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Borate esters
Borate esters
Borate esters
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In organic chemistry, borate esters are organoboron compounds which are conveniently prepared by the stoichiometric condensation reaction of boric acid with alcohols. There are two main classes of borate esters: orthoborates, B(OR)3 and metaborates, B3O3(OR)3. Metaborates contain 6-membered boroxine rings.

A dehydrating agent, such as concentrated sulfuric acid is typically added.[1] Borate esters are volatile and can be purified by distillation. This procedure is used for analysis of trace amounts of borate and for analysis of boron in steel.[2] Like all boron compounds, alkyl borates burn with a characteristic green flame. This property is used to determine the presence of boron in qualitative analysis.[3]

Trimethyl borate is a popular borate ester used in organic synthesis.

Borate esters form spontaneously when treated with diols such as sugars and the reaction with mannitol forms the basis of a titrimetric analytical method for boric acid.

Metaborate esters show considerable Lewis acidity and can initiate epoxide polymerization reactions.[4] The Lewis acidity of orthoborate esters, as determined by the Gutmann-Beckett method, is relatively low.

Trimethyl borate, B(OCH3)3, is used as a precursor to boronic esters for Suzuki couplings:[5] Unsymmetrical borate esters are prepared from alkylation of trimethyl borate:[6]

These esters hydrolyze to boronic acids, which are used in Suzuki couplings.

References

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Borate esters

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Borate esters are a class of organoboron compounds derived from the condensation of , B(OH)3, with alcohols, featuring a central atom bonded to three alkoxy groups in the general formula B(OR)3, where R denotes an alkyl, aryl, or other organic . These compounds adopt a trigonal planar at the center owing to its sp2 hybridization and an empty p-orbital, which imparts Lewis acidity and enables coordination with nucleophiles such as diols or amines. Borate esters are commonly synthesized through acid- or base-catalyzed dehydration of and the parent alcohol, often under to remove water, yielding colorless, volatile liquids or low-melting solids that are typically flammable and exhibit short B–O bond lengths (1.31–1.38 ) indicative of partial double-bond character. Many are hydrolytically labile in aqueous media but can be stabilized through steric hindrance or electron-withdrawing substituents on the alkoxy groups, enhancing their utility in non-aqueous environments; they are generally non-toxic to humans at low concentrations, comparable to itself. Variations in physical properties, such as boiling points and viscosities, depend on the R groups, with simple alkyl derivatives like triethyl borate boiling at 118 °C. These compounds find diverse applications in , , and due to their reactivity and . As Lewis acid catalysts, borate esters promote efficient bond formation from carboxylic acids and amines under mild conditions, offering sustainable alternatives to traditional agents with low mass intensity. In , they serve as sulfur- and phosphorus-free antiwear additives in lubricants, forming protective tribofilms on metal surfaces to reduce and . Additionally, borate esters of polyols enable the formation of dynamic hydrogels for biomedical uses, such as and , leveraging pH-responsive crosslinking with , while their natural occurrence in plant metabolites underscores roles in biological signaling and prebiotic chemistry.

Structure and bonding

Molecular geometry

Borate esters possess the general formula B(OR)3B(OR)_3, where R represents an alkyl or aryl substituent, with the central atom bonded to three oxygen atoms derived from alcohol groups. This arrangement results in sp² hybridization of the atom, yielding a around the center. The O-B-O bond angles are approximately 120°, consistent with the symmetry of sp²-hybridized systems. The trigonal planar configuration leaves an empty p-orbital perpendicular to the molecular plane, rendering the boron atom electron-deficient and exhibiting Lewis acidity. This electronic structure facilitates Lewis acid-base interactions, where nucleophiles can donate electron pairs to the empty p-orbital, forming dative bonds and tetrahedral adducts. A representative example is , B(OCH3)3B(OCH_3)_3, which displays B-O bond lengths of approximately 1.36 Å and O-B-O angles near 120°. In chelating or polymeric borate esters, such as those involving diols or extended networks, the can deviate from trigonal planar toward tetrahedral coordination upon binding additional ligands, as seen in reaction intermediates or stabilized complexes. These variations arise from the boron's ability to expand its through dative bonding, influencing reactivity while maintaining the core of the parent ester.

