Opening Access to New Chiral Macrocycles: From Allenes to Spiranes

Article pubs.acs.org/JPCA

Opening Access to New Chiral Macrocycles: From Allenes to Spiranes Silvia Castro-Fernández, María Magdalena Cid, Carlos Silva López,* and José Lorenzo Alonso-Gómez* Department of Organic Chemistry, Facultade de Quı ́mica, Universidade de Vigo, 36310 Vigo, Spain S Supporting Information *

ABSTRACT: Chiral macrocycles offer great potential and versatility regarding their applications. They have been employed in asymmetric catalysts, as chiral sensors, and as chiral supramolecular frameworks. For these reasons, they have been attracting increasing interest over the years. Despite all of 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 of 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 compare 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.

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INTRODUCTION Macrocyclic systems are very interesting structures because their overall cyclic shape provides them with key chemical advantages with respect to their linear counterparts. For example, the compact shape and smaller size make cycles more appealing for pharmaceutical chemistry because they cross the cell membrane easily, considerably increasing their bioavailability.1 On the other hand, their inherent pocket renders macrocycles as molecular receptors, making them suitable for molecular sensing,2 chiral resolution,3 and catalysis.4 As a consequence, among the wide variety of chemical structures, from small molecules to complex supramolecular frameworks, macrocycles have attracted increasing attention from the scientific community. Particularly, shape-persistent macrocycles stand out due to their highly selective molecular recognition capabilities.5 Moreover, when these systems are chiral, in addition to differentiating between pairs of enantiomers, they also present chiroptical responses. These optical properties however are very sensitive to conformational changes6 and complexations.7 Therefore, access to tunable, stable, and shape-persistent chiral macrocycles is of great interest for sensing applications. However, to date, the availability of chiral shapepersistent macrocycles is scarce. We have previously demonstrated that allenes constitute an appropriate building block for the construction of conformationally stable chiral macrocycles8 whose outstanding chiroptical responses were both theoretically predicted and experimentally measured.910 In order to broaden the applicability of these alleno-acetylenic macrocycles, we introduced aromatic units into their structures.11 These allenophanes however presented low conformational stability when bearing aromatic © XXXX American Chemical Society

Figure 1. Aromatic connectors with para substitution allow unhindered rotation, while the meta substitution locks the rotation gaining conformational stability.

connectors with para substitution. This is due to the free rotation of the aromatic rings attached to the sterically negligible acetylene moieties (Figure 1). To gain conformational stability and also to simplify the synthetic procedure, we synthesized a pyridoallenophane where the pyridine units are metasubstituted, blocking its free rotation around the acetylene axes.12 This chiral macrocycle, with only three possible conformers, presents strong chiroptical responses and acts as a receptor due to the functionality supplied by the pyridine units.13 Nevertheless, we could not exploit the formation of the host−guest complexes and their chiroptical responses due to the low association constants obtained. In addition, allenes Special Issue: 25th Austin Symposium on Molecular Structure and Dynamics Received: August 20, 2014 Revised: November 20, 2014

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dx.doi.org/10.1021/jp508414r | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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Scheme 1. Schematic Representation for Chiral Macrocyclesa

a

Vertices denote chiral axes, while circles denote aromatic rings used as connectors. In green are general structures for chiral macrocycles with only one possible conformation. In red are general structures for macrocycles with several possible conformations. In black are the representations of different possible conformations for a particular system.

Figure 2. Schematic representation of (P)-DEA and (P)-DES chiral motifs along with the geometries of the macrocycles included in this study (obtained at the cam-B3LYP/6-31G(d) level of theory). For the DEA derivatives, tert-butyl groups were replaced by methyl groups to reduce computational cost.

tackle these kinds of problems, particularly in the field of density functional theory.21,22 We have previously proved that the chiroptical properties of allene-acetylenic macrocycles can be satisfactory predicted at the gas-phase camB3LYP/6-31G(d)23 level of theory9,10 Therefore, we have employed this level of theory in order to study the potential of spiranes replacing allenes in the construction of new chiral macrocycles. Additionally, due to its ongoing popularity and the excellent results reported with the meta-hybrid formula belonging to the Minnesota suite, the M062X functional was also employed throughout this work.24 Both functionals yielded similar results in terms of geometries and relative energies, and more importantly, no significant differences were observed when comparing simulated spectra computed with either functional (vide infra). All structures presented in this work were fully optimized at the level of theory of choice. Harmonic analysis based on the computation of the second derivatives of the energy with respect to the displacement of the atomic positions confirmed that they are a minimum on the potential energy surface. Time-dependent density functional theory in the response regime25 was employed to compute electronic transitions of the macrocyclic structures included in this work. The UV/vis and ECD spectra were then simulated through a sum over states following the expression reported by Grimme.26 All of the calculations described in this work have been performed with the Gaussian 09 suite.27

