Regio- and stereoselective [2+2] photocycloaddition in Ba2+ templated supramolecular dimers of styryl-derivatized aza-heterocycles

The regio and stereoselective [2 þ 2] photocycloaddition of 15 crown 5 containing styrylheterocycles resulting in formation of only one cyclobutane isomer out of eleven possible is described. It was shown that the cycloaddition takes place solely in the case of the supramolecular preorganization of the reactive molecules provided by both p p stacking interaction of the heterocyclic fragments and sandwich type coordination of the crown ether residues by the barium cation. The results point out the importance of the supramolecular approach for the synthesis of cyclobutane derivatives with desired structure and conformation. for of 1a e with and Ba(ClO was studied in at data using


Introduction
Organic compounds comprising a cyclobutane motif represent an important class of natural metabolites that have been found in various terrestrial and marine species [1,2]. The majority of natu rally occurring cyclobutane derivatives demonstrates remarkable biological activity making them promising lead structures for the creation of novel anticancer, antibacterial and fungicidal drugs [2]. Besides that, cyclobutanes are important synthetic intermediates providing atom economic one step transitions from simple to complex structures that is especially important in the total syn thesis of natural products and other intricate molecules [3].
High practical importance of cyclobutane derivatives stimulated the development of synthetic approaches to these compounds. The most commonly applied method of the cyclobutane scaffold con struction is photochemical [2þ2] cycloaddition of two ethylene double bonds that may occur in the inter or intramolecular way [3c,4]. Nevertheless, a straightforward application of the photoin duced [2þ2] cycloaddition in the synthesis of complex biologically related molecules is hindered in many cases by its low regio and stereoselectivity. To manage this problem, three main methodolo gies allowing performance of [2þ2] cycloaddition reactions in a regio and stereoselective manner have been applied. The classic methodology is based on the attachment of appropriate sub stituents managing the preferred direction of the photochemical cycloaddition by steric and electronic effects [5]. In some cases, stereoselectivity of the photocycloaddition can be reached through chiral induction provided by introduction of chiral auxiliaries [5]. The second methodology relates to an emerging class of catalytic [2þ2] cycloadditions that have been extensively developing within the last decade [6]. This group of reactions includes both photo chemical and non photochemical transformations that are assisted by various catalysts, such as Lewis acids, amines or transition metal complexes, and lead to formation of enantiomerically enriched cyclobutanes. The third methodology is based on the supramolec ular preorganization of substrate molecules that provides fixation of the reactive bonds in the desired mutual position prior to the cyclization process. In this field, several reports are devoted to the solid state template directed synthesis of cyclobutanes managed 1 by confinement of the substrates in metal organic frameworks and hydrogen bond templates [7], zeolites [8], nanocages [9]. There are a few examples of the reversible single crystal to single crystal cyclobutane ring formation upon irradiation of stilbenoids in their crystal state [10]. In solution, supramolecular catalysis has been applied for complexes of macrocyclic hosts with photoactive guests that undergo stereospecific [2þ2] photocycloaddition directly inside the host cavity [11]. Another type of supramolecular preorganization for [2þ2] photocycloaddition was realized by self assembly of the substrates in solution assisted by the metal cation coordination, hydrogen bonding, pep stacking interaction and partial covalent linking. The examples of the latter approach, however, are still rather rare [12e20].
Herein, we present simple yet efficient variation of the latter method resulting in the regio and stereoselective light induced formation of cyclobutane derivatives in the series of crown containing styrylheterocycles 1ae4a (Chart 1). The key process providing the closure of a cyclobutane ring is an intermolecular [2þ2] photocycloaddition of the ethylene double bonds taking place upon irradiation of the supramolecular dimers of 1ae4a (Chart 1). Noticeably, the photoreaction yields only one of eleven possible isomers of the cyclobutane product. Formation of the head to head dimeric complexes of 1ae4a is driven by a delicate balance between the pep stacking interaction of the heterocyclic fragments and the sandwich type coordination of Ba 2þ cations by two 15 crown 5 ether residues. To estimate the role of the stacking interaction we varied the heterocyclic residue increasing the sur face area from pyridine to phenantroline. The effect of the sandwich type coordination with barium cations on the formation of dimers was studied by comparison with magnesium cations, whose size fits well to the 15 crown 5 ether cavity excluding the possibility of "sandwich" formation. Metal cation complexation, supramolecular dimerization and photocycloaddition reaction of compounds 1ae4a were investigated by absorption spectroscopy, steady state and time resolved fluorescence spectroscopy as well as 1D and 2D NMR techniques.

