Aromatic boron heterocycles

This contribution first surveys some of the attractive properties of boron, briefly describing applications that have been developed mostly with non-aromatic boron-containing compounds. It then examines many of the stable, formally aromatic boron heterocycles that have been reported to dale, covering much of the pertinent literature through the end of 1999. With the sum of these two parts, I hope the reader will gain an appreciation of the untapped potential held by boron heterocycles, especially for constructing new bioactive agents. 

G. E. Herberich, G. Greis, and H. F. Heil, Novel Aromatic Boron Heterocycle as Ligand in a Transition Metal rc-Complex, Angew. Chem. Int. Ed. Engl. 9, 805-806 (1970). 

Non-aromatic Boron Heterocyclics.—The range of cyclic dialkoxyboranes derived from monosaccharides has been extended to cover deoxyaldohexoses [by treatment of the sugar with ethyl(dimethoxy)borane] and derivatives of glucose (by a novel reduction process). If two dioxaborolane units are appropriately disposed in space, exchange of alkoxy groups between boron atoms is possible, as for example in the equilibration of compounds (17) and (18). ... 

Pseudo-aromatic Boron Heterocycles.—mass-spectrometric study of isotopic-ally labelled arylborazines has revealed a tendency to produce doubly charged ions. The compounds (24) and (25) are new boron heterocyclic systems. Boroxaphenanthrenes such as (26) are useful as antioxidants for lubricating oil.i ... 

The electronic similarity suggests that unsaturated boron heterocycles and aromatic systems have 7r-electron transitions in common. In fact, such similarities are abundant, and are one main reason for the statement that certain boron heterocycles possess aromatic properties. The UV spectrum of 5,6-dihydrodibenz[c,e][l,2]azaborine (15) is reminiscent... 

Mass spectral data have frequently been used in the structural determination of boron heterocycles. One paper has been devoted to the mass spectra of some six-membered boron-nitrogen systems. It was concluded that the spectra could be interpreted analogously to their hydrocarbon counterparts. In all cases the molecular peak was the base peak of the spectrum (68T6755). Doubly charged molecular ions, a feature typical of aromatic compounds, are often encountered. It should be noted, however, that some certainly non-aromatic aminoboranes give such doubly charged ions as well. 

Ferrocene, bis(cyclopentadienide)iron, was the first transition metal complex with aromatic ligands, and its discovery induced extensive research on complexes of different transition metals and different aromatic ligands. It is therefore not surprising that borinate ion complexes of this type are known. Some complexes with five-membered heterocycles were mentioned in Section 1.21.7. In this section borinate complexes are considered in greater detail because of their formal relationship to benzene. An extensive review on transition metal complexes with boron heterocycles has recently been published (80MI12100). 

Boron heterocycles bearing a hydroxy group on the boron atom are acidic. Boric, boronic and borinic acids are Lewis acids towards hydroxide ion. It was suggested that six-membered 5-hydroxy compounds behaved as Brdnsted acids. The experimental evidence for this was the similarity between the UV spectra of the hydroxy compounds in neutral and basic solution. The rationale for this fact was the maintained aromaticity which was supposed to favour the formation of, for example, ion (163) over ion (164). This suggestion was at first supported by nB NMR spectroscopy, which differentiates between anions of Lewis-acidic... 

The seven-membered fully unsaturated boron heterocycle is named l//-borepin or, more frequently, borepin. It has six 7r-electrons and is therefore a potential Hiickel aromatic, isoelectronic with the tropylium ion. This similarity has induced several groups to search for a synthesis. Recent theoretical calculations have predicted the -stabilization to be less than that of the tropylium ion. On the other hand, borepins are organoboranes and therefore expected to be sensitive to oxygen. 

The aromatic character of the unsaturated boron heterocycles has often been discussed. One must bear in mind that there are many properties indicative of aromatic stabilization and that many of the observed parameters can be explained by other factors as well. Statements concerning aromaticity must therefore not be based on the observation of only one property. 

One chemical property quoted in support of aromatic character is kinetic stability towards hydrolytic break-down. Many boron heterocycles have been stated to be more stable than expected for organoboranes. On the other hand, several saturated cyclic boron compounds are stable as well, suggesting that the cyclic structure itself is favorable. 

Although some points remain unclear, it is apparent from this summary that many unsaturated boron heterocycles have properties indicative of aromatic character. 

B-Aminoborazines are of particular interest for fundamental studies. In these compounds, boron is bonded to three nitrogen atoms with two different types of environment. B-Aminoborazines are also useful precursors for the synthesis of thermally stable polymers. Quite a few polycondensates of aminoborazines and copolymerisates with organic difunctional molecules have been described 4>. Of major interest are difunctional borazines yielding linear polycondensates. The condensation of l,3,5-tris(2,6-dimethylphenyl)-2,4-dichloroborazine (cf. Section II.2.5) with aliphatic, aromatic, and heterocyclic diamines, as well as the preparation of the same linear polyborazines by transamination of 1,3,5-tris(2,6-dimethylphenyl)2,4-bis(diethyl-amino)borazine with diamines was studied 139). 

Of all of the elements of the Periodic Table, only neighboring carbon and boron share the properties of self-bonding (catenation) and the support of electron-delocalized structures based upon these catenated frameworks. Carbon catenation, of course, leads to the immense field of organic chemistry. Boron catenation provides the nido-, arachno-, and /i p/io-boranes, which may be considered as the borane equivalents of aliphatic hydrocarbons, and the discrete families of c/oso-borane derivatives which bear a for nal resemblance to the aromatic hydrocarbons, heterocycles, and metallocenes. Aside from these analogies, boron and carbon chemistries are also important to each other through their extravagant ability to mix m ways not available to other element-pairs. Thus, the conflux of boron and carbon chemistries effectively provides an element-pair for exploitation in a variety of novel ways. 

Asymmetric hydrogenation of bromo-substituted aromatic a-enamides 14 affords the corresponding bromo-amino acid derivatives 15, which subsequently is subjected to Pd-catalyzed cross-coupling with aryl and vinyl boronic acids. In addition to diverse phenylalanine derivatives 16, a broad array of other novel aromatic and heterocyclic amino acids have been produced rapidly from a small number of bromo-functionalized intermediates [24], This same two-step process may be applied to the production of many other classes of aromatic and heterocyclic chiral building blocks, such as arylalkylamines, amino alcohols, diamines, and directly on peptides as well. 

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