Solvent-free synthesis of (poly)thiacalix[ n ]arenes: the evaluation of possible mechanism based on semi-preparative HPLC separation and mass-spectrometric investigation of the reaction products

The interaction between p - tert -butylphenol and elemental sulfur (S 8 ) in alkaline condition in absence of solvent at 150-170 °C have been studied. (Poly)thiacalix[ n ]arenes with the phenol units connected by bi-and trisulfide bridges have been detected and characterized based on the MS data


Introduction
Thiacalix [4]arenes 1 are relatively new class of the family of calix[n]arenes. 2Due to the presence of four sulfur atoms in their macrocycle and thus c.a. 15% larger cavity as compared to calix [4]arenes 3 these host molecules are of growing interest in terms of their advanced coordinations chemistry ("host-guest" complexes). 4For instance, thiacalix [4]arenes exhibits advanced coordination ability towards d-elements, especially toward Hg 2+ , 5 and, more importantly, 226 Ra 2+ cations. 6Recently, some thiacalix [4]arene-supported heterodinuclear Ni(II)-Ln(III) (Ln = Gd, Tb, Dy) complexes have been reported and the magnetic relaxation behavior for some of them have been described. 7In addition, thiacalix [4]arenes exhibit some physiological activity, for instance as potential inhibitors of protein tyrosine phosphatase 1B. 8 The very first p-tert-butylthiacalix [4]arene synthesis was reported by Sone and co-authors 9 via stepwise reactions of p-tert-butylphenol and SCl2.The most common approach for the thiacalix [4]arene synthesis so far reported is the condensation of p-R-phenol and elemental sulfur (S8) under basic conditions in high boiling solvent at the temperature above 170 °C.According to the accepted mechanism reported by Patel et al., 10 the formation of thiacalix [4]arenes occurs via their acyclic precursors (the kinetic products), i.e. polyphenolates, having one sulfur atom between the phenol moieties along with some less-stable polysulfide phenolates.As a last step the high-temperature cyclocondensation reaction affords the thiacalix [4]arene as a thermodynamic product. 10,11][14][15] On the other hand disulfide-and polysulfide-bridged polymers are commonly obtained in the reaction of S8 and polyenes. 16,17In addition, the di-and tetrasulfide-bridged macrocycles are obtained in the reaction of substituted phenols with disulfur dichloride. 18Recently, Dehaen and co-authors reported the preparation of disulfide-bridged [2 n ]pillararene-like macrocyles in good yields. 19erein, we report the synthesis of p-tert-butylthiacalix [4]arene as well as the formation of some di-and trisulfide-bridged macrocycles, i.e. (poly)thiacalix[n]arenes, via the one-pot solvent-free reaction of p-tertbutylphenol with S8 in the presence of NaOH.

