Boronic acids, RB

Boronic acids RB(OH)2 were first made over a century ago by the unlikely route of slow partial oxidation of the spontaneously flammable trialkyl boranes followed by hydrolysis of the ester so formed (E. Frankland, 1862) ... 

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. 

RELATIVE RATE COEFFICIENTS FOR THE SUBSTITUTION OF ESTERS OF BORONIC ACIDS, RB(OH)2, BY MERCURIC CHLORIDE10 11... 

Addition of a wide range of boronic acids [RB(OH)2] or esters [RB(OR )2] to the pinanediols gives the very stable pinanediol boronic esters. For example, propylboronic acid (available from the addition of propylmagnesium bromide to trimethyl borate followed by acid hydrolysis) and the (s) pinanediol combine to give a homochiral boronic ester as shown in Equation B6.3. 

Another, mechanistically closely related reaction is the cross-coupling reaction of aryl, benzyl, vinyl, or allyl halides with organometallic coupling reactions, such as Grignard reagents or, preferably, less reducing organometallics such as tin compounds or boronic acids RB(OH)2 the latter allow the reaction to be conducted even in aqueous media.83 The principal reaction steps are ... 

Boronic acids RB(OH)2, boronic esters RB(OR)2, and triorganoboranes R3B have been studied extensively for the selective recognition of anions. " Moreover, arylboronic acids have proven highly useful for sensing of saccharides based on the facile formation of esters with diols, thus providing an excellent alternative to more commonly employed synthetic molecular receptors based on hydrogenbonding interactions. Absorption and fluorescence spectroscopy, circular dichroism, and cyclic voltammetry have all been successfiilly used as detection methods for organoboron sensors. 

Boroxincs result from the condensation of boronic acids, RB(OH) . The cyclic Irimeric anhydride of methylboronic acid is (MeBO).v Give a balanced equation for the reaction of (MeBOlj with water. (See Brown, H. C. Cole, T. E. Oriftiiiometollic.i 1985, 4, 816.)... 

A different route employs organoborohydrides RBH2 or LiRBH3,31 obtained from the reaction of the corresponding boronic acids RB(OH)2 with LiAlH4,32 in the reaction with pyrazolate ion and excess pyrazole (Equation (3)).33... 

Physically safer as a mechanistic criterion is reversible inhibition by boronic acids, RB(0H)2, which add the active site serine to form tetrahedral species [RB(0H)20Ser] , which mimic the tetrahedral intermediate/transition state. They also mimic the tetrahedral intermediates in aspartic protease action, however, and are therefore not as definitive. 

There are also the so-called boronic and boronous acids, RB(OH)2 and R2B(OH), and their esters and anhydrides. The boronic acids can be made in various ways, for example ... 

Ketalization, however, requires acidic catalysis, with its potential for damage to some substrates. Boronic acids [RB(OH)2] have accordingly been advanced [205] as diol derivatization agents capable of functioning under neutral conditions. The... 

Methane (R = CH3), butane (R = C4H9), or benzene (R = CeHs), boronic acids, RB(OH)2 2-Methyl(butyl, phenylFl,3,2-dioxaborolanes (methyl, butyl, or phenyl boronates)... 

Heterocyclic boron chemistry is very rich. The boronic acids RB(OH)2 dehydrate to cyclic trimers, boroxines, and aminoboranes R2BNR2 also form cyclic trimers. The phosphorus and sulfur analogs of these cyclic structures are also known. 

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