Boroxines structure

Today, there exist a wide variety of boron-containing polymers including ring systems such as borazines (structure 11.41), boroxines (structure 11.42), and triphosphatoborins (structure 11.43),... 

Figure 46 The planar structures of the boron ring systems, borazine (71) and boroxine (72), with nonexistent aromaticities and of triphosphatriborin (73) with an appreciable degree of aromaticity. (Adapted from ref. 95.)...

Nitrone derivatives of 2-fonny phenylboronie acid have been prepared, and these exist in cyclic, trisubstituted boroxin fonn (35a-d) <83JOM247>. Interestingly, treatment with a 1,2-dio converts them into monomers with a rather unique 1,3-zwitterion structure (36a-d),... 

Electron diffraction experiments provide valuable information about structures in the gas phase. Consequently, this method of structural determination is important for inorganic ring systems that are volatile liquids or gases at room temperature. For example, the essentially planar structures of borazine (3.1, E = NH) and the isoelectronic boroxin ring (3.1, E = 0), ° the monomeric structure of the radical [CFgCNSSN] (3.2), and the all-cw arrangement of the... 

The spin-coupled method has now been applied to a large number of aromatic systems benzene and naphthalene azobenzenes, such as pyridine, pyridazine, pyrimidine and pyrazine five-membered rings, such as furan, pyrrole, thiophen, and thiazole and inorganic heterocycles, such as borazine ( inorganic benzene ) and boroxine, for which we find little evidence of aromaticity. Structural formulae are collected in Fig. 1. For all of these molecules we have included the effects of electron correlation for the Jt electrons but not for the a framework. This a-n separation is an approximation whose utility rests upon the chemistry of aromatic systems — to abandon it would be to ignore this entire body of experience. Furthermore, very extensive calculations [4] have demonstrated that rc-electron only correlation affords an excellent description of ground and excited states of benzene. 

Researchers at Sepracor later disclosed the use of a new class of chiral oxazaborolidines derived from r/. v-aminoindanol in the enantioselective borane reduction of a-haloketones.6,7 The 5-hydrogen oxazaborolidine ligand 10 was prepared in situ from d,v-aminoindanol 1 and BH3 THF.8 Stock solutions of 5-methyl oxazaborolidine 11-16 were obtained by reaction of the corresponding N-alkyl aminoindanol with trimethyl boroxine.6,7 5-Methyl catalyst 11 was found to be more selective (94% ee at 0°C) than the 5-hydrogen catalyst 10 (89% ee at 0°C), and enantioselectivities with 11 increased at lower temperatures (96% ee at -20°C). The catalyst structure was modified by introduction of A-a I kyI substituents. As a general trend, reactivities and selectivities decreased as the steric bulk or the chelating ability of the A -alkyl substituent increased (Scheme 17.4). 

Boroxins have properties and reactivities similar to orthoborate esters. The six-membered boroxin ring structure (2)... 

Tossell, J. A., and P. Lazzeretti (1990). Calculation of the structure, vibrational spectra, and polarizability of boroxine, H3B3O3, a model for boroxol rings in vitreous BjO,. J. Phys. Chem. 94, 1723-24. 

The electronic structures of borazine, BjNjHg, and boroxine, B3O3H3, have been compared, using ab initio SCF-LCAO-MO calculations. It was found that... 

Boroxines have trimeric, cyclic structures with planar rings of alternating boron and oxygen atoms. The alkyl groups are also in the plane of the ring. Boron can also be incorporated into numerous carbon ring systems. 

Scheme 28.7 Formation of periodic network structures (21 and 22) in solids using equilibrium boroxine and boronate formation with a monomer 23 and with monomers 23 and 24, respectively.

Reversible bond formation can, in principle, be used for the selfcorrection of product structures. For example, the formation of Schiff bases, boroxines and boronates has been applied to on-surface polymerizations, but has so far met with limited success. Linderoth and Gothelf employed trialdehyde 62 and diamine monomers 63 towards 2-D polymer synthesis on Au(lll) under UHV (Figure 28.28a) [134], but unfortunately obtained only branched or irregularly networked stmctures, a situation that was ascribed to the flexible monomer structure employed. In addition, UHV conditions caused an irreversible loss of water, the presence of which was essential to bring about the back-reaction. When Abel and coworkers synthesized boroxine/boronate networks on Ag(lll) under UHV [135] they were unable to achieve the expected periodic order this contrasted with the findings of Yaghi et al., who employed the solution approach (see Section 28.5.2). This difference was most hkely a consequence of irreversible bond formation. 

Figure 8.4 Covalent organic framework (COF) structure based on a condensation process followed by boroxine monomer extraction to give a 3D boroxine framework. (Reproduced from ref. 18 with the permission of Elsevier.)...

Despite their Hmited structure determination capabilities, ultraviolet and infrared spectroscopy were determinant characterization techniques in the early days of boronic acid research [332]. Notable IR absorptions are the strong H-bonded OH stretch (3300-3200 cm ), and a very strong band attributed to B-O stretch (1380-1310 cm ). IRis particularly diagnostic of the presence of boronic anhydrides. Upon anhydride (boroxine) formation, the OH stretch disappears and a new strong absorption appears at 680-705 cm [68]. 

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