Complexation of boronic acids with

Figure 3.24 shows the redox behavior of PABA thin films observed at neutral pH in the presence of NADH and NAD" ". The PABA film was redox inactive at neutral pH (Figure 3.24,a) due to deprotonation and loss of dopant as with polyaniline [150,151). However, in the presence of NADH (Figure 3.24, b) and NAD" " (Figure 3.24, c), PABA films became redox active due to complexation of boronic acid with cis-2,3-ribose diols and subsequent formation of self-doped polymer. In the presence of NADH, the cyclic voltammogram of PABA thin film exhibited a single redox couple at pa 0.05 and pc —0.10 V. In contrast, a second redox couple was observed in the presence of NAD+ at pa 0.34 and pc... 

The second reason is that the enthalpy-entropy compensation is generally limited to (i) acid complexes of specific base types (i.e. correlations are family dependent) and (ii) unhindered bases. An example is given for a set of complexes of boron acids with nifiogen and phosphorus bases [72, 91]. Table 1.10 shows the continuous increase in the quality of the AH-AG correlation when the sample of 31 nitrogen and phosphorus bases is gradually restricted to 21 unhindered primary and secondary amines. The limited correlation is displayed in Figure 1.3. 

A number of less hindered monoalkylboranes is available by indirect methods, eg, by treatment of a thexylborane—amine complex with an olefin (69), the reduction of monohalogenoboranes or esters of boronic acids with metal hydrides (70—72), the redistribution of dialkylboranes with borane (64) or the displacement of an alkene from a dialkylborane by the addition of a tertiary amine (73). To avoid redistribution, monoalkylboranes are best used /V situ or freshly prepared. However, they can be stored as monoalkylborohydrides or complexes with tertiary amines. The free monoalkylboranes can be hberated from these derivatives when required (69,74—76). Methylborane, a remarkably unhindered monoalkylborane, exhibits extraordinary hydroboration characteristics. It hydroborates hindered and even unhindered olefins to give sequentially alkylmethyl- and dialkylmethylboranes (77—80). 

Catalytic curing agents initiate resin homopolymerization, either cationic or anionic, as a consequence of using a Lewis acid or base in the curing process. The Lewis acid catalysts frequently employed are complexes of boron trifluoride with amines or ethers. 

Rhodium-catalyzed Heck-type coupling of boronic acids with activated alkenes was carried out in an aqueous emulsion.82 The couplings between arylboronic acids and activated alkenes catalyzed by a water-soluble tm-butyl amphosrhodium complex were found to progress at room temperature to generate Heck-type products with high yields and excellent selectivity. It was necessary to add two equivalents of the... 

Lopez Garcia et al. [2] have described a rapid and sensitive spectrophotometric method for the determination of boron complex anions in plant extracts and waters which is based on the formation of a blue complex at pH 1 - 2 between the anionic complex of boric acid with 2,6-dihydroxybenzoic acid and crystal violet. The colour is stabilised with polyvinyl alcohol. At 600 nm the calibration graph is linear in the range 0.3-4.5 xg boron per 25 ml of final solution, with a relative standard deviation of 2.6% for xg/l of boron. In this procedure to determine borate in plant tissues, the dried tissue is treated with calcium hydroxide, then ashed at 400 °C. The ash is digested with 1N sulfuric acid and heated to 80 °C, neutralized with cadmium hydroxide and then treated with acidic 2,6-dihydroxybenzoic acid and crystal violet, and the colour evaluated spectrophotometrically at 600 nm. Most of the ions present in natural waters or plant extracts do not interfere in the determination of boron complex anions by this procedure. Recoveries of boron from water samples and plant extracts were in the range of 97 -102%. 

Lewis Acids. Lewis acids, eg, boron trihalides, contain an empty outer orbital and therefore seek reaction with areas of high electron density. Boron trifluoride, BF3, a corrosive gas, reacts easily with epoxy resins, causing gelation within a few minutes. Complexation of boron trihalides with amines enhances the curing action. Reasonable pot lives using these complexes can be achieved because elevated temperatures are required for cure. Reactivity is controlled by the choices of the halide and the amine. The amine choice also affects other properties such as solubility in resin and moisture-sensitivity. Boron trifluoride monoethylamine (BF3 NH2C2H5), a crystalline material which is a commonly used catalyst, cures epoxy resins at 80-100°C. A chloride version is also commercially available. Other Lewis acids used in epoxy curing include stannic chloride and tin octanate. 

