en>Maxlittle2007: URL article and code link fixes

2011-07-06T19:41:21Z

URL article and code link fixes

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{{chembox
| Verifiedfields = changed
| verifiedrevid =
| ImageFile1 = Boroxine.png
| ImageSize1 = 250px
| ImageFile2 =
| ImageSize2 =
| IUPACName = 2,4,6-trihydroxy-1,3,5,2,4,6-trioxatriborinane
| OtherNames = Trihydroxy boroxine, cyclotriboroxane
| Section1 = {{Chembox Identifiers
| CASNo_Ref = {{cascite|changed|??}}
| CASNo = 289-56-5
| ChemSpiderID= 119911
| PubChem = 139461
| InChI=1S/B3H3O3/c1-4-2-6-3-5-1/h1-3H
| InChIKey= BRTALTYTFFNPAC-UHFFFAOYSA-N
| SMILES = O1BOBOB1
}}
| Section2 = {{Chembox Properties
| Formula = B3H3O3
| MolarMass = 83.455 g mol−1
| Appearance =
| Density =
| MeltingPt =
| BoilingPt =
| Solubility =
}}
}}
'''Boroxine''' ([[Boron|B]]3[[Hydrogen|H]]3[[Oxygen|O]]3) is a 6-membered, [[heterocyclic compound]] composed of alternating [[oxygen]] and singly-hydrated [[boron]] atoms. Boroxine derivatives (boronic [[anhydride]]s) such as trimethylboroxine and triphenylboroxine also make up a broader class of compounds called boroxines.<ref name="Brown">Brown, H.C. ''Boranes in Organoc Chemistry''; Cornell University Press: Ithaca, 1972; pp. 346–347.</ref> These compounds are solids that are usually in equilibrium with their respective [[boronic acid]]s at room temperature.<ref name="Brown"/><ref name="Hall">Hall, Dennis G. (2005). [http://www.knovel.com/web/portal/browse/display?_EXT_KNOVEL_DISPLAY_bookid=1450&Vertical%20IID=0 Boronic Acids – Preparation and Applications in Organic Synthesis and Medicine]. John Wiley & Sons ISBN 3-527-30991-8.</ref><ref name="Westcott">{{cite journal|author=Westcott, S.A.|title=BO Chemistry Comes Full Circle|journal=Angewandte Chemie, International Edition|year=2010|volume= 49|issue=48|pages=9045–9046|doi= 10.1002/anie.201003379}}</ref> Beside being used in theoretical studies, boroxine is primarily used in the production of optics.<ref name="Wu">{{cite journal|author=Wu, Q.G., G. Wu, L. Leon Branca, S. Wang|title=B3O3Ph3 (7-Azaindole): Structure, Luminescence, and Fluxionality|journal=Organometallics|volume= 18|year= 1999|pages=2552–2556|doi=10.1021/om990053t|issue=13 }}</ref>

==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.<ref name="Hall" /><ref name="Onak">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.</ref> 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.<ref name="Hall" /><ref name="Haberecht">{{cite journal|author=Haberecht, M.C. |title=A New Polymorph of Tri(''p''-tolyl)boroxine|journal=Journal of Chemical Crystallography|year=2005|volume=35|pages= 657–665|doi=10.1007/s10870-005-3325-y|last2=Bolte|first2=Michael|last3=Wagner|first3=Matthias|last4=Lerner|first4=Hans-Wolfram|issue=9}}</ref> Boron single-bonds on boroxine compounds are mostly s-character.<ref name="Onak" /> Ethyl-substituted boroxine has B-O bond lengths of 1.384 Å and B-C bond lengths of 1.565 Å.<ref name="Haberecht" /> 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.<ref name="Haberecht" />

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.<ref name="Haberecht" />

==Synthesis==
As discovered in the 1930s, boroxines are produced from their corresponding boronic acids by dehydration.<ref name="Brown" /><ref name="Hall" /><ref name="Westcott" /> This dehydration can be done either by a drying agent or by heating under a high vacuum.<ref name="Hall" /> A more recent synthesis of trimethylboroxine involves the reaction of [[carbon monoxide]] with [[borane]] (B2H6) and [[lithium borohydride]] (LiBH4) as a catalyst:<ref name="Onak" />

:<math>\rm \ 3 CO + 1.5 B_2H_6 \xrightarrow{LiBH_4} (CH_3BO)_3</math>

==Reactions==
One of the most utilized boroxines is trimethylboroxine, which is used in the methylation of various aryl halides through [[palladium]]-catalyzed [[Suzuki reaction|Suzuki-Miyaura coupling]] reactions:<ref name="Gray">{{cite journal|author=Gray, M.; Andrews, I.P.; Hook, D.F.; Kitteringham, J.; Voyle, M. |title=Practical Methylation of Aryl Halides by Suzuki-Miyaura Coupling|journal=Tetrahedron|year=2000|volume=41|pages= 6237–6240|doi=10.1016/S0040-4039(00)01038-8|issue=32}}</ref>

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

Another form of the Suzuki-Miyaura coupling reaction exhibits selectivity to aryl chlorides:<ref name="Song">{{cite journal|author=Song, C.; Ma, Y.; Chai, Q.; Ma, C.; Jaing, W.; Andrus, M.B. |title=Palladium Catalyzed Suzuki-Miyaura Coupling With Aryl Chlorides Using a Bulky Phenanthryl N-heterocyclic Carbene Ligand|journal=Tetrahedron|year=2005|volume= 61|pages= 7438–7446|doi=10.1016/j.tet.2005.05.071|issue=31}}</ref>

:[[File:Reaction of Boroxine.png|left|Reaction of Boroxine|600px]]{{clear-left}}

Boroxines have also been examined as precursors to monomeric oxoborane, HB≡O.<ref name="Westcott" /> This compound quickly converts back to the cyclic boroxine, even at low temperatures.<ref name="Westcott" />

==References==
{{reflist}}

[[Category:Boron compounds]]

Recurrence period density entropy - Revision history

en>Maxlittle2007: URL article and code link fixes