Boronic acids bis

Water-soluble poly(p-phenylene) 24, shown in Scheme 29, was prepared by the introduction of carboxylic acid pendant substituents along the p-phenylene chains [102]. In initial work in this area, a dicarboxy-substituted dibromobiphe-nyl was polymerized with 4,4f-biphenyl bis-boronic acid via Suzuki coupling... 

Thus, for our present purposes a similar approach was followed using Suzuki cross-coupling reactions as the key steps in the synthesis of our target compounds. Symmetrically substituted compounds were synthesized in a twofold Suzuki crosscoupling reaction from commercially available p-substituted phenylboronic acids or esters and 4,4 -dibromobiphenyl or 4,4 -biphenyl-bis-boronic acid ester and a p-substituted arylhalide, respectively, using tetrakis (triphenylphosphino) palladium as catalyst together with cesium fluoride as base in dry tetrahydrofurane as shown in Scheme 8.1. The desired products were obtained in respectable yields after heating at reflux for 50 h. 

A new synthetic approach to polycyclic aromatic compounds has been developed based on double Suzuki coupling of polycyclic aromatic hydrocarbon bis(boronic acid) derivatives with o-bromoaryl aldehydes to furnish aryl dialdehydes. These are then converted to larger polycyclic aromatic ring systems by either (a) conversion to diolefins by Wittig reaction followed by photocyclization, or (b) reductive cyclization with trifluoromethanesulfonic acid and 1,3-propanediol (Eq. (12)) [30]. 

Phenylene bis(lead triacetate) reagents, generated in situ from the corresponding bis(boronic acid) derivatives and lead tetraacetate, react with the a-methyl Meldrum s acid derivative to afford the meta-or para-phenylene bis(Meldrum s acid) derivatives in ca 45% yield. 1 Similarly to malonic acid compounds, the unsubstituted Meldrum s acid was very slow to react and the only observed product was the a,a-diarylated product in 7-17%. 

Later, the same methodology was applied by Wallow and Novak for the synthesis of water-soluble poly(p-phenylene) derivatives via the poly-Suzuki reaction of 4,4 -biphenylylene bis(boronic acid) with 4,4 -dibromodiphenic acid in aqueous di-methylformamide [26]. These aromatic, rigid-chain polymers exhibit outstanding thermal stability (decomposition above 500 °C) and play an important role in high-performance engineering materials [27] conducting polymers [28] and nonlinear optical materials [29]. 

Suzuki-Miyaura cross-coupling polymerization of 1,4-bis((Z)-2-bromovinyl)benzenes with aryl-bis-boronic acids. The interest has been in an alternative approach, where rather than building a PPV with a pre-ordained stereochemistry, a postpolymerization yyn-selective reduction on a poly(phenylene ethynylene) (PPE) is used [125]. This scheme has the advantage that high molecular weight PPEs can be synthesized using either Pd-catalysis or alkyne metathesis. This route could also potentially allow for the access to an additional array of PPVs that are uniquely accessible from PPEs. The transformation of the triple bonds in PPEs and other acetylene building blocks to alkenes has considerable potential. 

Circularly polarised emission is possible from polymers containing chiral groups. Scherf and coworkers have prepared a cyclophane-substituted PPP by the Suzuki route using the dibromocyclophane 35 and the corresponding bis-boronic acid 36 (Scheme 15) [98,99]. If racemic monomers were used the resulting polymer 37 was not chiral with the cyclophanes randomly distributed on either face of the polymer (atactic). If resolved enantio-pure monomers were used, then the stereoregular isotactic 38 or syndiotactic 39 polymers could be obtained depending upon which enantiomer of each monomer was used. The isotactic polymer is chiral and both enantiomers have been prepared. 

Cyano-substituted oligothiophene 2.57 (Chart 1.13) was prepared by Suzuki-type coupling reaction between 5-bromo-5"-cyanoterthiophene and the bis-boronic acid of the inner terphenyl building block... 

Example 22 Suzuki Cross-coupling Polymerization of 2,7-Dibromo-4,4,9,9-tet-raalkyl-4,9-digerma-4,9-dihydro-s-indaceno l,2- 5,6- )ditiIiophene (dibromo-digermaindacenodithiophene) (42) with 2,l 3-Benzothiadiazole-4,7-bis(boronic Acid Pinacol Ester) (43) to give P44-PGeTPTBT ... 

The first water-soluble, electroactive, self-doped sulfonatoalkoxy-substituted PPP, poly[2,5-bis(3-sulfonatopropoxy)-l,4-phenylene-aZf-l,4-phenylene] (55), was synthesized from 1,4-benzenediboronic acid [4612-26-4] (56) and the disodium salt of l,4-dibromo-2,5-bis(3-sulfonatopropoxy)benzene (57) by the homogeneous Suzuki coupling method (130). The polymer purified by dialysis of an aqueous solution was composed exclusively of 1,4-linkages. The abihty to utilize a variety of bis(boronic acids) in this polymerization with sulfonate containing aiyl hahdes will lead to a high degree of structural control of the optoelectronic properties of these water-soluble poljnners. 