Results and Discussion
The reaction between phenol in a form of phenolate anion and S8 affords linear oligomers with various degree of polysulfidity. 4 Subsequent homolytic cleavage of the -S-S-bonds in these oligomers affords monosulfide oligomers as well as some mercaptophenols.The chemical activity of polysulfide-containing macromolecules is stipulated by two main factors: (i) the mobility and electron donating ability of sulfur atoms in mercaptane end groups; (ii) the reduced bond energy in di-and polysulfide moieties compare to monosulfide ones, i.e. -S-S-vs.-C-S-bonds.The last factor sets the basic condition for exchange reactions in oligomer bulk and in the area of its contact with other compounds. 20][23][24][25] Based on the proposed mechanism, 10 tert-butyphenol reacts with S8 in a high-boiling solvent (tetraglyme or diphenyl ether) to afford a mixture of mono-and polysulfide bridged linear oligomers as kinetic products.While heating the reaction mixture to 230 °C affords the most thermodynamically stable thiacalix [4]arene.The use of lower-boiling solvents leads to lower yields of the thiacalix [4]arenes, and no solvent-free approaches have been reported so far.
Recently, during our efforts to prepare pillar[n]arenes, we found that under the solvent-free conditions the reaction afforded mainly kinetic products, i.e. pillar [6]arenes as major products. 26With this in mind that under the solvent-free conditions the kinetic products may be isolated, we studied the reaction between tertbutylphenol and S8 using two different reaction conditions.In our method, all reactions were carried out in the presence of 0.5 equiv. of NaOH as an important template.In a first case, to confirm the possibility of the formation of thiacalix [4]arenes the reaction mixture was heated at 170 °C for 3 h and then at 220-230 °C for 24 h (Scheme 1, way i) (a minimum amount of oxylene was used to remove any trace of water from the reaction mass by slow distillation to make the reaction mass super dry and after that o-xylene was removed).According to our expectation the dynamic nature of diand polysulfide bridges in initially formed linear oligomers was further explored in the transformation of the Ar-(S)n-S-Ar′ bridges to a Ar-S-Ar′ bridge at higher temperature (230 °C).At this temperature the ease of homolytic cleavage of the di-and polysulfide bridges increases with the number of sulfur atoms in a bridge.At higher temperature thiacalix [4]arene should be formed preferably as it is a thermodynamic product.The ESI-MS-analysis of the reaction mixture after 24 h at 230 °C revealed the presence of only thiacalix [4]arene 5 as major product with the trace amounts of unreacted tert-butylphenol and some linear oligomers.
In the second case the reaction mixture was heated at 170 °C for 3 h and then at 200 °C for 3 h (Scheme 1, way ii).The ESI-MS analysis of the reaction mixture after the first stage, i.e. heating at 170 °C for 3 h, reviled the presence of several types of linear oligomers (Scheme 1, Table 1).
After heating the reaction mixture for 200 °C for 3 h the ESI-MS-analysis of reaction mixture surprisingly revealed the unexpected cyclic oligomers with various ratios of aromatic fragments and sulfide moieties macrocycles 1-4 along with the "classic" thiacalix [4]arene 5 (Figures S1-S5, Tables S1-S5, ESI).The Table 2 indicates the detected products forming under these conditions.
The solvent-free condition's reaction for obtaining 5 was accomplished through way ii when the reaction continued at elevated up to 230 °C temperature (see Experimental Section, Method 3).Scheme 1. Solvent-free synthesis of thiacalix[n]arenes 1-5.Reagents and conditions: i: p-tert-butylphenol (1 equiv.),sulfur (2 equiv.),NaOH (0.5 equiv.),Ar, 150-170 °C, 3 h, then 230 °C, 24 h; ii: p-tert-butylphenol (1 equiv.),sulfur (2 equiv.),NaOH (0.5 equiv.),Ar, 150-170 °C, 3 h, then 200 °C, 3 h.S16, Tables S8-S9, ESI).We have achieved the separation of the reaction mixture to separate peaks in the analytical mode by using 70% acetonitrile and 30% 0.005 M citrate buffer as a mobile phase.Attempts to carry out the preparative separation to isolate the analytically pure samples failed: increasing the amount of the target components in the sample led to overlapping products peaks.However, by using the HPLC technique in manual semipreparative mode we were able to collect the minimum number of fractions of the eluate containing sufficiently pure products 1-5 to evaluate their ratio in the starting reaction mixture.In addition the massspectra of eluates in the direct input mode have been measured.The results of the HPLC and ESI-MS analysis of the products are summarized in Table 3.The stability of the obtained thiacalix[n]arenes 1-5 was computationally investigated using density functional theory (DFT) using dispersion corrected 27 B3LYP functional 28,29 and balanced polarized triple zeta basis set of Ahlrichs and co-workers. 