Benzene boronic, acid

Rate coefficients for protodeboronation of a number of substituted benzene-boronic acids were measured by Kuivila et al.621 at pH 6.70 and 6.42 at 90 °C, p = 0.14. The relative rates at the two pH values were reasonably constant, indicating that the same reaction was being studied for each compound (Table 200). The results indicate that all substituents increase the rate of reaction but... 

Rate coefficients have been determined for protodeboronation of benzene- and thiophene-boronic acids in a range of aqueous perchloric acid mixtures at temperatures between 25 and 90 °C630. First-order rate coefficients are given in Table 204, but not all of the rates were measured at the acidities indicated (see Table footnote), but were corrected to these making use of the linear relationship which was found, in preliminary experiments, to exist between log rate and H0. Similarly, the relative reactivities to thiophene-2-, thiophene-3-, and benzene-boronic acid (8.5 x 10s, 7.1 x 103, and 1.0, respectively) were obtained at an H0... 

Suzuki coupling chemistry of benzene boronic acid derivatives and haloben-zenes using a Pd(0) catalyst has also been employed for the synthesis of substituted PPPs as illustrated by the A-B type monomer 16 [67-73]. These initial syntheses were carried out under heterogeneous conditions at a basic pH as illustrated by Scheme 21. Such Suzuki coupling polymerizations are rather attractive alternatives as a wide variety of functional groups can be tolerated with minimal interference in the coupling scheme. 

Benzene boronic acids with hydrogen peroxide with perchloric ... 

In attempts to prepare oligo- and poly(aryl/heteroaryl) systems, the use of 2,5-dibro-mopyrimidine 81 was required in a sequential fashion. Treating 81 with three equivalents of benzene boronic acid in the presence of catalytic Pd(PPh3)4 in refluxing THF, gave only two isolable products. The mono-Suzuki product 83 resulting from reaction at the 2-position was obtained in 43% yield and the bis-Suzuki product 84 was obtained in 32% yield. The results of this reaction establish that the bromine at the 2-position is more reactive than the bromine at the 5-position [34]. 

Scheme 2 Synthesis of 4-cyano-benzene-boronic acid...

Experimental conditions 5-bromoindole 3(1 mmol), 4-cyano-benzene-boronic acid 4 (1.5 mmol), sodium carbonate (2 mmol) and the catalyst were heated in a 50 ml Schlenk-tube to 80 °C in toluene (6 ml), ethanol (1 ml) and water (1 ml) for the indicated time. The conversion was determined by GLC... 

Synthesis of 4-cyano-benzene-boronic acid 4. A vacuum isolated triple jacket reactor was charged with THF (76.5 g), triisopropylborate (49.8 g, 265 mmol), and toluene (88 g), and the mixture was cooled to —70°C. A solution of 4-bromo-benzonitrile 2 (40 g, 220 mmol) in toluene (210 g) was added within 20 min and a fine suspension is formed. BuLi (15% in hexane, 131 g, 308 mmol) was added dropwise within 4 h keeping the reaction temperature below —70°C. Hydrochloric... 

Synthesis of 5-(4-cyano-phenyl)-indole 1. A double jacket reactor was charged with 5-bromo-indole 3 (50 g, 250 mmol), 4-cyano-benzene-boronic acid 4 (38.7 g, 263 mmol), and isopropanol (393 g). The reactor was purged with nitrogen. Pd(PTol3)2Cl2 (0.2 g, 0.25 mmol) was added, and the reaction mixture was heated to reflux. Sodium carbonate solutimi (2 M, 250 ml, 500 mmol) was dosed within 2 h. The reaction was stirred for additional 90 min at reflux. The isopropanol was distilled off until the temperature of the distillation residue reached lOO C. The reaction mixture was cooled to room temperature and was extracted with dichloromethane (1,000 ml). The organic phase was extracted with water (250 ml) and was filtered slowly through a bed of silica (40 g) and charcoal (4 g). The filter bed was washed with dichloromethane (100 ml). To the combined dichloromethane phases, toluene (500 ml) was added and dichloromethane was distilled of until the distillation residue reached a temperature of 110°C. The product solution was cooled to 95°C and was seeded. The suspension was cooled slowly (l°C/min) to room temperature. The product is filtered, washed with toluene (50 ml), and dried at 60°C. 5-(4-cyano-phenyl)-indole 1 was obtained as colorless crystals (42.6 g, 188 mmol, 75%). 

A modification of the synthesis of the tetrarylindenofluorenes 186 shown in Scheme 86 has been used to prepare ladder-type pentaphenylenes (Scheme 88) [283]. The dibromoterephthalate was coupled with two equivalents of a flu-orene boronate instead of a benzene boronic acid to give a pentaphenylene 188. Reaction with an aryl lithium, followed by ring closure with boron trifluoride, gave the ladder-type pentamers 189, which were brominated to prepare dibromomonomers that were then polymerised using nickel(O). 

The design and synthesis routes of COF materials mentioned above have been widely used in practical applications, and a series of new materials have been developed. For example, the self-condensation reaction of 1,4-benzenediboronic acid (2D-C2) forms the 2D COF-1 (Figure 6.2a). The cocondensation reaction between 1,3,5-benzene boronic acid (BTBA 2D-C3) and 2,3,6,7,10,ll-hexahydro3ytriphenylene (HHTP 2D-C3) produced 2D COF-6 (Figure 6.2d), and the co-condensation reaction between HHTP and... 

Looking for other transition metal-catalyzed coupling reactions, the Mainz research group achieved an improved synthesis of poly(p-2,5-di-n-hexylphenylene) by using Pd catalysts. This procedure was adapted from the Suzuki [71] and Miller [72] reaction in which Pd catalysts are used to couple various bromobenzene derivatives with benzene boronic acid. This reaction was described in the literature as being highly selective and quantitative. The influence of substituents attached at the ortho position was reported to be negligible. 

Pd-Catalyzed Reactions Suzuki Coupling. The Suzuki [299] and Miller [300] reactions are based on the use of Pd catalysts to couple various bromobenzene derivatives with benzene boronic acid... 

The catalyst 4 was suspended in /7-xylene (20mL) and benzene boronic acid (0.914g, 7.5mmol) bromobenzene (0.80Sg, S.lmmol), potassium carbonate (1.382g, lOmmol) and 7t-dodecane (0.749g, 4.4mol) as GC internal standard were added. The reaction was then heated to reflux and followed by GC. 

In the propionic acid catalyzed hydroxyalkylation, benzene boronic acid directed the aldehyde into the ortho position via the boronic ester 449 and led to very good yields of the ortho-substituted phenols 450 [153]. 

---