Steady state optical spectroscopy studies
Absorption spectra of styryl derivatives 1ae4a exhibit intense long wavelength absorption bands at the spectral region of 300e400 nm ( Fig. 1; Table 1; Figures S1eS7, ESI) that are associated with an intramolecular charge transfer from the electron rich crown residues to the electron poor heterocyclic fragments. The absorption maxima of compounds 1ae4a shift to longer wave length with increasing electron withdrawing effect of the hetero cyclic part in the series 1a < 4a < 2a < 3a. Addition of Ba(ClO 4 ) 2 or Mg(ClO 4 ) 2 to the acetonitrile solutions of 1ae4a results in the blue shift of the absorption bands due to binding of Ba 2þ and Mg 2þ cations by crown ether oxygen atoms involved in the chromophoric system of the ligands ( Fig. 1    Mg 2þ , the inclusion complexes of 1:1 stoichiometry (metal/ligand) are formed due to the fine coincidence of the sizes of 15 crown 5 ether and magnesium cation [21,22]. At the same time, the cavity of 15 crown 5 ether is too small for binding of the barium cation. Therefore, in the presence of Ba 2þ formation of sandwich type complexes of 1:2 stoichiometry (metal/ligand) is expected (Chart 1) [21,22]. Spectrophotometric titrations of 1ae4a with Ba(ClO 4 ) 2 and Mg(ClO 4 ) 2 in acetonitrile and following treatment of the ob tained data confirmed the expected stoichiometries and allowed to determine stability constants of the complexes ( Table 1).
Irradiation of the acetonitrile solutions of styryl derivatives 1ae4a with filtered light (l 365 nm) resulted in the decrease of the absorption intensities along with a slight blue shift of the ab sorption maxima that is indicative of the formation of the photo stationary mixtures of E and Z isomers of 1ae4a (Scheme 1; Figs. S1 and S2, ESI) [23].
Photolysis of 1ae4a under the same conditions in the presence of 0.5 eq. of Ba(ClO 4 ) 2 led to more pronounced and more compli cated spectral changes. At first, a fast decrease of the absorption intensities of the long wavelength absorption bands and a slight blue shift of the absorption maxima were observed as in the case of E Z photoisomerisation of styryl derivatives 1ae4a. Prolonged irradiation of the barium complexes of 1ae4a (l 365 nm) caused the disappearance of the long wavelength absorption bands in the spectral region of 300e400 nm indicating destruction of the con jugated chromophoric system. At the same time, new significantly blue shifted bands appeared at 230e350 nm that is the charac teristic absorption region of non conjugated aromatic and hetero aromatic fragments ( Fig. 1; Figs. S5eS7, ESI). To explain this observation, we assumed that dimeric complexes L 2 $Ba 2þ undergo the [2þ2] photocycloaddition reaction of the ethylene double bonds resulting in the formation of cyclobutane derivatives 1be4b, in which conjugation between aromatic rings is disrupted by the cyclobutane ring closure. (Scheme 1). Kinetic studies on the photochemical reactions of the complexes of 1a and 3a with Ba(ClO 4 ) 2 were carried out upon irradiation at 313 nm or 365 nm depending on the position of the absorption maxima of the cor responding species. Analysis of the kinetic data allowed us to determine the values of the quantum yields of the forward and reverse photoisomerisation and photocycloaddition reactions (Table 2). No cyclobutane species was detected in control experi ments with Mg 2þ cation that does not form dimeric complexes with 1ae4a. Thus, irradiation (l 365 nm) of the complexes L$Mg 2þ just led to the decrease of the intensity of the long wave length absorption bands with a small blue shift of the absorption maxima (Figs. S3eS4, ESI), as it was observed for free 1ae4a.
Therefore, supramolecular preorganization of the ligands 1ae4a in the Ba 2þ templated supramolecular dimers is the key process managing the occurrence of [2þ2] photocycloaddition.