30The gas-phase thermodynamic enthalpies of formation were determined from harmonic frequency calculations in the same level.All the calculations were carried out using Gaussian 09 31 program.From the calculations we can observe that among macrocycles 1-5 thiacalix [4]arene 5 is the mostly thermodynamically stable, which is at first confirmed by the literature data, 10 and, at second, can be seen from the DFT calculation on the basis of enthalpy calculations (ΔfH°0 and ΔfH°298) (Table 4, entry 5).The stability of four other thiacalixarenes, namely 1-4, decreases in the following order 1 > 3 > 2 > 4, i.e. macrocycle 1 is the most stable, which can be seen from the basis of enthalpy calculations (ΔfH°0 and ΔfH°298).We can also conclude that among the macrocycles 1-4 the macrocycles with three phenol units connected by two sulfur atoms, i.e. 1 and 3, exhibit much lower enthalpy, which suggests these compounds to have higher stability than the macrocycles containing either two or three phenol units connected by three sulfur atoms.This observation is consistent with the experimental observations that show that the macrocycle with two phenol subunits, i.e. compound 2, can be initially detected in the reaction mixture, but it readily decomposes upon standing in DMSO solution.
The proposed mechanism for the formation of products 1-4 is presented below.Thus, like in case of solvent-mediated method 10 the homolytic cleavage of -S-S-bond in S8 affords the biradical A. Then interaction of A with phenolate anion and phenol affords connected by polysulfide bridges linear oligomers and their radicals, namely B-E, with various stoichiometry (Scheme 2).At the next step depending on (n = 1, 2…) the cyclocondensation of biradicals E1-4 under the solvent-free conditions affords thiacalix[n]arenes 1-4.The most important factor of this cyclocondensation is the template effect of sodium atom.Each of the biradicals E1-4 folds over the sodium atom to afford the cyclic products.Thus, the shortest biradical E1 (6 sulfur atoms, 2 phenol units) (n = 1, x = y = 3) affords thiacalix[n]arene 2 (Scheme 3).Scheme 3. Formation of thiacalixarene 2.
The linear oligomers with longer chains afford other thiacalixarenes under the same conditions.Thus, if n = 2, x = y = 2 (6 sulfur atoms, 3 phenol units) the cyclocondensation of biradical E2 affords the macrocycle 1. Due to its C3v-symmetry the formation of this macrocycle is preferred based on the DFT calculations (Table 1) and thus thiacalixarene 1 has the highest content in the reaction mixture (Scheme 4).Oligomer E3 (7 sulfur atoms, 3 phenol units) (n = 2, x = 3) and its cyclization product 3 are less symmetrical compare to oligomer E2 and 1. Due to the presence of one extra sulfur atom in the polysulfide bridge, macrocycle 3 is less stable than macrocycle 1 4).Worthy of note is that this macrocycle is geometrically very similar to macrocycle 1, but the template effect of sodium atom on the formation of the macrocycle 3 is less pronounced than for the macrocycle 1 (Scheme 5).Finally, oligomer E4 (n = x = 3) (9 sulfur atoms, 3 phenol units) affords macrocycle 4, which is also highly symmetric.However the sodium atom is less suitable as a template atom for the cyclization E4 into 4, therefore this cyclic product was observed as a minor product, which was confirmed based on both the experimental data and the theoretical calculations (Scheme 6).Scheme 6. Formation of thiacalixarene 4.
As kinetic products all these macrocycles 1-4 were observed in the reaction mixture at temperatures below 200 °C.To confirm the kinetic control for the formation of 1-4 at the final step we have increased the temperature of the reaction mixture to 230 °C.As expected, the most thermodynamically stable thiacalix [4]arene 5 was isolated as the only product in up to 80% yield (Scheme 7, Figure S17, Table S10, ESI).In this case the high-temperature cyclocondensation of the most geometrically suitable tetramer T1 (4 sulfur atoms, 4 phenol units) affords the thiacalixarene 5 (Schemes 7 & 8).Scheme 8. Formation of thiacalix [4]arene 5 from tetramer T1.
It is worthy to mention that, as we assumed, under the solvent-free conditions at lower temperature, i.e. less than 230 °C, the formation of macrocycles 1-4 was more favorable than thiacalix [4]arene 5.In this case the reaction mixture is enriched with S8 while it is poor in tert-butylphenol due to its continuous sublimation from the reaction mixture and that makes possible the formation of linear oligomers E1-E4 along with small amount of T1.The following sodium atom-mediated cyclization of E1-E4 and T1 affords the corresponding macrocycles 1-4 alone with small amount of classic thiacalix [4]arene 5.

Conclusions
Several di-and trisulfide-bridged thiacalix[n]arenes have been synthesized by means of solvent-free condensation between tert-butylphenol and sulfur in the presence of NaOH.The existence of these products was supported by mass-spectrometry (ESI-MS and semi-preparative HPLC) and DFT-studies.These macrocycles are suggested to form under the kinetically-controlled reaction conditions and their further conversion into the thermodynamically stable tert-butylthiacalix [4]arene is possible.

Figure 1 .
Figure 1.HPLC-analysis for the reaction mixture obtained according to way ii.

8 A 2 .
Scheme 2. The proposed pathway for the formation of radicals A-E under the solvent-free conditions.

Table 1 .
ESI-MS data for the proposed linear oligomers

Table 2 .
ESI-MS data for the proposed cyclic oligomers HPLC was then used to separate the reaction mixtures (Figures S6-

Table 3 .
Chromatographic separation in a semi-preparative mode