Pillar arenes, preparation

Figure 2.7 Synthesis of (a) pillar[5]arene 2.2 and (b) pillar[5-10]arenes from 1,4-dietho5ybenzene and (c) pillar[5-15]arenes prepared from ring-opening reaction of pillar[5]arene 2.2. (d) Electrospray ionization Mass Spectrometry (ESI-MS) spectrum of the mixture after the conversion reaction for 5 min. (e) Chromatogram of the mixture after the conversion reaction for 1 h. Reproduced with permission from ref. 9. Copyright 2014 American Chemical Society.

The starting material for assembly of asar[ ]arene macrocycles is tetramethoxybenzene (12.31), which can be prepared from commercial dihydrojybenzoquinone (12.33) in abundant quantities. We subjected (Scheme 12.11) a mixture of tetramethojybenzene and paraformaldehyde to Friedel-Crafts alleviation conditions at 80 °C in chlorinated solvents, with BF3 OEt2 as the Lewis acid. Influenced by the prior work on the synthesis of pillar[n]arenes - where pillar[5]arene is formed primarily under similar reaction conditions - we were expecting this reaction mixture to form asar[5]ar-ene as the major product. To our surprise, we found, however, that the reaction mixture produced only asar[6]arene and not even a trace of asar[5]arene. It is most likely that the increased steric demand imposed on the macrocyclic framework by the two additional methojyl groups not present in pillar[ ]arenes is responsible for this striking difference in reactivity between the asar[ ]arene and pillar[ ]arene families of macrocycles. Soxhlet extraction of the crude reaction mixture with acetonitrile as the solvent was then used as a scalable method of purification to access pure asar[6]arene (12.32a) in bulk quantities. 

In going from pillar[ ]arenes to asar[ ]arenes, two hydro g l groups are substituted onto each of the phenylene units in the host. Analogous pil-lar[n]arene derivatives can be prepared using different heteroatoms substituted at these positions to produce hosts with unique properties. Amino groups directly substituted on the pillar[n]arenes have the potential... 

Scheme 2.5 Proposed pathway for the preparation of para-bridged pillar[5]arene and pol5mier.

Scheme 2.6 Preparation of simple per-allg lated pillar[5]arenes and pillar[5]arenes with ten reactive moieties from l,4-dialko)gfbenzenes.

Scheme 2.10 Preparation of pillar[5]arene from trimer.

Scheme 3.2 shows per-functionalized pillar[5]arene derivatives prepared from per-hydro)ylated pillar[5]arene 3.6 as a starting compound. Applying an efficient organic reaction is required for the perfect introduction of functional groups at all 10 reaction sites. A straightforward and efficient functionalization process for per-hydroxylated pillar[5]arene is etherification, which has been used for functionalization of the lower rims of phenolic moieties in calix[ ]arene derivatives. Various functionalized pillar[5]arene derivatives can be obtained by etherification of per-hydroxylated pillar[5]-arene with an alkyl-halide in the presence of appropriate bases, such as NaH and K2CO3. Introduction of the substituents can change various physical... 

However, because of the low yields of the larger pillar[n]arenes, as these compounds were isolated as intermediates, they have not heen completely characterized. Water-soluble pillar[7]-, pillar[9]- and pillar[lO]arenes were also prepared by modification of the carbojgrlate moieties on the rims of per-hydro)g7lated pillar[7]- (3.27), pillar[9]- (3.28) and pillar[10]arenes (3.29). Tri(ethylene oxide)-substituted pillar[7]- (3.30) ° and pillar[10]arenes (3.31) were synthesized. As was observed with the tri(ethylene oxide)-substituted pillar[5]- (3.19) and pillar[6]arenes (3.26), 3.30 and 3.31 also showed tunable LCST behavior based on their host-guest systems. 

Wang and co-workers synthesized a reactive di-bromo-functionalized pillar[5]arene by co-cyclization of l,4-bis(2-bromoetho>y)benzene with DMB. The two bromide moieties on pillar[5]arene 3.78 were converted to amino moieties 3.79 by Gabriel synthesis. Using the diamine 3.79 as a key compound, di-ureidopyrimidinone (3.80) and di-urea (3.81) functionalized pillar[5]arene derivatives were prepared. 

Stoddart and co-workers synthesized the di-bromo-functionalized pillar[5]arene 3.82 by co-cyclization of l,4-bis(3-bromopropoxy)benzene with DMB. Elimination of bromide moieties afforded diallyl ether-substituted pillar[5]arene 3.83. De-protection of the allyl moiety afforded Al/A2-di-hydrojgrlated pillar[5]arene 3.84. Reaction of 3.84 with triflic anhydride gave a ditriflated pillar[5]arene (3.85). A ter-phenyl substituted pillar[5]arene (3.86) was prepared by Pd-catalyzed Suzuki coupling between ditriflated... 

