Boroxine

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Boroxine
Boroxine.png
Boroxin-3D-balls.png
Names
IUPAC name
1,3,5,2,4,6-Trioxatriborinane
Other names
cyclotriboroxane
Identifiers
3D model (JSmol)
ChemSpider
PubChem CID
  • InChI=1S/B3H3O3/c1-4-2-6-3-5-1/h1-3H
    Key: BRTALTYTFFNPAC-UHFFFAOYSA-N
  • O1BOBOB1
Properties
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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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]

Contents

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. With the vacant p-orbital on the boron atoms, they may possess some aromatic character. [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

The parent boroxine (cyclo-(HBO)3) is prepared in small quantities as a low pressure gas by high temperature reaction of water and elemental boron or reaction of various boranes (B2H6 or B5H9) with O2. It is thermodynamically unstable with respect to disproportionation to diborane and boron oxide. [7] Some reactivity studies and an IR spectrum are reported, but it is otherwise not well characterized. [8]

As discovered in the 1930s, substituted boroxines (cyclo-(RBO)3, R = alkyl or aryl) are generally 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]

Trimethylboroxine can be synthesized by reacting carbon monoxide with diborane (B2H6) and lithium borohydride (LiBH4) as a catalyst (or reaction of borane–tetrahydrofuran or borane–(dimethyl sulfide) in the presence of sodium borohydride): [5] [9]

Reactions

Trimethylboroxine is used in the methylation of various aryl halides through palladium-catalyzed Suzuki-Miyaura coupling reactions: [10]

Another form of the Suzuki-Miyaura coupling reaction exhibits selectivity to aryl chlorides: [11]

Reaction of boroxine Reaction of Boroxine.png
Reaction of boroxine



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]

Related Research Articles

<span class="mw-page-title-main">Boranes</span>

A borane is a compound with the formula BxHy or a related anion. Many such boranes are known. Most common are those with 1 to 12 boron atoms. Although they have few practical applications, the boranes exhibit structures and bonding that differs strongly from the patterns seen in hydrocarbons. Hybrids of boranes and hydrocarbons, the carboranes are also well developed.

<span class="mw-page-title-main">Diborane</span> Chemical compound

Diborane(6), commonly known as diborane, is the chemical compound with the formula B2H6. It is a toxic, colorless, and pyrophoric gas with a repulsively sweet odor. Given its simple formula, borane is a fundamental boron compound. It has attracted wide attention for its electronic structure. Several of its derivatives are useful reagents.

The Suzuki reaction is an organic reaction, classified as a cross-coupling reaction, where the coupling partners are a boronic acid and an organohalide and the catalyst is a palladium(0) complex. It was first published in 1979 by Akira Suzuki, and he shared the 2010 Nobel Prize in Chemistry with Richard F. Heck and Ei-ichi Negishi for their contribution to the discovery and development of palladium-catalyzed cross-couplings in organic synthesis. This reaction is also known as the Suzuki–Miyaura reaction or simply as the Suzuki coupling. It is widely used to synthesize polyolefins, styrenes, and substituted biphenyls. Several reviews have been published describing advancements and the development of the Suzuki reaction. The general scheme for the Suzuki reaction is shown below, where a carbon-carbon single bond is formed by coupling a halide (R1-X) with an organoboron species (R2-BY2) using a palladium catalyst and a base. The organoboron species is usually synthesized by hydroboration or carboboration, allowing for rapid generation of molecular complexity.

<span class="mw-page-title-main">Organoboron chemistry</span> Study of compounds containing a boron-carbon bond

Organoboron chemistry or organoborane chemistry studies organoboron compounds, also called organoboranes. These chemical compounds combine boron and carbon; typically, they are organic derivatives of borane (BH3), as in the trialkyl boranes.

<span class="mw-page-title-main">Borazine</span> Boron compound

Borazine, also known as borazole, is an inorganic compound with the chemical formula B3H6N3. In this cyclic compound, the three BH units and three NH units alternate. The compound is isoelectronic and isostructural with benzene. For this reason borazine is sometimes referred to as “inorganic benzene”. Like benzene, borazine is a colourless liquid with an aromatic odor.

In organic chemistry, hydroboration refers to the addition of a hydrogen-boron bond to certain double and triple bonds involving carbon. This chemical reaction is useful in the organic synthesis of organic compounds.

<span class="mw-page-title-main">Ammonia borane</span> Chemical compound

Ammonia borane, also called borazane, is the chemical compound with the formula H3NBH3. The colourless or white solid is the simplest molecular boron-nitrogen-hydride compound. It has attracted attention as a source of hydrogen fuel, but is otherwise primarily of academic interest.