Nomenclature and classification

Borate esters, also known as boric acid esters, are systematically named under IUPAC recommendations as trialkoxyboranes or triaryloxyboranes, reflecting their general formula B(OR)₃ where R represents alkyl or aryl groups. For example, the compound with three ethyl groups is designated triethoxyborane, though the retained common name triethyl borate is widely used for such simple derivatives. Similarly, the aryl analog with phenyl substituents is named triphenoxyborane or triphenyl borate. Classification of borate esters is based on structural variations, distinguishing monomeric forms from oligomeric and polymeric structures. Monomeric borate esters correspond to the simple orthoborate type B(OR)₃, typically formed with monohydric alcohols or , exhibiting a discrete trigonal planar geometry around . Oligomeric borate esters arise with polyols, such as diols, leading to cyclic structures like five- or six-membered rings (e.g., borate esters of ), or trimers known as metaborate esters with the B₃O₃(OR)₃ containing boroxine rings. Polymeric forms occur in cases involving multifunctional alcohols, resulting in extended networks through bridging oxygen atoms. A key distinction in classification separates borate esters B(OR)₃ from boronic esters RB(OR')₂, the latter featuring a carbon-boron bond and thus categorized under organoboranes rather than simple esters of . Substituent effects influence naming, with alkyl (e.g., tributyl borate) prefixed by the specific chain (n-, iso-, etc.) and aryl borates using arene names like cresyl for substituted phenyl groups. Historically, borate esters were commonly referred to as "boric acid esters" or "ortho borates," a convention originating from their derivation via esterification of , which evolved toward the more precise systematic proposed by the IUPAC Commission on Inorganic Nomenclature in the mid-20th century. This shift emphasized the parent structure over acid ester analogies, aligning with chemistry's unique features.

Physical and chemical properties

Physical characteristics

Borate esters, particularly simple alkyl variants such as , typically exist as colorless s at due to their monomeric nature and weak intermolecular forces. For instance, (B(OCH₃)₃) is a water-white with a of 0.915 g/cm³ and a of 68°C, reflecting its low molecular weight and volatility. Similarly, triisopropyl borate (B(OCH(CH₃)₂)₃) appears as a colorless with a of 0.815 g/mL at 25°C and a of 139–141°C. These compounds exhibit high solubility in organic solvents such as , , and acetone, attributed to their nonpolar character and ability to form monomeric solutions, while their in is limited, leading to upon contact. Low-molecular-weight borate esters are notably volatile, often possessing a fruity or characteristic ester-like ; for example, has a of 18 kPa at 25°C, and triisopropyl borate shows 76 mmHg at 75°C, enabling facile purification by . Spectroscopically, borate esters display characteristic absorption bands for the B–O stretch in the range of 1310–1350 cm⁻¹, confirming the presence of trigonal boron-oxygen bonds. In ¹¹B NMR , the trigonal boron centers resonate at chemical shifts typically between 15 and 20 ppm; trimethyl borate, for example, shows a signal at 18.5 ppm in CDCl₃. This trigonal planar geometry around also contributes to the low viscosity observed in these liquids.

Reactivity and stability

Borate esters are highly susceptible to , undergoing reaction with water to regenerate and the corresponding alcohols according to the equation: B(OR)3+3H2O→B(OH)3+3ROH\mathrm{B(OR)_3 + 3H_2O \rightarrow B(OH)_3 + 3ROH} This process proceeds via of water to the electrophilic center, followed by stepwise elimination of alcohol without cleavage of the C-O bond. The rate of hydrolysis varies with the alkyl chain length, occurring rapidly for methyl borate (complete in under one minute) and more slowly for longer chains like n-butyl or n-amyl (equilibrium in approximately two hours), with equilibrium constants decreasing from 15.81 for methyl to 1.805 for n-amyl. Hydrolysis is accelerated under acidic or basic conditions, though it proceeds even in neutral media, underscoring the inherent reactivity of the boron-oxygen bonds. In environments, borate esters remain stable, enabling their handling and storage without decomposition. As trivalent boron compounds, borate esters exhibit Lewis acidity due to the empty p-orbital on , though weaker than that of trialkylboranes owing to donation from oxygen lone pairs. This acidity facilitates coordination with s, forming tetrahedral adducts where the nucleophile binds to , increasing its to four. For instance, borate esters react with to form neutral adducts such as B(OR)3⋅NH3\mathrm{B(OR)_3 \cdot NH_3}, mimicking transition states in enzymatic inhibition. Similar coordination occurs with other Lewis bases like or anions, promoting further reactivity such as or stabilization. Borate esters demonstrate good thermal stability for straight-chain alkyl derivatives, which can be distilled at elevated temperatures without , while branched or tertiary variants decompose as low as 100°C to yield olefins, alcohols, and boron-containing fragments. Upon heating above approximately 200°C, they generally decompose to and alcohols, with sensitivity to sometimes leading to partial or in oligomeric species under controlled humid conditions. Regarding behavior, borate esters resist due to the stable +3 of but can be converted to trialkylboranes under specific conditions using excess organolithium or Grignard reagents, which displace the alkoxy groups stepwise.