undergo photoisomerization when conjugated with electronrich functional groups,8,9 limiting the exploration of new chiral macrocycles with complexating abilities. Consequently, analogous macrocycles bearing different functional groups should be explored in order to improve complexation and make possible their application as chiral sensors. To overcome the stability issues, we propose the construction of macrocycles using spiranes, a more stable structural motif to impose axial chirality. Spiranes serve as rigid scaffolds that present axial chirality, suitable as chiral building blocks for the construction of new chiral macrocycles. Particularly, diethynylspiranes (DESs) do not present photoisomerization14 and can be obtained in gram scale in the enantiopure form.15 In this work, we performed a theoretical study comparing the chiroptical reponses of conformationally stable shape-persistent macrocycles bearing allenes with their spiro counterparts. The analysis of the theoretically predicted UV/vis and ECD spectra presents spiranes as promising substitutes of allenes for the construction of new chiral macrocycles.

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COMPUTATIONAL METHODS The accurate description of vertical excitation energies and other optical properties through computational methods is a long-standing challenge of theoretical chemistry.16−20 As a consequence, a plethora of different methodologies exist to B



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Figure 3. Comparison of the UV/vis simulated spectra of (P4)-DEA (top) and (P4)-DES (bottom) computed at the cam-B3LYP/6-31G(d) (left) and M06-2X/6-31G(d) (right) levels.

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RESULTS AND DISCUSSION Conformational Stability. Chiroptical responses are very sensitive to conformational changes;6 for this reason, models with single conformations are desirable in order to facilitate the sensing process of chiral systems. In this respect, cyclic molecules are more attractive compared to their linear analogues. Moreover, chiral allenes have been used for the construction of macromolecules with defined topologies.28 In particular, we have synthesized the enantiopure allenoacetylenic macrocycle (P4)-DEA bearing four diethynylallenes (DEAs) (Figure 2), which shows outstanding chiroptical properties.9 This chiral macrocycle presents a single conformation (represented by a green square in Scheme 1), allowing the study of the mechanisms of its chiroptical response without the uncertainty produced by structural changes.10 The incorporation of functionality in these new chiral macrocycles opens access to enhancing both their recognition and sensing capabilities. For this reason, we have inserted anthracene8 and pyridine11 moieties within the structure of (P4)-DEA. However, this modification resulted in a loss of conformational rigidity due to the free rotation of the para-substituted aromatic spacers within the macrocycle framework (represented by the red squares in Scheme 1). In order to improve the conformational stability of the chiral macrocyles, we blocked the free rotation of the pyridine rings through a meta substitution (represented by the red hexagon in Scheme 1).12 Still, this pyridoallenophane bearing four axially chiral moieties has three possible conformations (represented by the black boat-, chair-, and twist-like structures in Scheme 1). To overcome this drawback, we have recently proposed allenic

macrocycles bearing only two chiral building blocks and an spacer as synthetically feasible and conformationally rigid molecules (green triangle in Scheme 1).29 Preliminary experimental results showed that (P2)-DEAp (Figure 2) can be synthesized, and it exhibits strong ECD; however, photoisomerization of the allene moieties8,30 under ambient light drove to the complete loss of the chiroptical responses within hours. To enable the use of macrocycles as chiral sensors, stable structural motifs imposing axial chirality are needed. From Allenes to Spiranes. DESs are stable and available in enantiopure form, and the topologies for (P4)-DES and (P2)DESp are very similar to their allenic counterparts (P4)-DEA and (P2)-DEAp (Figure 2). Additionally, (P2)-DEAbp and (P2)-DESbp are very appealing chiral macrocycles because the presence of the bipyridine moiety makes them more suitable for the formation of inclusion complexes. In order to evaluate the potential of spiranes to replace allenes in the construction of new chiral macrocycles with potent chiroptical responses, we compare the simulated UV/vis and ECD spectra of (P4)-DEA, (P2)-DEAp, and (P2)-DEAbp with those of (P4)-DES, (P2)DESp, and (P2)-DESbp. Cam-B3LYP versus M06-2X. As we indicated above, camB3LYP has been our method of choice when simulating the absorption and ECD spectra of allenophanic structures in the past.10 This functional yielded results in reasonable agreement with the experimental spectra for these systems. In this work, we decided to also include a modern functional employing a different approach to improve the description of electronic excitations within the DFT framework. The Minnesota meta-hybrid M06-2X was therefore compared with cam-B3LYP. C



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Figure 4. Simulated UV/vis (left) and ECD (right) spectra of (P2)-DEAp (top) and (P2)-DESp (bottom) at the cam-B3LYP/6-31G(d) level of theory.

