epimerization process, as indicated by variable-temperature NMR analysis as well as by DFT theoretical calculations.The syntheses of the chiral metal-fullerene hybrids were inspired by the preparation of half-sandwich complexes with aminocarboxylate ligands, [9] and take advantage ...

Chiral Molecular Carbon Nanostructures

“…Both new stereocenters feature a configuration that is determined by the chirality of the C2 stereocenter of the pyrrolidine ring, and therefore, the stereogenic metal center can be controlled by the suitable choice of the catalytic system (S M -or R M -selective) (Figure 1a). 9 Similar metallo[60]fullerene hybrids based on pyrrolino[60]fullerenes have also been prepared enantioselectively using both metal-mediated processes and organocatalysis (Figure 1b). These new half-sandwich metallo[60]fullerene complexes have been successfully used in photoelectrochemical cells, where they proved to be both good electron acceptor materials and excellent catalysts for photoinduced oxygen reduction reactions.…”

Separation of enantiomers of chiral fullerene derivatives through enantioselective encapsulation within an adaptable helical cavity of syndiotactic poly(methyl methacrylate) with helicity memory

“…Chiral fullerenes have attracted considerable attention due to their unique chiral nanocage structures and promising applications in materials science, biology, and medicine. [1][2][3][4][5][6][7][8][9][10][11] In the past two decades, the chirality of fullerenes has played a key role in the formation of photoconductive nanofibers by supramolecular self-assembly 12 and the detection of circularly polarized light in organic field-effect transistors 13 as well as in the formation of one-handed helical poly(phenylacetylene)s. 14,15 However, a practical and efficient method for the asymmetric synthesis of chiral fullerenes with high enantiomeric excess (ee) still remains limited to a few chiral fullerenes. [16][17][18][19][20] Most of the chiral fullerenes including the inherently chiral higher fullerenes, such as [76]fullerene (C 76 ), C 78 , and C 84 , and those composed of achiral C 60 and C 70 cores with chiral adducts or achiral adducts with inherently chiral addition patterns, 1,3 have been prepared by the chromatographic enantioseparation using chiral stationary phases in high-performance liquid chromatography (HPLC).…”