Monoboronic acids

Cao H, Diaz DI, DiCesare N, Lakowicz JR, Heagy MD. Monoboronic acid sensor that displays anomalous fluorescence sensitivity to glucose. Organic Letters 2002, 4, 1503-1505. 

Sun, X. Y. Liu, B. Jiang, Y. B. An Extremely Sensitive Monoboronic Acid Based Fluorescent Sensor for Glucose. Anal. Chim. Acta 2004, 515, 285-290... 

The only example of rod-rod copolymers with polyphenylene-based rods are the polymers 230 and 231, in which two polyfluorene blocks are separated by a short arylene vinylene unit (Scheme 105). These were prepared by Suzuki polycondensation of a fluorene monoboronic acid 232 in the presence of the bromo-substituted oUgo(arylene vinylene) units 233 and 234 [321], From the molecular masses of the polymers, the total number of fluorene units in both 230 and 231 is about 18. 

The D-fructose selective monoboronic acid-based sensor 5 was enhanced by James in 1995 with the introduction of a... 

Zhao and James have investigated a related monoboronic acid system J -14 and 5-14, which was found to be an... 

James has prepared a ferrocene monoboronic acid 64 and diboronic acid 65 as electrochemical saccharide sensors. The monoboronic acid system 64 has also been prepared and proposed as an electrochemical sensor for saccharides by Norrild and Sotofte. The electrochemical saccharide sensor 65 contains two boronic acid units (saccharide selectivity), one ferrocene unit (electrochemical read out), and a hexamethylene linker unit (for D-glucose selectivity). The electrochemical sensor 65 displays enhanced D-glucose (40 times) and D-galactose (17 times) selectivity when compared to the monoboronic acid 64. 

Compared to the monoboronic acid sensor, the diboronic acid displayed an enhanced selectivity for o-glucose and D-galactose. 

Zhang X, Chi L, Ji S et al (2009) Rational design of d-PeT phenylethynylated-carbazole monoboronic acid fluorescent sensors for the selective detection of a-hydroxyl carboxylic acids and monosaccharides. J Am Chem Soc 131 17452-17463... 

Figure 1.4 Typical pressure-area isotherms of monoboronic acid 26 at pH 10.0 and 293 K on a subphase containing 0.01 wt% PVI and D-fructose. Reproduced from ref. 32 with permission from The Chemical Society of Japan.

Figure 6.6 a-Methylbenzylamine based fluorescent chiral boronic acid chemosen-sors (i ,i )-(-)-5 and (S,S)-(+)-5. The monoboronic acid chemosensor 6 is also presented. Anthracene was used as fluorophore. 

Figure 6.8 Relative fluorescence intensity of sensors 5 and 6 versus concentration of D- or L-tartaric acid, (a) Bis-boronic acid 5 with d- and L-tartaric acid, Aex at 365 nm, Agm at 429 nm, pH 8.3 (b) monoboronic acid 6 with D- and L-tartaric acid, Ae at 373 nm, Ag, at 421 nm, at pH 7.0 3.0 x lO" mol dm" of sensors in 5.0 x lo mol dm" NaCl solution (52.1% methanol in water), 22 °C. (Reproduced by permission of the American Chemical Society.)...

In this case, the sensing mechanism with the bis-boronic acid is open to question. With monoboronic acid sensor 6, fluorescence enhancement was observed in the presence of both d- and L-tartaric acid (Figure 6.8b). For the combination of D-bis-boronic acid sensor and L-tartaric acid, however, no fluorescence enhancement was observed. Herein, we proposed that 1 1 cyclic binding complexes form, otherwise fluorescence enhancement should be observed with the 1 2 binding complexes. Interestingly, the putative 1 1 binding complexes is weakly fluorescent, therefore no significant fluorescence enhancement was observed. The reason for the weak fluorescence of the D-bis-boronic acid sensor/D-(or L)-tartaric acid is unclear. 

Interestingly, we observed a much larger fluorescence enhancement for (iS)-8 compared to the monoboronic acid chemosensor without an intramolecular boronate structure (Figure 6.14). An enhanced fluorescence... 

Anslyn et have developed a displacement assay to measure the e.e. values of diols (Scheme 6.1). The chiral monoboronic acid sensors were prepared by using o-formyl arylboronic acids and various pyrrolidine chiral secondary amines. Since no fluorophore was used for the construction of the... 

Figure 6.15 Molecular structure of the bis-boronic acid d-PET sensors (i ,i )-9 and (5,5)-9. The monoboronic acid sensor (5)-10 is also presented.

Figure 6.19 Molecular structure of the thienyl carbazole based d-PET bis-boronic acid sensors 11. The monoboronic acid sensors (S)-12 and (i )-12 are also presented.

James and co-workers have prepared 12a, a monoboronic acid fluorescent sensor that shows large shifts in emission wavelength on saccharide binding [50]. The dual fluorescence of 12a, can be ascribed to locally excited (LE) and twisted internal charge transfer (TICT) states of the aniline fluorophore [51]. When saccharides interact with sensor 12a in aqueous solution at pH 8.21 the emission maxima at 404 nm (TICT state) shifts to 362 nm 274 nm, LE state). The band at 404 nm is due to the TICT state of 12a containing a B-N bond i.e. the lone pair is coordinated with the boron and perpendicular to the jt-system. The band at 274 nm (LE state) corresponds to the situation where the B-N bond in 12a has been broken with formation of the boronate (Scheme 12.3). 

Figure12.3 Monoboronic acid photoinduced electron transfer (PET) fluorescent sensors.

Two Other systems are worthy of special mention since they differ from that outlined above The simple monoboronic acid 15 used by James and co-workers, which can selectively signal the fiiranose form of saccharides [61], and the on-ofP PET system 16 of Kijima, where steric crowding on saccharide binding breaks the B-N bond found in the free receptor [62]. 

D-melibiose 339 M , while the monoboronic acid 28a has a binding constant of 96 in 52.1 wt% methanol-water at pH 8.21 (phosphate buffer). The selectivity of 27a-f for D-melibiose mirrors that for D-glucose, indicating that these diboronic acid sensors form stable 1 1 cyclic structures with the furanose segment of D-melibiose [84]. These observations and those of Hall with polysaccharides complement the observations made by Norrild for monosaccharides [13,14,85]. 

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