Regio- and stereospecific [2+2] photocyclodimerization of a crown-containing butadienyl dyevia cation-induced self-assembly in solution

Abstract

Self-assembled pseudocyclic structures consisting of two molecules of a crown-containing butadienyl dye and two Mg²⁺ ions readily undergo regio- and stereospecific [2+2] photocycloaddition in MeCN to produce a single cyclobutane stereoisomer in almost quantitative yield.

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Self-organization into supramolecular structures via noncovalent interactions such as hydrogen bonding, host–guest interactions, ion association or stacking interactions, makes it possible to control excited-state reactions of organic molecules.¹ In the early nineties we reported a dramatic effect of molecular self-assembly on the photochemistry of a styryl dye containing a 15-crown-5 ether fragment and a benzothiazole residue with 3-sulfonatopropylN-substituent (dye 1, Scheme 1).²

Scheme 1 Self-assembly into dimeric complexes leading to stereospecific [2+2] photocycloaddition of styryl dye 1.

In aprotic solvents dye 1 binds Mg²⁺ ions to form pseudocyclic 2 : 2 complexes in which the sulfonate anion of one dye molecule coordinates with the metal cation located in the crown ether cavity of another and vice versa. The dye molecules in these complexes undergo selective anti-head-to-tail [2+2] photocycloaddition (PCA) to afford a single cyclobutane stereoisomer (2, Scheme 1). Irradiation at 365 nm led to almost quantitative conversion of dye 1 into cyclobutane 2. In metal-free solutions dye 1 does not undergo PCA even at a relatively high concentration (∼1 mM).

The effect of different structural factors on the cation-induced PCA of crown-containing styryl dyes has been studied.³ In particular, it was found that changing to a 4-styrylquinolinium chromophore altered the PCA pattern (syn-head-to-tail vs. anti-head-to-tail).^3d

Other supramolecular approaches that have been used to control intermolecular PCA of olefins in solution include duplex formation viahydrogen bonds,⁴dimerization within the cavities of γ-cyclodextrin,⁵cucurbit[8]uril⁶ and calixarenes,⁷ encapsulation within self-assembled coordination cages,⁸hydrogen bonding with molecular templates,⁹self-assembly into pseudorotaxane structures,¹⁰ preorganization via multiple noncovalent interactions.¹¹ In a few cases supramolecular assistance to PCA provided almost quantitative conversion of an olefin into a single cyclobutane isomer.

In comparison with olefins, 1,3-butadienes can be involved in a broader spectrum of photochemical reactions.¹² This makes it more difficult to control their photochemical behaviour by supramolecular methods. It is known that topochemical PCA of butadienes can result not only in cyclobutanes¹³ but also in [3]ladderanes through a stepwise process.¹⁴ A few studies have been published on supramolecular control of the solid-state intermolecular PCA of butadienes. For instance, the photochemical syntheses of [3]ladderanes from butadienes were reported¹⁵ in which proper molecular packing of the reactants were provided by weak noncovalent interactions. However, the potential of supramolecular chemistry with respect to controlling the solution-phase intermolecular PCA of 1,3-butadienes remains unexplored.

We assumed that for stereospecific PCA of two butadiene molecules to occur in solution, the reactant must be properly preorganized by means of relatively strong noncovalent interactions, such as two-centre ion pairing in complexes (1·Mg²⁺)₂.¹⁶ To test this assumption we synthesized a butadiene analogue of styryl dye 1, viz., the crown-containing butadienyl dye 5 (Scheme 2).

Scheme 2 Synthesis of butadienyl dye 5.

Dye 5 was obtained in 43% yield by condensation of cinnamaldehyde 3 (ref. 17) with quaternary salt 4 (ref. 3b) on long-term heating in absolute ethanol. The structure assigned to 5 is confirmed by elemental analysis and ¹H NMR spectroscopy.‡

Fig. 1 shows the absorption spectra of dye 5 in MeCN/0.01 M Bu₄NClO₄ as a function of the concentration of added magnesium perchlorate. The strong hypsochromic effect observed on addition of Mg(ClO₄)₂ indicates Mg²⁺ coordination to the 15-crown-5 ether moiety of dye 5.¹⁸ The spectrum measured at a 20% excess of Mg(ClO₄)₂ over the dye (spectrum B in Fig. 1) is attributable to a fully complexed form of 5 because a subsequent increase in the metal salt concentration by an order of magnitude did not induce any spectral changes. This spectrum has a specific profile with a pronounced long-wavelength shoulder, which points to H-type aggregation of dye molecules.¹⁹ To interpret this feature, we assume that dye 5, similarly to styryl dye 1, forms 2 : 2 complexes with Mg²⁺via two-centre ion pairing. Factor analysis²⁰ showed that other complexes most likely make an insignificant contribution to the spectra represented in Fig. 1. This enabled us to assign spectrum B to the dimers (5·Mg²⁺)₂.