Both functionals provided very similar results, to the extent that most spectra are qualitatively superimposable, disregarding minor shifts in the absorption bands. A couple of representative examples are illustrated in Figure 3. To avoid redundancy, the remainder of the discussion will be based on cam-B3LYP results only, but all spectra obtained with M06-2X are available in the Supporting Information. UV/Vis and ECD Simulations. When evaluating the strength of chiroptical responses, the g factor is commonly employed. This magnitude, also known as the anisotropy factor, is the ratio between the ECD and UV/vis intensities of the sample. This parameter therefore measures the efficiency of a chiral system in discriminating between right and left circularly polarized light. Because the ECD is often a very small fraction of the UV/vis, this ratio is extremely sensitive to even minute errors (either in terms of intensities or band shifting) when computing the simulated spectra. For this reason, the simulation of the g factor of a molecule is nowadays unreasonable and unreliable. Fortunately, we do have experimentally recorded ECD spectra for the allenophanes included in this work.9,31 These systems were rather promising in terms of chiral sensing due to their strong chiroptical response, and the simulated spectra computed in this work reproduced faithfully the sequence of Cotton effects observed experimentally. With this experimental reference at hand, a direct comparison of the ECD spectra produced by allenophanes and spirophanes is therefore practical, feasible, and should provide reliable information to either support or discard this structural motif as a potential candidate for the construction of new chiral macrocycles.

Typically, compounds bearing butadiyne moieties present a vibronic coupling resulting in a regular spacing of peaks in the UV/vis and ECD of ∼2000 cm−1, corresponding to the CC triple bond stretching. This phenomenon can be tackled computationally for small to medium sized systems; however, performing this detailed analysis for complex systems like (P4)DEA is not viable nowadays. Fortunately, the theoretical simulation of the ECD and UV/vis of this system considering pure electronic transitions yields qualitatively accurate spectra that reproduce quite faithfully the experimental results. Additionally, such strong vibronic coupling is not expected in spirane systems due to disruption of the conjugation between the triple CC bonds. On these grounds, a comparative analysis of the chiroptical responses of allenophanes versus spirophanes disregarding the fine structure of the spectra seems adequate.10 The UV/vis and ECD spectra of (P2)-DEAp and (P2)-DESp, bearing two chiral axes and one pyridine ring, were simulated (see Figure 4). The spirane system behaves as a similar chromophore when compared to the allenophanic parent system. They both absorb light in the near-UV region with comparable extinction coefficients. Actually, the spirophane is a slightly better chromophore, and its maximum of absorbance is slightly shifted toward the visible region. Nevertheless, the difference in the shape of the ECD spectra for these two analogous systems with the same absolute configuration but different chiral axes is remarkable. These results suggest that the origin of the chiroptical responses may be of very different nature. The maximum Cotton effect for (P2)-DEAp is positive, and it D



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Figure 5. Simulated UV/vis (left) and ECD (right) spectra of (P2)-DEAbp (top) and (P2)-DESbp (bottom) at the cam-B3LYP/6-31G(d) level of theory.

chiroptical response, albeit reduced, is still strong enough to be applied to sensing. Remarkable results arise from the comparison between the simulated spectra of (P4)-DEA and (P4)-DES. Orbital analysis performed on (P4)-DEA uncovered that electron rotation about the sp carbon of the allene moieties as well as electron displacement along the butadiene moieties is responsible for the chiroptical responses.10 Therefore, while the same behavior of the previous examples is reproduced (different shapes but similar intensities of the ECD profiles (Figure 6) along with comparable extinctions (Figure 3)), we expect that in (P4)DES, the absence of the sp allenic carbons simplifies the mechanism origin of the chiroptical responses. Due to the presence of quaternary carbons at the spiranic moieties, conjugation is lost, and the four independent chromophores along with the butadiene moieties should be responsible for the CD response via through-space coupling. Synthesis of (P4)DES is underway, and a detailed computational study to support this hypothesis and characterize the bands observed experimentally in this macrocycle will accompany the experimental work. The fact that the derivative based on the spirane structural motif shows a chiroptical response comparable to that of (P4)-DEA is very promising. (P4)-DEA was an enantiomerically pure alleno-acetylenic macrocycle with outstanding Cotton effects at the time of its synthesis and characterization in 2009.9 It is still one of the organic synthetic molecular structures with stronger chiroptical properties available. The fact that the more stable spirane analogue (P4)-DES shows a chiral response of similar magnitude places