Another new route for the selective synthesis of di-functionalized pillar[5]arenes is oxidation and reduction of pillar[n]arene units (Scheme 3.15). Our group and Huang s group reported that the oxidation of one pillar[5]arene unit afforded pillar[5]arenes containing one benzo-quinone unit (3.92 and 3.93). Reduction of the benzoquinone unit yielded di-hydrojylated pillar[5]arenes at the A1/A2 positions (3.84 and 3.95). This method avoids the formation of many constitutional isomers and can be used for pillar[5]arenes with one benzoquinone and two hydro)q l groups at the A1/A2 positions. A clickable di-functionalized pillar[5]arene was prepared by modification of the alkyne moieties. The A1/A2 di-all e... 

Huang and co-workers recently prepared two 2 3 co-pillar[5]arene constitutional isomers (Scheme 3.17). To synthesize co-pillar[5]arenes containing one different unit, a 1 4 feed ratio is suitable. The 1 3 feed ratio afforded two 2 3 co-pillar[5]arene constitutional isomers, A1/A2/B1/B2 (3.110) and A1/A2/C1/C2 co-pillar[5]arenes (3.111), in 8% yields. 

Penta-functionalized pillar[5]arenes can be prepared from non-symmetrical monomers with reactive moieties (Scheme 3.19). Huang et al. synthesized a pillar[5]arene with five allq nes on the same rim (3.116). A CuAAC reaction between 3.116 and a naphthalene containing an azide... 

We investigated alkane recognition by a monomeric per-methylated pil-lar[5]arene H5.2 and pillar[5]arene dimer H5.17, which was prepared by connecting two mono-reactive pillar[5]arenes through a linker. Determination of the K value of the complex between H5.2 and n-hexane was difficult owing to the weak host-guest complex. In contrast, the K value between the dimer H5.17 and n-hexane determined by NMR titration was 98 12 M (run 92). The dimerization is useful for enhancing CH/jt interactions between pillar[5]arenes and alkanes. 

Water-soluble pillar[6]arenes can be prepared using the same method as the synthesis of water-soluble pillar[5]arenes. The introduction of water-soluble substituents, such as cationic, anionic and nonionic moieties, allows pillar[6]arenes to solubilize in aqueous media. The first water-soluble pillar[6]arene was synthesized by Huang and co-workers, by introducing 12 carbojylate moieties (Figure 5.31, H5.36). H5.36 could capture paraquat (Table 5.9, run 145, G5.117) in water with a high association constant [ T= (1.02 0.10) x 10 M ], which is much higher than that of the anionic water-soluble pillar[5]arene H5.11 [Table 5.4, run 63, K= (8.20 1.70)xl0 M... 

Figure 7.21 The preparation of pillar[5]arene-based polyrotaxanes.

Scheme 8.1 The first pillar[w]arene-based [2]rotaxane (8.4) prepared by the end-capping method using the imidation between a diamine axle (8.1) and aldehyde stopper (8.2).

A catenane is one of the MIMs consisting of two or more interlocked macrocycles. We recently reported the first pillar[5]arene-based [2]catenane. A pyridinium salt with alkene moieties at both ends (8.44) was prepared. Pil-lar[5]arene formed a stable host-guest complex with the molecule... 

Figure 8.6 Nonionic polyrotaxanes (8.41 and 8.42) and a topological gel (8.43) prepared from liquid pillar[5]arenes.

The Al/A2-difunctionalized pillar[5]arene strut 1 (Scheme 10.1), with a rigid stmcture, has been sueeessfully prepared and incorporated into a MOF, i.e., P5A-MOF-1 (Figure lO.la), with Zn40 secondary building units (SBUs) in a conventional manner, via beating a DMF solution of a mixture of stmt 1 and Zn-(N03)2 6H2O at 100 °C for 24 hours. The cubic and transparent crystals of P5A-MOF-1 can be observed by optical microscopy and confirmed by powder X-ray diffraction (PXRD) (Figure 10.1b). Moreover, their thermal stability bas... 

Thus, homochiral MOFs containing enantiopure pillar[5]arene active domains have been obtained, which are early instances of porous frameworks incorporating active domains and possessing planar chiralities. Meanwhile, an efficient route to large-scale resolution of racemic pillar[5]-arene derivatives has been developed in the preparation of (5 p)-P5A-MOF-l and (J p)-P5A-MOF-l. 

The first water-soluble pillar[6]arene (WP6) (Schemes 10.8 and 10.9) with controlled solubility was prepared by Huang and co-workers. The solubility of WP6 can be reversibly adjusted by changing the pH of the solution. When the solution is acidic, all of the carbojylated groups will be insoluble... 

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