Emergent Organoboron Acid Catalysts

Organoboron acids are stable, organic-soluble Lewis acids with potential application as catalysts for a wide variety of chemical reactions. In this review, we summarize the utility of boronic and borinic acids, as well as boric acid, as catalysts for organic transformations. Typically, the catalytic processes exploit the Lewis acidity of trivalent boron, enabling the reversible formation of a covalent bond with oxygen. Our focus is on recent developments in the catalysis of dehydration, carbonyl condensation, acylation, alkylation, and cycloaddition reactions. We conclude that organoboron acids have a highly favorable prospectus as the source of new catalysts.

Facile amidinations of 2-aminophenylboronic acid promoted by boronate ester formation

Amidine synthesis by amine addition to nitriles normally requires high temperatures or harsh catalysts. Here, we report that boronate esters can facilitate amidination of proximal amines with moderate heating. With amidines present in a number of drugs and the synthetic handle provided by the boron, this chemistry should find useful applications.

Selective Extraction and Purification of Azaspiracids from Blue Mussels ( Mytilus edulis) Using Boric Acid Gel

Azaspiracids belong to a family of more than 50 polyether toxins originating from marine dinoflagellates such as Azadinium spinosum. All of the azaspiracids reported thus far contain a 21,22-dihydroxy group. Boric acid gel can bind selectively to compounds containing vic-diols or α-hydroxycarboxylic acids via formation of reversible boronate complexes. Here we report use of the gel to selectively capture and release azaspiracids from extracts of blue mussels. Analysis of the extracts and fractions by liquid chromatography-tandem mass spectrometry (LC-MS) showed that this procedure resulted in an excellent cleanup of the azaspiracids in the extract. Analysis by enzyme-linked immunoasorbent assay (ELISA) and LC-MS indicated that most azaspiracid analogues were recovered in good yield by this procedure. The capacity of boric acid gel for azaspiracids was at least 50 μg/g, making this procedure suitable for use in the early stages of preparative purification of azaspiracids. In addition to its potential for concentration of dilute samples, the extensive cleanup provided by boric acid gel fractionation of azaspiracids in mussel samples almost eliminated matrix effects during subsequent LC-MS and could be expected to reduce matrix effects during ELISA analysis. The method may therefore prove useful for quantitative analysis of azaspiracids as part of monitoring programs. Although LC-MS data showed that okadaic acid analogues also bound to the gel, this was much less efficient than for azaspiracids under the conditions used. The boric acid gel methodology is potentially applicable to other important groups of natural toxins containing diols including ciguatoxins, palytoxins, pectenotoxins, tetrodotoxin, trichothecenes, and toxin glycosides.

Crystallization-induced amide bond formation creates a boron-centered spirocyclic system

The 5-nitrosalicylate ester of 2-acetamidophenylboronic acid (C15H10BN2O6) is formed under crystallization conditions from the 5-nitrosalicylate ester of 2-aminophenylboronic acid. The boron at the center of this structure exists as a tetrahedral complex produced by a dative bond with the amide carbonyl. The perpendicular shape produces an unusual packing structure including a bifurcated hydrogen bond between the amide hydrogen and carbonyl groups on two neighboring molecules. We propose that this reaction occurs due to increased Lewis acidity of the nitrosalicylate ester of 2-aminophenylboronic acid.

6th Heron Island Conference on Reactive Intermediates and Unusual Molecules

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