Boroxine ring systems

Figure 45 Boronic acids (69 and 70) with multiple boronic acid functionalities that form boroxine ring systems on thermolysis through the elimination of water. (Adapted from ref. 93.)...

Boronic acids (69 and 70) (Fig. 45) with more than one boronic acid functionality are known to form a polymer system on thermolysis through the elimination of water.93 Specifically, they form a boroxine (a boron ring system) glass that could lead to high char formation on burning. Tour and co-workers have reported the synthesis of several aromatic boronic acids and the preparation of their blends with acrylonitrile-butadiene-styrene (ABS) and polycarbonate (PC) resins. When the materials were tested for bum resistance using the UL-94 flame test, the bum times for the ABS samples were found to exceed 5 minutes, thereby showing unusual resistance to consumption by fire.94... 

The best known boron-containing ring systems are the so-called inorganic benzenes, namely, borazine (71), boroxine (72), and triphosphatriborin (73) (Fig. 46), which are isoelectronic and isostructural with benzene. These ring systems are obtained by the interaction of boron with nitrogen, oxygen, and phosphorus, respectively.4... 

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.)...

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),... 

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... 

Semi-empirical MO calculations on bToroxine, B3O3H3, and some substituted derivatives show that the o--electron drift in the ring system is towards the oxygen, the TT-electron drift towards the boron. However, the latter is not as great as from N to B in borazine and hence, boroxine could be said to be less aromatic than borazine. 

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. 

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. 

The names borazole, boroxole and borthiole, respectively, for the three compounds in Examples 4, 5 and 6 have been abandoned long ago as they imply five-membered rings in the Hantzsch-Widman system. The names borazin(e), boroxin and borthiin indicate six-membered rings with unsaturation and only one boron atom and one other heteroatom (although the order of the element name stems is wrong) and are also not acceptable. 

The most prevalent boron-oxygen oligomeric systems are based on the six-membered (8—0)3 r ttg> formally a derivative of boroxine, (HBO)3. These compounds are synthesized directly, the tendency to ring formation usually precluding the isolation of any intermediate products. 

Other such system includes boroxine, silicazine, S-triazine, S-tetrazine, phosphazene, hexazine, hexasilabenzene, hexaphosphabenzene etc. Polyacene analogues of some of these inorganic ring compounds have also been stu-died (Fig. 7 and 8). 

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