appears on the UV side of the spectrum (200 nm), whereas it is negative and appears on the near-UV region for (P2)-DESp (300 nm). The relative chiroptical response, however, is of comparable strength when we consider the overall spectrum, making (P2)-DESp a reasonable substitute for the parent allenophane. With the aim of expanding the recognition abilities of these systems, we synthesized an analogue to (P2)-DEAp featuring a bipyridine connector instead of a single pyridine ((P2)-DEAbp, shown in Figure 2). This functional modification enlarges slightly the inner cavity of the macrocycle, and it provides two lone pairs directed toward this cavity, thus facilitating the anchoring of guest molecules. The spirane analogue to this improved allenophane, (P2)-DESbp, is also shown in Figure 2. The UV−vis and ECD spectra of these systems show similar trends to what we observed in the pyridine derivatives (Figure 5). Both systems are good chromophores, and the spirane counterpart extends its optical activity further into lower-energy wavelengths (200−300 nm for (P2)-DEAbp and 200−400 nm for (P2)-DESbp). We experimentally and computationally observed that the introduction of a bipyridine moiety replacing a pyridine in allenophanes ((P2)-DESp and (P2)-DESbp) improves the absorbance of the system, but it reduces significantly the chiroptical response.31 For instance, the largest Cotton effect in (P2)-DESp doubles the intensity of the most intense band (P2)-DESbp. This trend is also anticipated for the spirane macrocycles according the simulated spectra (compare spectra in Figures 4 and 5). Fortunately, the E



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Figure 6. Simulated ECD spectra of (P4)-DEA (left) and (P4)-DES (right) at the cam-B3LYP/6-31G(d) level of theory.

theory. This material is available free of charge via the Internet at http://pubs.acs.org.

DESs as promising building blocks for the construction of chiral sensors. In addition to exhibiting a strong absorption (Figure 3), the conformationally rigid chiral macrocycle (P4)-DES shows a considerable red shift compared to (P4)-DEA. This strong shift is quite valuable because it positions this chromophore at the door of visible light, which may exponentially broaden its applications. In summary, this study suggests that (P4)-DES could be a very promising system with outstanding optical and chiroptical activity. The synthesis and experimental characterization of the optical features of this spirane macrocycle are underway.

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*E-mail: [email protected] (J.L.A.-G.). *E-mail: [email protected] (C.S.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Spanish Ministerio de Economıa y Competitividad (CTQ2011- 28831) and Xunta de Galicia (EM2013/ 017) for research funding. J.L.A.-G. thanks Xunta de Galicia for a “Parga Pondal” research contract. All authors are thankful to the Supercomputing Center of Galicia (CESGA) for generous allocation of computer time.

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CONCLUSIONS We have simulated ECD and UV/vis spectra for conformationally rigid chiral macrocycles bearing two different axially chiral motifs. 1-DEA has been used for the construction of chiral macrocycles with outstanding chiroptical properties; however, its photoisomerization limits the exploration of new chiral structures. 2-DES, in contrast, is a photostable chiral axis and, thus, a desirable building block for the construction of new chiral macrocycles. Our calculations show that the chiroptical responses of models bearing DEAs or DESs and a pyridine or bipyridine moiety are of comparable intensities. This is of particular relevance in (P4)-DES because the allenophanic parent system, (P4)-DEA, features outstanding chiroptical properties that are hard to match. As a result, the use of DES should provide access to new chiral macrocycles with the conformational rigidity, chemical stability, and strong chiroptical properties that make promising candidates for chiral sensing. Additionally, the presence of the quaternary carbon of the DES moieties will disrupt the electronic conjugation along the cyclic system. Thus, allowing the analysis of the chiroptical properties by the well-established exciton-coupled method because of a multichromophoric system with noninteracting individual chromophores may be assumed.

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AUTHOR INFORMATION

Corresponding Authors

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Cartesian coordinates, SCF energies and the number of imaginary frequencies, and simulated UV/vis and ECD spectra for all of the structures included in this work computed at the cam-B3LYP/6-31G(d) and the M06-2X/6-31G(d) levels of F



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Opening Access to New Chiral Macrocycles: From Allenes to Spiranes

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