Figure 5.22 Six carbohydrate receptors consisting of cationic bis-boronic acid appended benzyl viologens [3,3 -olm-, 4,3 -olm-, and 4,4 -o/m-BBV) and the anionic fluorescent dye 8-hydroxypyrene-l,3,6-trisulfonic acid trisodium salt (HPTS). Three corresponding benzyl viologens (3,3 -, 4,3 -, and 4,4 -BV) were used as controls.

In 2004, Zhao and James et used the same Binol-based boronie acid chemosensors 4 (Figure 6.2) for enantioseleetive reeognition of D- and L-tartarie acids (Figure 6.3). As there are two a-hydroxy earboxylic acid units in tartaric acid, a cyclic adduction with the Binol based bis-boronic acid sensor can be formed. pH titrations showed that the fluores-eence response of the chemosensor is significant in the acidic pH range. 

Figure 6.2 Binol-based fluorescent chiral bis boronic acid chemosensors (4).

Figure 6.3 Chiral polyol substrates that can be selectively recognized by chiral bis-boronic acid sensor 4. ...

Interestingly, the chiral bis-boronic acid sensor 4 show enhanced fluorescence emission towards one enantiomer of the tartaric acid, whereas fluorescence decreased in the presence of another enantiomer (Figures 6.4 and 6.5). This enhancement/reduction in fluorescence response for chiral recognition of enantiomers was rarely reported. Both the fluorescence... 

In 2004, Zhao and James et developed the anthracene-based chiral boronic acid fluorescent sensors 5 and 6, with the a-methylbenzylamine as the chirogenic centre (Eigure 6.6). It is supposed that a cyclic adduct can be formed between the bis-boronic acid sensor and tartaric acids. Based on the fluorescence response of the bis-boronic acid sensor, drastically different fluorescence responses toward the two enantiomers were observed in the acid/ neutral pH range (Figure 6.7). For example, the (i )-sensor shows substantially increased fluorescence in the presence of o-tartaric acid at pH 8.3. In the presence of L-tartaric acid, however, there was no fluorescence enhancement (Figure 6.8a). 

Figure 6.7 Fluorescence intensity-pH profile of sensors 5 and 6. (a) Bis-boronic acid with D- and i>tartarlc acid, ex at 365 nm, at 429 nm. (b) Mono-boronlc acid 6 with d- and L-tartaric acid, at 373 nm, at 421 nm 3.0 X 10" mol dm" of sensors in 5.0 x 10" mol dm" NaCl ionic buffer (52.1% methanol in water), [l- and D-tartaric acid] = 5.0 x lO" moldm", 22 (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. 

Figure 6.9 Single-crystal structure of the binding complex of bis-boronic acid sensor (5,5)-5 complex with L-tartaric acid. (Reproduced by permission of the American Chemical Society.)...

Determination of the binding constants confirmed the enantioselec-tivity. For instance, at pH 5.6, in the presence of o-tartaric acid, the fluorescence enhancement for Rfi]-2 and (S,S)-2 are 9.05- and 3.61-fold, respectively. The binding constants are log AT = 5.78 and 4.20, respectively. At pH 2.5, the fluorescence intensity of (Rfi]-2 and (5,S)-2 increased by 3.31-and 1.49-fold, respectively, whereas the a-methylbenzylamine-based chiral bis-boronic acid chemosensors hardly give any fluorescence enhancement (Figure 6.11). 

The UV-vis absorption spectra of a mixed solution of the chiral bis-boronic acid and tartaric acids shows that the UV-vis absorption band becomes more structured for the optimal combination of the (7 ,7 )-sensor 7 and the D-tartaric acid. For (l ,i )-sensor 7 and L-tartaric acid, however, the UV-vis absorption spectrum does not change (Figure 6.12). Thus it was proposed... 

Figure 6.10 a-Methylnaphthylamine based chiral fluorescent bis-boronic acid sensors (R,i )-(-)-7 and (S,5 )-(+)-7. 

In 2009 a d-PET boronic acid was devised by Zhao and James et al (Figure 6.15). An electron-rich carbazole was used as the fluorophore of the chiral bis-boronic acid sensor 9 and a-methylbenzylamine was used as the chirogenic centre of the sensor. To achieve chiral recognition a bis-boronic acid system was used in designing the fluorescent sensor. The fluorescence response of the sensor toward protonation is in contrast to the normal a-PET chemosensors. The fluoreseenee of the sensor is weaker in the acid pH region (Figure 6.16). 

The d-PET effeet has been proposed for these carbazole-based bis-boronie aeid sensors, where d-PET indieates that the fluorophore is the eleetron donor at the photoexeited state, with the protonated N-atom and the binding site as the eleetron aceeptor. Henee, in the acidic pH region, with the protonated N-atom, eleetron transfer from the carbazole fluorophore to the protonated amine and the boronie aeid moiety occurs. As a result, the fluorescence is actually quenched at aeidie pH, which is in stark contrast to the fluorescence-pH relationship of normal a-PET fluorescence sensors. With the diminished background fluorescence of the chiral bis-boronic acid... 

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.

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