Fig. 1 Absorption spectra of dye 5 in MeCN/0.01 M Bu₄NClO₄ as a function of the concentration of added magnesium perchlorate (C_M): 1 cm cell, the concentration of 5C_D = 1.2 × 10⁻⁵ M, C_M/C_D = 0 (A), 0.18, 0.36, 0.53, 0.71, 0.87 and 1.2 (B); spectrum B is assigned to complex (5·Mg²⁺)₂.

The main absorption and luminescence characteristics of dye 5 and complex (5·Mg²⁺)₂ are listed in Table 1. Note that the formation of 2 : 2 complexes between 5 and Mg²⁺ leads not only to significant changes in the absorption spectrum but also to luminescence quenching.

Table 1 Electronic absorption and emission spectroscopy data for dye 5 and complex (5·Mg²⁺)₂ in MeCN²¹

Compound	λ max ^abs/nm	ε/dm^3 mol^−1 cm^−1	λ max ^em/nm	φ lum
φ lum – luminescence quantum yield.
5	459	47000	626	0.017
(5·Mg^2+)2	417	106000	587	0.0008

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Fig. 2 shows the effect of irradiation with low-intensity 436 nm light on the absorption spectra of (5·Mg²⁺)₂ in MeCN. The irradiation results in rapid disappearance of the long-wavelength absorption band associated with (5·Mg²⁺)₂. The product obtained after 15 min photolysis was tested for the presence of Z photoisomers of 5 using a four-stage procedure, as described previously for styryl dyes.^3d This test showed that the photolysis led to complete consumption of dye 5, presumably owing to very efficient PCA reaction between dye molecules in the dimeric complexes (5·Mg²⁺)₂. The PCA quantum yield was estimated to be about 0.3.

Fig. 2 Photolysis of dye 5 (9.1 × 10⁻⁶ M) in MeCN in the presence of Mg(ClO₄)₂ (1 × 10⁻⁴ M), as monitored by spectrophotometry: 1 cm cell, irradiation at 436 nm, light intensity 5 × 10¹⁴ cm⁻²c⁻¹, irradiation time 0 (B), 0.5, 1.0, 1.5, 2.5 and 15 (C) min; spectrum C is assigned to complex 6·(Mg²⁺)₂.

The photolysate obtained from (5·Mg²⁺)₂ was studied in a MeCN-d₃/D₂O solution by ¹H NMR spectroscopy.§Water was added in order to preclude complex formation between the photolysate and Mg²⁺. The ¹H NMR spectrum of the uncomplexed photolysate (Fig. 3) revealed that the PCA reaction in complexes (5·Mg²⁺)₂ occurs regio- and stereospecifically to afford a single cyclobutane isomer (cyclobutanertct-6, Scheme 3). The photolysate contained no other detectable components, i.e., the cyclobutane 6 is obtained from dye 5 in almost quantitative yield (>95%).

Fig. 3 ¹H NMR spectrum of cyclobutanertct-6 in a MeCN-d₃/D₂O solution (10 : 1 v/v): (a) aromatic and (b) aliphatic regions.

Scheme 3 Regio- and stereospecific synthesis of cyclobutanertct-6.

In the aromatic region, the ¹H NMR spectrum of 6 exhibits four signals from the ethylene protons H(a′)–H(d′) and a set of signals from the protons of two non-equivalent benzothiazole residues and two non-equivalent benzo-15-crown-5 ether fragments. The protons of cyclobutane ring, H(a)–H(d), give four signals between 3.6 and 4.8 ppm.²² These facts indicate that the substituents in the 1,2,3,4-tetrasubstituted cyclobutane 6 differ from each other in structure. The relative position of these substituents was derived from analysis of the 2D COSY and NOE spectra of 6. The cyclobutane protons H(a) and H(b) show two triplet signals at 4.73 and 4.01 ppm with ³J = 9.5 and 9.7 Hz, respectively. The triplet structure of these signals as well as the magnitudes of vicinal coupling constants are indicative of the rtct-configuration of the cyclobutane moiety in 6.^3a–c The structure assigned to cycloadduct 6 suggests an anti-head-to-tail alignment of the two dye molecules in the precursor complexes (5·Mg²⁺)₂ (Scheme 3). This alignment is confirmed by molecular mechanics and DFT calculations of (5·Mg²⁺)₂ (see ESI†).

