Hydrogen bonds/bonding boronate assemblies

There are many kinds of capsules but few have the capacity to position co-guests in predictable orientations. We used the ability of 1.24.1 to do so, and applied it to evaluate hydrogen-bonding interactions between boronic acids, carboxylic acids, and primary amides [52], The phenyl boronic acids are useful as components of covalently self-assembled systems [53], and we found that the p-methyl, methoxy, ethyl, and isopropyl derivatives all fit as symmetrical dimers inside I.24.I. The structure of the boronic acid dimer has been debated but a recent theoretical study found the doubly hydrogen-bonded exo/endo conformer (Fig. 10) to be lowest in... 

The absolute stereo-preference in the Diels-Alder reaction can be easily understood in terms of the most favorable transition-state assembly 5, in which an attractive donor-acceptor interaction favors coordination of the dienophile at the face of boron which is cis to the 2-hydroxyphenyl substituent. At this time, the conformation of a,y3-enal has a strong s-trans preference. We believe that the coordination of a proton of the 2-hydroxyphenyl group with an oxygen of the adjacent B-O bond in complex 5 plays an important role in asymmetric induction this hydrogen-bonding interaction via a Brpnsted acid would cause the Lewis acidity of boron and the jr-basicity of the phenoxy moiety to increase, and the transition-state assembly 5 would be stabilized. The jr-basic phenoxy moiety and the jr-acidic dienophile could then assume a parallel orientation at the ideal separation (3 A) for donor-acceptor interaction. In this conformation, the hydroxyphenyl group blocks the si face of the dienophile, leaving the re face open to approach by diene. 

Custelcean and Jackson have demonstrated that anionic boron hydrides can also serve as hydrogen bond acceptors to assemble novel lattices held together by hydridic-protonic bonds [107, 108]. Cations with hydrogen bond donors include A -[2-(6-aminopyridyl)]acetamidine and triethanolamine(sodium). The former is an NH donor while the latter is an OH donor. These scientists provided evidence for the topochemical conversion of the hydridic-protonic bonds into B-N and B-0 covalent bonds, respectively, when the materials are heated and dihydrogen is driven from the H-H bonds. 

There is abundant information available on supramolecular assemblies that incorporate boronic acids. The vast majority of these ensembles have been observed primarily in the solid state, though several are stable in solution. This section begins with a brief discussion of hydrogen bonded assemblies, followed... 

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... 

Beyond planar sheet assemblies, three-dimensional COFs have also been synthesized. The tetrahedral boronic acids used to create porous, hydrogen bonded networks were condensed with 2,3,6,7,10,11-hexahydroxytriphenylene to produce COF-105 and COF-108 (Fig. 35). The resulting materials have a cubic structure with an open pore system. It is interesting that these two COFs do not have identical structures, even though they differ only in that COF-105 has a carbon tetrahedral center and COF-108 has a silicon atom at the tetrahedral centers. While no surface area measurements were reported for these boronate COFs, they were reported as low-density crystals with densities of 0.17 to 0.18 g/cm. ... 

Yamamoto developed a conceptually new class of chiral boron-Lewis acids 174 (Equation 20) that can readily be assembled from a tetraphenol ligand and B(OMe)3 [93, 94]. These were shown to be especially effective with a wide range of a-substituted enals in enantioselective Diels-Alder reactions. One illustrative example is the cycloaddition between aldehyde 173 and cyclopentadiene to give 175 in 99% ee, > 99 1 exo/endo selectivity, and quantitative yield [93]. Yamamoto suggested that these catalysts 174 were Bransted-assisted chiral Lewis acids (termed BLAs), in which the phenol proton activates the enal substrates by hydrogen bonding, as depicted in the proposed transition state assembly 176. 

---