<span class="mw-page-title-main">Borohydride</span>

Borohydride refers to the anion [BH4]−, which is also called tetrahydridoborate, and its salts. Borohydride or hydroborate is also the term used for compounds containing [BH4−nXn]−, where n is an integer from 0 to 3, for example cyanoborohydride or cyanotrihydroborate [BH3(CN)]− and triethylborohydride or triethylhydroborate [BH(CH2CH3)3]−. Borohydrides find wide use as reducing agents in organic synthesis. The most important borohydrides are lithium borohydride and sodium borohydride, but other salts are well known. Tetrahydroborates are also of academic and industrial interest in inorganic chemistry.

<span class="mw-page-title-main">Boronic acid</span> Organic compound of the form R–B(OH)2

A boronic acid is an organic compound related to boric acid in which one of the three hydroxyl groups is replaced by an alkyl or aryl group. As a compound containing a carbon–boron bond, members of this class thus belong to the larger class of organoboranes.

<span class="mw-page-title-main">Phenylboronic acid</span> Chemical compound

Phenylboronic acid or benzeneboronic acid, abbreviated as PhB(OH)2 where Ph is the phenyl group C6H5-, is a boronic acid containing a phenyl substituent and two hydroxyl groups attached to boron. Phenylboronic acid is a white powder and is commonly used in organic synthesis. Boronic acids are mild Lewis acids which are generally stable and easy to handle, making them important to organic synthesis.

<span class="mw-page-title-main">Borane dimethylsulfide</span> Chemical compound

Borane dimethylsulfide (BMS) is a chemical compound with the chemical formula BH3·S(CH3)2. It is an adduct between borane molecule and dimethyl sulfide molecule. It is a complexed borane reagent that is used for hydroborations and reductions. The advantages of BMS over other borane reagents, such as borane-tetrahydrofuran, are its increased stability and higher solubility. BMS is commercially available at much higher concentrations than its tetrahydrofuran counterpart and does not require sodium borohydride as a stabilizer, which could result in undesired side reactions. In contrast, BH3·THF requires sodium borohydride to inhibit reduction of THF to tributyl borate. BMS is soluble in most aprotic solvents.

Borane, also known as borine, is an unstable and highly reactive molecule with the chemical formula BH
3. The preparation of borane carbonyl, BH3(CO), played an important role in exploring the chemistry of boranes, as it indicated the likely existence of the borane molecule. However, the molecular species BH3 is a very strong Lewis acid. Consequently, it is highly reactive and can only be observed directly as a continuously produced, transitory, product in a flow system or from the reaction of laser ablated atomic boron with hydrogen. It normally dimerizes to diborane in the absence of other chemicals.

<span class="mw-page-title-main">Borinic acid</span> Chemical compound

Borinic acid, also known as boronous acid, is an oxyacid of boron with formula H
2BOH. Borinate is the associated anion of borinic acid with formula H
2BO−
; however, being a Lewis acid, the form in basic solution is H
2B(OH)−
2.

Metal-catalyzed C–H borylation reactions are transition metal catalyzed organic reactions that produce an organoboron compound through functionalization of aliphatic and aromatic C–H bonds and are therefore useful reactions for carbon–hydrogen bond activation. Metal-catalyzed C–H borylation reactions utilize transition metals to directly convert a C–H bond into a C–B bond. This route can be advantageous compared to traditional borylation reactions by making use of cheap and abundant hydrocarbon starting material, limiting prefunctionalized organic compounds, reducing toxic byproducts, and streamlining the synthesis of biologically important molecules. Boronic acids, and boronic esters are common boryl groups incorporated into organic molecules through borylation reactions. Boronic acids are trivalent boron-containing organic compounds that possess one alkyl substituent and two hydroxyl groups. Similarly, boronic esters possess one alkyl substituent and two ester groups. Boronic acids and esters are classified depending on the type of carbon group (R) directly bonded to boron, for example alkyl-, alkenyl-, alkynyl-, and aryl-boronic esters. The most common type of starting materials that incorporate boronic esters into organic compounds for transition metal catalyzed borylation reactions have the general formula (RO)2B-B(OR)2. For example, bis(pinacolato)diboron (B2Pin2), and bis(catecholato)diborane (B2Cat2) are common boron sources of this general formula.

<span class="mw-page-title-main">1,2-Dimethyldiborane</span> Chemical compound

1,2-Dimethyldiborane is an organoboron compound with the formula [(CH3)BH2]2. Structurally, it is related to diborane, but with methyl groups replacing terminal hydrides on each boron. It is the dimer of methylborane, CH3BH2, the simplest alkylborane. 1,2-Dimethyldiborane can exist in a cis- and a trans arrangement. 1,2-Dimethyldiborane is an easily condensed, colorless gas that ignites spontaneously in air.

