Boroxine

Boroxine

Names
IUPAC name 2,4,6-trihydroxy-1,3,5,2,4,6-trioxatriborinane
Other names Trihydroxy boroxine, cyclotriboroxane
Identifiers
CAS Number	289-56-5
ChemSpider	119911
Jmol interactive 3D	Image
PubChem	139461
InChI InChI=1S/B3H3O3/c1-4-2-6-3-5-1/h1-3H Key: BRTALTYTFFNPAC-UHFFFAOYSA-N
SMILES O1BOBOB1
Properties
Chemical formula	B3H3O3
Molar mass	83.455 g mol^−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

Boroxine (B₃H₃O₃) 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.^[1] These compounds are solids that are usually in equilibrium with their respective boronic acids at room temperature.^[1][2][3] Beside being used in theoretical studies, boroxine is primarily used in the production of optics.^[4]

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.^[2][5] These compounds are isoelectronic to benzene and, with the vacant p-orbital on the boron atoms, have partial aromatic character with a π-ring system.^[2][6] Boron single-bonds on boroxine compounds are mostly s-character.^[5] Ethyl-substituted boroxine has B-O bond lengths of 1.384 Å and B-C bond lengths of 1.565 Å.^[6] 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.^[6]

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.^[6]

Synthesis

As discovered in the 1930s, boroxines are produced from their corresponding boronic acids by dehydration.^[1][2][3] This dehydration can be done either by a drying agent or by heating under a high vacuum.^[2] A more recent synthesis of trimethylboroxine involves the reaction of carbon monoxide with borane (B₂H₆) and lithium borohydride (LiBH₄) as a catalyst:^[5]

$\rm \ 3 CO + 1.5 B_2H_6 \xrightarrow{LiBH_4} (CH_3BO)_3$

Reactions

One of the most utilized boroxines is trimethylboroxine, which is used in the methylation of various aryl halides through palladium-catalyzed Suzuki-Miyaura coupling reactions:^[7]

$\rm \ C_6H_5X + (CH_3BO)_3 \xrightarrow[dioxane]{K_2CO_3, Pd(PPh_3)_4} C_6H_5CH_3 (X = Br, I)$

Another form of the Suzuki-Miyaura coupling reaction exhibits selectivity to aryl chlorides:^[8]

Boroxines have also been examined as precursors to monomeric oxoborane, HB≡O.^[3] This compound quickly converts back to the cyclic boroxine, even at low temperatures.^[3]

References

1. Brown, H.C. Boranes in Organoc Chemistry; Cornell University Press: Ithaca, 1972; pp. 346–347.
2. Hall, Dennis G. (2005). Boronic Acids – Preparation and Applications in Organic Synthesis and Medicine. John Wiley & Sons ISBN 3-527-30991-8.
3. Westcott, S.A. (2010). "BO Chemistry Comes Full Circle". Angewandte Chemie International Edition 49 (48): 9045–9046. doi:10.1002/anie.201003379.
4. Wu, Q.G., G. Wu, L. Leon Branca, S. Wang (1999). "B3O3Ph3 (7-Azaindole): Structure, Luminescence, and Fluxionality". Organometallics 18 (13): 2552–2556. doi:10.1021/om990053t.
5. 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.
6. Haberecht, M.C.; Bolte, Michael; Wagner, Matthias; Lerner, Hans-Wolfram (2005). "A New Polymorph of Tri(p-tolyl)boroxine". Journal of Chemical Crystallography 35 (9): 657–665. doi:10.1007/s10870-005-3325-y.
7. Gray, M.; Andrews, I.P.; Hook, D.F.; Kitteringham, J.; Voyle, M. (2000). "Practical Methylation of Aryl Halides by Suzuki-Miyaura Coupling". Tetrahedron 41 (32): 6237–6240. doi:10.1016/S0040-4039(00)01038-8.
8. Song, C.; Ma, Y.; Chai, Q.; Ma, C.; Jaing, W.; Andrus, M.B. (2005). "Palladium Catalyzed Suzuki-Miyaura Coupling With Aryl Chlorides Using a Bulky Phenanthryl N-heterocyclic Carbene Ligand". Tetrahedron 61 (31): 7438–7446. doi:10.1016/j.tet.2005.05.071.