Borate complexes effects

Chiral boron catalysts had already been widely used in a variety of reactions before they were applied in Diels-Alder reactions220. Boron catalysts were first employed in the Diels-Alder reactions of quinones with electron-rich dienes. Kelly and coworkers221 found that stoichiometric amounts of a catalyst prepared from BH3, acetic acid and 3,3 -diphenyl-l,l/-bi-2-naphthol (344) catalyzed the reaction of 1-acetoxy-l,3-butadiene (341) with juglone (342) to afford cycloadduct 343 with 98% ee (equation 96). The reaction was supposed to proceed via a spirocyclic borate complex in which one face of the double bond of juglone was effectively shielded from attack by the diene. 

Polyhydroxy compounds are known to form borate complexes that can interfere with the subsequent acetylation step. In essence, two methods of acetylation are employed. In one, the conditions are sufficiently vigorous that borate complexes are cleaved during the acetylation, and, in the second, the complexes are decomposed prior to acetylation. In one example of the former method, the crude, dry alditols were refluxed for 4 hours with a mixture containing equal amounts of acetic anhydride and pyridine ( 1 ml/100 mg), and the cooled solution was injected directly into the gas chromatograph.217 The effect of borate on the rate of acetylation of glucitol by this... 

Speculation is made about why epi-inositol flips into the triaxial form to form a tridentate complex with borate, but complexes without flipping with metal ions. The distances between the three axial oxygen atoms are similar to those between the three oxygen atoms in the ax-eq-ax sequence. However, in the triaxial borate complexes (3) the bonds are tetrahedral if one were formed at the ax-eq-ax oxygen atoms, the bonds would be considerably distorted. However, the angles have much less effect on complex formation with cations which does not involve covalent bonds. 

Borate complexes have been utilized by Brigl and Griiner47 to effect partial esterification. Anhydrous D-glucose and metaboric acid dissolved in acetone give a complex which exhibits the analysis of a diborate. Reaction of the latter with an excess of benzoyl chloride gives 2,6-di-O-benzoyl-D-glucose (XL). D-Mannitol likewise forms a diborate, which produces the 1,6-di-O-benzoyl derivative (XLI) upon benzoylation. In the presence of boric acid, D-glucose diethyl thioacetal yields the 6-benzoate (XLII). In the non-aqueous medium the formation of complexes... 

Shiroishi and Hayakawa (18) have described the effect of sunlight irradiation on pyridoxine and related compounds in aqueous solution at various pH s. Pyridoxine and pyTidoxamine are relatively stable in acidic medium, while pyridoxal decomposes independently of pH. The authors suggested that the aldehydic 0 participates in the photolysis of pyridoxal. The presence of Cu++ had no effect on the stability of these substances. A borate complex of pyridoxine is stable to sunlight irradiation, heating, or autoclaving. Acetic acid and a reddish brown substance were separated as the decomposition products from an irradiated solution of pyridoxine (18). 

Until recently, very little had been reported on the important area of metal borate complexation in aqueous solution. The effect of salts on the ionization of boric acid (358, 375) has been mentioned above, and subsequent research suggests that complexation of borate with, for example, calcium ions can account for the enhanced acidity of H O Literature on cationic complexes of boron was reviewed in 1970 (376). 

The effect of neutral salts (e.g., NaCl) on the composition of borates precipitated from, or in equilibrium with, aqueous solutions doubtless arises from a reduction in water activity, metal borate complexation, and a shift in polyborate equilibria (Sections IV,A, B). The "indifferent or inert component method has frequently been used for the synthesis of borates. Potassium and sodium chlorides can be used to enhance the precipitation of specific nickel (48), aluminum (51), iron (49), and magnesium (151) borates. In the K20-B203-H20 system at 25°C (248), the presence of potassium chloride results in a reduced boric acid crystallization curve, lower borate solubilities, lower pH, and an extended B203 K20 range over which the pentaborate crystallizes. 

The solid-state structure of the heteroleptic compound [Tp Bal (HMPA)2] is shown in Fig. 21.111 The steric bulk of the poly(pyrazolyl)bo-rate ligand is apparent in this figure, effectively blocking oligomerization from occurring. This is even more apparent in the bis(3,5-dimethylpoly (pyrazolyl)borate) complex Ba[HB(3,5-Me2pz)3]2 shown in Fig. 22, where two poly(3,5-dimethylpyrazolylborate) ligands have sterically saturated the coordination sphere of the barium center. The shortest intermolecular distance is 5.23 A.112... 

Detection of side products generated by Rubisco is accomplished by various means. XuBP (30) and pyruvate (36) are both conveniently detected spectrophotometrically by coupling to NADH oxidation with appropriate enzymes. Alternatively, our chromatographic procedure (27) gives a complete profile of all RuBP-derived products. Resolution of these compounds is enhanced by inclusion of 10 mM sodium borate, which complexes vic-diols, in elution buffers. Since our initial report of the separation of borohydride-reduced misprotonation products (27), we have observed that borate also effects complete separation of unreduced RuBP and XuBP (37). Thus, the analysis is simplified by circumventing the necessity to deduce the amounts of misprotonation-derived bisphosphate based on ratios of ribitol-, arabinitol-, and xylitol-1,5-bisphosphates. 

The borate buffer is the most effective buffer for the CZE separation of native (and derivatized) carbohydrates. Borate complexes with adjacent hydroxyl groups on carbohydrates to form a negatively charged complex, which has a 2- to 20-fold increased UV absorbance at 195 nm. ... 

Both of the steric effects mentioned above are operative in the o-manno-pyranosides. Thus, in methyl a-o-mannopyranoside, the 4,6-borate complex is hindered, but the 2,3-complex is not. The reverse situation obtains for the /3 anomer. Since the 2,3-complex is formed more readily than the 4,6-complex, the a anomer has the greater Mg value. 

The reversible formation of borate esters by carbohydrates in aqueous solution has been known for over a century, and played a role in early conformational studies.Currently it is an ancillary in the separation of carbohydrates, and with its aid very fine separations can be achieved - the secondary tritum isotope effects at H2, H3, H5 and H6 (but not HI) of glucose on borate complexation permit complete isotopic separation to be achieved by ion-exchange chromatography. ... 

Fig. 4 Role of xylulose-borate complexation in the co-iimnobilized enzyme system. When sodium tetraborate (borax) is added to solution, it dissociates into tetrahydroxyborate (borate, B) ion and boric acid. In the pellet interior, higher pH favors tighter xylulose-borate binding, which effectively reduces the xylulose concentration in the interior and forces the isomerization forward. In the bulk, the lower pH has an uncoupling effect on the Xu-B complex, making the dissociated xylulose readily available to the yeast. Removal of xylulose via fermentation further forces dissociation of the xylulose-borate complex. Dashed lines represent transport of species solid lines represent reactions... 

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