Strong Chiroptical Responses
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The unique combination of geometric and electronic properties explains the remarkable magnitude of the Cotton effects in the circular dichroism spectra of new enantiomerically pure alleno-acetylenic macrocycles (see picture). The macrocycles (P,P,P,P)-(−)-1 (red) and (M,M,M,M)-(+)-1 (blue) were prepared in three steps starting from optically pure 1,3-di-tert-butyl-1,3-diethynylallenes.
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New enantiomerically pure alleno–acetylenic macrocycles were prepared by oxidative homocoupling of optically active 1,3-diethynylallenes. Enantiomer separation resulted from a combined strategy of synthesis and chiral HPLC techniques. Two other achiral stereoisomers were also isolated and fully characterized. In addition, the X-ray structures of the chiral D4- and C2-symmetric macrocycles are reported. The chiroptical properties of these macrocycles are discussed on the basis of quantum chemical calculations, by using the CAM-B3LYP functional. Studies were carried out to investigate the vibronic fine structure observed experimentally in the UV/Vis and CD spectra. The origin of the intense chiroptical response of the chiral alleno–acetylenic macrocycles is explained by considering the topology of the molecular orbitals involved, thus relating electronic properties to structural features. Further analysis of the canonical molecular orbitals and the electron localization function (ELF) shows that these macrocycles belong to a relatively rare class of highly stable and formally anti-aromatic Hückel compounds.
Enantiomerically pure alleno-acetylenic oligomers of defined lengths were synthesized by the palladium-mediated oxidative homocoupling of optically pure 1,3-diethynylallenes. The large amplification of their chiroptical properties strongly suggests the formation of helical secondary structures. This assignment is supported by time-dependent quantum chemical calculations.
Covalent Organic Helical Cages
A Covalent Organic Helical Cage with Remarkable Chiroptical Amplification
From collagen to DNA, helical morphologies are ubiquitous in nature. These fascinating chiral structures have led researchers to mimic them with helicates and helicenes. However, even when covalent organic helical cages are of great interest due to their potential as artificial chiral receptors, they have not been explored so far. We propose a general and broad methodology for the construction of covalent organic helical cages through axial chirality by connecting the opposite bases of a prism with loop-like lateral edges. We used this approach to design and synthesize a covalent organic helical cage with high efficiency. Crystal structure analysis, NMR, and electronic circular dichroism of this novel cage certified its helical morphology, inclusion complex formation, and outstanding chiroptical responses. We believe that these results pave the way for the construction of a broad variety of covalent organic helical cages in the near future.
From collagen to DNA, helical morphologies are ubiquitous in nature. These fascinating chiral structures have led researchers to mimic them with helicates and helicenes. However, even when covalent organic helical cages are of great interest due to their potential as artificial chiral receptors, they have not been explored so far. We propose a general and broad methodology for the construction of covalent organic helical cages through axial chirality by connecting the opposite bases of a prism with loop-like lateral edges. We used this approach to design and synthesize a covalent organic helical cage with high efficiency. Crystal structure analysis, NMR, and electronic circular dichroism of this novel cage certified its helical morphology, inclusion complex formation, and outstanding chiroptical responses. We believe that these results pave the way for the construction of a broad variety of covalent organic helical cages in the near future.
Covalent Organic Helical Cages as Sandwich Compound Containers
A covalent organic helical cage (COHC) with D3 symmetry bearing two 1,3,5-trimethylphenyl cores and six di-tert-butyldiethynylallene moieties was synthesized and fully characterized. This molecular structure cage, unlike a previously reported one, favors inclusion-complex formation with organometallic sandwich compounds due to the presence of methyl groups on the aryl rings. The strong chiroptical responses of these COHCs, along with their ability to entrap guest molecules, enabled the detection of a ruthenium sandwich compound by means of electronic circular dichroism (ECD) spectroscopy.
