View of the H-bonded layers linked by the charge-assisted carboxylate–pyridinium hydrogen bonds in the structure of 6 . 

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... rctt mixture in different ratios, depending on the starting compound, except for the dimers from 2,2 0 -bpe and 2,4 0 -bpe (h–t), where the partial formation of the rtct -isomers was also observed in the 1 H-NMR spectra. The compositions of the resulting mixtures were determined by 1 H-NMR analysis and are summarised in Table 1. In all the cases the isomerisation degree was always slightly higher in the presence of Al 3+ than Mn 2+ solution. It is important to note that the formation of rcct isomers is not commonly observed, to our knowledge, from direct photoreaction of stilbenes in solution, 4 f ,7 b ,8 the rctt -isomer always being the favoured one. Nevertheless, under such conditions it is possible to achieve moderate yields (36–52%), such a possibility can be viewed as a useful route to drive the synthesis of these stereoisomers. In addition, the rcct -isomer obtained from the isomerisation of 1b yields a chiral isomer (Fig. S6, ESI w ). This result suggests that this route can also be exploited in order to induce chirality from achiral cyclobutane derivatives. 12 Encouraged by these results, we evaluated the effect of Al 3+ salt on the same transformations under hydrothermal conditions at 140 1 C for 24–48 h, in order to achieve quantitative conversions. On the other hand, the Mn 2+ ion was discarded in the study under hydrothermal conditions due to the effect of a paramagnetic ion on NMR line broadening, which is difficult to be removed completely from the solution. The 1 H-NMR spectrum of the products obtained after the hydrothermal treatment either of the mixture obtained from the study upon reflux in the presence of Al 3+ or using fresh rctt -dimers did not show the presence or formation of asymmetrical stereoisomers ( rcct ), in contrast to those cases described above. The signals observed display different chemical shift values compared to the corresponding protons in the starting rctt -isomers. These values are characteristic of different symmetrical isomers ( rtct -configuration). 5 All the spectra revealed a quantitative isomerisation (100%). In order to gain better insight about this process, a new set of experiments in the presence of a Brønsted-acid were also evaluated. Thus, taking advantage of the use of 1,2,3,4 benzenetetracarboxylic acid (bta) as an efficient template to direct reactivity of olefins in the solid state and its ability to give interesting supramolecular assembly directed by charge-assisted hydrogen bonds, 11 here, we extended the use of bta either to assist the isomerisation process or as a potential crystallisation agent of the resulting isomers. The dimers were heated at 140 1 C in the presence of bta under hydrothermal conditions, using bta : dimer in a molar ratio 1 : 1. The 1 H-NMR spectra showed similar results to those observed for the isomerisation in the presence of Al 3+ (quantitative isomerisation into the rtct -isomer). As anticipated, the resulting stereochemistry for rtct -4,4 0 -tpcb and rtct -2,4 0 -tpcb-ht isomers was confirmed by single crystal X-ray diffraction analysis. Crystals of [(bta 2 À ) Á ( rtct -4,4 0 -H 2 tpcb) 2+ ] Á 5H 2 O ( 5 ) and [(bta 2 À ) Á ( rtct -2,4 0 -H 4 tpcb-ht) 0.54+ ] Á 1.28H 2 O ( 6 ) were obtained by crystallisation of corresponding bta– rtct -isomer mixture after heating. The asymmetric unit of 5 contains half stilbenium ( rtct -4,4 0 tpcb) 2+ cation lies on a twofold-axis, one bta 2 À anion and five disordered lattice water molecules. z The crystal structure of 5 forms a 3D-hydrogen bonded network (Fig. 1(a)), which can be described as 2D-hydrogen bonded layers built-up from self- assembly of bta 2 À anions linked by charge-assisted hydrogen bonding between carboxylic and carboxylate groups [O1 Á Á Á O5: 2.492(4) and O7 Á Á Á O3: 2.526(4) A ̊ ] (Fig. 1(b)). Such H-bonded networks are extended in the bc -plane and display rhomboidal windows with distances ca. 7.2 Â 8.7 A ̊ . In each layer, every window is penetrated by one 4-pyridyl unit from ( rtct -4,4-H 2 tpcb) 2+ cations, whereas the remaining pyridyl moieties are oriented parallel to 2D-hydrogen networks. These cations are distributed above and below these layers in an alternate fashion. These layers are sustained through multiple hydrogen bonding interactions among pyridinium, pyridyl, carboxylate and carboxylic moieties and disordered lattice water molecules located into intricate channels that run along the b -direction. The percentage of solvent-accessible space in these channels in the crystal structure corresponds to 21.8% (2132 A ̊ 3 per unit cell) of the total volume (9780 A ̊ 3 ). The asymmetric unit of 6 contains one half stilbenium ( rtct - 2,4 0 -tpcb) 4+ cation lies about a twofold axis, two halves of crystallographically independent bta 2 À anions, one is located on a twofold axis, while the other lies about an inversion centre and one water of crystallisation