Synthesis

Preparation from boric acid

Borate esters are primarily synthesized in laboratory and industrial settings through the esterification of with alcohols, following the general equilibrium reaction: B(OH)3+3ROH⇌B(OR)3+3H2O\mathrm{B(OH)_3 + 3 ROH \rightleftharpoons B(OR)_3 + 3 H_2O} where R represents an . This reversible process favors the reactants due to the weak acidity of and the stability of , necessitating the continuous removal of to drive the reaction forward. Commonly, is employed using solvents such as or , which form low-boiling azeotropes with , allowing its separation while refluxing the alcohol back into the reaction mixture. Yields for simple primary alcohols can reach 87–92% under optimized conditions. The reaction conditions vary based on the alcohol type. For primary alcohols, heating the mixture of and excess alcohol (typically 4–5 equivalents) in the presence of a Dean-Stark apparatus or equivalent setup facilitates efficient water removal at temperatures around 80–110°C. , such as with , is occasionally used to enhance the rate, particularly in starting from (sodium tetraborate decahydrate) as the source, where the acid liberates . However, tertiary alcohols present significant challenges due to steric hindrance around the hydroxyl group and a propensity for to alkenes under heating, resulting in low yields or incomplete esterification without specialized water removal techniques like columns with immiscible entrainers. A representative example is the synthesis of tributyl borate (B(OC₄H₉)₃) from and n-. In a typical procedure, 124 g (2 mol) of is mixed with 666 g (9 mol) of n- in a flask equipped for . The mixture is heated to distill the -water (b.p. ~91°C) at a rate of 90–100 mL/h for 3–3.5 h, with dried returned to the pot periodically. Heating continues until the vapor temperature rises to 110–112°C, indicating water removal completion. Excess is then removed by (b.p. 103–105°C at 8 mmHg), and the residue is redistilled to afford 400–425 g (87–92% yield) of tributyl borate as a colorless (b.p. 114–115°C at 15 mmHg). Purification relies on under reduced pressure to separate the product from any unreacted materials. Industrial production often scales this esterification using as a cost-effective source, reacting it with alcohols under acidic conditions to generate in situ, followed by azeotropic dehydration. This approach has been refined since the mid-20th century for large-scale manufacture of alkyl borates, such as , with annual productions exceeding thousands of metric tons for applications in and flame retardants.

Alternative synthetic routes

One alternative route to borate esters involves the reaction of with alcohols under inert atmospheric conditions to minimize by moisture. The process follows the stoichiometry BCl₃ + 3ROH → B(OR)₃ + 3, liberating as a byproduct, and is conducted at low temperatures such as -80 °C in solvents like for sensitive substrates. This method affords high yields, particularly with aryl alcohols like phenol to form triphenyl borate. Transesterification provides another approach, enabling the exchange of alkoxy groups between borate esters or with excess alcohol. A representative example is the reaction of with a higher alcohol: B(OMe)₃ + 3ROH → B(OR)₃ + 3MeOH, where the volatile is readily distilled off to drive the equilibrium forward. This route is advantageous for incorporating complex or sterically hindered substituents, as it leverages the and differences of the alcohols involved. Recent developments include microwave-assisted esterification and reactions in ionic liquids, which facilitate removal and improve yields under milder conditions compared to traditional heating. These routes generally proceed faster than direct esterification from by avoiding prolonged dehydration steps, but they necessitate careful handling of toxic and reactive intermediates like BCl₃, which poses safety and equipment challenges. equilibria resemble those in the method but allow greater flexibility in substituent selection.

