Boroxine

Boroxine () is a 6-membered heterocyclic compound composed of alternating oxygen and singly-hydrogenated boron atoms. Boroxine derivatives (boronic anhydrides) such as trimethylboroxine and triphenylboroxine also make up a broader class of compounds called boroxines.Brown, H.C. ''Boranes in Organoc Chemistry''; Cornell University Press: Ithaca, 1972; pp. 346–347. These compounds are solids that are usually in equilibrium with their respective boronic acids at room temperature.Hall, Dennis G. (2005)
Boronic Acids – Preparation and Applications in Organic Synthesis and Medicine
John Wiley & Sons . Beside being used in theoretical studies, boroxine is primarily used in the production of optics.

Structure and bonding

Three-coordinate compounds of boron typically exhibit trigonal planar geometry, therefore the boroxine ring is locked in a planar geometry as well.Onak, T. in ''Organoborane Chemistry''; Maitles, P.M., Stone, F.G.A., West, R., Eds.; Academic Press: New York, 1975; pp. 2,4,16,44. These compounds are isoelectronic to benzene. With the vacant p-orbital on the boron atoms, they may possess some aromatic character. Boron single-bonds on boroxine compounds are mostly s-character. Ethyl-substituted boroxine has B-O bond lengths of 1.384 Å and B-C bond lengths of 1.565 Å. Phenyl-substituted boroxine has similar bond lengths of 1.386 Å and 1.546 Å respectively, showing that the substituent has little effect on the boroxine ring size.

Substitutions onto a boroxine ring determine its crystal structure. Alkyl-substituted boroxines have the simplest crystal structure. These molecules stack on top of each other, aligning an oxygen atom from one molecule with a boron atom in another, leaving each boron atom between two other oxygen atoms. This forms a tube out of the individual boroxine rings. The intermolecular B-O distance of ethyl-substituted boroxine is 3.462 Å, which is much longer than the B-O bond distance of 1.384 Å. The crystal structure of phenyl-substituted boroxine is more complex. The interaction between the vacant p-orbitals in the boron atoms and the π-electrons in the aromatic, phenyl-substituents cause a different crystal structure. The boroxine ring of one molecule is stacked between two phenyl rings of other molecules. This arrangement allows the phenyl-substituents to donate π-electron density to the vacant boron p-orbitals.

Synthesis

As discovered in the 1930s, boroxines are produced from their corresponding boronic acids by dehydration. This dehydration can be done either by a drying agent or by heating under a high vacuum. A more recent synthesis of trimethylboroxine involves the reaction of carbon monoxide with borane (B₂H₆) and lithium borohydride (LiBH₄) as a catalyst:

:$\ce + 1.5\ \overbrace\ce^\ce\ \ce$

Reactions

Trimethylboroxine is used in the methylation of various aryl halides through palladium-catalyzed Suzuki-Miyaura coupling reactions:

:\overset + (CH3BO)3 -> ce\ce] C6H5CH3

Another form of the Suzuki-Miyaura coupling reaction exhibits selectivity to aryl chlorides:

:

Boroxines have also been examined as precursors to monomeric oxoborane, HB≡O. This compound quickly converts back to the cyclic boroxine, even at low temperatures.

References

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Boron heterocycles
Inorganic compounds
Six-membered rings