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.

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”, since they are cylindrical or "pillar" in shape and composed of aromatic or "arene" moieties.¹ Multiple chemists refer to the macrocycle as "pillarene" orally as this is often easier to pronounce and remember.

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. Due to the close proximity of many electron-rich hydroquinones, the cavity of pillararenes are able to form strong association complexes with electron-poor species. Also, derivatives of the pillararenes can be generated by modifying the hydroxyl groups at all positions or selectively on one or two positions.

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.

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