Linked Macrocyclic and Cage Assemblies

It is well established that 1,2- and 1,3 diols form cyclic boronate esters, whereas 1,4- and 1,5-alkane diols principally give rise to dimeric macrocycles. While the crude product is a mixture of various size cycles and oligomeric materials, distillation, which likely shifts this dynamic equilibrium toward the compounds with higher volatility, can be used to isolate the 2 2 complex (Fig. 15a). Analogously if dithioboronate esters are heated with bisphenols, cyclic dimeric diboronates are isolated (Fig. 15b).  

Taking advantage of the more traditional cyclic ester formation between boronic acids and 1,2- and 1,3-diols can also lead to macrocyclic structures. It has been shown by NMR, FTIR, and size exclusion chromatography (SEC) that reaction between phenylene-l,3-diboronic acid and pentaerythritol principally produces 6 6, hexameric cycles (Fig. 16a). As before, the crude reaction mixture was a dynamic mixture of cyclic and oligomeric products, but the 

FIGURE 15. (a) Long-chain a, to-alkane diols form acyclic, 2 2 boronate ester 

FIGURE 16. (a) When phenylene-l,3-diboronic acid was coupled with the bis-l,3-diol, pentaerythritol, traditional cyclic boronate ester formation resulted in macrocyclic structures, (b) The bis-l,2-diol 1,2,4,5-tetrahydroxybenzene forms similar structures. 

FIGURE 17. Solvent can form templates of (a) 2 2 and (b) 3 3 cyclic adducts from phenylene-l,4-diboronic acid with a racemic bis-diol. 

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