In summary, the self-assembly of butadienyl dye 5 into 2 : 2 complexes with Mg²⁺via two-centre ion paring in MeCN leads to very efficient regio- and stereospecific [2+2] photocycloaddition affording cyclobutanertct-6 in almost quantitative yield. This result demonstrates for the first time that intermolecular PCA of 1,3-butadienes can be effectively controlled in solution by supramolecular methods. The approach we developed can be used to design novel supramolecular systems with high functionality.

This work was supported by the Russian Academy of Sciences and the Russian Foundation for Basic Research.

Notes and references

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16. For dye 1 and its analogues having a sulfonatobenzylN-substituent, the dissociation of dimers (dye·Mg²⁺)₂ into monomers dye·Mg²⁺ in anhydrous MeCN at a temperature of ca. 20 °C occurs within a characteristic time ranging from a few tens of seconds to a few minutes, which attests relatively strong intermolecular bonding in the dimers: (a) E. N. Ushakov, O. B. Stanislavskii, S. P. Gromov, O. A. Fedorova and M. V. Alfimov, Crown-containing styryl dyes. 6. Photoinduced and dark cation-dependent dimerization of betaines of photochromic 15-crown-5 ethers, Dokl. Phys. Chem., 1992, 323, 164; (b) E. N. Ushakov, S. P. Gromov, A. V. Buevich, I. I. Baskin, O. A. Fedorova, A. I. Vedernikov, M. V. Alfimov, B. Eliasson and U. Edlund, Crown-containing styryl dyes: Cation-induced self-assembly of multiphotochromic 15-crown-5 ethers into photoswitchable molecular devices, J. Chem. Soc., Perkin Trans. 2, 1999, 601.
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19. H-type dye aggregation commonly results in absorption band splitting; in many cases, the weaker, lower-energy band is unresolved and reveals itself as a shoulder of the more intense, higher-energy band: (a) V. I. Yuzhakov, Association of dye molecules and its spectroscopic manifestation, Russ. Chem. Rev., 1979, 48, 1076; (b) E. N. Ushakov, S. P. Gromov, O. A. Fedorova and M. V. Alfimov, Crown-containing styryl dyes. 19. Complexation and cation-induced aggregation of chromogenic aza-15-crown-5 ethers, Russ. Chem. Bull., 1997, 46, 463.
20. The set of experimental spectra shown in Fig. 1 was reconstructed within a residual error of 0.0012 optical density units by a factor analysis method under the assumption that there are only two absorbing components in the system; the small value of residual error suggests that contribution from a third component to the total spectral variation can be neglected: (a) J. J. Kankare, Computation of equilibrium constants for multicomponent systems from spectrophotometric data, Anal. Chem., 1970, 42, 1322; (b) E. N. Ushakov, S. P. Gromov, O. A. Fedorova, Yu. V. Pershina, M. V. Alfimov, F. Barigelletti, L. Flamigni and V. Balzani, Sandwich-type complexes of alkaline-earth metal cations with a bisstyryl dye containing two crown ether units, J. Phys. Chem. A, 1999, 103, 11188.
21. MeCN solutions of dye 5 and complex (5·Mg²⁺)₂ were prepared in a darkroom under red light. Spectral experiments were conducted in quartz cells with ground stoppers to avoid MeCN evaporation during the experiments. The atmospheric oxygen contained in solutions was not removed. Luminescence spectra were corrected taking into account the spectral sensitivity of the photomultiplier tube. Luminescence quantum yields φ_lum were determined using [Ru(bpy)₃]Cl₂ in air-equilibrated water as the reference compound. The φ_lum value for the reference is 0.028: K. Nakamaru, Synthesis, luminescence quantum yields, and lifetimes of trischelated ruthenium (II) mixed-ligand complexes including 3,3-dimethyl-2,2-bipyridyl, Bull. Chem. Soc. Jpn., 1982, 55, 2697.
22. The signals from cyclobutane protons H(c) and H(d) are overlapped by an intense signal from the protons of CH₂O groups.

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Footnotes

† Electronic supplementary information (ESI) available: Absorption spectroscopy data for a cationic analogue of dye 5; global analysis of spectrophotometric titration data (factor analysis, discussion on complexation stoichiometry, parameterized modelling of absorption spectra); molecular mechanics and DFT modelling of dimers (5·Mg²⁺)₂. See DOI: 10.1039/c0pp00227e