NMR spectroscopy studies
Since binding of the barium cation to the crown ether residue has an intermediate rate in the NMR timescale, the addition of Ba(ClO 4 ) 2 to the solutions of styryl derivatives 1a-4a in CD 3 CN resulted in severe broadening of their 1 H NMR spectra at room temperature, owing to chemical exchange between various possible complexes. While at lower temperature (233 K) the intermolecular exchange is slow enough to allow observing the signals of complexes with different barium to ligand ratio (Fig. S15, ESI), the irradiation of these mixtures with 365 nm light produced heavily overlapped NMR spectra of the photocycloaddition and E Z photoisomerisation products and their barium complexes. The spectra are dramatically simplified if a small amount of D 2 O (15% v/ v) is added to the sample and if the measurements are performed at room temperature, as the presence of water shifts the dynamic exchange processes to the high rate regime (Fig. 2).
In all cases, the main photochemical process was the formation of the only one cyclobutane derivative of 1be4b, as revealed by the presence of two doublets in a characteristic region of the cyclo butane ring protons (4.5e5.5 ppm), and the only side process was the E Z isomerization of the initial compounds. Notably, irradiation (l 365 nm) of compounds 1ae4a in the absence of barium species resulted in the E Z isomerization only, and no products of photo cycloaddition were detected that is in agreement with the steady state optical spectroscopy data (vide supra).
The processes occurring upon addition of barium ions and following photoirradiation were analyzed by NMR in more details for 4 quinoline derivative 2a. The addition of 0.5 eq. of Ba 2þ to a solution of 2а in CD 3 CN caused two distinctive sets of signals to appear in the 1 Н NMR spectrum (Fig. S15b, ESI); those are assigned to two isomers of sandwich (2a) 2 $Ba 2þ complexes with different Scheme 1. Photochemical transformations of styryl derivatives 1a 4a and their complexes with Ba 2þ and Mg 2þ .