Chiral complexes 27 were tested in oxidative Heck-type reactions of boronic acids with acyclic alkenes (Equation (9.2)). Product yields were modest however, enantioselectivities were excellent (9(C98% ee). Other examples of functionalised benzimidazol-2-ylidene Pd complexes include 28 and 29 (Figure 9.6), which required high temperatures and long reaction times to afford reasonable conversions. ... 

Scheme 12 While the binding of diols by trigonal boronic acids can be considered almost negligible, this is not the case when boronic acids complex with more acidic poly-hydroxyl species. Here the complexation of trigonalphenylboronic acid with oxalic acid proceeds with a rate constant kcooHcooH of 2000 s With trigonal boron acids complexation is generally discussed in terms of the overall reaction proceeding with a change in geometry from trigonal to tetrahedral at boron on complexation.

Figure 10 The series of boron acids, with their respective pK s, as documented by Pizer. It was found that for a given ligand the stability constants for complex formation increased with increasing boron acid acidity.

Scheme 16 As the reaction kinetics of tetrahedral borates with most ligands cannot be followed due to the problems associated with kinetically indistinguishable pathways and proton ambiguity, studies have examined the biruling of boric acid with bidentate ligands in a 1 2 complex. By considering the change from a 1 1 to a 1 2 complex a tetrahedral structure is enforced at boron. This allows parallels to be made with the complexation of other tetrahedral boronate anions. Yoshimura s proposed transition state is depicted here, illustrating the complexation of boric acid with chromo tropic acid. ...

Scheme 17 In addition to the pair-wise interaction of boronic acids with polyhydroxyl species discussed, boronic acids also form stable complexes with buffer conjugate bases. These complexes can be formed between both the free boronate anion and Lewis bases as well as between saccharide boronate complexes and Lewis bases. Not recognised until 2004, these species persist into acidic solution and under certain stoichiometric conditions can become the dominant component in the solution. The two modes of interaction between the phenylboronate anion and phosphate are illustrated here.

Since the discovery of the complex H3N-BF3 by Gay-Lussac in 1809 [1], a great number of complexes of boron trifluoride with Lewis bases have been smdied. Indeed, BF3 is an archetypical Lewis acid. It is clear that the central boron is electron deficient since it has only six electrons in its outer shell and the highly electronegative fluorine atoms further decrease its o electron density. 

The preparation of functionalized aryl boronic reagents can be achieved by directed metallation followed by a transmetallation of aryllithiums with organoboron compounds. Thus, Caron and Hawkins have described a directed ortho-metalla-tion of aryl neopentyl esters such as 1 for the synthesis of substituted ortho-horo-nyl neopentyl benzoates using lithium diisopropylamide (LDA) as the base and B(OiPr)3 as an in situ trap [3]. The crude boronic acids obtained by acidic hydrolysis were subsequently treated with ethanolamine and converted to stable diethanolamine complexes such as 2. This methodology allows the preparation of a new class of boronic acids with ortho-carbonyl substituents and other functionalities... 

The solid appears to be a mixture of the complexes CH,COOH.BF, and 2CH COOH.BF,. The latter appears to be a liquid and is alone soluble in ethylene dichloride the former is a solid. The solid moiioocetic acid complex is obtained by saturating an ethylene dichloride solution of acetic acid with boron trifluoride, filtering and washing the precipitate with the solvent it is hygroscopic and should be protected from moisture. It may be used as required 0-75 mol is employed with 0-26 mol of ketone and 0 6 mol of anhydride. 

Coumarone—Indene Kesins. These should be called polyindene resins (17) (see Hydrocarbon resins). They are derived from a close-cut fraction of a coke-oven naphtha free of tar acids and bases. This feedstock, distilling between 178 and 190°C and containing a minimum of 30% indene, is warmed to 35°C and polymeri2ed by a dding 0.7—0.8% of the phenol or acetic acid complex of boron trifluoride as catalyst. With the phenol complex, tar acids need not be completely removed and the yield is better. The reaction is exothermic and the temperature is kept below 120°C. When the reaction is complete, the catalyst is decomposed by using a hot concentrated solution of sodium carbonate. Unreacted naphtha is removed, first with Hve steam and then by vacuum distillation to leave an amber-colored resin. It is poured into trays, allowed to cool, and broken up for sale. 

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