Boronic acid dehydration

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

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. 

Mercurioferrocenes (290) and (291) are separable by Soxhlet extraction and these may be used to prepare bromoferrocene or dibromoferrocene or the iodo analogs. Alternatively, these halogenated ferrocenes are prepared from the ferrocene boronic acids or from anions (322) or (323). Bromoferrocene is the starting material for cyanoferrocene, azidoferrocene, or aminoferrocenes, generally in the presence of a copper salt. Amtnoferrocene can be acylated to produce an amide, or converted to isocyanoferrocene by a formylation/dehydration sequence (Scheme 96). The 1,T-diisocyanoferrocene is available from the bis(acyl)azide, itself derived from ferrocene dicarboxylic acid. ... 

Significant features of these active borinic acid catalysts are that they are strong Lewis acids and have a hydroxy group on the boron atom. Dehydration is strongly favored in THF. The reaction usually proceeds smoothly, and a,/3-enones are obtained in high yields as ( ) isomers. In reactions of a-substituted-/3-hydroxy carbonyl compounds, a,/3-enones are preferentially obtained from anti aldols, whereas most syn aldols are recovered. This dehydration thus represents a useful and convenient method for isolating pure syn aldols from syn anti isomeric mixtures (Eq. 106). 

Fig. 6 Synthesis of aromatic boronic acid flame retardants, showing the dehydration process that can take place in formulated products when subjected to heat.

Li et al. studied the thermal dehydration of boronic acids (Fig. 9.23(a), [54]). Crosslinking considerably improved the photoluminescence stability. Multilayer capabilities were demonstrated. 

Organo derivatives of boroxine are much more stable than boroxine or its halo derivatives. They are formally anydrides of boronic acids, and most are prepared nearly quantitatively by dehydration of the corresponding acid ... 

It is of interest to note in passing that In more recent work (ref.72), use has been made in synthesis of the transformation of chalcones to isoflavones with thallium(lll) nitrate. Thus, 6-acetyl-2,2-dimethyl-7-hydroxy-5-methoxychromanone was converted to the chalcone with 2,4-dibenzyloxybenzaldehyde. The O-acetyl derivative by treatment with the thallium reagent followed by acidic cyclisation gave a bischromanone structure. Selective reduction of the least hindered carbonyl group in the bischromanone, acidic dehydration of the resultant alcohol and final debenzylation with boron trichloride gave the linear isofiavone. 

Carbonylation of cw, ra j-perhydro-9b-boraphenalene (3b) in the presence of (CH20H)2 results in the formation of essentially a single product identified as an ethylene glycol ester of the boronic acid (56) (90%). Oxidation of this ester produces c ,c ,tranj-perhydro-9b-phenalenol (55) (70%) <67JA5478, 83OS(61)i03>. a//-tra j-Perhydrophenalene (58) was prepared via dehydration-hydrogenation of (55) via mixed alkenes (57) (Scheme 20). 

The higher homologs of methylboroxine cannotbepreparedfrom B2O3 and the corresponding trialkylborons, because the latter decompose at elevated temperatures. Dehydration of the alkyl-boronic acids is preferable. 

In 2012, Sharma and coworkers reported the direct arylation of tautom-erizable heterocycles with azoles under microwave conditions. This Pd/Cu catalyzed dehydrative phosphonium coupling via C—OH and C—H bond activation elegandy produced the diheteroaryl compounds via direct arylation of the tautomerizable heterocycle with boronic acids using PyBroP in the presence of Pd(OAc)2 and Cul catalysts (120L1854). 

Figure 3.17 Emeraldine salt form of PABA (A), self-doped in the presence of fluoride. Proposed cross-link (B), resulting from an interchain dehydration reaction between a boronic acid-imine and a boronic acid moiety, hence maintaining a self-doped state. (Reprinted with permission from Chemistry of Materials, 17, 3803. Copyright (2005) American Chemical Society.)...

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