Boron ring systems

B. Polymers Containing Boron Ring Systems in the Backbone... 

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... 

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.)...

The area of organoboron polymers containing borazine and its derivatives is covered in Chapter 5 of this book by Miele and co-workers. Miele and Bernard also describe the utilization of these polymers in ceramics, fibers, and so on, in Chapter 3 of this book. In this section, the utilization of polymers containing borazine or in some cases the bicyclic boron ligand, 9-BBN, for the production of SiC or Si/C/B fibers is briefly described. Recent advances in polypyrazolylborate or pyrazabole-containing polymers and other boron ring system-derived polymers also have been briefly described. 

Organic ring systems are named by replacement nomenclature. Three- to ten-membered mono-cyclic ring systems containing uncharged boron atoms may be named by the specialist nomenclature for heterocyclic systems. Organic derivatives are named as outlined for substitutive nomenclature. The complexity of boron nomenclature precludes additional details the text by Rigaudy and Klesney should be consulted. 

Oxiranes exhibit 1,3 [e,n] capacity. Therefore, seven-membered ring systems can be synthesized on reaction with hetero-1,3-dienes. The reaction is catalyzed by 4-dimethylaminopyridine. On catalysis with boron trifluonde, the regioche-mistry is reversed [263] (equation 58). 

One of the first reports related to the preparation of macrocyclic ring systems with boron atoms deals with the transformation of m-difunctionalized borazines into cyclic di-, tetra-, or hexaborazines [16-19]. 

A further example of a macrocyclic ring system with boron that is related to the structures of the metallocyclophanes mentioned before is compound 68 that can be prepared via condensation between two bis(borazaphenanthrene) molecules. 

In compound 91 the diborahexasiloxane ring system 89 can be identified, which has been expanded to a three-dimensional structure by an additional (R2Si0)20 fragment bridging now, in contrast to compound 90, two boron atoms [127], This cage can be obtained in yields of 45% from tetraphenyldisiloxanediol and boric acid when reacted in a 6 1 stoichiometry. The molecule contains a... 

These examples and those in Scheme 2.6 illustrate the key variables that determine the stereochemical outcome of aldol addition reactions using chiral auxiliaries. The first element that has to be taken into account is the configuration of the ring system that is used to establish steric differentiation. Then the nature of the TS, whether it is acyclic, cyclic, or chelated must be considered. Generally for boron enolates, reaction proceeds through a cyclic but nonchelated TS. With boron enolates, excess Lewis acid can favor an acyclic TS by coordination with the carbonyl electrophile. Titanium enolates appear to be somewhat variable but can be shifted to chelated TSs by use of excess reagent and by auxiliaries such as oxazolidine-2-thiones that enhance the tendency to chelation. Ultimately, all of the factors play a role in determining which TS is favored. 

Entry 8 was used to create the central nine-membered ring system found in the diterpene jatrophatrione. Entry 9 is an example of a boronate fragmentation (see p. 899). Entry 10 illustrates enolate fragmentation. The reaction presumably proceeds... 

Turner, H. S., and R. J. Warne A New Boron-Nitrogen Ring System. The Tetrameric Borazynes, Boron-Nitrogen Chemistry. Advances in Chemistry, Series 42. Amer. Chem. Soc. p. 290 (1964). 

Lappert, M. F., and M. K. Majumdar The Three-coordinate Boron-Nitrogen Four-membered Ring System (l,3-Diaza-2,4-boretane). Proc. chem. Soc. [London] 1963, 88. 

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.)...

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... 

Three types of compounds have been synthesized in this class [1.1.6] bicyclics, [1.1.3] bicyclics, and [1.1.1] bicyclics which used boron as part of the ring system. Compound 235 was synthesized in high yield and the partially (compound 236) and fully (compound 237) oxidized products produced via the action of DIAD and MCPBA, respectively <2005CC5396>. Compound 238 was formed slowly over 15 h from compound 239. This then gradually transformed into the thermodynamically more stable compound 240 (Scheme 22) <2002ZFA1903>. 

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