Azacalix pyridine

Another application of the single-step procedure tvas made by Miyazaki et al, who utilized the Buchwald-Hartwig aryl amination reaction for the preparation of azacalix[6]pyridine 7d [18], in which all the benzene rings of the above azacalix[6]arene 4d were replaced by pyridine rings (Scheme 3). Azacalix[ ]pyridines 8a-f (n = 3-8) with tolyl groups on the nitrogen bridges... 

Also interesting are the fullerene-complexation properties of azacalix[ ] pyridines 7b, 7c, 7f, and 7g as well as azacalix[m]arene[ra]pyridines 20b and 20f [24-26]. Depending on the size of the macrocyclic ring, aza-calix[5]pyridine 7c, azacahx[8]pyridines 7f, azacahx[10]pyridine 7g, and azacalix[4]arene[4]pyridine 20f strongly interacted with fullerenes Ceo and C70 in toluene, whereas smaller analogues 7b and 20b had no affinity towards them. The interaction between 7f or 20f and Ceo was detectable even with the naked eye the color of a toluene solution of C60 changed from its... 

The gas phase basicities and pKa values of tris(phosphazeno) substituted azacalix[3](2,6) pyridine in acetonitrile and some related compounds were examined by the density functional theory (DFT) computational method. It was shown that the hexakis(phospha-zeno) derivative of azacalx[3](2,6)pyridine is a hyperstrong neutral base, as evidenced by the absolute proton affinity of 314.6kcal/mol and pKa (MeCN) of 37.3 units. It is a consequence of the very strong bifurcated hydrogen bond (32kcal/mol) and substantial cationic resonance effect [14]. 

Despotovic, I., Kovacevic, B. and Maksic, Z.B. (2007) Hyperstrong neutral organic bases phosphazeno azacalix[3](2,6)pyridine. Organic Letters, 9, 4709-4712. 

Figure 11.2 Typical structures of N-substituted azacalix[n](2,6)pyridine derivatives...

The fragment coupling approach has been successfully applied in the preparation of azacalix[4]pyridines bearing different substituents on the bridging nitrogen atoms [19, 20]. Illustrated in Scheme 14.4 are examples of (NAUyl) ,(NMe)4. ,-... 

Scheme 14.5 Fragment coupling synthesis of azacalix[n]pyridines (n = 4—10) [21-23]...

Scheme 14.12 shows another example of the stepwise synthesis of azacalix[n] pyrazines that has been reported by Zhao and Wang [48]. Reaction of 2,6-bis (methylamino)pyrazine 38 and 2,6-dichloropyrazine 37 produced trimer 39 effectively. Attempted convergent macrocyclic cross coupling reaction of 39 with 38 failed to afford the targeted azacahx[4]pyrazine. Instead, a linear tetramer 40 was produced in 47 % yield along with the recovery of 31 % yield of reactant 39. Catalyzed by Pd2(dba)3, moderate yields of azacalix[4]pyrazine (21 %) and its macrocychc octamer, namely, azacalix[8]pyrazine (31 %) were achieved from 40 when the reaction was carried out in 1,4-dioxane in the presence of DavePhos as a ligand and CS2CO3 as a base (Scheme 14.12). Following the same strategy, the same authors reported very recently the preparation of azacalix[3n]pyrazine[n] pyridines (n = l, 2) when the linear trimer 39 was reacted with 2,6-bis (methylamino)pyridine [49].

Scheme 14.13 Site-specifical deuteration and foimylation of azacalix[2]arene[2]pyridines [20, 50]...

Scheme 14.17 Regiospeciflc functionalization of azacalix[l]arene[3]pyridine [52, 54-59]...

F. 14.1 Crystal structure of azacalix[3]pyridine 62. All hydrogtai atoms are omitted for clarity [40]... 

Fig. 14.4 Crystal structure of azacalix[2]arene[2]pyridine 9a top) [15], azacalix[2]arene[2] triazine 4c middle) [14] and azacalix[2]pyridine[2]triazine bottom) [18], Alkyl groups and hydrogen atoms are omitted for clarity...

Fig. 14.6 Cavity structures of mrato- (left), di- (middle) and tri-protonated (right) azacalix[4] pyridines [21]...

Fig. 14.7 Crystal structures of azacalix[3]arene[3]pyridine (top) and azacalix[3]pyridine[3]...

Fig. 14.8 Crystal structures of azacalix[4]arene[4]pyridine (left) [15] and azacalix[8]pyridine (right) [21]. All hydrogen atoms are omitted for clarity...

It should be noted that detailed conformational structures of larger azacalix[n] aromatics (n > 4) in solution still remain unknown. For example, the single sets of resonance signals are recorded in NMR spectra for azacalix[n]pyridines (n = 5—10) and azacalix[n]arenes (n = 5—8). The outcomes can probably only suggest the formation of a mixture of conformers which are able to undergo very rapid interconversions at the probe temperature in relative to NMR time scale. Further studies, especially, new strategy and physical methods, are needed to solve the problem. 

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