Boron-oxygen rings

6 BORON-CHALCOGEN RINGS 9.6.1 Boron-Oxygen Rings 

Cyclotriboroxanes (RBO)3 (also known as boroxines) are formally anhydrides of the corresponding boronic acids RB(OH)2 only mild heating is required to convert these dibasic acids into a wide range of cyclic trimeric anhydrides [eqn (9.18)]. This dehydration process can be adapted for the synthesis of B3O3 rings with different aryl groups on the boron atoms by using two or more arylboronic acids in the appropriate stoichiometric ratio. 

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Supplement to Mellor s Comprehensive Treatise on Inorganic and Theoretical Chemistry , Vol. V, Boron Part A, Boron-Oxygen Compounds , Longman, London, 1980, 825 pp. See also J. R. Bowser and T. P. Fehlner, Chap. 1 in H. W. Roesky (ed.). Rings, Clusters and Polymers of Main Group and Transition Elements, Elsevier, Amsterdam, 1989, pp. 1-48. 

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... 

Dewar185-187 reported the first six-membered aromatic rings containing boron, oxygen, and nitrogen heteroatoms, and later Gronowitz188 prepared similar systems annelated with thiophene. The chemical reactivity and NMR data indicate electronic delocalization. 

Oxygen-containing rings, boron-containing ring complexes, 3,44... 

Phosphorus—phosphorus bonds, addition, to alkynes, 10, 751 Phosphorus rings, chromium carbonyl link arsenic rings, 5, 226 As, Sb, Se, Te, Sn rings, 5, 226 boron rings, 5, 222 carbon rings, 5, 224 characteristics, 5, 221 oxygen rings, 5, 227... 

Ring Systems Containing Boron, Oxygen, and Nitrogen Atoms 646... 

Table 3 The boron-oxygen bond distances (A) of five-membered ring internai esters of boronic acids, which have been characterized by X-ray crystaiiographic studies...

As variously mentioned, considerable parts of the subject described in this chapter, especially concerning boron-oxygen compounds, have been treated in the literature before 1980. However, as some important aspects, particularly those concerning syntheses, were not considered in CHEC-I, synthetic principles, describing the route to the various types of rings, will be taken into consideration. 

Boron—sulphur ring compounds are not as commonly occurring as those of oxygen. The chemical shifts of a four- and six-membered ring compound of the type (R2NBS) have been reported by Forstner and Muetterties. The two compounds and their shifts [p.p.m. from (MeO)3B] are shown in (28) and (29). Rather interestingly, the ethyl-thioborane EtSBH2 exists as a trimer and has a shift of -1-14 5 (p.p.m. 

There is an extensive and complex structural chemistry of boron-oxygen anionic species (polyborates), in aqueous or nonaqueous solution and in the melt or solid state, Six-membered ring formation dominates, but the structural chemistry of the species is complicated since boron exists in either 3- or 4-coordinate environments, or various combinations of these. Thus most polymeric species consist of (6—0)3 rings joined by boron atoms linked to an intervening oxygen atom, or y rings sharing a common boron atom. 

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. 

Ab initio and DFT calculations on l,2-dihydro-l,2-oxaborine suggested that this boron-oxygen heterocycle has considerable aromatic stabilization <2001MC43>. To test this hypothesis experimentally, a new synthesis of 1,2-dihydro-2-phenyl-l,2-oxaborine has been developed. Preliminary investigation of the chemistry of this compound suggests that the l,2-dihydro-l,2-oxaborine ring is aromatic, as had been predicted by calculations <20070M1563>. 

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