A covalent organic helical cage (COHC) with D3 symmetry bearing two 1,3,5-trimethylphenyl cores and six di-tert-butyldiethynylallene moieties was synthesized and fully characterized. This molecular structure cage, unlike a previously reported one, favors inclusion-complex formation with organometallic sandwich compounds due to the presence of methyl groups on the aryl rings. The strong chiroptical responses of these COHCs, along with their ability to entrap guest molecules, enabled the detection of a ruthenium sandwich compound by means of electronic circular dichroism (ECD) spectroscopy.
From Allene to Spirane
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Diverse Chiral Scaffolds from Diethynylspiranes: All-Carbon Double Helices and Flexible Shape-Persistent Macrocycles
State-of-the-art chiroptical spectroscopies are valuable tools for structural elucidation. However, the potential of these spectroscopies for everyday applications has not been exploited to date partially due to the lack of sufficiently stable and efficient chiroptical systems. To this end, the development of suitable chiroptical structures is essential. Herein, we present the synthesis of spiro-compounds (P2)-1 and (P4)-2 as well as (M2)-1 and (M4)-2 exhibiting remarkable chiroptical responses. Theoretical simulations show that (P2)-1, constituted by two (P)-configured spiranic chiral axes, presents an all-carbon double helix structure with (M)-helicity. On the other hand, molecular dynamic simulations reveal (P4)-2 to have a single path for geometry-modification along its flat conformational space, certifying it as a chiral flexible shape-persistent macrocycle. Geometric quantification of chirality has been used to compare the spiranic derivatives presented herein. |
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Opening Access to New Chiral Macrocycles: from Allenes to Spiranes
Chiral macrocycles offer great potential and versatility regarding their applications. The have been emplyed in asymmetric catalysts, as chiral sensors and chiral supramolecular frameworks. For these reasons they have been attracting increasing interest over the years. Despite all the work developed in this area, most of the reported chiral macrocycles are not conformationally stable and present weak chiroptical responses. Such features substantially limit the scope of applications for these compounds. On the other hand, we have shown that axially chiral allenes can be introduced into macrocycles, conferring conformational stability and outstanding chiroptical responses. However, these allenes photoisomerize when conjugated with electron-donating groups, hampering the possibility of synthesizing systems with tuned optical properties. To overcome all these limitations with a single structural motif, we propose the use of spiranes to construct new stable, conformationally rigid and chemically functionalizable macrocyclic structures with strong chiroptical responses. As a first step in this new direction, we theoretically predict the chiroptical responses for macrocycles bearing spiranes to be as strong as with their allenic counterparts. As a side product we also test the popular Minnesota functional, M06-2X, and compared it with cam-B3LYP, which has been previously analyzed with respect to experimental data in our laboratory. Thus, we hereby propose that spiranes are a good alternative to allenes for the construction of new chiral macrocycles.
Chiral macrocycles offer great potential and versatility regarding their applications. The have been emplyed in asymmetric catalysts, as chiral sensors and chiral supramolecular frameworks. For these reasons they have been attracting increasing interest over the years. Despite all the work developed in this area, most of the reported chiral macrocycles are not conformationally stable and present weak chiroptical responses. Such features substantially limit the scope of applications for these compounds. On the other hand, we have shown that axially chiral allenes can be introduced into macrocycles, conferring conformational stability and outstanding chiroptical responses. However, these allenes photoisomerize when conjugated with electron-donating groups, hampering the possibility of synthesizing systems with tuned optical properties. To overcome all these limitations with a single structural motif, we propose the use of spiranes to construct new stable, conformationally rigid and chemically functionalizable macrocyclic structures with strong chiroptical responses. As a first step in this new direction, we theoretically predict the chiroptical responses for macrocycles bearing spiranes to be as strong as with their allenic counterparts. As a side product we also test the popular Minnesota functional, M06-2X, and compared it with cam-B3LYP, which has been previously analyzed with respect to experimental data in our laboratory. Thus, we hereby propose that spiranes are a good alternative to allenes for the construction of new chiral macrocycles.
