Macrocyclic assemblies boronate

FIGURE 19. (a) Acyclic boronate diesters form macrocyclic assemblies with coordinate amines for added stability, (b) These macrocyles bind paraquat in a 1 1 manner. 

As demonstrated by examples, these relatively novel assemblies hold promise in a number of different venues. The simple phenyl-boron-phenyl binding motif provides a unique supramolecular assembly strategy for researchers to take advantage of. Macrocyclic assemblies have been shown to serve as host molecules for molecular recognition events. Linear polymers have shown self-repair capabihties as well as serving as wide band-gap semiconductors. And porous COFs provide stable network geometries for the inclusion of guests. 

Christinat, N. ScopeUiti, R. Severin, K. Multicomponent assembly of boronic acid based macrocycles and cages. Angew. Chem. Int. Ed. 2008, 47, 1848-1852. 

Macrocycles. This reagent was used twice in an assembly of aplasomycin (5), a boron-containing macrolide with antibiotic properties. It was used to effect alcrification of 1 with 2 in 98% yield and again for cyclization of 3 to the macrocycle 4 ill 71% yield under conditions of only moderate dilution. After minor adjustments ol the functional groups, the metal was introduced by reaction with trimethylborate hi give 5. 

Slow crystallization of 1-trifluoroacetoxybenziodoxaborole 188 from methanol afforded the tetrameric macrocyclic structure 192 resulting from self-assembly of the initially formed 4-fluoro-l,3-dimethoxy-l//-lX -benzoM[l,2,3]iodoxoborole (191) (Scheme 2.59). The structure of macrocycle 192 was established by a single-crystal X-ray analysis [243]. The driving force for formation of the eight-membered cyclic system 192 is the transformation of initial trigonal-planar sp hybrid boron atoms in 191 into tetrahedral sp hybridized atoms. Indeed, each boron atom in tetramer 192 forms one covalent bond with carbon and three covalent... 

Therefore, for the construction of boron-based pentam-eric macrocycles 9a-f, they performed a condensation of aryl- and alkylboronic acids with 3,4-dihydroxypyridine 8 in which 5 boronate esters are connected by dative B—N bonds (Scheme 4.6) [11]. This procedure was inspired in then-previous work [12]. The boron atoms of these complex structures represent chiral centers, and the assembly process is highly diastereoselective. 

It was found that the presence of an amino or aldehyde groups in the meta-position of the arylboronic acid did not interfere in the macrocyclization reaction. This allowed the effective assembly of functionalized boronic acids, dihydroxypyridine ligands, and amines or aldehydes, respectively, in a multicomponent procedure affording pen-tameric boronic acid macrocycles (Scheme 4.7) and tetramer derivatives (Scheme 4.8). 

FIGURE 5. Bis-diol functionalized porphyrins assemble into macrocycles based on boronate ester formation coupled with metal ligation to a Lewis basic pyridine. 

Beyond the coordinative interactions described, boronate ester formation by itself has been shown to facilitate the generation of macromolecular and polymeric structures. Taking advantage of the covalent yet reversible nature of boronate ester formation has allowed for the formation of thermodynamically stable assemblies. Of course, simple monoesters, diesters, and triesters have been readily synthesized and characterized. Numerous reports have taken advantage of the formation of cyclic diesters as a means to define selectivity for molecular sensors, but these small cycles will not be discussed here. Attempts to synthesize and characterize macrocyclic, polymeric and extended covalently... 

Incorporation of this coordinative amine ligand within the cycle structure is an alternative approach for generating stable macrocycles. In this regard, alkyl bridged bis(2-hydroxyphenylmethyl) amines have been reacted with 4,4 -biphenyldiboronic acid (Fig. 19a). Esterification proceeds with concomitant coordination of the amine to the resulting ester. The paraquat dication is complexed by this assembly in a 1 1 manner (Fig. 19b) through ionic interactions (anionic boronate with pyridyl cation), attractive n-n interactions (between biphenyl components) and C —H —O hydrogen bonds (paraquat... 

FIGURE 18. (a) A tripodal technetium chelator is based on three oxime ligands tethered through an acyclic boronate triester, (b) Borosilicate macrocycles and (c) cages have been formed as building blocks and (d) assembled via coordinative ditopic Lewis bases. 

One of the unique architectural motifs that truly relies on the covalent yet reversible nature of boronate ester formation is the creation of complex hierarchical structures referred to as COFs. If boronate ester formation can be used to obtain discrete macrocyclic structures, it is then reasonable to expect that linked macrocyclic networks may be generated. These networks assemble... 

In Chapter 6, B. M. Rombo et al. delve into the formation of boronate-linked supramolecular architectures based on boronate ester formation—for example, small molecule diesters form supramolecular self-assemblies in the solid state based on a phenyl-boron-phenyl sandwich motif in which these small oligomers link together to generate macrocycles and other polymers. The polymeric macrocyclics and linear structures demonstrate self-repair capabilities and constitute a new class of wide band-gap semiconducting materials. Through the incorporation of polyvalent boronates, covalent organic frameworks are described, which create highly crystalline, porous network materials. 

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