Applications

Role in organic synthesis

Borate esters serve as effective protecting groups for diols, particularly in chemistry, where they form reversible complexes with cis-diol moieties, allowing selective manipulation of other functional groups. These complexes, often cyclic, are stable under basic conditions but can be readily deprotected via mild acidic , enabling orthogonal protection strategies without disrupting sensitive glycosidic bonds. For instance, isopropylidene borates derived from and 1,2-diols in sugars provide temporary masking that tolerates subsequent or steps. In cross-coupling reactions, trialkyl borate esters act as boron sources for borylation, facilitating the preparation of organoboron intermediates used in Suzuki-Miyaura couplings. Lithium trialkyl borates, generated from organolithium reagents and trialkyl borates like triisopropyl borate, undergo with catalysts to form arylboronate species that couple with aryl halides, yielding biaryls in high yields under mild conditions. This one-pot lithiation-borylation-coupling sequence avoids isolation of unstable boronic acids and has been applied to diverse heteroaryl systems, enhancing efficiency in pharmaceutical synthesis. Their Lewis acidity contributes to activation in these catalytic cycles. Borate esters also function as catalysts in reactions, promoting ester exchange in the synthesis of polyesters and other macromolecules by coordinating to carbonyl oxygens and facilitating nucleophilic attack. The mechanism proceeds via a tetrahedral intermediate, where the lowers the activation barrier for alcohol addition to the , enabling equilibrium shifts toward desired products under solvent-free or mild heating conditions. Borate-zirconia composites, for example, catalyze the of β-ketoesters with high selectivity, minimizing side reactions in chain extension. Recent developments have expanded the utility of chiral borate esters in asymmetric synthesis, particularly as ligands or catalysts for enantioselective reductions. Spiroborate esters derived from 1,1'-bi-2-naphthol and anhydrides promote the borane-mediated reduction of prochiral ketones and imines, achieving enantiomeric excesses up to 99% through a chair-like that directs delivery. In Chan-Lam couplings, borate-derived boronic esters have been employed post-2000 for N-arylation of amines, with catalysis enabling mild conditions for C-N bond formation from alkylboronates. These advances underscore the versatility of chiral borates in constructing stereocenters for synthesis.

Industrial and material uses

Borate esters serve as effective retardants in various polymeric materials, particularly in plastics like (PVC), where they enhance fire safety by promoting char formation during . This mechanism involves the of the ester to release , which acts as a barrier to heat and oxygen, suppressing spread and reducing production. Triaryl borates, such as triphenyl borate, are particularly valued for their compatibility with halogenated polymers, often synergizing with to achieve V-0 ratings without excessive generation. In the ceramics industry, borate esters function as precursors in sol-gel processes for producing boron-doped glasses and hybrid materials. For instance, triethyl borate (B(OCH₂CH₃)₃) is hydrolyzed alongside tetraethoxysilane to form silica-borate hybrids, enabling precise control over incorporation and resulting in materials with tailored thermal expansion and refractive indices suitable for optical and electronic applications. These sol-gel-derived borosilicate glasses exhibit high homogeneity and , facilitating their use in advanced coatings and fibers. Alkyl borates are widely employed as anti-wear additives in lubricants and fuels, particularly engine oils, where they form protective tribofilms on metal surfaces under high-pressure conditions. By reacting with metallic surfaces to generate complexes, these esters reduce and while improving oxidation stability, extending oil life in automotive and industrial applications. Representative examples include trialkyl borates derived from C8-C12 alcohols, which demonstrate superior performance compared to traditional dialkyldithiophosphates in reducing . Borate esters have emerged as key additives in advanced battery electrolytes, particularly for lithium- and calcium-metal batteries, as of 2024. Fluorinated borate esters enhance salt dissolution and ionic conductivity, forming stable solid-electrolyte interphases that improve cycle life and safety in high-voltage systems. For example, borate anion receptors enable reversible Ca-I₂ electrolytes for calcium-organic batteries, addressing challenges in multivalent transport. In biomedical materials, dynamic boronate ester hydrogels, developed through 2023-2024 research, offer pH-responsive crosslinking for and . These reversible networks, formed from borate esters and polyols like , provide self-healing properties and controlled release, advancing applications in and . The market for borate esters has seen notable growth in applications since 2015, driven by patents for bio-based variants synthesized from renewable alcohols such as those derived from vegetable oils and bioethanol. These sustainable additives align with environmental regulations by reducing reliance on petroleum-derived feedstocks, with innovations focusing on enhanced biodegradability and lower toxicity in lubricants and polymers. For example, boron-containing esters from epoxidized fatty acids have been patented for use as multifunctional additives, supporting the expansion of eco-friendly formulations in the global chemicals sector.

