Boron heterocycles compounds

Similarly, an acetoxyphenyl-subsdtuted boron heterocyclic compound, resulting from a cyanoborate reaction, lost the phenolic acetate group even when bu ered hyc gen peroxide was used. However, MCPBA gave the required ketone retaining the labile ester group (equation 41). ... 

The first chapter covers organometallic complexes of boron, silicon, and phosphorus analogs of azoles. This chapter continues the series by A. P. Sadimenko (Fort Hare, South Africa) in which he is treating comprehensively organometallic complexes of heterocyclic compounds. So far, he has covered, in the volumes of AHC indicated, complexes of the following heterocycles ... 

Thus, existing methods give the possibility of obtaining various heterocyclic compounds containing the P—C—O—B fragment with phosphorus and boron atoms being in a different coordination. 

Tris(dimethylamino)borane similarly fails to 3deld a trivalent boron heterocycle, but gives the spiro-compound (CLXV). A monocyclic system of type (CLXVI) does arise from diethylaminodiphenylborane, and of type (CLXVII) from phenylboronic acid and its analogues 436). 

Another boron heterocycle with a formula apparently appropriate for a nido-pyramidal structure is the compound H4C4B2F2 (193) (Fig. 28). The fluorine substituents in this molecule, however, can 7r-bond to the boron atoms, thereby generating a quinonoid electronic structure. Elsewhere in boron cluster chemistry, the presence of halogen substituents appears to modify the skeletal electron requirements of the cluster. 

Other unsaturated boron heterocycles, such as borazines and borabenzenes, form transition metal complexes with the expected nido geometry, as exemplified by compounds (Et3N3B3Et3)Cr(CO)3 (123) and (CBH5BPh)Mn(CO)s (110) (Fig. 29). 

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. 

Our laboratory conducted the most extensive investigation of the 2,3,1-benzodiazaborines reported to date. We analyzed 25, l,2-dihydro-l-hydroxy-2,3,l-benzodiazaborine (26), and certain derivatives related to 26 by multisolvent H, C, "B, and NMR using isotopically-enriched ( C, N) compounds <97JA7817>. The X-ray crystal structures of 25 and 26 were obtained first, and that of the 2-methyl derivative 39 was determined soon thereafter <98AX(C)71>. The topography (internal geometry, intramolecular associations) of 39 was found to be most similar to 26, but some subtle 25-like characteristics were found. All three boron heterocycles were shown to exist in planar form in protic solution just like they do in the... 

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. 

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... 

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. 

Boron has two isotopes, 10B (20%) and nB (80%), of which the former strongly absorbs neutrons to give a-particles according to equation (21). One aim of the work with boron heterocycles was to find nontoxic compounds, stable in the human body, for use in neutron capture therapy of cancer. It was hoped that boron-nitrogen analogues of benzene or pyridine should have derivatives with these properties, but no such compound has been found. 

A number of patents have appeared on the preparation of biologically active sulfonyl derivatives of 2,3-dihydro-l,2,3-diazaborines. The boron heterocycle is fused to benzene, furan, pyrrole or thiophene e.g. 79GEP2809212). Apparently the compounds have bactericidal as well as fungicidal and trichomonacidal properties. 

For the last four decades, there has been an exponential increase in the synthesis and usage of a number of organic and inorganic boron compounds in industry and academia. Due to the explosive growth of heterocyclic boron-containing compounds, this topic is covered as a separate chapter here. Previously, the subject of this chapter was covered in a subsection of Chapter 4.24 in CHEC-II(1996) (Sections 4.24.1.3.6 and 4.24.1.3.7). This chapter covers as much literature as possible, but the space restraints did not permit a comprehensive coverage of all literature. 

The heterocyclic compounds occupy an important place amongst boron-containing ligands [8], It is well known that borazines, having a six-member ring, form ri6(7i)-complexes, for example 554 [953], It was recently shown that cr(B)-metal-coordinated compounds of type 555 may also be obtained on the basis of this ligand system [954a] ... 

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