‡ A mixture of cinnamaldehyde 3 (100 mg, 0.31 mmol) and quaternary salt 4 (76 mg, 0.28 mmol) in abs. EtOH (12 mL) was heated at 80–85 °C (oil bath) for 100 h and then cooled to 5 °C. The precipitate formed was filtered, washed with abs. EtOH (3 mL), benzene (2 × 10 mL), hexane (15 mL) and dried in air to yield compound 5 (72 mg, 43%) as a brown-red powder (Found: C, 56.87; H, 5.79; N, 2.30. Calc. for C₂₈H₃₃NO₈S₂·H₂O: C, 56.64; H, 5.94; N, 2.36%); mp 213–216 °C; ¹H NMR (Bruker DRX500; DMSO-d₆; 25 °C): δ 2.16 (m, 2H, CH₂CH₂SO₃), 2.64 (t, J = 6.8 Hz, 2H, CH₂SO₃), 3.63 (m, 8H, 4 CH₂O), 3.80 (m, 4H, 2 CH₂CH₂OAr), 4.13 (m, 4H, 2 CH₂OAr), 4.91 (m, 2H, CH₂N), 7.04 (d, J = 8.4 Hz, 1H, H(17)), 7.21 (br.d, J = 8.4 Hz, 1H, H(16)), 7.29 (s, 1H, H(14)), 7.30 (dd, J = 15.3 Hz, J = 10.7 Hz, 1H, H(c)), 7.43 (d, J = 15.3 Hz, 1H, H(d)), 7.59 (d, J = 14.7 Hz, 1H, H(a)), 7.74 (m, 1H, H(6)), 7.84 (m, 1H, H(5)), 8.02 (dd, J = 14.7 Hz, J = 10.7 Hz, 1H, H(b)), 8.35 (d, J = 8.8 Hz, 1H, H(4)), 8.37 (d, J = 8.4 Hz, 1H, H(7)). The spin–spin coupling constants J_H(a),H(b) = 14.7, J_H(b),H(c) = 10.7 and J_H(c),H(d) = 15.3 Hz point to E,E configuration and s-trans conformation of the butadiene fragment in 5.

§ Dye 5 (10.0 μmol) and Mg(ClO₄)₂ (10.5 μmol) were dissolved in 50 mL of MeCN (water content < 0.03% v/v). The solution was irradiated with 436 nm light (glass-filtered light of a high-pressure Hg lamp); after complete consumption of the dye (monitoring by spectrophotometry), the solvent was evaporated in vacuo. The solid residue was dissolved in a MeCN-d₃/D₂O mixture (10 : 1 v/v) and analyzed by ¹H NMR spectroscopy. This analysis revealed that the solute contained a single detectable component, viz., cyclobutane isomer rtct-6 (Scheme 3); ¹H NMR (Bruker DRX500; MeCN-d₃/D₂O (10 : 1 v/v); 25 °C): δ 2.08 and 2.25 (2 m, 2H, CH₂CH₂SO₃), 2.27 (m, 2H, CH₂CH₂S′O₃), 2.67 (br.t, J = 6.3 Hz, 2H, CH₂SO₃), 2.91 (t, J = 6.2 Hz, 2H, CH₂S′O₃), 3.60–3.70 (m, 17H, 8 CH₂O, H(d)), 3.72 (m, 1H, H(c)), 3.82 (m, 4H, 2 CH₂CH₂OAr), 3.86 (m, 4H, 2 CH₂CH₂OAr), 4.01 (t, J = 9.7 Hz, 1H, H(b)), 4.08 (m, 2H, α′′′-CH₂OAr), 4.10 (m, 2H, α′-CH₂OAr), 4.17 (m, 2H, α′′-CH₂OAr), 4.23 (m, 2H, α-CH₂OAr), 4.67 and 4.77 (2 m, 2H, CH₂N), 4.73 (t, J = 9.5 Hz, 1H, H(a)), 4.89 (m, 2H, CH₂N′), 6.62 (dd, J = 15.8 Hz, J = 7.9 Hz, 1H, H(c′)), 6.68 (d, J = 15.8 Hz, 1H, H(d′)), 6.88 (d, J = 8.3 Hz, 1H, H(17′)), 6.98 (br.d, J = 7.5 Hz, 2H, H(17), H(16′)), 7.13 (br.d, J = 8.2 Hz, 1H, H(16)), 7.20 (br.s, 1H, H(14′)), 7.22 (br.s, 1H, H(14)), 7.46 (d, J = 15.5 Hz, 1H, H(a′)), 7.58 (dd, J = 15.5 Hz, J = 7.6 Hz, 1H, H(b′)), 7.78 (m, 1H, H(6′)), 7.79 (m, 1H, H(6)), 7.87 (m, 1H, H(5′)), 7.89 (m, 1H, H(5)), 8.18 (d, J = 8.6 Hz, 2H, H(4), H(4′)), 8.21 (d, J = 8.1 Hz, 1H, H(7′)), 8.30 (d, J = 8.2 Hz, 1H, H(7)).

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