Table 2
Quantum yields of the E-Z-photoisomerisation (4 1 ), Z-E-photoisomerisation (4 2 ), forward (4 3 ) and reverse (4 4 ) [2þ2] photocycloaddition reactions of the supramolecular dimers L 2 Ba 2þ at different irradiation wavelengths (l orientations of the styryl fragments ( Fig. 3a and b) [19]. In both cases, the downfield shift of the signals of the heterocyclic protons suggests the de shielding influence of a neighboring aromatic fragment of the second styrylheterocyclic fragment, which is possible only in a sandwich like coordination geometry. Further increase of the concentration of the metal ions results in a new set of signals (Fig. S15, ESI) assigned to a conventional 1:1 complex (Fig. 3c). While the irradiation of different isomers of sandwich complexes would result in different products of photo cycloaddition, in our experiments, only one cyclobutane isomer out of eleven possible [24] (Fig. 4) is formed. The presence of only two signals in the region of cyclobutane protons (4.5e5.5 ppm) limits the plausible structures to the symmetric ones, thus excluding isomers 2, 3, 8 and 9. Given the origin of the product, i. e. the ge ometry of the sandwich dimers, it leaves only four structures of the cyclobutane products with crown ether moieties located on the same side of the cyclobutane ring (compounds 1, 6, 7, 11 on Fig. 4). Indeed, the formation of all other isomers is less probable, as the corresponding L 2 $Ba 2þ sandwich complexes will not be stabilized by the p p stacking of heteroaromatic fragments. The L 2 $Ba 2þ   complexes shown on Fig. 3a and b would lead to cyclobutane de rivatives 6 (rctt) and 11 (rtct), respectively, in the absence of E Z photoisomerisation, and to isomers 1 (rccc) and 7 (rccc), respec tively, if E Z photoisomerisation would occur. The relative positions of the protons in the cyclobutane structure can be identified with the aid of the two dimensional NMR ROESY (Rotating frame nu clear Overhauser effect spectroscopy) pulse sequence, in which the intensity of cross peaks in 2D spectra depends on the distance between the corresponding nuclei [25]. Generally, rссс and rtct isomers would have stronger through space interactions between protons H a,a' and H b,b' than for rctt one (Fig. 5): in rссс, all protons H a , H a' , H b , H b' are close to each other, as they are located on the same side of the cyclobutane ring, whereas in rtct isomer there are twice as many H a H b interactions as in rctt isomer. On a more quantitative level, the relative intensity of the ROE type cross peaks between different protons may be estimated from the ratio of distances between them; the pair of protons with a well determined inter nuclear distance may be used as a reference. [26] The approximate geometries of three possible isomers, as obtained by quantum chemical calculations using MM2 force field, were used to estimate expected H a (H a' ) e H b (H b' ) cross peak intensities relative to the cross peak of the neighboring aromatic protons H 5 and H 6 (~2.5 Å). The comparison with the experimental data (Table 3; Fig. S19, ESI) shows that the product 2b is an rctt isomer (Fig. S20,  ESI). Moreover, the value of the scalar coupling constant 3 J Ha-Hb 6.6 Hz obtained from lineshape simulation of 1 Н NMR signals using DAISY module of Topspin 3 software (Fig. S21, ESI) is also indicative of a transoid geometry of H a and H b protons in 2b [27].

Time resolved fluorescence spectroscopy studies
Time resolved fluorescence measurements showed the mono exponential fluorescence decay for free ligands 1a, 3a, 4a, whereas for ligand 2a two distinct fluorescence lifetimes were observed (Table 1). Most likely, the existence of two lifetimes for 2a corre sponds to the occurrence of two photoinduced processes in the exсited state, namely, the equilibration of syn and anti populations (reflecting orientation of the 4 quinoline moiety with respect to the double bond) and following E Z photoisomerisation [28]. Complexation of 1ae4a with Mg 2þ shortened the first fluorescence lifetime for all compounds and additionally led to the appearance of the second component for 3a and 4a. We assume that the second lifetime arises from the occasional formation of labile sandwich like structures of 2ae4a complexes with Mg 2þ due to the stack ing interactions between the heterocyclic fragments and the ex change of magnesium cations between the two crown ether units [18]. Complexation of the ligands with Ba 2þ resulted in the appearance of new very characteristic long living components in the timescale of 1000e3600 ps for compounds 2ae4a that were assigned to the formation of the tight syn sandwich dimers with the Ba 2þ cation (Table 1). Shorter lifetimes were associated with the relaxation of the monomeric and/or loose syn complexes as well as possible anti conformers of 4 quinoline derivative 2a.
The value of the longest relaxation time of the exited state correlates with the bulkiness of the moving molecular fragments on the E Z isomerization pathway. In other words, formation of the sandwich complexes in the presence of Ba 2þ cations hinders large amplitude rotations due to the tight stack arrangement of the molecules in the supramolecular dimers. This fact results in the increase of the excited state (fluorescence) lifetimes. Surprisingly, no long living component was detected for styrylpyridine deriva tive 1a upon complexation with the barium cation. The most possible reason for this is quite weak pep stacking interaction between the small aromatic systems of the pyridine rings that does not allow a long living sandwich dimer of 1a to be formed. On the other hand, irradiation of the complex (1a) 2 $Ba 2þ still yields the single isomer of the cyclobutane species 1b as it does in the cases of 2ae4a. We assume, therefore, that the sandwich dimer of (1a) 2 $Ba 2þ is formed, but it possesses high conformational lability that is reflected by the absence of the long living fluorescence lifetime. However, the lifetime of (1a) 2 $Ba 2þ in the favorable conformation appears to be sufficient for the occurrence of the regio and stereoselective [2þ2] photocycloaddition.