Safety and toxicology

Health hazards

Borate esters generally exhibit low to moderate via oral exposure, with representative examples showing LD50 values exceeding 2,000 mg/kg in animal models. For instance, the oral LD50 for monomethyl ether borate ester in rats is greater than 2,000 mg/kg, while for , it is reported as 6,140 mg/kg. These compounds can also cause skin and eye irritation upon direct contact, attributed to their rapid to and alcohols in moist environments, leading to local inflammatory responses. Chronic exposure to borate esters may result in accumulation, which has been associated with , including reduced quality and infertility risks in occupationally exposed individuals. and similar borate esters are classified under the European Union's Classification, Labelling and Packaging ( as reproductive toxicants (Category 1B) with hazard statement H360FD (may damage ; may damage the unborn child). A study of workers in a boric acid production facility in , with mean blood levels of 1.69 mg/L, demonstrated statistically significant decreases in concentration, , and morphology compared to controls, supporting links to impaired at elevated exposure levels. The U.S. (OSHA) establishes a (PEL) of 15 mg/m³ (total dust) for , the primary product, as an 8-hour time-weighted average to mitigate such risks. Inhalation of volatile borate esters poses risks of irritation, with symptoms ranging from coughing and throat discomfort to more severe in high-concentration scenarios. Pre-1990s industrial accident reports involving compounds, including esters used in synthesis processes, documented acute respiratory distress in exposed workers, often exacerbated by poor ventilation. Under the European Union's , and (, is classified as acutely toxic dermal (Category 4) with hazard statement H312 (harmful in contact with skin), rendering it harmful but generally less corrosive than halides like . protocols emphasize immediate medical consultation: for , avoid inducing and administer or milk if conscious; for skin contact, wash thoroughly with soap and ; for eye exposure, flush with for at least 15 minutes; and for , relocate to fresh air while monitoring for breathing difficulties.

Environmental impact

Borate esters exhibit rapid in aqueous environments, breaking down primarily into and alcohols, which contributes to their overall biodegradability. This process occurs readily under neutral to basic conditions, minimizing persistence in water bodies and facilitating . However, the resulting can lead to boron accumulation in , particularly in arid or semi-arid regions where leaching is limited, potentially affecting growth and soil microbial communities over time. Aquatic toxicity of simple borate esters is generally low, with acute LC50 values exceeding 500 mg/L for fish species such as and EC50 values greater than 100 mg/L for , indicating minimal direct harm to most aquatic organisms at typical environmental concentrations. Nonetheless, boron-sensitive ecosystems, including certain freshwater and in boron-limited habitats, may experience subtle chronic effects from prolonged exposure to hydrolysis products. Solubility variations influence dispersal in , but overall, borate esters pose limited risk to broad aquatic populations due to their transformation into less persistent forms. Industrial waste management for borate ester production involves treating effluents through chemical to remove , often using calcium or magnesium compounds to form insoluble borates that can be separated and disposed of safely. In the , REACH regulations implemented since 2007 require registration, evaluation, and risk assessment of compounds, including limits on emissions to to prevent environmental release exceeding safe thresholds, typically aligned with standards below 1 mg/L total . These measures ensure controlled discharge and reduce ecological risks from manufacturing activities. Sustainability initiatives post-2020 have focused on developing recyclable borate ester-based materials, leveraging dynamic bonds in polymers for applications like vitrimers and adhesives that enable reprocessing without loss of performance. These green manufacturing approaches, often incorporating bio-based feedstocks, aim to minimize waste and promote principles in industries such as coatings and composites. Efforts include hybrid systems for efficient , reducing reliance on virgin resources and lowering the overall environmental footprint of borate ester use.

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