Boron tetrahedral borate

Organoboranes undergo fragmentation if a good leaving group is present on the 8-carbon.102 The reactive intermediate is the tetrahedral borate formed by addition of hydroxide ion at boron. 

Isotope labeling experiments indicate that the B—O bond is broken and not the C—O bond in the formation of the diol-boric acid complexes (18). This indicates that the initial step in the mechanism may be an attack on the boron atom by an oxygen of the diol, followed by the release of water. This could occur without developing any charge separation. If such a mechanism were correct, it would seem that an attack on the boron atom would be easier for trigonal boric acid than for the tetrahedral borate anion (Figure 1). 

Guaran is a polygalactomannan containing T-OH groups. It is well known that the tetrahedral borate ions readily form tetrahedral complexes with the cw-OH groups. Thus the borate immobilized polymer (boron content 0.7 mmol/g) was used for enantioseparation of racemates such as 1,2- or 1,3-dihydroxy compounds or a-hydroxy acids.This appears to be the first example of the use of boron as a complexing ion in CLEC. 

Tetrahedral borate or boronate complexes have been shown to be involved in enzyme inhibition. Serine proteases were proposed to be inhibited by boric acid [6], and simple borates have been patented as protease stabilizers in liquid detergent formulations [7, 8]. 

This relationship held quite well for a range of neutral trigonal planar boranes BXYZ and analogous carbocations (CX YZ), though it proved less precise for systems in which there was conjugation between the substituent (X, Y, or Z) and three-coordinate boron. When data for tetrahedral borate anions BXt and neutral carbon species CX4 were considered, a similar relationship [of slightly different slope, Eq. (5.36)] was found ... 

Boronic acid derivatives have been shown to inhibit -lactamases [168]. The boronic acid function, which is isoelectronic with a protonated carboxyl group, is presumed to block enzymatic activity by forming a stable tetrahedral borate complex with the active serine residue. Boronic acids 319, structurally related to penicillin G and methicillin, were synthesised in three steps from dibutyl iodomethaneboronate [169] (Scheme 97). They are inhibitors of P-lactamase I from B. cereus. There are presently no reports of boronic acid derivatives acting as inhibitors of bacterial D,D-peptidases. 

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. ...

While unknowns still remain, Pizer has developed a plausible explanation for the observed increase in rate with tetrahedral borates over trigonal boron... 

Figure 9. High-resolution nB-MAS-NMR spectra (128A MHz) of borate glass systems. The trigonal to tetrahedral boron ratios, Bs/Bh are calculated from the oxide formula of the glass.

Two triborate moieties may link together by sharing a common tetrahedral boron atom to form the pentaborate anions shown in Fig. 5. The pentaborate monoanion shown in Fig. 5a is observed in solution (see Fig. 2) and occurs in synthetic and mineral borates. Examples are the synthetic commercial ammonium pentaborate, NH4[Bs06(0H)4] 2 H2O [ = (NH4)20 5 B2O3 ... 

Most of the isolated borate anions shown above may polymerize in a variety of ways to form extended chains, sheets, and networks. Polymerization occurs by either sharing an exocyclic oxygen atom or by sharing an intracyclic tetrahedral boron atom. These are exemplified by the structures of the important industrial minerals colemanite [29], Ca[6304(0H)3] H2O... 

Other borate ester chelates containing tetrahedral boron, shown in Fig. 16a, have been prepared as precursors to solvent-supported borate cations (Fig. 16b) that show catalytic activity toward olefin polymerization [54]. 

B3 O7 )5 group or other borate groups in which one or more boron atoms are tetrahedrally coordinated. 

The most common units found in borate structures are shown in Fig. 1. Many of the known borate structures can be rationalized by assigning appropriate charges, protons, or hydroxyl groups (which change the coordination of boron from trigonal to tetrahedral) to these basic units. Thus the B03 unit can become BOj -, B(OH)3, or B(0H)4, respectively. Polymerization by elimination of one water molecule between two hydrated units results in chain formation, and further water elimination gives sheets or networks. These operations are illustrated in Fig. 2 for the triborate ring unit. 

Fig. 1. Basic units of borate structures. All trigonally coordinated boron atoms shown can theoretically become tetrahedrally coordinated.

Infrared spectroscopy is the favored technique for characterizing borates, and several compilations of data have been made (171, 333,417, 423,431,432). The intense absorption at about 970 cm-1 is characteristic of tetrahedrally coordinated boron, as measured for the magnesium borates MgO B203 nH20 (153), and bands at 700 to 900 and 1200 to 1500 cm-1 can correspond to triangular boron coordination. Absorptions in the region 400 to 700 cm-1 have been attributed to chain structures (423). 

Borates can be formed with all metals, although those of groups 1 and 2 are best known. The structural features are complex and rival those of silicates. Boron can occur as planar B03 or tetrahedral B04... 

Characterization of Substituted Boron. We used solid state -B NMR and X-ray diffraction data to distinguish occluded borates from boron substituted into the zeolite framework. When an element replaces aluminum or silicon in a zeolite structure, the local coordination environment changes to accommodate the new ion. Since B + is a much smaller ion than Al "1", the unit cell axes contract when boron replaces aluminum in the framework. The ionic radii of trivalent B and A1 in a tetrahedral environment are 0.25 and 0.53, respectively (1). The magnitude of the contraction is dependent upon the level of substitution (17). 

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