Conclusions
In summary, we have demonstrated that the regio and ster eoselectivity of [2þ2] photocycloaddition can be controlled in so lution in an all supramolecular way by means of the appropriately balanced non covalent interactions. Thus, the [2þ2] photo cycloaddition of the crown derivatized styrylheterocycles takes place only in their supramolecular dimeric complexes, in which a favorable arrangement of the reactive C]C double bonds is pro vided. The supramolecular dimerization is managed by pep stacking interaction of the heterocyclic fragments and sandwich type coordination of Ba 2þ cations by two 15 crown 5 ether  Notably, for all studied compounds only one cyclobutane isomer out of eleven possible is formed. The obtained results represent a promising synthetic approach towards construction of the cyclobutane ring of the desired structure and conformation.

Experimental section
Crown derivatized styrylheterocycles 1ae4a were synthesized according to published protocols [29,30]. 1 H and 13 C NMR spectra were recorded from CD 3 CN solutions using Bruker Avance 500 and Avance 600 spectrometers. The chemical shifts were referenced using the residual signals of the deuterated solvents. The tempera ture was calibrated using standard methanol sample. Samples in CD 3 CN were irradiated in NMR tubes (5 mm) at 295 K in a home built apparatus that has been previously described [31]. The filtered emission spectrum of a 1000 W XeeHg high pressure short arc lamp (Oriel) (optical filter: L.O.T. Oriel 365FS10e50: 354<l < 378 nm with l max 365 nm and T 25) was focused on the end of a silica light pipe (length 6 cm, diameter 8 mm), leading the light to the spinning sample tube, inserted into a quartz Dewar. After irradiation, the tube was transferred into the NMR spectrometer. To monitor the pathways of the photoreactions, the spectra were recorded at regular time intervals. The concentration of each photoproduct was obtained from the ratio of peak intensities. Absorption spectra were recorded on a Varian Cary 100 double beam spectrophotometer. Preparation and handling of the solu tions were carried out under red light. Photochemical reactions were carried out with a high pressure Hg vapor lamp (120 W) and an immersed Hg photoreactor (125 W). Kinetic studies on the phototransformations of the complexes were performed using a diode array fiber optic spectrometer AvaSpec 2048. Complex for mation of 1ae4a with Mg(ClO 4 ) 2 and Ba(ClO 4 ) 2 was studied by a spectrophotometric titration method in acetonitrile at 20±1 C; the stability constants of the complexes were determined from the photometric data using the SpecFit 32 program.
The fluorescence excitation light pulses for the time resolved measurements were obtained by frequency doubling and tripling of a Ti:sapphire femtosecond laser system (Femtopower Compact Pro) output. The depolarized excitation light was used to excite the samples. The highest pulse energies used to excite fluorescence did not exceed 100 nJ and the average power of excitation beam was 0.1 mW at a pulse repetition rate of 1 kHz focused into a spot with a diameter of 0.1 mm in the 10 mm long fused silica cell. The fluo rescence emitted in the forward direction was collected by reflec tive optics and focused with a spherical mirror onto the input slit of a spectrograph (Chromex 250) coupled to a streak camera (Hamamatsu 5680) equipped with a fast single sweep unit M5676, temporal resolution 2 ps. The convolution of a rectangular streak camera slit in the sweep range of 250 ps with an electronic jitter of the streak camera trigger pulse provided a Gaussian (over 4 de cades) temporal apparatus function with a FWHM of 20 ps (for more details see ESI).

Note
Authors declare no competing financial interests.