Stereodivergent Synthesis of Chiral Fullerenes by [3 + 2] Cycloadditions to C60

Stereodivergent Synthesis of Chiral Fullerenes by [3 + 2] Cycloadditions to C60

Maroto, E. E.; Filippone, S.; Suárez, M.; Martínez-Álvarez, R.; de Cózar, A.; Cossío, F. P.; Martín, N. J Am Chem Soc 2014. ASAP.

DOI: 10.1021/ja410408c 

A wide range of new dipoles and catalysts have been used in 1,3-dipolar cycloadditions of N-metalated azomethine ylides onto C60 yielding a full stereodivergent synthesis of pyrrolidino[60]fullerenes with complete diastereoselectivities and very high enantioselectivities. The use of less-explored chiral α-iminoamides as starting 1,3-dipoles leads to an interesting double asymmetric induction resulting in a matching/mismatching effect depending upon the absolute configuration of the stereocenter in the starting α-iminoamide. An enantioselective process was also found in the retrocycloaddition reaction as revealed by mass spectrometry analysis on quasi-enantiomeric pyrrolidino[60]fullerenes. Theoretical DFT calculations are in very good agreement with the experimental data. On the basis of this agreement, a plausible reaction mechanism is proposed.

nazario

During the last two decades, new carbon forms have been discovered by the scientific community with a sensational pace. First, fullerenes as carbon molecular allotropes, then carbon nanotubes and, more recently, graphene have emerged as suitable materials with promising and outstanding properties. Simultaneously, the chemical modification of these new carbon nanoforms has been developed to process and make these materials available for different applications. However, despite the high level of knowledge reached specially in the molecular chemistry of fullerenes, a fundamental issue of chirality control has been scarcely addressed so far.  Recently, the direct synthesis of optically active [60]fullerene derivatives by using chiral metal catalysis by Martin and Itami. In 2009, Martin group reported report the enantioselective catalytic synthesis of chiral pyrrolidinofullerenes.[1] Recently, Itami group found that chiral phosphoramidites are effective chiral ligands for the enantioselective tetraallylation of C60. [2]

In this paper, Nazario group have carried out a thorough and systematic study to expand the scope of this highly versatile stereoselective catalytic cycloaddition onto C60 to other catalysts and dipoles, using also complementary mass spectrometry (MS) methodologies. Additionally, they have done  theoretical DFTcalculations by collaborating with Cossio group.

  •  References

[1] “An efficient approach to chiral fullerene derivatives by catalytic enantioselective 1,3-dipolar cycloadditions ”

Filippone, S.; Maroto, E. E.; Martín-Domenech, Á.; Suárez, M.; Martín, N. Nat. Chem. 2009, 1, 578–582. DOI: 10.1038/NCHEM.361

nchem.361_toc

 

Fullerene chirality is an important but undeveloped issue of paramount interest in fields such as materials science and medicinal chemistry. So far, enantiopure fullerene derivatives have been made from chiral starting materials or obtained by separating racemic mixtures. Here, we report the enantioselective catalytic synthesis of chiral pyrrolidinofullerenes (the most widely studied fullerene derivatives), which proceeds in high yields under very mild conditions at low temperatures. The combination of a particular metal catalyst—Ag(I) or Cu(II)—and a chiral ligand is able to direct the cycloaddition of buckminsterfullerene C60, the first non-coordinating dipolarophile used in such reactions, to opposite enantiofaces of N-metallated azomethine ylides. This methodology has proven to be quite general, affording enantiomeric excesses of greater than 90%. Furthermore, well-defined chiral carbon atoms linked to the fullerene sphere are able to perturb the inherent symmetry of the fullerene π-system as revealed by circular dichroism measurements

[2] “Palladium-catalyzed tetraallylation of C60 with allyl chloride and allylstannane: mechanism, regioselectivity, and enantioselectivity”

Nambo, M.; Wakamiya, A.; Itami, K. Chem. Sci. 2012, 3, 3474. DOI: 10.1039/c2sc21126b 

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We have established a unique Pd-catalyzed tetraallylation of C60 with allyl chloride andallylstannane that likely proceeds by the action of amphiphilic bis(π-allyl)palladium. Mechanistic analysis has revealed that both steric (for the first diallylation) and electronic (for the second diallylation) factors are responsible for high regioselectivity. The ring-closing metathesis reaction and hydrogenation of the tetraallylated product took place in the presence of Ru and Rh catalysts. Moreover, we found that chiral phosphoramidites are effective chiral ligands for the enantioselective tetraallylation of C60. Pronounced enantioselectivity up to 88% ee was realized in the production of tetraallylated C60.

 

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