Aromatic boron compounds

Schleyer, P. v. R. Subramanian, G. Jiao, H. Najafian, K. Hofmann, M. Are boron compounds aromatic An analysis of their magnetic properties. In Advances in Boron Chemistry, Ed. W. Siebert, 3, The Royal Society of Chemistry Cambridge, 1997. 

Oxidative reactions frequently represent a convenient preparative route to synthetic intermediates and end products This chapter includes oxidations of alkanes and cycloalkanes, alkenes and cycloalkenes, dienes, aromatic fluorocarbons, alcohols, phenols, ethers, aldehydes and ketones, carboxylic acids, nitrogen compounds, and organophosphorus, -sulfur, -selenium, -iodine, and -boron compounds... 

Fuel, oxygen, and high temperature are essential for the combustion process. Thus, polyfluorocarbons, phosphazenes, and some composites are flame-resistant because they are not good fuels. Fillers such as alumina trihydrate (ATH) release water when heated and hence reduce the temperature of the combustion process. Compounds such as sodium carbonate, which releases carbon dioxide when heated, shield the reactants from oxygen. Char, formed in some combustion processes, also shields the reactants from a ready source of oxygen and retards the outward diffusion of volatile combustible products. Aromatic polymers, such as PS, tend to char and some phosphorus and boron compounds catalyze char formation aiding in controlling the combustion process. 

Dewar MJS, Rogers H (1962) Pi-complexes. 2. Charge transfer spectra of pi-complexes formed by tetracyanoethylene with polycyclic aromatic hydrocarbons and with heteroaromatic boron compounds. J Am Chem Soc 84 395... 

Char also shields the reactants from oxygen and in addition retards the outward diffusion of volatile combustible products. Aromatic polymers tend to char, and some phosphorus and boron compounds tend to catalyze char formation. 

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. 

Fungicides -m aquaculture [AQUACULTURE CHEMICALS] (Vol 3) -arsenic compds as [ARSENIC COMPOUNDS] (Vol 3) -borates as [BORON COMPOUNDS - BORON OXIDES, BORIC ACID AND BORATES] (Vol 4) -from carbon disulfide [CARBON DISULFIDE] (Vol 5) -containing fluorine [FLUORINE COMPOUNDS, ORGANIC - FLUORINATED AROMATIC COMPOUNDS] (Vol 11) -coordination compounds as [COORDINATION COMPOUNDS] (Vol 7) -copper compounds as [COPPER COMPOUNDS] (Vol 7) -copper phosphates as [PHOSPHORIC ACID AND PHOSPHATES] (Vol 18)... 

Aromatic boron compounds, in zinc halide preparation, 9, 92... 

A ring current model has been shown to correlate proton and boron-11 chemical shifts in a number of pyramidal boron compounds including pentaborane(9) and its derivatives 180>. This model is analogous to the model used to explain proton chemical shifts in benzene and similar aromatic compounds. 

So far, practical applications of 1,3,2-diazaboracycloalkanes appear to be rather limited. However, the recent utilization of an aromatic boron derivative as a tranquilizer 32,33) may have an important bearing on increasing the pharmocodynamic studies of boron compounds for clinical uses in general. It remains to be seen whether or not this event will stimulate a wider interest in 1,2,3-diazaboracycloalkanes. 

The mechanism proposed for aromatic C-H borylation of aromatic compounds 1 by B2pin2 3 catalyzed by the Ir-bpy complex is depicted in Scheme 3 [6-9]. A tris(boryl)Ir (III) species [5, 6, 11] 6 generated by reaction of an Ir(I) complex 5 with 3 is chemically and kinetically suitable to be an intermediate in the catalytic process. Oxidative addition of 1 to 6 yields an Ir(V) species 7 that reductively eliminates an aromatic boron compound 4 to give a bis(boryl)Ir(III) hydride complex 8. Oxidative addition of 3 to 8 can be followed by reductive elimination of HBpin 2 from 9 to regenerate 6. 2 also participates in the catalytic cycle via a sequence of oxidative addition to 8 and reductive elimination of H2 from an 18-electron Ir(V) intermediate 10. Borylation of 1 by 2 may occur after consumption of 3, because the catalytic reaction is a two-step process - fast borylation by 3 then slow borylation by 2 [6],... 

Aryl boron compounds, Ar,B(OH)3 (n = 1-3), bearing electron-withdrawing aromatic groups such as triarylboranes,... 

The aromatic borepins and benzoborepins have been prepared primarily as compounds of theoretical interest and no significant applications have been reported to date. Saturated 1-alkylborepanes, such as 23 and similar large ring boron compounds, are of use primarily as precursors in the synthesis of cyclic ketones by treatment with carbon monoxide followed by alkaline hydrogen peroxide <1982JOC1792>. 

Many aromatic boron compounds are readily oxidized to phenols, as illustrated in equation (11) for the oxidation of aryldihydroxyboranes (arylboronic acids). This is a useful synthetic pathway as arylboronic acids can resist both alkaline permanganate and nitric acid, so that many derivatives are available. ... 

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