Esters, boronate assemblies

Assemblies relying on the dative B-N interaction in conjunction with boronate ester formation to form linear polymers have also been identified. Unlike the stepwise phenanthroline-boronate assembly, esterification coupled with ligation occurs simultaneously. In fact, it is likely that coordination... 

Fig. 18. Cooperative closed dimer assembled using boronic esters as well as zinc-pyridine coordination.

Severin and coworkers reported (146) the reaction of tris(2-aminoethyl)amine and 4-formylphenylboronic acid with penta-erythritol to give, via multicomponent assembly, the boronic acid based macrobicyclic cage 35 (Fig. 25). The cage has the form of an ellipsoid with a diameter of 20.5 A and binds two Cud) ions in a fashion similar to the smaller tren-based cryptands. The reversible formation of boronic esters has also been employed to build other hollow structures such as nanotubes (147) and porous covalent organic frameworks (148,149). 

Figure 5.2 Self-assembly of chiral tetra boronic ester clathrates from a...

Iwasawa, N. Takahagi, H. Boronic esters as a system for crystallization-induced dynamic self-assembly equipped with an on-off switch for equilibration. J. Am. Chem. Soc. 2007,129, 7754-7755. 

This methodology has been developed by Matteson and coworkers, with particular emphasis on chiral boronic esters derived from pinanediol. Sequential chiral centers can be assembled by an iterative procedure with excellent enantiomeric excess at each step. The two chiral centers in eldanolide (37), for example, were assembled in this manner (Scheme 36). ... 

Shinkai reported the formation of the cyclic dimer 21 upon mixing a 1 1 mixture of a meso-dicatechol zincporphyrin and pyridine-3-boronic acid dimethyl ester in CH2CI2 [78]. The dimerization constant iCsa = 1.1x10 mol 1 and the reference constant iCinter = 340 mol" 1, yield an EM 0.1 moll The authors claimed complete formation of the dimer 21, however, the product EMiCinter/ 17 does not satisfy the condition for complete self-assembly expressed by Eq. 22. According to the plot in Fig. 13 the weight fraction of the dimer can be in no case higher than ca. 0.9. 

Kobayashi investigated a self-assembled boronic ester cavitand capsule for photochemical reaction of 2,6-diacetoxyanthracene. Tsuda found a self-assembled helical anthracene nanofiber in a vortex. Chou examined rotational behaviors and fluorescence energy transfer of N-l- and N-2-anthryl succinimide derivatives. " Reversible photoinduced twisting of molecular crystal microribbons via [4 - - 4] photocycloaddition of 9-anthra-cenecarboxylic acid. Reversible single walled carbon nanotubes of 1,3-bis(9-anthracenylmethyl)imidazolium chloride was examined as a functionalized anthracene salts.Karatsu reported the intramolecular photodimerization of 9-substituted anthracene derivatives (253) tethered by oligosilanes giving [4 + 4] and [2 - - 4]cycloadducts (254), (255), and (256). ... 

In comparison to abundant examples of noncovalent supramolecular assemblies,relatively few structures have been built with strong covalent bonds. This is due to the fact that normal covalent bonds are not easily broken and reformed. One can manipulate the temperature and conditions so that the formation of stable imines, esters, disulfides, hydrazones, and boronate esters is reversible. Some examples of these covalent self-assembled systems include disulfide hosts and covalent organic frameworks. ... 

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 is challenging to determine the e.e. value of simple chiral hydrox-ylamines because there is only one binding site. However, based on the B-N interaction, the e.e. value of simple amines can also be determined by using H NMR analysis based on the three-component assembly of 2-formylphenylboronic acid, racemic BINOL and the chiral hydroxyl-amines to afford a mixture of diastereomeric nitrono-boronate esters whose ratio is an accurate reflection of the enantiopurity of the parent hydroxylamine (Scheme 6.3). ... 

Electroanalytical innovation and development of novel sensor electrodes are often driven by progress in materials chemistry. In particular for boron-based structures and assemblies the detection of saccharides plays a very prominent role. Monomeric as well as polymeric borate and boronic esters are reactive towards diols and lead to novel electroanalytical tools as well as new signal amplification strategies. There are many new opportunities arising and this overview will cover some of the recent developments in solid state, surface, and molecular boron structures with application in electroanalysis. 

FRET donor and acceptor can also be incorporated into the self-assembled nanofibers of supramolecular hydrogels. In this way Ikeda et developed a gel-based fluorocolorimetric sensor for polyols. The acceptor is boronic acid-appended. In the presence of polyols, it forms boronate ester complexes with the polyols, becomes negatively charged and more hydrophilic, and therefore migrates from the hydrophobic site. The increased distance between the donor and acceptor results in the fluorescence color change, from which the presence of polyols can be detected. 

A judicious combination of plural dihydroxyboryl and multiple hydroxyl components can lead to the formation of various boronate-based self-assemblies. Furthermore, the surfaces of such ensembles are composed of boronate esters with Lewis-acidic characteristics, and this opens up new possibilities for the use of scaffolds in various applications. In this context, we envisaged that such self-assemblies might have applications in the fabrication of organic-inorganic hybrid catalysts. The development of new types of catalyst for fine chemical synthesis is urgently required from the viewpoint of sustainable chemistry and green organic synthesis in particular, Au nanoparticle (Au NP)-catalysed fine chemical synthesis has the potential for practical use in industry. ... 

An a-lithioether could combine with 2-(l-chloroetl l)-l,3,2-dioxaborolane 11 in order to accomplish the assembly of all three stereogenic centers of 17 with a single chiral director. However, use of a racemic lithioether would result in a diastereomeric mixture at the alkoxy substituent, which would complicate purification and NMR identification of the boronic ester intermediates. An enantiopure a-lithioether, whether prepared via our own route (10) or others, would be extra effort. 

If more substituents are desired, the precursor to the cyclobutane can be assembled with more chain extensions with (dichloromethyl)lithium. For example, the (bromomethyl)boronic ester 50 is easily prepared in lots >300 g 26). Alkylation with lithiopropionitrile yielded 51, which was transesterified to astereomeric mixture 52, converted via 53 to in the usual manner, and again treated with (dichloromethyl)lithium to form 55. An analogous series without the methyl substituent was also prepared from lithioacetonitrile. Treatment with LDA led via 56 to cyclobutaiKS 57. This woric was done before the role of magnesium salts in the ring closure was understood, and yields of 57 were consequently not optimized (Scheme 13). 

Scheme 8.1 General assembly of asymmetric carbon chains from boronic esters of C2-symmetrical diols.

Scheme 8.3 General assembly of asymmetric carbon chains via pinanediol boronic esters. Structures 13-20 are drawn as slightly distorted planar projections of a three-dimensional computer model of the rigid terpenoid unit. For detailed reaction conditions, see Scheme 8.1.

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