with partial occupation. The crystal structure of 6 forms a 3D-hydrogen bonded network (Fig. 2), which can be seen as a sinusoidal 2D-hydrogen bonded network self-assembled from bta 2 À anions linked by charge-assisted carboxylate–pyridinium supramolecular synthons. This array is constructed from zigzag ribbons connected via charge assisted hydrogen bonding between one bta 2 À anion and 4-pyridinium units from the ( rtct -2,4 0 -H 4 tpcb-ht) 4+ unit along the c -direction [N1 Á Á Á O1: 2.657(3) A ̊ ]. These ribbons are linked by the second bta 2 À anion through remaining 2-pyridyl units from ( rtct - 2,4 0 -H 4 tpcb-ht) 4+ cations also via carboxylate–pyridinium interactions [N2 Á Á Á O8: 2.405(3) A ̊ ]. These interactions generate H-bonded layers parallel to the ac -plane. Adjacent layers are stacked via self-complementary hydrogen bonding between water molecules of crystallisation and oxygen atoms of carboxylic groups. In this structure the preferred formation of intra- molecular hydrogen bonds between carboxylate and carboxylic groups is observed, in contrast to that observed for 5 . Encouraged by the above results, we have expanded the tolerance of this approach in relation to stilbazole derivatives. In particular, the isomerisation from the rctt -head to tail dimer of 4-Cl-Stb ( rctt -4-Cl-dpcb-ht) under hydrothermal conditions at 140 1 C leads to the formation exclusively of the asymmetrical stereoisomer ( rcct ) in high yield ca. 86%, in contrast to those cases found using stilbene derivatives. A remarkable feature in these processes is the apparent isomerisation of the pyridyl ring. Suitable crystals bearing bta- rcct -4-Cl-dpcb-ht isomer mixture ( 7 ) were also obtained. The asymmetric unit of 7 contains one molecule of the resulting dimer located in a general position with no imposed symmetry; one half of bta lies about an inversion centre, and one water of crystallisation with partial occupation. The crystal structure of 7 forms 1D-hydrogen bonded chains self-assembled via heterodimeric hydrogen- bonded synthons based on the interaction between carboxylic and pyridyl groups in an alternate fashion, producing hydrogen bonded rings along the (101)-direction [N1 Á Á Á O3: 2.594(3) and N2 Á Á Á O2: 2.725(3) A ̊ ] (Fig. 3). Such interactions are neutral in contrast to those observed for 5 and 6 . A striking feature of the isomerisation processes observed for the different tpcb is that these did not occur when the reactions were carried out in the absence of metal centres or bta under identical experimental conditions. In addition, in these assays the formation of side products due to the cyclo- reversion reaction and/or other asymmetrical stereomutations was not observed, in contrast to the thermal isomerisation of cyclobutanes at elevated temperatures. 12 These results reveal the regio- and stereospecific nature of all the isomerisations upon hydrothermal control. At the present stage, based on these observations and further examination of our previously reported works on isomerisation of pyridyl cyclobutanes some insight can be drawn: (a) the isomerisation of this kind of compounds is closely related to the intrinsic strain associated with a four membered ring combined with repulsive electrostatic charges generated for the presence of ionisable groups (pyridyl and –COOH) on the rings. 6,13 In particular, a remarkable electronic effect either of the relative position of the N atom on the pyridyl ring or the presence of neutral substituent groups is observed. (b) Either acidity or temperature plays a fundamental role in the stereoselective control of the resulting isomers, where the rcct -isomer is a metastable intermediate, which is formed in a first step upon mild heating, then is transformed to the most stable isomer ( rtct ) at highest temperatures. Similar results have been observed by Vittal et al. , 7 in the isomerisation of rctt -4,4 0 -tpcb to rtct -isomer promoted by HCl. (c) The intelligent choice of the substituent groups on the cyclobutane ring can be exploited to fine-tune quantitative and regio- and stereoselective synthesis either of rcct or rtct isomers, depending on temperature. In summary, we have demonstrated the ability either of a Lewis or Brønsted acid combined with an increase in temperature to promote the partial or total isomerisation in solution of rctt isomers either to rcct or rtct . We have established a non-photochemical route to prepare quantitatively and regio- and stereo- selectively rtct -tpcb compounds via hydrothermal-assisted isomerisation. This novel approach opens a window to develop efficient routes for the preparation and/or improvement of the yield of new and conventional cyclobutane-like stereoisomers that are difficult or impossible to access either in solution or by known solid state routes, including the possibility of obtaining chiral cyclobutanes. This alternative can be very helpful in order to overcome the limitations imposed by the topochemical postulate 14 for obtaining regioselective photoproducts with such stereochemical ...