Pillararene
Pillararenes are macrocycles composed of hydroquinone units (5 to 10) linked in the para position. They are structurally similar to the cucurbiturils and calixarenes that play an important part in host-guest chemistry. The first pillar[n]arene macrocycle was the five membered derivative, Pillar[5]arene.
History
1,4-Dimethoxypillar[5]arene (DMpillar[5]arene), the first pillararene, was reported in 2008 Journal of the American Chemical Society by Tomoki Ogoshi et al. Ogoshi et al catalyzed 1,4-dimethoxybenzene by Lewis Acid with paraformaldehyde to obtain 1,4-dimethoxypillar[5]arene. The methoxy groups of DMpillar[5]arene were deprotected using boron tribromide to give pillar[5]arene. Ogoshi named the new family of host macrocycles “pillararene”, para-bridged pillar-shaped novel hosts.[1][1]
Structure
Pillararenes are composed of hydroquinone units linked by methylene bridges at para-positions. Its features a symmetrical pillar architecture with two identical cavity gates. Pillar[5]arene is the most conformationally stable member in this family. Also, derivatives of the pillararenes can be generated by modifying the hydroxyl groups at all positions or selectively on one or two positions.[2]
Planar Chirality
The orientation of the hydroquinone oxygens on both rims of the pillararene allow the macrocycle to exhibit planar chirality. When the substituent on the hydroquinone oxygen is small enough to fit through the cavity of the pillararene, allowing for oxygen-through-the-annulus rotation to occur, racemization occurs. If this substituent is large enough to prevent rotation, optically active pillararene macrocycles can be isolated.[3][4]
Synthesis
Homopillararenes
Three strategies are mainly used to obtain pillararenes (Scheme 1).[5] All of three strategies use an acid as catalyst.
- The Lewis acid or Trifluoromethanesulfonic acid catalyzed condensation of 1,4-dialkoxybenzene and paraformaldehyde at room temperature.
- The condensation of 1,4-dialkoxy-2,5bis(alkoxymethyl)benzene catalyzed by p-toluenesulfonic acid.
- Cyclooligomerization of 2,5-dialkoxybenzyl alcohols or 2,5-dialkoxybenzyl bromides with an appropriate Lewis acid as the catalyst.
Mechanism
Pillararenes are traditionally formed through a thermodynamically controlled Friedel-Crafts cyclooligomerization.
Copillararenes
In 2010, Zibin Zhang et al. introduced three new ways to synthesize copillararenes, which are composed of different repeating units. It is easier to selectively functionalize copillararenes, helping to generate interesting physical properties, conformations, and host-guest binding interactions. There are two possible ways to make copillararenes: to selectively modify repeating monomers of homopillararenes, or to use two different monomers to carry out co-oligomerization.
Potential Applications
Pillararenes have been shown to have potential application in sensing, nanoparticle synthesis, artificial transmembrane channels, liquid crystals and ionic liquids.
References
- ^ Ogoshi, T.; Kanai, S.; Fujinami, S.; Yamagishi, T.-a.; Nakamoto, Y., para-bridged symmetrical pillar 5 arenes: Their Lewis acid catalyzed synthesis and host-guest property. Journal of the American Chemical Society 2008, 130 (15).
- ^ Strutt, N. L.; Forgan, R. S.; Spruell, J. M.; Botros, Y. Y.; Stoddart, J. F. Monofunctionalized Pillar[5]arene as a Host for Alkanediamines. J. Am. Chem. Soc., 2011, 133, 5668-5671.
- ^ Ogoshi, T.; Masaki, K.; Shiga, R.; Kitajima, K.; Yamagishi, T.-a. Org. Lett. 2011, 13, 1264
- ^ Strutt, N. L., Fairen-Jimenez, D.; Iehl, J.; Lalonde, M. B.; Snurr, R. Q.; Farha, O. K.; Hupp, J. T.; Stoddart, J. F. J. Am. Chem. Soc., 2012, 134, 17436.
- ^ Xue, M.; Yang, Y.; Chi, X.; Zhang, Z.; Huang, F., Pillararenes, A New Class of Macrocycles for Supramolecular Chemistry. Accounts of Chemical Research 2012, 45 (8)