Cyclobuteno[60]fullerenes as Efficient n-Type Organic Semiconductors.

Cyclobuteno[3,4:1,2][60]fullerenes have been prepared in a straightforward manner by a simple reaction between [60]fullerene and readily available allenoates or alkynoates as organic reagents under basic and mild conditions. The chemical structure of the new modified fullerenes has been determined by standard spectroscopic techniques and confirmed by X-ray diffraction analysis. Some of these new fullerene derivatives exhibit a remarkable intrinsic electron mobility (determined by using flash-photolysis time-resolved microwave conductivity (FP-TRMC) measurements), which surpasses that of the well-known phenyl-C61-butyric acid methyl ester, thus behaving as promising n-type organic semiconductors.

Organocatalysis for new chiral fullerene-based materials.

The cycloaddition reaction of 3-alkynoates to fullerenes has been studied under different experimental conditions and phosphines in the presence of a base which is responsible for the previous isomerization to the corresponding allenoate. The versatility of this reaction allows chiral fullerenes to be obtained at will, thus opening up a new avenue in the use of enantiomerically pure carbon nanostructures in materials science.

Chiral fullerenes from asymmetric catalysis.

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 that are 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 devoted 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 chiral fullerenes at will in an efficient manner using asymmetric catalysis has not been previously addressed in fullerene science. Thus, despite the fact that the chirality of fullerenes 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 usually been obtained only 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, which have allowed the highly efficient synthesis of enantiomerically pure derivatives with total control of the stereochemical result using metallic catalysts and/or organocatalysts under very mild conditions. Density functional theory 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 a two-π-electron 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 noncoordinating 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 nonexpensive asymmetric catalysts should provide access to chiral fullerenes and their further application in a variety of different fields. In this regard, in addition to biomedical applications, chiral fullerenes are of interest in less-studied areas such as materials science, organic electronics, and nanoscience, where control of the order and morphology at the nanometer scale are critical issues for achieving better device efficiencies.

Enantioselective cycloaddition of münchnones onto [60]fullerene: organocatalysis versus metal catalysis.

Novel chiral catalytic systems based on both organic compounds and metal salts have been developed for the enantioselective [3 + 2] cycloaddition of münchnones onto fullerenes and olefins. These two different approaches proved to be efficient and complementary in the synthesis of optically active pyrrolino[3,4:1,2][60]fullerenes with high levels of enantiomeric excess and moderate to good conversions. Further functionalization of the pyrrolinofullerene carboxylic acid derivatives has been carried out by esterification and amidation reactions.

Chiral hypervalent iodine reagents: synthesis and reactivity.

Chiral hypervalent iodine chemistry has been steadily increasing in importance in recent years. This review catalogues enantioselective transformations triggered by chiral hypervalent iodine(III/V) reagents, in stoichiometric or catalytic quantities, highlighting the different reactivities in terms of yield and enantioselectivity. Moreover, the synthesis of the most remarkable and successful catalysts has been illustrated in detail.

Asymmetric synthesis of cyclobutanes by a formal [2+2] cycloaddition controlled by dienamine catalysis.

Trap it: A combination of aminocatalysis with H-bonding activation is used in two new approaches to carry out formal enantioselective organocatalyzed [2+2] cycloaddition reactions. This cooperative catalysis solves the inconveniences associated with this transformation. These two new reactions will open opportunities to find reactivities involving other organocatalytic cycloadditions.

Metallic organophosphates as catalysts in asymmetric synthesis: a return journey.

This perspective article provides a general overview of the most relevant topics in the applications of chiral metallic organophosphates. A brief introduction along with a historic comparative profile of the BINOL and phosphoric acid analogues are given. Next, a selection of the most outstanding uses of the catalysts according to the employed metal is presented.

An amine-catalyzed enantioselective [3+2] cycloaddition of azomethine ylides and α,ß-unsaturated aldehydes: applications and mechanistic implications.

The catalytic enantioselective [3+2] cycloaddition between azomethine ylides and α,ß-unsaturated aldehydes catalyzed by α,α-diphenylprolinol has been studied in detail. In particular, the reaction has been extended to the use of 2-alkenylidene aminomalonates generated in situ as azomethine ylide precursors. These reactions lead to the formation of pyrrolidines containing a 5-alkenyl side chain with potential for chemical manipulation. Moreover, a detailed and concise computational study has been carried out to understand the exact nature of the mechanism of this reaction and especially the consequences derived from the incorporation of the chiral secondary amine catalyst on the reaction pathway.