<span class="mw-page-title-main">Tetramethyldiborane</span> Chemical compound

Dimethylborane, (CH3)2BH is the simplest dialkylborane, consisting of a methyl group substituted for a hydrogen in borane. As for other boranes it normally exists in the form of a dimer called tetramethyldiborane or tetramethylbisborane or TMDB ((CH3)2BH)2. Other combinations of methylation occur on diborane, including monomethyldiborane, trimethyldiborane, 1,2-dimethylborane, 1,1-dimethylborane and trimethylborane. At room temperature the substance is at equilibrium between these forms. The methylboranes were first prepared by H. I. Schlesinger and A. O. Walker in the 1930s.

<span class="mw-page-title-main">Trimethyldiborane</span> Chemical compound

Trimethyldiborane, (CH3)3B2H3 is a molecule containing boron carbon and hydrogen. It is an alkylborane, consisting of three methyl group substituted for a hydrogen in diborane. It can be considered a mixed dimer: (CH3)2BH2BH(CH3) or dimethylborane and methylborane. called 1,2-dimethyldiborane. Other combinations of methylation occur on diborane, including monomethyldiborane, 1,2-dimethyldiborane, tetramethyldiborane, 1,1-dimethylborane and trimethylborane. At room temperature the substance is at equilibrium between these forms, so it is difficult to keep it pure. The methylboranes were first prepared by H. I. Schlesinger and A. O. Walker in the 1930s.

<span class="mw-page-title-main">Methyldiborane</span> Chemical compound

Methyldiborane, CH3B2H5, or monomethyldiborane is the simplest of alkyldiboranes, consisting of a methyl group substituted for a hydrogen in diborane. As with other boranes it exists in the form of a dimer with a twin hydrogen bridge that uses three-center two-electron bonding between the two boron atoms, and can be imagined as methyl borane (CH3BH2) bound to borane (BH3). Other combinations of methylation occur on diborane, including 1,1-dimethylborane, 1,2-dimethyldiborane, trimethyldiborane, tetramethyldiborane, and trimethylborane (which is not a dimer). At room temperature the substance is at equilibrium between these molecules.

<span class="mw-page-title-main">Borane carbonyl</span> Chemical compound

Borane carbonyl is the inorganic compound with the formula H3BCO. This colorless gas is the adduct of borane and carbon monoxide. It is usually prepared by combining borane-ether complexes and CO. The compound is mainly of theoretical and pedagogical interest.

<span class="mw-page-title-main">1,1-Dimethyldiborane</span> Chemical compound

1,1-Dimethyldiborane is the organoboron compound with the formula (CH3)2B(μ-H)2BH2. A pair of related 1,2-dimethyldiboranes are also known. It is a colorless gas that ignites in air.

References

  1. 1 2 3 Brown, H.C. Boranes in Organoc Chemistry; Cornell University Press: Ithaca, 1972; pp. 346–347.
  2. 1 2 3 4 5 Hall, Dennis G. (2005). Boronic Acids – Preparation and Applications in Organic Synthesis and Medicine. John Wiley & Sons ISBN   3-527-30991-8.
  3. 1 2 3 4 Westcott, S.A. (2010). "BO Chemistry Comes Full Circle". Angewandte Chemie International Edition. 49 (48): 9045–9046. doi:10.1002/anie.201003379. PMID   20878818.
  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. 1 2 3 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. 1 2 3 4 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. ↑ Barton, L.; Grimm, F. A.; Porter, Richard F. (1966). "Boroxine: A Simplified Preparation". Inorganic Chemistry. 5 (11): 2076–2078. doi:10.1021/ic50045a062. ISSN   0020-1669.
  8. ↑ Lee, George H.; Porter, Richard F. (1966). "Gaseous Boroxine: Mechanisms of Reactions with Oxygen and Carbon Monoxide". Inorganic Chemistry. 5 (8): 1329–1333. doi:10.1021/ic50042a007. ISSN   0020-1669.
  9. ↑ Rathke, Michael W.; Brown, Herbert C. (1966). "A New Reaction of Diborane with Carbon Monoxide Catalyzed by Sodium Borohydride. A Convenient Synthesis of Trimethylboroxide". Journal of the American Chemical Society. 88 (11): 2606–2607. doi:10.1021/ja00963a054. ISSN   0002-7863.
  10. ↑ 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.
  11. ↑ 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.
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