Boronic acid anhydrides

The phenyl ether oxygen atoms allow the two borazaphenanthrene rings to pivot with respect to each other, therefore this dimeric boronic acid anhydride can potentially exist in two isomeric forms, either face-to-face or helical (Fig. 18). In the face-to-face form the boron atoms of the bis(borazaphe-nanthrene) moieties have syn-orientation, while they have approximate anti-orientation in the helical form. Compound 68 has been characterized by X-ray crystallography in the helical form [109]. The dimensions of the cavity can be described by the transannular C C contacts between the carbon atoms in 2-position of the phenyl ether units, which have values of 5.12 and 6.21 A. 

Fig. 18. The dimeric boronic acid anhydride 68 can potentially exist in a face-to-face or a helical form. X-ray crystallography proves the helical conformation...

Y. Li, J. Ding, M. Day, Y. Tao, J. Lu, and M. D iorio, Novel stable blue-light-emitting oligofluorene networks immobilized by boronic acid anhydride linkages, Chem. Mater., 15 4936-4943, 2003. 

The conductivity of self-doped PABA without heat treatment was observed to be around 0.96 S/cm. This conductivity value is consistent with the 21 % doping suggested by NMR based on the conductivities of other forms of self-doped polyanilines [110, 113]. However, the conductivity was lower than HCl doped polyaniline, probably due to distortion of the polymer backbone by the presence of the boronic acid substituent [116-118]. After heat treatment at 100 and 500°C, a decrease in conductivity from 0.96 S/cm (without heat treatment) to 0.094 and 0.009 S/cm, respectively was observed. However, in the case of polyaniline, the conductivities of the heat treated polyaniline declined significantly compared with that of the self-doped PABA. The relative decrease in conductivity of heat treated PABA was less than that of HCl-doped polyaniline, probably due to the formation of a thermally stable boronic acid anhydride crosslink. In the case of polyaniline, the dramatic decrease in conductivity was a result of the decomposition of the backbone above 420 °C, as seen in the thermograms. In contrast, the process of crosslinking in the PABA polymer above 100 °C may make... 

Boronic. . . (s. a. Mercuride-boronation) 20, 488 Boronic acid anhydrides s. Boroxines ---- esters 16, 156... 

Without additional reagents Boronic acid anhydrides from chloroborinic acid esters... 

The acylation of ketones with acid anhydrides may be effected by means of the acid reagent boron trifluoride, for example ... 

A mixture of an acid anhydride and a ketone is saturated with boron trifluoride this is followed by treatment with aqueous sodium acetate. The quantity of boron trifluoride absorbed usually amounts to 100 mol per cent, (based on total mola of ketone and anhydride). Catalytic amounts of the reagent do not give satisfactory results. This is in line with the observation that the p diketone is produced in the reaction mixture as the boron difluoride complex, some of which have been isolated. A reasonable mechanism of the reaction postulates the conversion of the anhydride into a carbonium ion, such as (I) the ketone into an enol type of complex, such as (II) followed by condensation of (I) and (II) to yield the boron difluoride complex of the p diketone (III) ... 

Ketone Synthesis. In the Friedel-Crafts ketone synthesis, an acyl group is iatroduced iato the aromatic nucleus by an acylating agent such as an acyl haUde, acid anhydride, ester, or the acid itself. Ketenes, amides, and nittiles also may be used aluminum chloride and boron ttitiuotide are the most common catalysts (see Ketones). 

Boronic acids readily dehydrate at moderate temperatures (or over P4O10 at room temperature) to give trimeric cyclic anhydrides known as trialkyl(aryl)boroxines ... 

A simple method for the synthesis of maleic anhydride derivative 165 starting from 2,5-dimethyl-3-thienylboronic acid and commercially available inexpensive mucobromic acid 168 was reported (04IZV2238) (Scheme 52). The starting boronic acid was prepared in good yield from 2,5-dimethyl-3-iodothiophene. 

The product consists of a mixture of (3,4,5-trifluorophenyl)boronic acid and varying amounts of (3,4,5-trifluorophenyl)boronic anhydride. 

The polyborate oxoanions can be regarded as acid anhydrides, formed by elimination of water following nucleophilic attack of a B-OH oxygen on a trigonal boron center. The polyborate anions contain one or more tetrahedral boron centers and have a negative charge resulting from acceptance of the OH ion that is shared by boron atoms within the polyborate. Polyborate... 

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. 

Preparation of Amorphous Boron from Boric Acid Anhydride. (Each student performs only one of the experiments for preparing amorphous boron.) Put 2 g of boric acid anhydride into an iron crucible or onto an iron plate and roast it if the boric acid anhydride contains moisture, then when it is heated with magnesium an explosion may occur ). Triturate the boric acid anhydride as rapidly as possible (why ) in 4 mortar, weigh 1 g and mix it with 2 g of a magnesium powder. Place the mixture into a refractory test tube and fasten it in an inclined position in a clamp of a stand in a fume cupboard . First carefully heat the test tube with the mixture with the flame of a burner, and then strongly heat the bottom part of the tube. What do you observe ... 

To prepare boron trifluoride, mix 50 g of potassium fluoborate and 9 g of boric acid anhydride in flask 3, and pour 25 ml of anhydrous sulphuric acid into the mixture. Prepare the anhydrous sulphuric acid by adding the relevant amount of 65 % oleum to a 96 % sulphuric acid solution carefully, wear eye protection ). 

The procedure described4 is a modification of the method of Khotinsky and Melamed,8 who first reported the preparation of boronic acids from Grignard reagents and borate esters. Benzeneboronic acid and the corresponding anhydride also have been prepared by reaction of phenylmugnesium bromide with boron... 

Triflates of boron, aluminum, and gallium were found to be efficient catalysts in Friedel-Crafts acylations.46 However, these are water-sensitive materials and were required to be used in equimolar quantities to be effective. More recently various water-tolerant and recyclable triflate salts, which were also tested in alkylation, were found to exhibit similar good characteristics in Friedel-Crafts acylations. Although benzene cannot be acylated, Sc triflate,47 48 lanthanum triflates,48-51 and Hf triflate52 usually give high yields of aryl ketones in acylation with acid anhydrides. In many cases, Li perchlorate was found to accelerate the reactions.48 52... 

Boroxines result from the condensation of boronic acids. RBfOH). The cyclic Irimeric anhydride of methylboronic acid is (IVfeBO),. Give a balanced equation for the reaction of (MeBO)j with water (See Brown, H. C. Cole, T. E. Organometallics 1985.4. 816.)... 

Boron oxide is an acid anhydride. Aluminum shows some... 

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